First Assessment of Cloud-Land Coupling in LASSO Large-Eddy Simulations

To enhance our understanding of cloud simulations over land, this study provides the first assessment of coupling between cloud and land surface in the Large-Eddy Simulation (LES) Atmospheric Radiation Measurement Symbiotic Simulation and Observation (LASSO) activity for the shallow convection scenario. The analysis of observation data reveals a diurnal cycle of cloud-land coupling, which co-varies with surface fluxes. However, coupled (or decoupled) cumulus clouds are inadequately simulated, manifesting as a too-high (or low) occurrence frequency during the afternoon. This discrepancy is mirrored by the overestimated cloud liquid water path and cloud-top height. These overestimations are linked to the overpredicted boundary-layer development and the easier trigger of shallow convection misrepresented in LES runs. Our study underscores the need to improve the representations of boundary-layer processes and cloud-land interactions within LES to better simulate shallow clouds in the future. Cloud-surface coupling is crucial in weather and climate predictions, yet its processes, particularly over the land surface, remain poorly understood. This study examined the evolution of coupled cloud occurrence frequency and cloud macro-physical properties [e.g., cloud fraction, liquid water path (LWP), and cloud-top height (CTH)] in LASSO Large-Eddy Simulations (LESs) for 77 shallow convection case days against observations. We found that LES runs erroneously simulate a too-high occurrence frequency of coupled cumulus clouds during the afternoon. In accord with that, shallow convection development is also markedly overestimated during the same period, manifested by overestimated cloud LWP and CTH. These errors are possibly linked to the misrepresentation of a key physical process, namely, the shallow convective trigger, which needs to be improved to achieve more accurate cloud simulations. Observed cloud-land coupling has notable diurnal variations but is inadequately simulated by Large-Eddy Simulations (LESs) LESs overestimate the frequency and vertical extent of coupled cumuli in the afternoon These overestimations by LESs are related to the misrepresentation of triggering shallow convection