Macroscopic impacts of cloud and precipitation processes on maritime shallow convection as simulated by a large eddy simulation model with bin microphysics

摘要

This paper discusses impacts of cloud and precipitation processeson macrophysical properties of shallow convective clouds as simulatedby a large eddy model applying warm-rain bin microphysics. Simulationswith and without collision–coalescence are considered with cloud condensationnuclei (CCN) concentrations of 30, 60, 120, and 240 mg. Simulationswith collision–coalescence include either the standard gravitationalcollision kernel or a novel kernel that includes enhancements dueto the small-scale cloud turbulence. Simulations with dropletcollisions were discussed in Wyszogrodzki et al. (2013) focusingon the impact of the turbulent collision kernel. The current paperexpands that analysis and puts model results in the context ofprevious studies. Despite a significant increase of the drizzle/rainwith the decrease of CCN concentration, enhanced by the effects ofthe small-scale turbulence, impacts on the macroscopic cloud fieldcharacteristics are relatively minor. Model results show a systematicshift in the cloud-top height distributions, with an increasingcontribution of deeper clouds for stronger precipitating cases. Weshow that this is consistent with the explanation suggested inWyszogrodzki et al. (2013); namely, the increase of drizzle/rainleads to a more efficient condensate offloading in the upperparts of the cloud field. A second effect involves suppression ofthe cloud droplet evaporation near cloud edges in low-CCN simulations,as documented in previous studies (e.g., Xue and Feingold, 2006).We pose the question whether the effects of cloud turbulence ondrizzle/rain formation in shallow cumulican be corroborated by remote sensingobservations, for instance, from space. Although a clear signal isextracted from model results, we argue that the answer is negativedue to uncertainties caused by the temporal variability of theshallow convective cloud field, sampling and spatial resolution ofthe satellite data, and overall accuracy of remote sensing retrievals.