Land–atmosphere coupling around ARM SGP site in the presence of non-precipitating shallow clouds

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Description

Continental shallow clouds are an important component of the land-atmosphere coupled climate system because of their impact on the energy and water cycle. The parameterization of these small clouds in climate models presents a significant challenge. In this study, we evaluate the potential impact of subgrid-scale (for climate models) land-atmosphere interactions on shallow clouds using large-eddy simulations (LESs) whose domain size is comparable to that of a typical climate model grid box. We compare LESs that allow for land-atmosphere interactions to ones that artificially suppress all or part of them by smoothing out surface heterogeneities within the domain. The LES domains are coupled to an interactive land surface model and nested inside a mesoscale domain to allow for realistic winds and surface conditions. Three cases of summertime non-precipitating shallow convection over the Department of Energy Atmospheric Radiation Measurement (ARM) Facility’s Southern Great Plains (SGP) site are examined. Low-level wind, soil moisture and cloud amount vary significantly among these cases. It is found that removing surface heterogeneities and the land-atmosphere interactions they induce in the LES domain leads to a consistent increase in cloud water content in all cases. Also, by comparing experiments where only surface flux heterogeneities induced by cloud shadows are removed to ones where all surface flux heterogeneities are removed, we show that the impact of cloud shading-induced surface heterogeneities on shallow clouds is dominant compared to that of static land surface heterogeneities in our cases. Furthermore, the impact of cloud shading on cloud water content and cloud size is found to depend sensitively on the solar zenith angle. The implications of our findings for shallow cloud and land surface parameterizations in climate models are also discussed.