Investigation of negative cloud radiative forcing over the Indian subcontinent and adjacent oceans during the summer monsoon season

摘要

Radiative properties of clouds over the Indian subcontinent and nearbyoceanic regions (0–25° N, 60–100° E) during the Asiansummer monsoon season (June–September) are investigated using the Clouds andEarth's Radiant Energy System (CERES) top-of-the-atmosphere (TOA) flux data.Using multiyear satellite data, the net cloud radiative forcing (NETCRF) atthe TOA over the Indian region during the Asian monsoon season is examined.The seasonal mean NETCRF is found to be negative (with its magnitudeexceeding ~30 Wm−2) over (1) the northern Bay of Bengal(close to the Myanmar–Thailand coast), (2) the Western Ghats and (3) thecoastal regions of Myanmar. Such strong negative NETCRF values observed overthe Indian monsoon region contradict the assumption that near cancellationbetween LWCRF and SWCRF is a generic property of all tropical convectiveregions. The seasonal mean cloud amount (high and upper middle) andcorresponding cloud optical depth observed over the three regions showrelatively large values compared to the rest of the Indian monsoon region. Usingsatellite-derived cloud data, a statistical cloud vertical model delineatingthe cloud cover and single-scattering albedo was developed for the threenegative NETCRF regions. The shortwave (SW), longwave (LW) and net cloudradiative forcing over the three negative NETCRF regions are calculatedusing the rapid radiative transfer model (RRTM) with the cloud vertical model asinput. The NETCRF estimated from CERES observations show good comparisonwith that computed using RRTM (within the uncertainty limit of CERESobservations). Sensitivity tests are conducted using RRTM to identify theparameters that control the negative NETCRF observed over these regionsduring the summer monsoon season. Increase in atmospheric water vaporcontent during the summer monsoon season is found to influence the negativeNETCRF values observed over the region.