Sensitivity Analysis of Outgoing Radiation at the Top of the Atmosphere in the NCEP/MRF Model

Z. Li, I. M. Navon

The outgoing radiation at the top of the atmosphere (TOA) determines the net radiative heating of the earth's surface and atmosphere system. Variabilities of the TOA outgoing radiation depend on features of the earth's surface and atmosphere. The fundamental goal of the present study is to quantify the sensitivity of the outgoing radiation to those features in a sophisticated radiation model used in the National Center of Environment Prediction (NCEP) medium range forecasting system. The technique of adjoint sensitivity analysis is employed.

The results obtained show that the TOA outgoing radiation is highly sensitive to spatial structures and temporal variations of water vapor, atmospheric and earth's temperature, and cloud cover. The outgoing short wave radiation variabilities are much more sensitive to the water vapor in the middle troposphere at high latitudes in the summertime hemisphere than at the tropical and subtropical areas. The outgoing long wave radiation variabilities are one order of magnitude more sensitive to specific humidity in the upper troposphere at mid- and lower altitudes than to that in other areas. In the tropics, more than 80\% of a perturbation increase in the earth's surface emission is trapped by the clear sky column atmosphere, while only about 60 to 70\% is trapped at middle and high latitudes; a rapid decrease band of trapping is found within the subtropics. For the outgoing short wave radiation, the lower clouds are more sensitive than the middle and high clouds. The most sensitive lower clouds tend to be located at mid-latitudes rather than in the tropical regions. Deep clouds and thin clouds display different impacts. For the outgoing long wave radiation, as expected, the high clouds are the most sensitive, but do not differ significantly from the middle clouds in the deep cloud case. The net radiation ( shortwave and longwave radiation) is most sensitive to lower clouds at mid-latitudes in the summertime hemisphere with a cooling effect on the atmosphere. Implications of the obtained results for understanding supper green house effect over tropical oceans and the influence of cloud feedback on the climate change are explored. Our analyses reveal some deficiencies in the model when the calculated results are compared with relevant observations.