Outgoing longwave radiation and its diurnal variations from combined Earth Radiation Budget Experiment and Meteosat observations :2. Using Meteosat data to determine the longwave diurnal cycle

Abstract

For April and July 1985, applying the narrow to broadband conversion of part 1 to Meteosat observations obtained at 3‐hour intervals (ISCCP B2 data), we determine the monthly mean radiant exitance as well as the mean diurnal variation, over 2.5°×2.5° latitude‐longitude regions of tropical Africa and the neighboring Atlantic Ocean. We compare these determinations with those obtained directly from the ERBS and NOAA 9 Earth Radiation Budget Experiment (ERBE) data for this month, for which the time sampling is sparser and not so uniform. Excellent agreement is obtained in most cases, in particular for the overall monthly means. However, for the monthly mean diurnal variation there are situations in which the ERBE time sampling, the nature of the ERBE diurnal modeling scheme and the convolution of weather system changes with the diurnal cycle, combine to produce significant differences between the ERBE determination and the Meteosat result. These differences would mostly have been much smaller had the third ERBE instrument package been in operation (at 0730/1930 LT) as originally planned. We consider possible improvements in diurnal interpolation procedures, but note that there is no general way to remove bias resulting from inadequate time sampling.