SIMULATED SENSITIVITY OF THE EARTHS RADIATION BUDGET TO CHANGES IN-CLOUD PROPERTIES

Simulations are made of the long-term mean clear-sky and total-scene c omponents of the earth's radiation budget (ERB), using detailed solar and thermal infrared radiative transfer schemes. These employ as input a wide range of multi-year observations of atmospheric, surface and c loud parameters. For clear skies, differences between the simulations and results from the Earth Radiation Budget Experiment (ERBE) are gene rally within the expected accuracy of the observations and calculation s, with the most notable disagreement at high latitudes. For cloudy sk ies, the agreement is less good, but still generally within the system atic error bounds. The absorbed shortwave radiation (ASR) is simulated least well over oceanic regions, with the pattern of disagreement res embling the pattern of response when low cloud cover or liquid water p ath is perturbed; this suggests that the representation of low cloud c over or liquid water path dominates the ASR discrepancy between the si mulation and ERBE. The simulated outgoing longwave radiation (OLR), wh ich is sensitive to the high-cloud emissivity, differs most from the E RBE OLR in deep convective regions; whilst assuming the high cloud to be black lessens agreement between the simulation and observations in some areas, agreement is substantially improved in others. This indica tes that better knowledge of high-cloud emissivity remains an importan t issue. The spatial and temporal variations of the sensitivity of the ERB to perturbations to low, mid and high cloud properties are invest igated. Principal results include (a) the dominant importance of cloud amount, as compared to cloud temperature, in determining the sensitiv ity of the ERB to high-cloud properties, and (b) the greater sensitivi ty of the net ERB to low-cloud properties than to those of mid and hig h cloud. A major uncertainty in the calculations is the degree of clou d overlap; the inclusion of overlapping clouds is found to be capable of reversing the net high-cloud sensitivities in certain locations. Gl obal mean sensitivity to low cloud perturbations are compared with pre vious results obtained using a GCM. The simulated sensitivity of the E RB to low cloud cover is substantially larger than in the previous res ults, although the effects of changes in liquid water path are similar .