Tesis Doctorals - Departament - Física Aplicada

URI permanent per a aquesta col·leccióhttps://hdl.handle.net/2445/106785

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Variability and Source Apportionment of Ultrafine Particles in Urban Europe
(Universitat de Barcelona, 2026-02-12) Garcia Marlès, Meritxell; Querol Carceller, Xavier; Alastuey, Andrés; Universitat de Barcelona. Departament de Física Aplicada
[eng] Atmospheric particulate matter (PM) is recognised as a major air pollutant with significant impacts on human health and the Earth’s climate. The nature and severity of its effects depend, among other factors, on particle size. Ultrafine particles (UFPs), defined as particles with diameters below 100 nm, contribute only a small fraction of PM mass concentrations (µg m-3) but represent a substantial portion of particle number concentrations (PNCs, # cm-3). Particle number size distributions (PNSDs) categorise particle into three size modes: Nucleation (< 25 nm), Aitken (25–100 nm) and accumulation (100–1000 nm). Due to their nanoscale size, UFPs can penetrate deep into the respiratory tract and translocate through the alveolar barrier into the bloodstream, reaching other organs. However, evidence on their health impacts remain inconsistent, likely due to the scarcity of long-term UFP datasets, the lack of standardised monitoring methods, the strong spatial variability of UFPs, and the diversity of sources contributing to PNCs in different regions. Currently, UFPs have no legally binding standards at the EU or global levels. The new Ambient Air Quality Directive (European Union) 2024/2881 introduced UFPs into European legislation for the first time. Although binding limits are not yet defined, it mandates monitoring of PNC at urban supersites, rural supersites, and pollution hotspots such as traffic-dense areas, airports, harbours, and industrial locations, while PNSD monitoring is required only at urban supersites, covering particles down to 10 nm. The goal is to obtain harmonised data to subsequently evaluate health associations, thereby supporting decisions on evidence-based air quality (AQ) management. The objective of this thesis is to improve understanding of the spatial and temporal variability and sources of UFPs in urban Europe. Long-term PNC and PNSD data (2009–2019) from twenty-nine European cities were compiled, harmonised, and analysed. Spatial variability and long-term trends of UFPs across diverse environments were investigated, and sources were identified and quantified through source apportionment (SA) analyses using Positive Matrix Factorisation (PMF). In addition, aviation-related emissions near a major European airport were characterised, and meteorology-driven dispersion effects on source contributions were assessed. The results presented provide one of the most comprehensive assessments of UFP concentrations and SA across European urban environments to date. Harmonisation of PNSDs within the 10–800 nm size range allowed reliable inter-site comparisons and minimised artefacts arising from different lower size detection limits. PNCs increased progressively from Northern to Southern Europe, with substantially higher levels at traffic sites, pointing to road traffic as the dominant contributor to UFPs in cities. Long-term trend analyses (2009–2019) revealed marked decreases in Aitken and accumulation mode particles, driven by reductions in traffic-related emissions following successive European road traffic emission policies. In contrast, nucleation mode particles remained stable or increased slightly, influenced by new particle formation (NPF) events and non-traffic sources such as industry, shipping, and aviation. SA using PMF identified up to ten distinct factors contributing to PNCs, with road traffic as the major contributor, followed by photochemical nucleation and domestic heating. The source contributions varied a lot among cities, in both type and magnitude. Surprisingly, NPF contributions were not the highest in Southern Europe (with higher insolation and biogenic emissions of precursors) but in Central Europe, likely due to SO2 emissions from coal-fired power plants. Domestic heating was a major source of UFPs in a relevant number of cities. Aviation-related emissions were characterised separately in a dedicated case study at Brussels Airport, where aircraft operations were found to dominate local UFP levels and remained detectable several kilometres downwind. The application of dispersion normalisation in a Barcelona analysis (2013–2024) further improved SA results by separating emission-driven changes from meteorological variability. The 2013–2024 UFP decreasing trend was more pronounced than for 2013–2019, reflecting the effect of European, national, regional, and local measures on road traffic, the decrease of shipping emissions, and the COVID-19 lockdown. The insights derived from this work contribute to a better understanding of UFP behaviour in urban environments, support evidence-based monitoring recommendations, and inform targeted mitigation measures to improve AQ and protect public health across Europe.
Advanced microscopy for real-time imaging of biological samples
(Universitat de Barcelona, 2026-04-10) Alexandropoulos, Christos; Duocastella Solà, Martí; Universitat de Barcelona. Departament de Física Aplicada
[eng] A central challenge in modern optical microscopy is imaging biological processes that occur on very fast timescales. Ultimately, imaging speed is constrained by the available signal-to-noise ratio (SNR), which is typically low for biological samples, particularly in fluorescence-based imaging. Label-free techniques exist that obviate the need for fluorophores and provide image contrast of optically transparent samples – as in most biological specimens – by measuring the phase changes of light. In this case, a much higher SNR is available, enabling faster imaging, but imaging speed is still limited by technical bottlenecks. These include camera readout times in widefield implementations and the reliance on mechanical scanning in point-by-point detection schemes. As a results, current microscopes remain suboptimal tools to fully characterize dynamic processes as relevant as molecular diffusion, neuronal communications or the rapid evolution of microorganisms. This thesis focuses on the development of three optical strategies to overcome these speed barriers, with the goal of enabling ultrafast visualization of dynamic biological samples. Two of the systems are based on quantitative phase imaging (QPI), a label-free technique that measures how transparent samples shift the phase of light. Within QPI, the Transport of Intensity Equation (TIE) is a widely used approach: it reconstructs phase information from a set of slightly defocused images. However, acquiring those defocus planes normally requires either mechanically moving the sample or the objective, which is typically too slow for many biological applications. My first system addressed this limitation by introducing a tunable acoustic gradient (TAG) lens and pulsed monochromatic illumination to achieve electronic focus shifting at microsecond time scales. This eliminates the need for mechanical movement, enabling fast sequential acquisition of defocused images and, consequently, much faster TIE phase reconstructions. My second system tackles the same problem from a different angle: it uses chromatic aberration and pulsed red-green-blue (RGB) illumination so that a single snapshot from a color camera already contains the necessary defocus information in its color channels. This makes TIE essentially a single-shot method, achieving reconstructions at up to 150 frames per second. My third contribution goes beyond a specific imaging technique and directly addresses one of the main bottlenecks of any widefield optical microscope: slow camera readout time. In this case, instead of relying on cameras with millions of pixels and imaging rates of hundreds of frames-per-second (fps), I developed a frequency-encoded microscope which features a camera with a single pixel. Here, acousto-optic deflectors scan the sample with rows of light, each tagged with a unique modulation frequency. A single detector records the combined signal, which is then demultiplexed computationally. This bypasses the speed limits of cameras and, in principle, enables imaging at hundreds of thousands of fps — opening new possibilities for ultrafast studies, particularly in label-free systems with an intrinsically high SNR. Taken together, these three systems demonstrate a consistent theme: using acousto-optic devices and clever optical encoding to achieve speed gains that traditional microscopes cannot reach. They point toward a new generation of ultrafast microscopy tools, combining the quantitative insights of phase imaging with light-control strategies for characterizing biological samples with an unprecedented spatiotemporal detail.
