SHORTWAVE RADIATIVE CLOUD FORCING IN THE TROPICAL PACIFIC INCLUDING THE 1982-1983 AND 1987 EL-NINO

The 'cloud radiative forcing' concept has been used extensively to stu dy radiative effects of clouds on the earth-atmosphere system. Until n ow it has been applied primarily to the top of the atmosphere (TOA). I t is of interest to apply it to the surface, where the absorbed radian t energy regulates the surface temperature. In this study we derived a nd compared the shortwave cloud forcing (SWCF) at the surface of the t ropical Pacific during two El Nino years (1982-1983 and 1987) with two regular years (1984 and 1985). The surface SWCF during an El Nino yea r averaged over the entire tropical Pacific region was found to exceed that of a normal year by 10 W m(-2). The areas with strongest SWCF in El Nino years are located in the 'warm pool' region, where they can e xceed those of normal years by 40 W m(-2). To evaluate the range of po ssible error in these estimates, the SWCF was derived from several ava ilable sources of information, such as: satellite methods driven with the International Satellite Cloud Climatology Project (ISCCP) Cl data; Earth Radiation Budget Experiment (ERBE) information at the TOA; and the National Meteorological Center (NMC) model output, both at the sur face and at the TOA. Because the clear-sky component of the radiative flux that reaches the surface is affected by the amount of precipitabl e water in the atmosphere, a comparison was made between different sou rces of such information (e.g. European Centre for Medium Range Weathe r Forecasts (ECMWF); TIROS-N Operational Vertical Sounders (TOVS); and the Special Sensor Microwave/Imager (SSM/I)). The impact of such diff erences on the SWCF was estimated using LOWTRAN 7. We were also able t o examine two widely discussed 'greenhouse effects' hypotheses during El Nino conditions. One was proposed by Lindzen, suggesting possible d ecrease of the atmospheric greenhouse effect as a result of drying of the upper atmosphere during intensified direct thermal circulation. Th e other was proposed by Ramanathan and Collins, regarding the 'thermos tat' effect of clouds, preventing the rise of ocean surface temperatur e above 305 K. During the April 1987 peak El Nino period, Lindzen's hy pothesis was found to be valid, and a relationship between the sea-sur face temperature and the surface SWCF was obtained similar to that fou nd by Ramanathan and Collins for the SST and TOA SWCF.