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Johannes Schmetz, Mohamed Mhita, and Leo Van De Berg

Abstract

Outgoing longwave radiative fluxes (OLR) and the longwave cloud-radiative forcing at the atmosphere are retrieved from METEOSAT radiance observations in the thermal infrared window (IR: 10.5–12.5 μm) and water vapor (WV: 5.7–7.1 μm) channels for April 1985. The analysis exploits an operationally preprocessed radiance dataset that includes a scene identification of clear sky, low level, medium level and high level clouds. Monthly means of the OLR and the longwave cloud-radiative forcing are inferred for areas of about 200 km × 200 km. Extended regions with a forcing larger than 60 W m−2 are found within the intertropical convergence zone (ITCZ) over southern Sudan and around 5°S over Brazil and the adjacent Atlantic Ocean.

The contribution of three levels of cloud to the longwave radiative forcing is estimated: high level coulds (≤400 hPa) contribute about 80% to the total longwave forcing in regions with strong convective activity (ITCZ). Medium level coulds (700 ≤ cloud top < 400 hPa) induce a maximum forcing of 15–20 W m−2 over the Ethiopian highland, while low level cloud forcing reaches values of 5–10 W m−2 over the marine stratocumulus regions and within the midlatitude westerlies.

Systematic errors in the longwave cloud-radiative forcing due to calibration errors, cloud contamination of clear sky radiances and a dry bias in the humidity of the upper troposphere, which may occur as a result of minimizing the cloud contamination, are discussed; it is concluded that the present study underestimates maximum values of the longwave cloud-radiative forcing by about 10 W m−2.

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