Millora de la predicció i seguiment de la convecció per a la presa de decisions i alerta primerenca en entorns crítics
(Universitat de Barcelona, 2026-02-04) Esbrí Corbella, Laura; Llasat Botija, María del Carmen; Rigo, Tomeu; Universitat de Barcelona. Departament de Física Aplicada
[cat] Aquesta tesi aborda el seguiment i el nowcasting de tempestes convectives intenses en entorns crítics, amb especial atenció a l’Àrea Metropolitana de Barcelona (AMB) i als principals aeroports del nord d’Itàlia. El fil conductor és la utilització del producte radar de densitat de VIL (DVIL) com eina d’identificació i pronòstic de sistemes convectius. La DVIL combina l’estimació del contingut d’aigua integrat verticalment amb l’alçada de la columna com a magnitud optimitzada tant per a la diagnosi de la magnitud de la intensitat de la precipitació convectiva com pel seguiment i la predicció a molt curt termini (nowcasting). Malgrat no oferir una informació detallada sobre el desenvolupament vertical de les tempestes, la DVIL incorpora aquesta dimensió en un producte bidimensional de menor complexitat. Això el fa més fàcil de processar pels algoritmes que s’utilitzen, per exemple, en l’aviació, i en facilita les prediccions en temps quasi real. Aquestes prediccions poden ser molt útils en l'ajuda a la presa de decisions envers la trajectòria, aterratge i enlairament d'un avió o la gestió de la xarxa de drenatge d'una gran ciutat. El treball s’articula al voltant de quatre eixos principals: la definició de criteris òptims de detecció i seguiment; el desenvolupament i l’optimització de l’eina de seguiment i pronòstic RaNDeVIL; aplicant-lo a situacions de temps violent en els principals aeroports del Nord d'Itàlia, en tres casos d’estudi del 2019 a partir de les dades de la Rete Radar Nacionale (RRN) i observacions en superfície; l’anàlisi dels nius de tempesta a Barcelona i aplicació del RaNDeVIL a l'AMB, amb especial incidència en l'aeroport de Barcelona, a partir de dades radar del Servei Meteorològic de Catalunya (SMC), així com d'observacions en superfície incloent-hi la xarxa de pluviòmetres de Barcelona Cicle de l'Aigua S.A. (BCASA); i la dimensió de transferència de coneixement i cocreació amb la societat a través del Barcelona Living Lab on Extreme Events (BLLEE) del projecte I-CHANGE. En essència el RaNDeVIL consisteix en la identificació i seguiment dels nuclis de tempesta més intensos a partir de la DVIL per poder-los caracteritzar i realitzar el pronòstic de la seva evolució en els pròxims minuts (fins a 60 min). El primer requisit de l’algoritme és definir el llindar de DVIL associat a la convecció o intensitat del fenomen que es pretén identificar. En els entorns aeroportuaris del nord d’Itàlia, les proves de configuració van apuntar a llindars de DVIL d’1 g/m3 i una àrea mínima de 20 km2 pel seguiment òptim de les tempestes amb convecció profunda (a vegades associada a temps violent) dels casos d’estudi seleccionats. A Catalunya, en canvi, ha estat necessari relaxar els llindars d’intensitat (DVIL = 0.5 g/m3) i d’extensió (4 km2). Aquests llindars permeten capturar episodis convectius de precipitació intensa o amb temps violent sense fragmentar en excés el seguiment. La validació per objectes aplicada (MODE i mètriques d’àrea i distància entre centroides), mostra prediccions robustes fins a 18–30 minuts i útils fins a 60 minuts en alguns casos pel que fa a trajectòries, amb una degradació de l’habilitat a mesura que s’allarga l’horitzó. Sobre dades pel període 2014–2022 a l’AMB, el mòdul del tracking del RaNDeVIL assoleix una probabilitat de detecció amb anticipació de precipitació convectiva del 72.8%, amb un índex crític d’encerts de 0.53 i una taxa de falses alarmes del 33%, demostrant potencial com a predictor de convectivitat. A partir de la combinació dels valors de CAPPI 6-minutals (SMC), de la pluja en superfície 1-minutal (BCASA) i de les incidències (BCASA) ha estat possible identificar i classificar els episodis de precipitació significativa pel municipi de Barcelona. Les distribucions de centroides intraciutat identifiquen punts negres o àrees que concentren els valors de reflectivitat més elevats i es comparen amb aquelles que resulten d’aïllar els episodis amb incidències de drenatge a la ciutat. Els resultats mostren la combinació de les dades anteriors amb informació sobre inundacions i retards en l'aeroport de Barcelona han permès identificar els episodis que han tingut un major impacte en l’AMB. Aquests es caracteritzen mitjançant el mòdul de tracking del RaNDeVIL. L’anàlisi revela patrons clars: a l’estiu dominen trajectòries més heterogènies, amb centroides distribuïts terra endins, i un màxim horari ampli entre la matinada i primera hora de la tarda; en canvi, a la tardor predominen intensificacions de tempesta sobre el mar amb desplaçaments paral·lels a la costa i pics horaris de tarda. Operativament, el RaNDeVIL s’ha integrat satisfactòriament en una cadena de gestió d’arribades (AMAN, Arrival Manager) desplegada sobre l’aeroport de Milà-Malpensa pel cas d’estudi, aportant informació immediata i rellevant per a la gestió de seqüenciacions en presència de convecció. Alhora, el BLLEE ha demostrat que la combinació d’instrumentació de baix cost i processos participatius pot potenciar l’alfabetització dels impactes del canvi climàtic i risc hidrometeorològic, facilitant el retorn de coneixement i suggerint incorporar indicadors de percepció i criteris de confort en la planificació urbana i els protocols d’alerta. Les limitacions identificades a la tesi inclouen la fragmentació dels seguiments per manca de superposicions d’àrea (segons el llindar de DVIL establert) entre productes consecutius de radar, que comporten prediccions de molt curta durada, i les propagacions anòmales nocturnes. Aquestes obren línies futures concretes: incorporació d’una variació en la mida de l’àrea del seguiment (en funció de les tendències recents) i de la intensitat en l’extrapolació, combinació amb mòduls probabilístics o basats en aprenentatge automàtic, disseny de llindars adaptatius condicionats pel context i integració de fonts complementàries com descàrregues elèctriques i productes 3D per a diagnosi i verificació creuada. En conjunt, la tesi ofereix una nova proposta per millorar la predicció de tempestes intenses a alta resolució en entorns de decisió. Connecta diagnosi física, necessitats operatives i dimensió social, i contribueix a enfortir els sistemes d’alerta primerenca i la resiliència urbana davant la precipitació convectiva en escenaris mediterranis.
Selective contacts for crystalline silicon solar cells
(Universitat de Barcelona, 2023-07-13) Tom, Thomas; Bertomeu i Balagueró, Joan; López-Vidrier, Julià; Universitat de Barcelona. Departament de Física Aplicada
[eng] Global energy use has increased, causing a rapid shift in the climate. Renewable energy sources such as solar energy must replace fossil fuels to slow down this trend. In order to achieve this, the amount of power generated by solar energy must increase by a factor of 40 by 2050. As silicon solar cells account for 90% of the global photovoltaic industry, addressing their existing limiting factors such as efficiency and production cost can help to make a big difference. Therefore, the primary objectives of this thesis are to minimize or replace indium (In) in transparent conducting layers (due to rising In costs) and to develop efficient hole and electron transport layers utilizing more industrial and less expensive ways for silicon heterojunction (SHJ) solar cells. In efficient solar cells, the used transparent conducting oxides must exhibit high mobility and high infrared transparency. Fluorine-doped indium oxide (IFO) films were developed with DC sputtering, achieving a highest mobility of the films of 93 cm2V–1s–1 and an efficiency of 22.6% when used in SHJ solar cells. To reduce the usage of In, stacks containing IFO (with ever decreasing thickness) and transparent conducting aluminum-doped zinc oxide (AZO) on top were prepared, resulting in improved electrical and antireflection properties. In addition, In-free transparent conducting layers were developed using dielectric-metal- dielectric (DMD) structures based on AZO as the dielectric with an ultra-thin silver aluminum metallic interlayer. The optimized DMD structures were used as a front contact for n-type silicon solar cells by introducing a hole-selective vanadium pentoxide (V2O5) dielectric layer. Second, hole transport layers (HTL) for SHJ solar cells were fabricated using the industrially scalable technique of sputtering. Molybdenum oxide (MoOx) films were fabricated either employing reactive sputtering using a metallic target or using conventional sputtering from a ceramic target. Whereas the former led to SHJ solar cells exhibiting a power conversion efficiency (PCE) of 8.8%, the latter allowed for sub-stochiometric MoOx films when using a reducing hydrogen atmosphere and applying temperature to the substrate. Finally, organic dipole interlayers as electron transport layers (ETL) were developed in dopant-free SHJ solar cells. Polyethylenimine (PEI), Poly(amidoamine) (PAMAM) dendrimer, and Deoxyribonucleic Acid (DNA) were the selected organic layers, and the spin coating technique was adopted to fabricate these films. Among the different benefits exhibited by the polymeric layers, three of them must be highlighted: (i) increased interface passivation completely eliminated Fermi level pinning at the metal/semiconductor junction; (ii) the dipole layers demonstrate a significant shift in work function of more than 1 eV in certain cases; (iii) the contact resistance was dramatically decreased, accompanied by an increase of PCE up to 13.8%, 14.5% and 15.5%, when using PEI, PAMAM and DNA as dipole layers, respectively. In sum, high-efficiency SHJ solar cells with high-mobility transparent conductors were achieved, and the electron selective contacts were successfully replaced by organic molecules. Furthermore, the potential to utilize these transparent layers and organic compounds in various electronic devices extends far beyond solar cell applications and should be seen as an attractive trend.
