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SHORTWAVE RADIATIVE CLOUD FORCING IN THE TROPICAL PACIFIC INCLUDING THE 1982–1983 AND 1987 EL NIÑOs
Author(s) -
ZHOU LIHANG,
PINKER R. T.,
LASZLO I.
Publication year - 1996
Publication title -
international journal of climatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.58
H-Index - 166
eISSN - 1097-0088
pISSN - 0899-8418
DOI - 10.1002/(sici)1097-0088(199601)16:1<1::aid-joc990>3.0.co;2-5
Subject(s) - shortwave , environmental science , cloud forcing , international satellite cloud climatology project , atmosphere (unit) , climatology , shortwave radiation , radiative transfer , radiative flux , precipitable water , forcing (mathematics) , atmospheric sciences , satellite , radiative forcing , cloud cover , atmospheric radiative transfer codes , meteorology , geography , radiation , cloud computing , geology , water vapor , physics , aerosol , quantum mechanics , astronomy , computer science , operating system
The ‘cloud radiative forcing’ concept has been used extensively to study radiative effects of clouds on the earth‐atmosphere system. Until now it has been applied primarily to the top of the atmosphere (TOA). It is of interest to apply it to the surface, where the absorbed radiant energy regulates the surface temperature. In this study we derived and compared the shortwave cloud forcing (SWCF) at the surface of the tropical Pacific during two El Niño years (1982–1983 and 1987) with two regular years (1984 and 1985). The surface SWCF during an El Niño year 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 Niño years are located in the ‘warm pool’ region, where they can exceed those of normal years by 40 W m −2 . To evaluate the range of possible error in these estimates, the SWCF was derived from several available 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 surface and at the TOA. Because the clear‐sky component of the radiative flux that reaches the surface is affected by the amount of precipitable water in the atmosphere, a comparison was made between different sources of such information (e.g. European Centre for Medium Range Weather Forecasts (ECMWF); TIROS‐N Operational Vertical Sounders (TOVS); and the Special Sensor Microwave/Imager (SSM/I)). The impact of such differences on the SWCF was estimated using LOWTRAN 7. We were also able to examine two widely discussed ‘greenhouse effects’ hypotheses during El Niño conditions. One was proposed by Lindzen, suggesting possible decrease of the atmospheric greenhouse effect as a result of drying of the upper atmosphere during intensified direct thermal circulation. The other was proposed by Ramanathan and Collins, regarding the ‘thermostat’ effect of clouds, preventing the rise of ocean surface temperature above 305 K. During the April 1987 peak El Niño period, Lindzen's hypothesis was found to be valid, and a relationship between the sea‐surface temperature and the surface SWCF was obtained similar to that found by Ramanathan and Collins for the SST and TOA SWCF.