How changing cloud water to rain conversion profile impacts on radiation and its linkage to a better Indian summer monsoon rainfall simulation

Abstract

Effect of modifying cloud water to rain conversion factor above freezing level on radiation flux is studied. The linkage of cloud-radiation interaction with a better Indian summer monsoon (ISM) rainfall simulation has also been investigated. Two simulations were performed with Climate Forecast System version 2 (CFSv2), one with default revised simplified Arakawa-Schubert (RSAS) convection scheme and another with modified RSAS (RSAS-mod) in which the cloud water to precipitation conversion factor is revised. It is observed that both top of the atmosphere (TOA) and surface energy imbalances are improved in the RSAS-mod simulation. The major contribution seems to come from the enhanced reflected shortwave radiation at the TOA. In RSAS-mod, more shortwave radiation is getting reflected at the TOA and thus less shortwave is available to reach at the surface. It has happened as a consequence of enhanced planetary albedo in RSAS-mod. It is also observed that high cloud cover has increased in RSAS-mod and there is a decrease in low cloud cover. As a result of increased high cloud cover, outgoing longwave radiation has decreased significantly in RSAS-mod. It is also found from the multivariate empirical orthogonal function (MV-EOF) analysis that the composite of longwave cloud radiative forcing corresponding to phases 4 and 5 of boreal summer intraseasonal oscillation (BSISO), which signifies the active phase of ISM, is better simulated in RSAS-mod. This has helped to simulate a better seasonal rainfall.