Towards a better understanding of shallow convection over land using ground-based observation and large-eddy simulation

It is very important and challenging for global climate models to represent a correct diurnal cycle of shallow cumuli over land. In this study, we build a new composite case for fair-weather shallow cumulus over land based on observational statistics. We aim to answer questions: 1) how well the can large-eddy simulation (LES) reproduce the observed cloud metrics? And 2) how well can the composite case represent the shallow cumuli regime compared with one “golden” day case study? 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 large-scale models on simulating continental shallow cumulus.

First, a case library of continental shallow cumulus clouds was established based on longterm observations at Southern Great Plains ARM site (Zhang and Klein 2010, 2013), which includes days of forced- and active-shallow cumulus clouds, and days of shallow convection that transits to deep convective precipitating events in late afternoon. Observational statistics was developed for each regime on cloud macrophysics, meteorological atmospheric and surface conditions. 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. Secondly, LES of the composite case is performed for active shallow cumulus clouds and it shows significantly less cloud fraction than observations. Sensitivity tests are then performed on grid resolution, domain size, microphysics scheme, surface fluxes and so on. LES is also done for each individual shallow cumulus day to investigate the possible non-linear effect due to the composite case. Thirdly, 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.

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-658121