Anàlisi i caracterització de la precipitació i dels camps de vents en zones d’orografia complexa: La Cerdanya (Pirineu Oriental)
(Universitat de Barcelona, 2023-07-13) Rosell Puig, M. Àngela; Bech, Joan; Fàbrega Gallego, Cristian; Universitat de Barcelona. Departament de Física Aplicada
[cat] Aquesta tesi, relacionada amb el projecte ATMOUNT (Análisis de las interacciones atmósfera-superficie en zonas de montaña y el conocimiento de los impactos del cambio global), i més concretament en el subprojecte ATMOUNT-III (Ondas de gravedad precipitación orografica y procesos asociados a las áreas de montaña), se centra a la zona Pirinenca, concretament a la Vall del Segre a la Cerdanya (Pirineu Oriental). La motivació i procediment es resumeixen seguidament de forma abreujada. Totes les circulacions atmosfèriques en orografia complexa depenen de molts factors, un dels principals és la situació sinòptica. Aquesta hi pot influir notòriament quan no és de calma, produint circulacions de vent que típicament no es donen quan no existeix aquesta situació. Una manera de dur a terme aquest estudi en una vall és estudiar les dades estació per estació segons la seva situació, ja sigui en el fons de la vall o en els vessants, amb la missió d’entendre les circulacions de vent globalment, tenint en compte no tan sols la situació sinòptica sinó també l’hora del dia i el dia de l’any. Per tant, s’han estudiat generalment les circulacions de vent a la Vall del Segre a la Cerdanya amb dades històriques de 2006 a 2015, ambdós inclosos i pels mesos de desembre de 2016, gener, febrer, març i abril del 2017 amb les dades de la campanya Cerdanya-2017. Amb les dades històriques s’ha fet un breu estudi de precipitació on queda reflectit que a Das hi precipita menys que a la resta de les estacions de la vall. L’aplicació de diferents mètodes estadístics a l’hora de relacionar simultàniament el mòdul i la direcció del vent amb les estacions ha permès demostrar que treballar amb la màxima resolució temporal possible és fonamental a l’hora d’obtenir unitat en els resultats tractant les dades amb qualsevol dels mètodes. L’estudi de quatre episodis, un anticiclònic i tres amb precipitació, ha permès explicar una de les circumstàncies del perquè a Das hi precipita menys.
Classifying synthesized optical codes using polarimetric information and machine learning algorithms for optical security applications
(Universitat de Barcelona, 2023-01-20) Ahmadi, Kavan; Carnicer González, Arturo; Universitat de Barcelona. Departament de Física Aplicada
[eng] Given the increasing use of the internet and the transfer of information in this era, it is crucial to focus on encryption and data security. According to the technological advances in optics and photonics and their multiple applications, many researchers have been urging to apply optics to encrypt and authenticate information in the last decades. In other words, optical waveforms involve many complex degrees of freedom, such as polarization, amplitude, phase, large bandwidth, quantum properties of photons, and multiplexing that can be combined in many ways to produce high-security information systems. In this thesis, we have been investigating different photonics techniques appropriate for optical security applications. This interdisciplinary investigation includes photonics techniques such as digital holography, beam shaping, Fourier optics, polarization optics, diffractive imaging system, and interferometry. Besides, our applied approaches demanded extensive research in computational methods such as pattern classification by means of machine learning algorithms, computer simulation, fringe analysis, statistical analysis, and binary encoding. However, despite the defined thesis title, our achievement has not been limited to optical security. Classifying synthesized (unique) optical codes can be mainly split into two categories. The first one is an approach for obtaining unique optical codes. The second one is a method or technique for classifying and distinguishing synthesized optical codes. Regarding the first category, in this thesis, on the one hand, we propose a method to obtain unique optical codes (polarimetric signature codes) from illuminating 3D printed samples by linearly polarized beams. Also, the ability of 3D printed samples to be considered as Physical Unclonable Functions based on polarimetric information is discovered in this thesis. Hence, we consider 3D printed samples as physical keys able to produce unique polarized optical codes. On the other hand, we obtain unique polarized optical codes by synthesizing a laser beam at the entrance pupil of a highly focusing system. Accordingly, we developed a binary approach for encoding character codes into holographic cells appropriate for transferring information in free space. Regarding the second category, on one side, we classify the polarimetric signature codes obtained by a physical key (3D printed sample) by means of the Support Vector Machine classifier using feature vectors extracted from statistical analysis on speckle patterns. On the other side, we introduce polarimetric mapping images as multidimensional arrays to be inputs of a convolutional neural network model for the autodetection of character codes obscured in the longitudinal component of a highly focused electromagnetic field. This approach might be considered as an alternative method, which eliminates the necessity of phase retrieval algorithms in particular cases.
Precipitation phase discrimination: diagnosing and nowcasting
(Universitat de Barcelona, 2022-05-30) Casellas Masana, Enric; Bech, Joan; Universitat de Barcelona. Departament de Física Aplicada
[eng] Precipitation phase discrimination at ground level constitutes a fundamental variable in many meteorological and hydrological applications, including avalanche hazards, winter road safety, and flooding from rain on snow events. Discrimination of the precipitation phase at surface level has been widely studied following different approaches ranging from decision tree algorithms based on vertical temperature profiles parameters, to machine learning algorithms through schemes relying on microphysical parameterisations. However, precipitation phase discrimination is still challenging, specially at temperature close to freezing point. Several studies pointed out research gaps regarding this topic and the present thesis aims to make its small contribution to some of them. In addition, this thesis comes from the need to provide the Meteorological Service of Catalonia with an adjusted and verified precipitation phase discrimination product for diagnosing and nowcasting purposes. In order to achieve both kind of requirements six specific objectives were set and upon which this thesis was structured. These are the following: • SO1. Obtention of a dynamic interpolation scheme suitable for complex terrain, and high spatial and temporal resolution. • SO2. Evaluation and adjustment of different schemes and meteorological variables to diagnose discrimination of the surface precipitation phase. • SO3. Assessment of citizen science and crowd sourced observations for monitoring snow events. • SO4. Development and evaluation of different schemes to nowcast discrimination of the precipitation phase. • SO5. Evaluation of ensemble techniques to nowcast discrimination of the precipitation phase. • SO6. Implementation of a precipitation phase product in an operational chain. The present thesis is based in a compendium of three scientific publications and three major blocks were defined following each publication. The storyline of the thesis is first based on obtaining spatial surface information from point meteorological observations. Then, the spatial information is used to estimate precipitation phase for diagnosing purposes. And finally, include extrapolation techniques and numerical weather prediction models to nowcast the precipitation phase with a forecast lead time of 180 minutes. The first block of the thesis presents a methodology to interpolate high spatially and temporally resolved meteorological observations. Interpolation techniques have been widely studied and verified for daily and monthly observations, but limited for hourly or sub-hourly time scales. At these scales, observations tend to be more irregular and present higher variability as they are influenced by weather conditions, such as the presence of fog banks or thermal inversions. For this reason, an adaptive interpolation system was proposed. It is based on the combination of three elements: clustering, multiple linear regression, and residual correction. Meteorological observations are first divided in several clusters of variable size to separate areas prone to be affected by different weather conditions. A multiple linear regression is calculated for each cluster and then compared against an MLR that considers all data. It is in this step where the proposed system plays its role. The system, based on interpolation errors, decides which MLR uses in each cluster: that calculated using the stations of the cluster only or that using all stations available. The adaptive character of the system lays on using different number of clusters and test all them every time an interpolation is conducted. The system was successfully applied in three European regions, and results indicate a reduction of RMSE when the proposed interpolation system is used compared to using a single MLR considering all stations. Once the step to interpolate point meteorological observations is achieved, the thesis focuses on discrimination of the precipitation phase in the following two blocks. The second block evaluates different precipitation phase discrimination schemes based on surface observations for diagnosing purposes. These schemes set thresholds on meteorological variables upon which precipitation is classified as rain, mixed or snow. In order to perform the evaluation of the schemes around 7700 quality-controlled observations of precipitation phase were gathered from different sources concerning Catalonia. According to the verification results, the schemes including air saturation conditions perform best, that is wet bulb temperature or combining air temperature with relative humidity. When analysing the schemes for specific snowfall events, a certain variability among the optimum thresholds was identified. This lead to suggest a range of thresholds when monitoring snowfall events. In addition, apart from the quality-controlled observations, citizen science and crowd sourced observations were also collected and evaluated showing both advantages and limitations. The third block of the thesis is also focused on precipitation phase determination, but in this case for nowcasting purposes. Apart from considering surface precipitation phase discrimination schemes, algorithms based on vertical temperature profiles, which play a key role on determining precipitation phase at ground level, were also considered. According to the threshold and performance variability observed when diagnosing precipitation phase and based on previous studies, combinations of algorithms were also taken into account in this block. The performance of the different algorithms and their combinations was assessed in eight low-altitude snowfall events reported in Catalonia between 2010 and 2021. Verification results showed that a combination of algorithms is preferable as it may provide a wide perspective to forecasters during precipitation phase transitions. In addition, this block included the implementation of a probabilistic methodology to nowcast the precipitation field. The results obtained in the present thesis allowed to adjust and improve the real- time precipitation phase discrimination at the Meteorological Service of Catalonia. In addition, a nowcasting of precipitation phase product was also developed and operationally implemented. The results may also contribute to add a new verification dataset for precipitation phase discrimination purposes, together with the evaluation of precipitation phase schemes with interpolated meteorological variables and the development of spatially resolved products.
