Response of the current climate to the land-ocean contrast in cumulus entrainment

One of the largest uncertainties in modern climate models is the representation of cumulus entrainment, or the ingestion of environmental air through the lateral and top boundaries of cumulus clouds. Entrainment induces cloud dilution, which reduces buoyancy and the depth reached by ascending towers. This modification of cloud properties, in turn, impacts the global radiation and precipitation fields. Recently, studies have identified a sharp land--ocean contrast in cumulus bulk entrainment, with about 50% smaller entrainment over land. Motivated by that finding, we investigate the response of the current climate to the land-ocean contrast in cumulus entrainment using the High-Resolution Atmospheric Model (HIRAM). Climate simulations varying the parameterized entrainment rates (within the cumulus scheme) by 25% over land and/or ocean indicate that global circulation systems and precipitation patterns are substantially affected by the land-ocean entrainment contrast. Reduced entrainment rates over land facilitate deeper and more prevalent storms over the continents, enhancing precipitation there by a large margin. This increase is reinforced by a positive soil moisture-precipitation feedback, where increased precipitation enhances surface latent-heat fluxes and moist instability, which further invigorates the storms. Reducing the entrainment rate over oceans, in contrast, leads to a notable eastward shift in deep convection in the central and western Pacific, owing to the more ready development of deep convection within the Pacific easterly trade-wind belt. Among global circulation systems, the Pacific Walker circulation is found to be more sensitive to the land-ocean entrainment contrast than the Hadley circulation. It strengthen in response to reduced land entrainment and weakens in response to reduced ocean entrainment, the latter by 14%. These changes in global circulation systems are physically interpreted through an analysis of the dry potential temperature budget.