Sea surface temperature updating impacts on WRF simulations during a heatwave period

Abstract

This study investigates the effects of sea surface temperature (SST) updating strategy in the Weather Research and Forecasting (WRF) model during a heatwave event over the Northwestern Mediterranean Sea in July 2019. The MM5 revised Surface Layer and Yonsei University Planetary Boundary Layer (PBL) schemes were used and wind field at 10 m, air potential temperature, surface fluxes, and planetary boundary layer height were examined. Generally, the greatest impacts over the sea were observed within 20 km of the shoreline. We found an underestimation of the modeled SST in non-updated SST simulations during the heatwave episode that was propagated into the atmosphere, leading to a cold bias of 2-m potential temperature up to 2.5 K onshore. In heatwave conditions the most common surface layer stability class was very unstable, and its frequency increased when the SST was updated, particularly near the coast, revealing that SST updating leads to greater dominance of thermal turbulent mixing of surface fluxes in the heatwave period studied. Sensible and latent heat fluxes across stability regimes were analyzed, and latent heat flux showed greater sensitivity to SST updating and the highest magnitudes. However, PBL height variations between SST-updated and non-updated simulations presented a greater sensitivity to sensible heat flux. On average, during the heatwave period, the planetary boundary layer height in simulations with updated SST increased by 75 m onshore, compared to a smaller increase of 26 m offshore, which highlights the greater sensitivity in the onshore region and their impact on vertical modeled profiles. These results emphasize the importance of an accurate representation of boundary layer conditions on numerical weather prediction models, as well as illustrating the nonlinear behavior on the surface layer and PBL scheme, particularly important under heatwaves.