Holographic light shaping through acousto-optic deflectors
(2021-05-18) Treptow, Dorian; Martín Badosa, Estela; Montes Usategui, Mario; Universitat de Barcelona. Departament de Física Aplicada
[eng] The invention of the laser in the 1960s triggered a new era of optical technologies which revolutionized many fields of industry and research. At the same time, a new demand for technologies emerged that allow to dynamically control the optical properties of a laser beam and to shape laser light into arbitrary patterns. This need led to the development of spatial light modulators, which are programmable diffractive elements that shape laser light into well-defined intensity distributions through optical phase or amplitude modulation. Their capability to display computer generated holograms made them an indispensable tool in a broad range of optics and photonics applications, because it allows to reconstruct optical wavefronts from digital models without the need of physical counterparts. This most versatile control over an optical wavefront enables complex functionalities such as holographic beam shaping, aberration correction, adaptive optics applications and optical micro-manipulation, to name only a few. Various spatial light modulator technologies exist with inherent capabilities that define their possible applications. The high light efficiency and outstanding modulation capabilities of spatial light modulators based on liquid crystals (LC-SLMs) found a myriad of applications and represent the current standard for dynamic light modulation based on digital holograms. There are, however, several applications that require other capabilities than those offered by LC-SLMs. Ultra-fast light modulation, high laser power applications and high-quality pattern formation are difficult to achieve with liquid crystal devices, so that alternatives to this prevailing technology are needed. An interesting candidate for high performance spatial light modulation are acousto-optic deflectors (AODs). These devices are commonly used for high-speed beam deflection, but it has also been demonstrated in few scattered precedents that they are capable of holographic light modulation. Acousto-optic deflectors are based on a considerably different technology than LC-SLMs, which complicates their use as full-fledged spatial light modulators. But at the same time, they provide promising light modulation capabilities, such as very high optical power thresholds, modulation rates of several kilohertz and a continuous (non-pixelated) wavefront modulation, which potentially make them a complement or even competitor to existing spatial light modulator technologies. The scope of this thesis is the investigation and application of the holographic modulation capabilities of a spatial light modulator system based on acousto-optic deflectors. Such a system is realized in the frame of this work as a common optical Fourier transform setup with off the shelf hardware. The principal effort in the implementation is the development of calculation methods for acousto-optic holograms and the corresponding electronic driving signals, which on the one hand need to consider the specific capabilities and constraints of AODs, and on the other hand have to provide high light efficiencies of the displayed holograms and a high reconstruction quality of the formed patterns. Thus, the one-dimensionality of the acousto-optic modulation and the resulting separability restrictions for two-dimensional AOD modulation are discussed, and different reconstruction strategies for arbitrary two-dimensional patterns are investigated. Furthermore, various image degrading effects are analyzed, and corresponding correction methods are proposed. Especially the inherent reduction of coherent artifacts through the motion of acousto-optic holograms is analyzed in detail. The capabilities of such implemented acousto-optic SLM are demonstrated by reconstructing arbitrary laser patterns with very high perceived image fidelity. Through the combination of different correction methods, also high-speed reconstructions of separable patterns at rates of several kilohertz are achieved, which eventually come into play in a structured illumination microscope.
Tornadic events in the Iberian Peninsula and Balearic Islands: characteristics and environmental conditions
(Universitat de Barcelona, 2021-05-06) Rodríguez i Ballester, Oriol; Bech, Joan; Universitat de Barcelona. Departament de Física Aplicada
[eng] Tornadoes are the meteorological phenomenon which can produce the strongest surface wind on Earth, causing damage, injuries and fatalities. Several studies show that severe storm environments could be more frequent due to global warming, especially during the second half of the century. Thus, it is necessary to deep on tornado knowledge. The main aim of this thesis is to study tornadic events in the Iberian Peninsula and Balearic Islands both to characterise them and to describe favourable conditions for tornadogenesis. The work is divided into three different blocks. In the first part, a methodology to conduct wind damage field surveys for high-impact weather events of convective origin has been proposed. Moreover, orthophotographs have been used to identify areas affected by damaging winds, through the comparison of images taken before and after case studies. With both methods it is usually possible to know which phenomenon occurred (i.e. tornado or downburst) by studying the damage pattern, and to provide relevant data to complete severe weather databases. In the second part, a tornado and waterspout database for Catalonia is built up, containing 434 cases for the period 2000-2019. In addition, an analysis of characteristics of tornadic events is also performed. In the third part, sounding data has been used to describe different weather types in Catalonia, including days with tornadoes, by the calculation of several thermodynamic, kinematic and composite parameters. Furthermore, vertical temperature, humidity and wind profiles from tornado and waterspout events registered in the Iberian Peninsula and Balearic Islands between 1980 and 2018 have been analysed using ERA5 reanalysis Finally, Szilagyi nomogram has been tested to detect waterspout-related environments. Results may contribute to enhance the detection and analysis of damaged areas due to strong- convective winds in the Iberian Peninsula and Balearic Islands. The tools presented here help to build up robust and homogeneous databases, and the analysis of favourable conditions for tornadogenesis carried out may be useful for forecasting and surveillance tasks. The improvement of typical tornadic environments detection is especially interesting for the area of study, as contains some of the regions where tornadoes are the most frequent in southern Europe.
