Atmospheric longwave radiation under cloudy skies for HAM simulation programs

Atmospheric longwave radiation is one of the needed climatic boundary conditions in Heat-Air-Moisture (HAM) simulation programs. The paper presents principal procedures for the computation of atmospheric longwave radiation under clear and cloudy skies by means of simple near-ground weather data. Several approaches from literature have been calculated, compared and validated against measured data from Schleswig (D). For the clear sky data a new parameterisation has been found. Special attention has been paid to the determination of atmospheric radiation under cloudy skies. Therefore three cases of initial data situation have been distinguished: 1 There is cloud cover data available. In this case, the determination of atmospheric radiation is easily done by given formulae. Nevertheless, an improved parameterisation is proposed. 2 There is no cloud cover data, but shortwave direct and diffuse radiation data available. Sky radiative properties are expressed by means of the cloud cover index n' which is usable like the real cloud cover index. 3 There is no cloud cover data, but shortwave total radiation data available. Sky radiative properties are expressed by means of the cloud cover index n'' which is usable like the real cloud cover index. Of course, the usage of the cloud cover indices is possible for daytime data only. Therefore, a simple approach for the night time data has been found. The determination of the atmospheric longwave radiation during night depends therewith only on near-ground temperature, relative humidity and daytime. Although accuracy is markedly reduced by including night time data, the approach gives still acceptable results (correlation coefficient of 0.80). The paper presents therewith an integrated approach for the determination of atmospheric longwave radiation. Contrary to other approaches, all common cases of data availability and daytime are included. The approach makes therefore an important contribution to the proper provision of climatic boundary conditions in HAM simulation programs.