An Earth Outgoing Longwave Radiation Climate Model. Part II: Radiation with Clouds Included

Abstract

An Earth outgoing longwave radiation (OLWR) climate model was constructed for radiation budget studies. The model consists of the upward radiative transfer parameterization of Thompson and Warren, the cloud cover model of Sherr et al., and a monthly average climatology defined by the data from Crutcher and Meserve, and Taljaard et al. The required water vapor climatology was developed by Yang et al. Cloud top temperature was adjusted so that the calculation agreed with NOAA scanning radiometer measurements. Cloudy sky cases were calculated and discussed for global average, zonal average and worldwide distributed cases. The results agreed well with the satellite observations. Although the zonally averaged OLWR has a minimum in the tropics, this minimum is essentially contributed by a few very low flux regions. There are regions in the tropics where the OLWR is as large as that in the subtropics. Gradients of OLWR occur in the tropics that are as large as those from the tropics to the poles. In the high latitudes, where cold air contains less water vapor, OLWR is basically modulated by the surface temperature. Thus, the topographical heat capacity becomes a dominant factor in determining the seasonal variation. The two very different regimes of OLWR can be easily identified using the time history of the zonal average OLWR. Clouds enhance the water vapor modulation of OLWR. Tropical clouds have very cold cloud top temperatures, which increases the longitudinal variation in the region. However, in the polar region, where temperature inversion is prominent, cloud top temperature is warmer than surface temperature. Hence, clouds have the effect of increasing OLWR. The implications of this are that the latitudinal gradient of net radiation is further increased, and that the forcing of the general simulation is substantially different due to the increased additional available energy. A set of cloud top temperature maps were compiled using the cloud region classification of Sherr et al. (1968), with the aid of the LW radiation measurement by NOAA sunning radiometer. The results also suggest that a simple parameterization of the longwave cooling should include a water vapor absorbing term.