Cubiertas urbanas y comportamiento térmico en escenarios de temperaturas extremas: del dato al geoservicio
(Universitat de Barcelona, 2021-05-15) Gilabert Mestre, Joan; Llasat Botija, María del Carmen; Corbera Simon, Jordi; Universitat de Barcelona. Departament de Física Aplicada
[spa] Las tendencias de los episodios de temperaturas extremas en las ciudades están aumentando (en frecuencia, magnitud y duración) debido al cambio climático en interacción con el efecto urbano. Otro factor relevante es que más de la mitad de la población mundial reside actualmente en áreas urbanas y se espera que esta tendencia demográfica aumente hasta un 68% a mediados de siglo. Ante esta creciente urbanización, los posibles impactos del cambio climático en las zonas urbanas se han convertido en una de las principales preocupaciones y, a su vez, un reto. Las morfologías urbanas y las propiedades térmicas de los materiales utilizados para construirlas son factores que influyen en la variabilidad climática espacial y temporal y se convierten en uno de los principales motivos de la singularidad urbana. El principal impacto en el microclima de las ciudades se caracteriza generalmente por el fenómeno de isla de calor urbano, que se refiere a que las áreas urbanas tienden a ser más cálidas que sus alrededores periurbanos y rurales, particularmente durante la noche, a consecuencia del calentamiento diferencial debido a su propia morfología y materiales. Se espera que el calentamiento global amplifique esta vulnerabilidad térmica, haciendo que los habitantes de las ciudades estén más expuestas a sufrir patologías asociadas a las elevadas temperaturas. Barcelona y su área metropolitana constituyen un buen ejemplo de megaciudad costera mediterránea (ciudades portuarias con una población superior a 1 millón de habitantes) y ya están siendo gravemente afectadas por estos efectos de acumulación de calor y al mismo tiempo de contaminación lumínica, acústica y en particular la atmosférica asociada a la actividad socioeconómica en un ecosistema con muy alta densidad de población y movilidad. El objetivo general de la tesis consiste en entender y modelizar la complejidad del comportamiento térmico urbano como herramienta de soporte a la toma de decisión. Una complejidad singular que modifica el clima provocando, en nuestras latitudes, una elevada vulnerabilidad a las altas temperaturas que se agravará debido al cambio climático global. Un segundo objetivo asociado al hecho de que se trata de un doctorado industrial, consiste en la elaboración de productos en base a las propiedades térmicas de las cubiertas urbanas y a los distintos escenarios de temperatura, transferibles como geoservício preoperacional o productivo al Institut Cartogràfic i Geològic de Catalunya. En primer lugar, se presentan diferentes metodologías para evaluar el efecto urbano y periurbano sobre la exposición a temperaturas extremas en Barcelona. Para ello se parte de la clasificación de Zonas Climáticas Locales o Local Climate Zones (LCZ) como enunciado base. Las LCZ consisten en un sistema de estandarización propuesto por Stewart y Oke (2012) para áreas urbanas y periurbanas clasificadas según sus respuestas térmicas. Cada categoría o uso del suelo es medible y comparable a partir de una combinación de parámetros geométricos, térmicos, radiativos y metabólicos que la caracterizan. En esta clasificación, las propiedades definidas de cada uso del suelo están directamente ligadas a una perspectiva térmica que permite estudiar los efectos del clima urbano con más detalle espacial y temporal. Las LCZ cartografiadas para nuestra región de interés, se introdujeron como entrada del modelo climático UrbClim, de alta resolución (100x100m), para crear series de temperaturas diarias (mediana y máxima) para el verano (1987-2016), con el fin de elaborar una cartografía de extremos asociada a las altas temperaturas. Utilizando la relación entre la mortalidad asociada a temperaturas elevadas y la distribución de la temperatura, se obtuvo la exposición al calor para cada LCZ que se combinó con datos poblacionales con el fin de crear mapas de vulnerabilidad climática. Todo ello se aplicó para el período climático observado y para finales de siglo 2071-2100 teniendo en cuenta los escenarios RCP 4.5 y RCP 8.5. En segundo lugar, y aprovechando las cartografías obtenidas de las LCZ, el trabajo aquí presentado se centró en mejorar los estándares propuestos en el ADN de las LCZ de Stewart y Oke (2012). La finalidad era la de mejorar los parámetros de entrada del modelo de dosel urbano WRF BEP+BEM y así mostrar la efectividad de los techos fríos y la vegetación en la reducción de la temperatura en el Área Metropolitana de Barcelona. Para hacerlo posible, se creó un flujo de trabajo centrado en la obtención de parámetros urbanos tales como el consumo de energía de los edificios y el calor antropogénico generado por los sistemas de aire acondicionado, la geometría de los cañones de las calles y los edificios, así como las características de la superficie (por ejemplo, albedo, capacidad calorífica, emisividad, fracción urbana y de vegetación). Los métodos se validaron para la ola de calor ocurrida en julio de 2015 durante la cual las temperaturas se mantuvieron entre 30 y 40°C durante cinco días consecutivos. Luego se simularon tres escenarios potenciales: 1) aumentando el albedo de cubiertas a 0.85 para ciertas clases urbanas, 2) aumentando el verde urbano en 255.64 ha adicionales de acuerdo con la propuesta del Plan Director Urbanístico de 2030 con dos esquemas de riego diferentes y 3) combinando las dos estrategias de mitigación (1 y 2). Los resultados, han permitido comprender muy bien el detalle del entramado urbano de nuestra región de estudio. Se han integrado en este estudio capas cartográficas de geoinformación que nos ha permitido comprender y modelizar íntegramente el comportamiento térmico, ya sea desde un enfoque climático o meteorológico. Gracias a estas mejoras en las cubiertas urbanas y sus propiedades, se ha podido caracterizar el entramado con el objetivo de poder hacer propuestas de mitigación y mejorar la resiliencia de la ciudad (disminuir su vulnerabilidad) en un entorno de cambio climático presente y futuro.
Radar-based nowcasting of severe thunderstorms: A better understanding of the dynamical influence of complex topography and the sea
(Universitat de Barcelona, 2020-07-20) del Moral Méndez, Anna; Llasat Botija, María del Carmen; Rigo, Tomeu; Universitat de Barcelona. Departament de Física Aplicada
[eng] Natural disasters of hydro-meteorological origin are the biggest risk worldwide. In Catalonia (NE of the Iberian Peninsula), severe weather and flash floods occur each year, resulting in major damage to property, losses in agriculture, and also of human lives. To reduce its impact, we need to improve the early warning systems and storm short-term forecasting. There’s a need to gain in-depth knowledge of severe thunderstorm dynamics, since the current accused conditions of global warming can impact in factors triggering these storms. The main objective of the present thesis is to enhance the knowledge of severe storms dynamics and to improve their identification and monitoring in real time, in order to help prevent their surface effects on the citizens. The project addresses the unresolved problem of storm anomalous motion, as it becomes a great challenge to predict their evolution and impact in the next few hours. For this purpose, the area of Catalonia has been chosen as the study region of this project, due to the proximity to the sea and complex topography, which are often key factors in varying the weather at a local scale. There is also the advantage of having good radar coverage, which will be the essential tool for characterizing storms. We first propose a methodology that identifies potentially convective days from daily cumulative rainfall fields, selects them to search for storms, and determines if their motion is anomalous. We have found that the area with the highest convective activity between 2008-2015 in Catalonia was located in the eastern Pre-Pyrenees, due to the possible creation of a convergence line. It has also been identified that there are more convective structures with possible anomalous propagation in summer and spring, with the main patterns being related to splitting, merging, stationarity and elongated storms. Once the study sample is defined, we have developed an algorithm to improve the identification and tracking of these thunderstorms, especially those with anomalous propagations. The keys of improvement have been based on proposing new techniques in the three main modules; 2D, 3D identification and tracking. In addition, it incorporates alerts before possible cell splitting or merging. These changes have shown that the algorithm is able to faithfully reproduce storm life cycle, correctly identify in advanced anomalous motion, and correctly distinguish storms in highly dense convective situations. The algorithm has been verified first over 30 severe cases, proving that it can identify anomalous movements with a mean 30-min lead-time, being the splitting, the easiest one to do. It has also been demonstrated a good ability at not only identifying these movements but also separating cases with and without anomalous motion. On the other hand, the algorithm has demonstrated a good performance in cases of heavy rainfall on a Catalan flood-prone coastal area of touristic interest. It is identified that storms are usually organized in convergence lines, and that topography and the sea play a very important role, whether affecting the movement, the time of exposure, or the amount of precipitable water causing flash floods. Finally, the dual-Doppler technique is applied in Catalonia for the first time. This allows getting complete information of the internal dynamics of a thunderstorm, without the need of running idealized models, and then, getting to know the local topographic influence on the evolution and organization. It is demonstrated that the complex local topography changes and/or amplifies the wind flow inside and near thunderstorms, modifying completely their life cycle and their possible interactions with their neighbor cells. It is also shown that this qualitative improvement into storm-scale dynamic knowledge can improve the nowcasting techniques and the early warning systems in the future.
