Abstract
Abstract. Trade wind cumulus clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual clouds as well as the mesoscale organization on scales of O(10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulation experiments, the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization.
Highlights
The effect of precipitation on the formation of downdrafts and cold pools is, most well established for mesoscale deep convective systems for which details can be found in standard textbooks on cloud dynamics (e.g. Cotton et al, 2011)
The sensitivities to grid spacing and cloud microphysical assumptions are discussed in Sect. 5, and the paper ends with conclusions and an outlook for future research (Sect. 6)
We conclude that, in the regimes characterized by large-scale conditions similar to the forcing assumed for the RICO simulations, the formation of cold pools by evaporation of rain in the sub-cloud layer is the dominant feedback for the organization of precipitating trade wind cumulus
Summary
In this study we apply the University of California, Los Angeles large-eddy simulation (UCLA-LES) model which solves the Ogura-Phillips anelastic equations (Stevens et al, 1999, 2005). Following Savic-Jovcic and Stevens (2008) the cloud microphysical processes are parameterized in UCLA-LES based on the two-moment warm rain scheme of Seifert and Beheng (2001) with some refinements described in detail in Stevens and Seifert (2008, SS08 hereafter). The moister initial condition leads to a faster development of precipitating shallow convection and higher rain rates of roughly 1 mm d−1 compared to the drier case which rains only marginally, i.e, of order 0.1 mm d−1. This allows us to study these two different regimes using very similar model configurations
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