SIMULATIONS OF THE EFFECTS OF WATER-VAPOR, CLOUD LIQUID WAITER, AND ICE ON AMSU MOISTURE CHANNEL BRIGHTNESS TEMPERATURES

Radiative transfer simulations are performed to determine how water va por and nonprecipitating cloud liquid water and ice particles within t ypical midlatitude atmospheres affect brightness temperatures T-B's of moisture sounding channels used in the Advanced Microwave Sounding Un it (AMSU) and AMSU-like instruments. The purpose is to promote a gener al understanding of passive top-of-atmosphere T-B's for window frequen cies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176. 31, 180.31, and 182.31 GHz by documenting specific examples. This is a ccomplished through detailed analyses of T-B's for idealized atmospher es, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distrib ution phase, liquid or ice water content, altitude, and thickness. Eff ects on T-B of changing surface emissivity also are addressed. The bri ghtness temperature contribution functions are presented as an aid to physically interpreting AMSU T-B's. Both liquid and ice clouds impact the T-B's in a variety of ways. The T-B's at 23.8 and 89 GHz are more strongly affected by altostratus liquid clouds than by cirms clouds fo r equivalent water paths. In contrast, channels near 157 and 183 GHz a re more strongly affected by ice clouds. Higher clouds have a greater impact on 157- and 183-GHz T-B's than do lower clouds. Clouds depress T-B's of the higher-frequency channels by suppressing, but not necessa rily obscuring, radiance contributions from below. Thus, T-B's are les s closely associated with cloud-top temperatures than are IR radiometr ic temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 G Hz. The Rayleigh approximation is found to be adequate for typical dro plet size distributions; however, Mie scattering effects from liquid d roplets become important for droplet size distribution functions with modal radii greater than 20 mu m near 157 and 183 GHz, and greater tha n 30-40 mu m at 89 GHz. This is due mainly to the relatively small con centrations of droplets much larger than the mode radius. Orographic c louds and tropical cumuli have been observed to contain droplet size d istributions with mode radii in the 30-40-mu m range. Thus, as new ins truments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds.