Large-scale changes in the cloud radiative forcing over the Indian region

Based on earth radiation budget experiment (ERBE) data, earlier studies have shown that in tropical deep-convective regions there is a near cancellation between shortwave and longwave cloud radiative forcing and El Nino event affects the cloud radiative forcing in the Pacific Ocean. The present study investigates these features over the Indian region (0–30°N, 60–120°E) in the peak monsoon month July, (being a representative month of the southwest monsoon season) using satellite measurements of ERBE and International Satellite Cloud Climatology Project (ISCCP) cloud data during the period 1985–1989. It has been observed from the study that a unique imbalance is seen between shortwave cloud radiative forcing (SWCRF) and longwave cloud radiative forcing (LWCRF). It is found to be prominent when the magnitude of LWCRF is higher than 50 W m−2. Net cloud radiative forcing (NCRF) is highly negative in the Indian region from Arabian sea to Indochina and near zero in the equatorial Indian Ocean. The ratio N  , i.e., N=-SWCRF/LWCRFN=-SWCRF/LWCRF, is showing significantly higher values for all the years, the variation of observed N   against SWCRF also shows higher values with SWCRF during average of 1985–1989, 1987 and 1988 because it is more than (N=∼1)(N=∼1) that mentioned by Kiehl and Ramanathan (1990, Comparison of cloud forcing derived from the earth radiation budget experiment with that simulated by NCAR community climate model. Journal of Geophysical Research 95, 11679–11698) suggesting more imbalance between SWCRF and LWCRF and the lowering of the cloud top pressure particularly in 1987. Cloud radiative forcing (CRF) components are undergoing year-to-year variability with maximum magnitude in 1988 and minimum in 1987 similar to rainfall variation over the region, indicating an association between monsoon rainfall activity and CRF. Large-scale reduction occurred in the magnitudes of CRF and cloud physical properties in the Arabian Sea and south Bay of Bengal especially in 1987, and NCEP/NCAR-vertical velocity for 700 h Pa is also found considerably weak in 1987 in the Arabian Sea and Bay of Bengal i.e., the reduction in the vertical velocity might have affected the cloud formation and their vertical growth and ultimately changed the CRF. We attribute this reduction to the anomalous subsidence associated with the 1987 El Nino event.