Large Eddy Simulation of Continental Shallow Cumulus: Challenges and Insights

Description

A new composite case for active fair-weather shallow cumulus over land is built upon long-term observational statistics and set up for large eddy simulation (LES). There are three targeted questions: 1) How well can the LES generate the observed cloud metrics? 2) What are the key configurations and model physics for the LES to simulate a more realistic cloud statistics? 3) What are the dominant environmental factors that control the cloud macrophysics? We find that in LES, the cloud base is sensitive to the surface evaporative fraction. The onset time of first cloud is affected by the residual layer in the initial sounding due to daytime mixed layer development in the previous day. With the default bulk 1-moment microphysics scheme in System for Atmospheric Modeling (SAM), cloud fraction is significantly underestimated by LES, while a spectral bin microphysics improves the total fraction but still has difficulties to produce cloud deep enough. Particularly for active shallow cumulus clouds, in-cloud vertical velocity retrieval based on long-term radar measurements is used to derive cloud updraft and downdraft mass fluxes. Such data together with other cloud macrophysics statistics are compared against LES results. In addition, LES is used to study the effect of large-scale environmental controls, such as relative humidity and atmospheric stability, on the vertical extent of clouds, the transition between forced and active shallow cumulus and the transition from shallow to deep convection in the afternoon. These LES results, and the associated forcing and observations, will further serve as a benchmark test bed for the performance of the single column model version of CAM5 on simulating continental shallow cumulus. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-666347