Séminaire
Stratocumuli are prevalent over the eastern boundary of the subtropical oceans (e.g. northeast and southeast Pacific). Due to their shortwave properties, these low clouds significantly impact the regional and global climate. However, shortwave fluxes in stratocumulus regions are often poorly simulated in climate models. In this presentation, we investigate in a first part the processes contributing to the simulated errors. We use the Weather Research and Forecasting (WRF) model to carry out a multi-parameterization analysis. Our results show that errors in shortwave fluxes are traceable to errors in liquid water path (LWP). Two mechanisms controlling LWP in our simulations are diagnosed. The first mechanism involves boundary layer and shallow cumulus schemes, which control moisture available for cloud by regulating boundary layer height. The second mechanism involves microphysics schemes, which control LWP through the production of drizzle. Our study demonstrates that when parameterizations are appropriately chosen, stratocumuli are reasonably well represented in WRF. Our results also highlight the importance of representing accurately the aerosol indirect effect on LWP for accurate shortwave fluxes simulations. In a second part, we investigate the possible anthropogenic changes in marine stratocumulus using a suite of dynamically downscaled experiments in the California region. Our results show a systematic decrease in low cloud cover, which is mostly imputable to a reduction of the coupling between boundary layer top and surface. Our analysis suggests that these changes are at least partially driven by the drying of the free troposphere in comparison to the boundary layer in future climate.