Longwave Cloud Radiative Forcing as Determined from Nimbus-7 Observations

Abstract

Collocated and coincident cloud and outgoing longwave radiation observations taken by experiments on board the Nimbus-7 satellite have been used to infer the daytime longwave cloud-radiative forcing. Through the specification of a time-series of daily values of cloud amount, cloud-top temperature, surface temperature, and outgoing longwave radiation, the clear-sky flux is obtained for both the summer (June, July, and August 1979) and winter (December 1979. January and February 1980) seasons. The longwave component of the cloud-radiative forcing is then computed by subtracting the observed outgoing longwave flux from the inferred clear-sky longwave flux. The results are compared to independent cloud forcing estimates produced using high spatial resolution radiometers and found to agree closely. The resultant cloud forcing is analyzed regionally, zonally, and globally for each season to quantify, through observation, the role that clouds play in modulating the outgoing longwave radiation. The largest cloud forcing is found over regions of tropical convection, and reaches peak values of about 80 W m?2 in the vicinity of the summer and winter monsoon. Cloud forcing values of less than 10 W m?2 are evident over the deserts the subtropical oceans, and in the polar latitudes, Zonally, the cloud forcing reaches maxima over the Intertropical Convergence Zone (40 to 50 W m?2) and over the polar frontal zones of both hemispheres (25 to 30 W m?2), and minima in the subtropical belts and at the poles. Globally, the cloud forcing is found to be 24 W m?2. The globally averaged cloud cover for the same period is 50%.