Flood damage assessment in two western Mediterranean regions. Present conditions and future scenarios
(Universitat de Barcelona, 2020-01-21) Cortés Simó, Maria; Llasat Botija, María del Carmen; Turco, Marco; Universitat de Barcelona. Departament de Física Aplicada
[eng] Flooding is one of the main natural hazard in the world causing huge economic and human impacts. Assessing the flood damage in the Mediterranean region is of great importance, mainly due to its pronounced sensitivity to climate change. A large number of floods affecting the western Mediterranean region of study are surface water floods that can cause catastrophic damage. These floods are caused by intense precipitation events, thus, in order to understand properly these type of events, the analysis of the relationship between precipitation and flood damage is crucial. The overall objective of this thesis is to analyse flood damages in two Mediterranean regions, namely Catalonia and the Valencian Community, frequently affected by intense precipitation events, as well as to estimate their changes when future climate change projections and different socioeconomic scenarios are considered. To do this, the relationship between heavy precipitation and flood damage estimates from insurance datasets, provided by the Spanish Insurance Compensation Consortium (Consorcio de Compensación de Seguros, CCS), have been analysed. Other than for the above mentioned two areas of study, this analysis was performed for the Metropolitan Area of Barcelona (MAB), a highly vulnerable urban area. The study period covers 1996-2015. Several regression models have been tested in order to gauge the probability of large damaging events occurring given a certain precipitation amount and taking into account other variables related to the exposure of the territory. Results have shown that generalized linear mixed models are the most appropriate tool for studying the relationship between precipitation and flood damage. The probability of a damaging flood event increases with precipitation and population of the basin. Moreover, 30-min precipitation data proved to be a better predictor of the probability of large damages than daily precipitation, however, this type of information is not always available. On the other hand, we have analysed the projected changes in precipitation extremes in the Iberian Peninsula considering the Representative Concentration Pathway (RCP) 8.5 scenario and an ensemble of seven EURO-CORDEX simulations spanning the period 1976-2100. In order to do this, different climate indices were calculated to estimate the changes in precipitation assuming global warming scenarios of 1.5, 2 and 3 °C above preindustrial levels. Results show a general decrease of the total annual precipitation and an increase in the length of dry spell in most of the Peninsula. This increase accentuates with higher levels of global warming and during summer months. In terms of heavy precipitation, we have found increases with global warming in the maximum 1-day and consecutive 5-day precipitation indices as well as in the number of days with precipitation exceeding 40 mm, especially during the months of autumn and winter and in the north and north-east of the Iberian Peninsula. Finally, changes in the probability of occurrence of damaging flood events have been assessed for both regions when considering a global warming of 1.5, 2 and 3 °C and taking into account different climate projections and socioeconomic scenarios. To do this, the previously developed statistical climate model that links precipitation, population and flood damage estimates, has been used to assess future climate conditions. The daily precipitation data from the seven climate models used previously and population projections based on five different socioeconomic scenarios (Shared Socioeconomic Pathways, SSPs) were incorporated into the model. Results have shown a general increase in the probability of a damaging event for most of the cases and in both regions of study, being higher in the case of Catalonia. This change is usually larger when greater warming is considered and for higher percentiles of damage. Moreover, the increase in probability is larger when both climate and population changes are included. Our findings highlight that, when it comes to flood damage analysis, it is crucial not only to account for climate change but also consider socioeconomic conditions. Furthermore, results show that limiting global warming is a must in order to minimise the consequences of flood events in the study area.
Mathematical work on the foundation of Jones-Mueller formalism and its application to nano optics
(Universitat de Barcelona, 2019-12-19) Kuntman, Ertan; Arteaga Barriel, Oriol; Canillas i Biosca, Adolf; Universitat de Barcelona. Departament de Física Aplicada
[eng] Jones matrix and nondepolarizing Mueller matrix are the basic elements of the calculus of polarization optics. In this thesis we discus other forms that can be used to represent optical properties of deterministic systems. We investigate four different forms that we interpret as the states of deterministic optical systems. Vector state |h⟩ is the basic element of our formalism. Coherent parallel combination of deterministic optical systems can be most conveniently expressed as a linear combination of vector states. In other words, any nondepolarizing optical system can be considered as a coherent combination of other deterministic systems that serve as basis systems. Vector states are not suitable for representing serial combination of optical systems, because |h⟩ vectors cannot be multiplied as |h1⟩|h2⟩|h2⟩···. We observe that there exists a matrix state Z that mimics all properties of Jones matrices. Z matrices are also akin to the nondepolarizing Mueller matrices by the relation, M=ZZ∗. We show that Z matrices transform the Stokes matrix S into another Stokes matrix S′ according to the relation, S′=ZSZ†, where S corresponds to the Stokes vector |s⟩ and S′ corresponds to the transformed Stokes vector |s′⟩ (|s′⟩=M|s⟩). Z matrices also transform Stokes vectors, |s⟩ into complex vectors| ̃s⟩,| ̃s⟩=Z|s⟩. It can be shown that| ̃s⟩vectors bears the phase introduced by the optical system. We observe that |h⟩vectors and Z matrices are different representations of quatenion states h. Quaternion states can be added or multiplied to yield new quaternion states, therefore they are suitable for representing any coherent combination of deterministic optical systems. Z matrix and quaternion formulations are especially useful for describing the emergence of depolarization effects. But, density matrix approach is more convenient when we want to find the original constituents of a depolarizing Mueller matrix. Density matrices that associated with deterministic (pure) optical systems are defined in terms of |h⟩vectors as H=|h⟩⟨h|. A depolarizing Muller can be written as a convex sum of nondepolarizing Mueller matrices. The associated H matrix (density matrix of the mixture) can also be written as a convex sum of density matrices corresponding to the pure component systems. It can be shown that if there exists some knowledge about the anisotropy properties of component systems it is possible to find the nondepolarizing Mueller matrices of original constituents uniquely by means of the rank conditions of Hmatrices. Weapply our formalism to several phenomena. We study the interference effects in a Young’s double slit experiment with complete polarimetric methods. We show that our formalism can be useful for the analytic formulation of interacting dipole systems. We apply the vector state decomposition method to analyze plasmon hybridization, Fano resonances and circular effects in chiral and achiral geometries.
Z-scan methods for ultrashort pulsed laser microprocessing of transparent materials
(Universitat de Barcelona, 2019-10-15) Caballero Lucas, Francesc; Serra Coromina, Pere; Fernández Pradas, Juan Marcos; Universitat de Barcelona. Departament de Física Aplicada
[eng] The use of femtosecond lasers has recently gained attention as a result of the recognition to Gérard Mourou and Donna Strickland with the award of the Nobel Prize in Physics 2018 "for their method of generating high-intensity, ultra-short optical pulses". The innumerable areas of application of ultrashort laser pulses have not yet been completely explored, but their possibilities for accessing the microworld are considered highly valuable. Following this spirit, the objective of this thesis consisted in proposing and implementing feasible solutions to the challenges involved in the microfabrication of materials with ultrashort laser pulses for diverse advanced applications. To that end, attention was put in laser ablation of transparent polymers with spatial resolutions that transcend sharpness limitations due to light diffraction. The results presented here were obtained during the development of the doctorate studies of the PhD program in Nanosciences at the Departament de Física Aplicada of the Universitat de Barcelona. It is structured as follows: INTRODUCTION AND OBJECTIVES: This introductory chapter contains a description of the most significant microfabrication techniques, centered on laser-based methods. Having a key role in this thesis, the interaction between laser radiation and matter is shortly reviewed. The physical phenomena motivate the use of femtosecond lasers for the precise processing of transparent materials, where focus is put on their superficial laser ablation, the entailing challenges and its applications. The objectives pursued in this work close this first chapter. EXPERIMENTAL: A description of the experimental setups implementing femtosecond laser systems, methods and materials applied during the trials constituting the developed research is presented in this chapter. The features of the laser sources together with the corresponding laser direct-write setups form the sections of this chapter, followed by some comments on Gaussian beams and their focusing. These found the presentation of the z-scan focusing technique. To close this chapter, some remarks and background about the employed materials are delivered. Z-SCAN FOCUSING METHOD: The results obtained by putting to work the z-scan focusing technique introduced in the previous chapter are presented here. The theme is the development and characterization of the z-scan focusing technique as a method to address the issue of securing surface ablation of transparent material with femtosecond laser pulses. Its successful implementation in surface ablation of the transparent polymer polymethyl methacrylate (PMMA) with high spatial resolution is given as proof of the viability of the proposed strategy for a precise focusing of laser beams onto transparent materials. The contents of this chapter include studies on transmittance and reflectance measurements at different focusing distances between the laser beam waist and the processed material surface through single laser pulse surface ablation and laser surface scanning for channel microfabrication (comparing their results), the beam waist position determination thanks to the transmittance measurements and analysis of the produced surface ablation. APPLICATIONS IN LASER MICROFABRICATION OF MATERIALS: the implementation of the developed z- scan focusing technique was put to use in laser microfabrication of materials with diverse applications. The applications include the irradiation with femtosecond laser pulses of biodegradable polymers for profound hole ablation in polylactic acid (PLA) and study of the its influence in biodegradability of polylactic-co-glycolic acid, the laser perforation for leakage studies on medical use polypropylene bags, and the laser fabrication of microfluidic guides for conductive line printing. Owing to their diversity the chapter is divided in four sections, one for each topic. The various processed materials are briefly introduced, with some background supporting their study. CONCLUSIONS: The last chapter sums up the most relevant results and main achievements that have been obtained during the development of this thesis in the form of closing remarks.
