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Outgoing longwave radiation and its diurnal variation at regional scales derived from Meteosat

The outgoing longwave radiative flux (hereafter OLR) is estimated from radiance observations in the infrared (IR: 10.5–12.5 μm) and water vapor (WV: 5.7–7.1 μ) channels aboard the geostationary satellite Meteosat 2. The OLR is obtained from the IR and WV radiance and the satellite viewing angle via a regression scheme. The scheme involves two steps. First, the narrow-band radiances are converted into narrow-band fluxes, using empirical limb darkening functions. Then, the OLR is computed from the narrow-band fluxes with a third-order polynomial. Regression coefficients were determined with a radiative transfer model. It is shown that inclusion of the WV information more than halves the rms error of the OLR retrieval, as compared to a single IR channel approach. The OLR estimation technique is applied to four regional areas: (1) marine stratocumulus in the South Atlantic; (2) Saharan desert; (3) Intertropical Convergence Zone (ITCZ) over central Africa; and (4) ITCZ over the Atlantic Ocean. All areas extend over about 800 × 800 km. The satellite data comprise observations for the full month of July 1983, with eight samples per day. Time series and monthly mean diurnal variations of the OLR are analyzed. For the stratocumulus regime we find a highly coherent diurnal range of the OLR of 6 W m−2. Over the Sahara the monthly mean diurnal range is 62 W m−2, with maximum values of 75 W m−2. In multilevel cloud situations within the ITCZ, we attempt to segregate the individual OLR contributions of the clear-sky, low-, medium- and high-level cloud scenes. The height attribution of clouds is based on their brightness temperature (corrected for semitransparency) and climatology or forecast temperature profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF), in Reading, England. We find pronounced diurnal variations over the continental ITCZ for the altitude of high-level cloud and the coverage with medium-level cloud. Over the oceanic area the signal is not conclusive for high- and medium-level cloud, whereas the OLR from the low-cloud scene undergoes a clear diurnal cycle resembling the observation for marine strato-cumulus.