SENSITIVITY OF THE EARTH RADIATION BUDGET TO INTERANNUAL VARIATIONS IN-CLOUD AMOUNT

Using two pairs of coincident long-term satellite derived cloud and ea rth radiation budget data sets (Nimbus-7 ERB/Nimbus-7 Cloud Climatolog y and ERBE Scanner/ISCCP-C2), estimates are made of the sensitivity of the top of the atmosphere radiation budget to interannual variations in the total cloud amount. Both sets of analyses indicate that the lar gest net warming due to interannual cloud cover changes occurs over de sert regions, while the largest net cooling occurs in areas of persist ent marine stratiform cloud. There is generally a large amount of canc ellation between the large shortwave cooling and longwave warming effe cts in tropical convection regions. However, the Nimbus-7 analysis ide ntifies an area of net warming in the tropical eastern Pacific Ocean w hich is shown to be associated with the 1982-83 ENSO event. In the zon al mean the Nimbus-7 data sets indicate that interannual cloud cover c hanges lead to a net warming at low latitudes and net cooling poleward s of 25 degrees in both hemispheres. In contrast, the analysis of the ERBE and ISCCP data sets indicates net cooling everywhere except for t he Northern Hemisphere equatorwards of 20 degrees N. For the spatial a verage between 60 degrees N and 60 degrees S the ratio of the shortwav e and longwave effects is 0.94 in the Nimbus-7 analysis (i.e. clouds c ause a small net warming) and 1.21 in the ERBE-ISCCP analysis (i.e. a net cooling). Given their improved spatial and temporal sampling the a nalysis using the ERBE and ISCCP data sets should be the more reliable . However, the large differences between the two analyses still raises some issues concerning the confidence with which the sign of the effe ct of clouds on the radiation budget at these time scales is currently known.