Carbon nanotubes grown on stainless steel for supercapacitor applications
(Universitat de Barcelona, 2019-09-20) Pantoja Suárez, Luis Fernando; Bertrán Serra, Enric; Amade Rovira, Roger; Universitat de Barcelona. Departament de Física Aplicada
[eng] The ability of humans to study, manipulate and understand matter at the nanoscale has enabled us to develop materials that can combine physical, chemical, optical, magnetic and mechanical properties that bulk materials do not possess. One of the materials that triggered interest in the world of Nanoscience and Nanotechnology was carbon nanotubes (CNTs). These nanostructures had already been reported more than forty years ago, but it is not until the beginning of the 90s that Dr. Sumio Iijima manages to produce them under stable conditions in his laboratory. From that time onwards, the resources devoted to the research and production of these carbon-based materials were on the rise. Although today they do not capture the same scientific interest as they did until 2010, their importance in the scientific world and especially in the market is relevant. In fact, since the technology for the production of CNTs on an industrial scale has matured, they are found in an infinite number of applications, such as reinforcing polymers, acting as scaffolds for the growth of artificial tissue, in the manufacture of conductive inks or as part of new generation battery electrodes and supercapacitors. It is precisely in this last application that scientific interest has been focused with special attention. Together with other carbon-based materials, such as graphene, they are excellent support materials for materials with high capacitance. Research groups and companies around the world are spending a lot of resources to obtain electrodes that have a three-dimensional architecture at the nanoscale and whose specific surface is high. In that sense, the objective of this work was to synthesize CNTs on the surface of a flexible and conductive material: 304 stainless steel. We focused on optimizing the growth processes by plasma enhanced chemical vapor Deposition (PECVD) and water assisted chemical vapor deposition (WACVD) with and without the contribution of external catalyst material. In addition, as will be seen in the development of this work there is an important effort to understand the effects that thermal processes, necessary for CNTs growth, produced on the properties of steel. Especially the influence on corrosion resistance, since the final use of stainless steel CNTs is the manufacture of electrodes exposed to corrosive environments.
Precipitació i circulacions mesoescalars en zones d’orografia complexa
(Universitat de Barcelona, 2019-07-11) González Herrero, Sergi; Bech, Joan; Universitat de Barcelona. Departament de Física Aplicada
[cat] L’orografia té un impacte molt important en la circulació del vent i en la precipitació a totes les escales, des de la microescala fins a la macroescala. La complexitat i la singularitat de les interaccions entre el relleu i l’atmosfera fan que el seu estudi sigui complicat. En aquesta tesi es pretén aportar nous coneixements sobre aquestes interaccions, posant èmfasi en l’estudi de la microescala i la mesoescala. En la primera part s’ha realitzat una caracterització dels extrems absoluts de precipitació a Espanya a diverses escales temporals. S’ha estudiat que l’escalat d’aquestes dades és compatible amb una llei potencial del tipus R = a Db on R és la precipitació per a una pluja extrema de durada D. Els resultats mostren que la proximitat al mar i la influència orogràfica són elements claus en la localització d’aquests extrems a Espanya. En la segona part s’ha estudiat la precipitació hivernal i les circulacions de muntanya a la vall de la Cerdanya (Pirineus Orientals) des del punt de vista microfísic, utilitzant dades de sensors no convencionals, tant remots com in-situ. En concret, s’ha analitzat les mesures del disdròmetre Parsivel i del Micro Rain Radar (MRR) obtingudes durant la campanya experimental Cerdanya-2017, i s’han desenvolupat i aplicat noves metodologies per a l’anàlisi d’aquest tipus de dades descrivint i examinant els episodis de precipitació ocorreguts durant la campanya, tant de la neu com de la pluja des del punt de vista microfísic. En la tercera part s’ha estudiat com interacciona la circulació atmosfèrica a nivells baixos davant l’orografia costanera del litoral català. En concret, s’ha identificat i analitzat la resposta ageostròfica mesoescalar de l’aire en forma de corrents de densitat atrapades per la costa (CTDs en les seves sigles en anglès). Aquests estudis pretenen contribuir en l’aprofundiment del coneixement de les interaccions orogràfiques tant en la precipitació com en les circulacions mesoescalars induïdes pel relleu, de forma que a la llarga puguin contribuir a la millora les estimacions de precipitació en zones d’orografia complexa així com els models conceptuals i numèrics de predicció.
Implementation of the direct force measurement method in optical tweezers
(Universitat de Barcelona, 2018-12-18) Català i Castro, Frederic; Martín Badosa, Estela; Montes Usategui, Mario; Universitat de Barcelona. Departament de Física Aplicada
[eng] Mechanics is the branch of physics that studies movement and force, and plays an evident role in life. The swimming dynamics of bacteria in search of nutrients, organelle transport by molecular motors or sensing different kinds of stimuli by neurons, are some of the processes that need to be explained in terms of mechanics. At a human scale, distance and force can be measured with a ruler and a calibrated spring. However, assessing these magnitudes may become an important challenge at a micron scale. Among several techniques, optical tweezers stand out as a non-invasive tool that is capable of using light to grab micron-sized particles and measuring position and force with nanometer (10(-9) and femto-Newton (10(-15) accuracy. Small specimens, such as a bacterium or a cell membrane, can be trapped and effectively manipulated with a focused laser beam. Light momentum exchanged with the trapped sample can be used for eventually measuring the otherwise inaccessible forces that govern biological processes. Optical tweezers have enabled, after trapping cell vesicles in vivo, to measure the pulling force exerted by molecular motors, such as kinesin. Flagellar propulsion forces and energy generation have been investigated by optically trapping the head of a bacterium. Cell membranes have been deformed with optical tweezers and the underlying tension determined. However, the exact forces exerted by optical tweezers are difficult to measure beyond the in vitro approach. In order to calibrate the optical traps, the trapped samples often need to be spherical or present some degree of symmetry, it is important to bear information on the experimental parameters, and one needs high control of several variables that determine the trapping dynamics, such as medium homogeneity and temperature. A cutting-edge method, developed in the Optical Trapping Lab – BiOPT, from the Universitat de Barcelona, targets the light-momentum change as a direct reading of the force exerted by an optical trap. This frees experiments from the necessity of calibrating the optical traps, and makes possible to perform accurate force measurement experiments in vivo and involving irregular samples. In my PhD thesis, the direct force detection method for optical tweezers has been implemented and tested in some of such situations. I first give a technical description of the set-up used for the experiments. The use of a spatial light modulator (SLM) for holographic optical tweezers (HOTs), a piezo-electric platform to induce drag forces, and the trapping laser emission characteristics, are explained in detail. The light-momentum set-up is tested against certain situations deviating from the ideal performance and some steps for optimization of several effects are analyzed. Backscattering light loss is quantified through experiments and numerical simulations and finally assessed to account for an average ±5% uncertainty in force measurements. Then, the method is used to measure forces on irregular samples. First, arbitrary systems composed of microspheres of different kinds are collectively treated as irregular samples, in which the global momentum exchanged with the trapping beam coincides with the total Stokes-drag force. Second, pairs of optical tweezers are used to stably trap cylinders of sizes from 2 milimicras to 50 milimicras and measure forces in accordance with slender-body hydrodynamic theory. Another aspect of the thesis deals with the temperature change induced by water absorption of IR light, which is one of the major concerns within the optical trapping community. As main reasons, accurate knowledge of local temperature is needed for understanding thermally-driven processes, as well as eventual damage to live specimens. Here we use direct force measurements to detect changes in viscosity that are due to laser heating, and compare the results with heat transport simulations to discuss the main conclusions on this effect. The last goal of my thesis has been the implementation of the method inside tissue. The laser beam is affected by the scattering structures present in vivo, such as refractive index mismatches throughout different cells, nuclei, cell membranes or vesicles. As a primary result, despite the trapping beam is captured beyond 95%, I quantified this effect to result in an increase in the standard deviation of force measurements around ±20%. The approach has consisted in comparing the trapping force profiles of spherical probes in vitro (water) and in vivo (zebrafish embryos). To conclude, I here demonstrate that the direct force measurement method can be applied in an increasing number of experiments for which trap calibration becomes intricate or even impossible. Quantitative measurements become feasible in samples with unknown properties, the more important examples being arbitrary, non-spherical samples and the interior of an embryonic tissue.
Impacto de la asimilación radar en el pronóstico de precipitación a muy corto plazo usando el modelo WRF
(Universitat de Barcelona, 2018-11-08) Cáceres León, Richard Humberto; Codina, Bernat; Universitat de Barcelona. Departament de Física Aplicada
[spa] El propósito de esta investigación es maximizar el impacto de la asimilación de datos radar sobre el pronóstico de precipitación a muy corto plazo usando el modelo Weather Research and Forecasting (WRF), de tal manera que pueda ser implementado de modo operativo en los Servicios Meteorológicos Nacionales. En una primera fase se utiliza un episodio de precipitación extrema que tuvo lugar en Cataluña (NE de España) para probar varias configuraciones de predicción inmediata, basadas en el modelo WRF con la asimilación de datos radar, y en una segunda fase, se profundizan los resultados a través de una serie de experimentos basados en diez eventos de precipitación extrema ocurridos en Cataluña en el periodo 2015 - 2017. Las configuraciones consideradas se generan modificando: 1) el ciclo de inicialización del WRF, 2) los procedimientos de preprocesamiento de datos radar utilizados por el Centro Nacional de Investigación Atmosférica (NCAR) y por el Servicio Meteorológico de Cataluña (SMC), 3) datos convencionales y de radar asimilados dentro del sistema variacional tridimensional (3DVAR), y 4), otros parámetros tales como la escala de longitud de las observaciones, el número de bucles externos y la parametrización de la convección húmeda. El efecto de los anteriores parámetros se evalúa mediante la habilidad de una serie de experimentos para simular la cantidad y localización de la precipitación usando técnicas estadísticas convencionales, índices categóricos y el Receiver Operating Characteristic (ROC). Sin embargo, en este estudio la construcción de las tablas de contingencia está condicionada al desplazamiento del patrón de precipitación y la cercanía entre los valores observados y pronosticados, sin considerar los umbrales de precipitación. El método de control de calidad desarrollado por el SMC es confiable y en el caso particular del 12 de octubre de 2016, produjo mejores resultados que el método de NCAR. La predicción inmediata de precipitación logra mejores resultados cuando el modelo WRF es ejecutado con dos ciclos de asimilación, uno en frío y otro en caliente con un length scale de 0.75 y 0.50 respectivamente, asimilando en cada ciclo datos radar y datos convencionales en un mismo momento y con el error del background CV7. Los largos ciclos de inicialización, las actuales parametrizaciones usadas en el SMC y el uso de tres bucles externos no mejoran los resultados del pronóstico.
Precipitation measurements with polarimetric radio occultations
(Universitat de Barcelona, 2017-06-22) Padullés Rulló, Ramon; Cardellach Galí, Estel; Universitat de Barcelona. Departament de Física Aplicada
[eng] In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric (Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims to capture ROs using a two orthogonal linear polarization antenna. The experiment has been named Radio Occultations and Heavy Precipitation with PAZ (ROHP-PAZ). The objective is to measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced when heavy precipitation flattened raindrops are present in the ray-path. This effect, widely studied in weather radar community, will be measured from space using GNSS signals for the first time with PAZ, which is planned to be launched in 2017. The main objective of this new concept is to enhance the RO capabilities by providing vertical precipitation information along with the current standard RO thermodynamic products (i.e. temperature, pressure and moisture). Until now, no other observing system has been able to provide simultaneous thermodynamic and precipitation information under extreme conditions. The high vertical resolution, global coverage and all-weather capability properties of the RO observations combined with vertical indication of precipitation intensity can be of great value for heavy rain characterization, and therefore for climate and weather forecast and research. The theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. The theoretical basis has been developed combining electromagnetic propagation theory and cloud and precipitation microphysics. Forward scattering simulations at L-band have been obtained in order to relate the microphysics parameters with the expected Pol-RO observables. The feasibility has been addressed using coincident (in space and time) RO profiles and space-based precipitation observations. Such simultaneous observations allow for the characterization of actual RO measurements according to the coincident precipitation information. Finally, the applications have been investigated through realistic end-to-end simulations of the Pol-RO observations, which provide the anticipated Pol-RO products for different precipitation situations, regions, and seasons. Before the launch of the satellite, a field campaign has been conducted with the aim of starting the characterization of the polarimetric measurements. The engineering model of the PAZ antenna was placed at the top of a mountain peak in order to capture, for the first time, linear polarimetric GNSS signals at low grazing angle. This campaign has been useful to start identifying the hardware internal effects and unexpected precipitation features that will be affecting the Pol-RO observations. These effects have been incorporated to the simulations, hence providing valuable feedback to obtain more realistic Pol-RO products. Besides feedback, the data from the field campaign have shown the first observational evidence that precipitation and other hydrometeors induce a noticeable effect on the GNSS polarimetric signals. All these exercises yielded several relevant results. The noise level analysis from actual RO observations sensing precipitation scenarios has allowed to set a detectability threshold for the technique, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. Nevertheless, the integrated nature of the Pol-RO observable does not allow to distinguish between the contributions from the rain's intensity and extension, leaving an ambiguity in the provided product. In an attempt to solve such ambiguity, a tomographic approach has been proposed, which has yielded promising theoretical results. Moreover, it has been shown how the Pol-RO observables can be linked to physical precipitation parameters, such as the along-ray averaged rain rate, in a probabilistic way. The end-to-end simulation has also revealed that the ionosphere will induce a non-negligible depolarization, that will require calibration. Finally, the collocated data has shown the potential applications for Pol-ROs products.
A study of the shortwave schemes in the Weather Research and Forecasting model
(Universitat de Barcelona, 2017-02-09) Montornès Torrecillas, Alex; Codina, Bernat; Zack, John W.; Universitat de Barcelona. Departament de Física Aplicada
[eng] The radiative transfer cannot be explicitly resolved in the atmospheric models for two reasons: i) a full treatment of the radiative transfer equation (RTE) requires a high amount of computational resources and ii) the radiative transfer fields such as the optical thickness are not a direct solution of the Euler equations and hence, they must be parameterized as a function of the meteorological fields. Consequently, the physical processes related with radiation are simplified and approximated in physical schemes. In the particular case of the solar radiation, the use of these parameterizations were reduced for many years to represent the day/night cycle inside the model. Therefore, the accuracy of the solar schemes was left in the background and the computational resources were prioritized. With the growth of the solar energy industry during the last decade, a paradigm shift has occurred. Now, the solar irradiance (i.e. global horizontal GHI, direct horizontal DHI and diffuse DIF) becomes an important product for resource assessment as well as for forecasting applications. The main objective of this thesis is the identification and quantification of the sources of error that have a direct or an indirect contribution to the accuracy of the solar schemes, particularly, in those available in the Weather Research and Forecasting (WRF-ARW) model, widely used in the sector. First, the thesis presents a review of the set of physical approximations considered in six solar parameterizations available in the WRF-ARW model: Dudhia, Goddard, New Goddard, Rapid Radiative Transfer Model for General Circulation Models (RRTMG), Climate Atmospheric Model (CAM) and Fu-Liou-Gu (FLG). The sources of error are limitations in the representation of the radiative transfer as a conse- quence of the set of approximations assumed by one scheme. In this thesis three sources of error are analyzed: i) errors due to the vertical discretization of the atmosphere in a set of layers that are assumed to be homogeneous (truncation error), ii) errors due to the misrepresentation of the layer between the top of the model (TOM) and the top of the atmosphere (TOA), called TOM error and iii) errors due to the physical simplifications and parameterizations in the RTE, named physical error. In order to avoid the uncertainty introduced by the other components of the model, the source code of each one of the six solar schemes has been separated of the model and adapted for working with 1-dimensional vertical profiles. The studies of the truncation and TOM errors are performed by using ideal vertical profiles under four scenarios: a dry atmosphere, a wet cloudless sky, low water cloud and a high ice cloud. The results for the ETOM show that for the typical range of TOM values in mesoscale appli- cations (i.e. 10 hPa), the error with respect to a full atmospheric column is less than 0.5% and hence, the TOM error can be neglected. The analysis of the Etrun reveals that the sensitivity of the solar schemes on the vertical config- uration (i.e. number of vertical levels and their distribution) is directly related with the method used for the vertical integration of the multiscattering processes. For the typical mesoscale config- urations, the Etrun under clear-sky conditions is determined around 1.1%, 0.9% and 4.9% for the GHI, DHI and DIF, respectively. In both cloudy scenarios, the Etrun increases significantly, being more important for the high clouds. The Ephys is analyzed under clear-sky conditions using real soundings from the Integrated Global Radiosonde Archive data-set and comparing the irradiance outcomes with the Baseline Solar Radiation Network measurements. With the exception of Dudhia, the behavior for all the parameterizations is the same. A large overestimation of the DHI with a large underestimation of the DIF that leads to a near-zero bias for the GHI. Polar sites show the lowest errors with a mean MAE of 2.1%, 5.2% and 3.7% for GHI, DHI and DIF, respectively. Midlatitude sites show the worst results with a mean MAE of 3.4% in GHI, 11.6% in DHI and 7.8% in the DIF.

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