CLOUD MODEL-BASED SIMULATION OF SPACEBORNE RADAR OBSERVATIONS

Simulations of observations from potential spaceborne radars are made based on storm structure generated from the three-dimensional (3D) God dard cumulus ensemble model simulation of an intense overland convecti ve system. Five frequencies of 3, 10, 14, 35, and 95 GHz are discussed , but the Tropical Rainfall Measuring Mission precipitation radar sens or frequency (14 GHz) is the focus of this study. Radar reflectivities and their attenuation in various atmospheric conditions are studied i n this simulation. With the attenuation from cloud and precipitation i n the estimation of reflectivity factor (dBZ), the reflectivities in t he lower atmosphere in the convective cores are significantly reduced. With spatial resolution of 4 km X 4 km, attenuation at 14 GHz may cau se as large as a 20-dBZ difference between the simulated measurements of the peak (Z(mp)) and near-surface reflectivity (Z(ms)) in the most intense convective region. The Z(mp) occurs at various altitudes depen ding on the hydrometeor concentrations and their vertical distribution . Despite the significant attenuation in the intense cores, the presen ce of the rain maximum is easily detected by using information of Z(mp ). fn the stratiform region, the attenuation is quite limited (usually less than 5 dBZ), and the reduction of reflectivity is mostly related to the actual vertical structure of cloud distribution. Since Z(ms) s uffers severe attenuation and tends to underestimate surface rainfall intensity in convective regions, Z(mp) can be more representative for rainfall retrieval in the lower atmosphere in these regions. In the st ratiform region where attenuation is negligible, however, Z(mp) tends to overestimate surface rainfall and Z(ms) is more appropriate for rai nfall retrieval. A hybrid technique using a weight between the two rai n intensities is tested and found potentially useful for future applic ations. The estimated surface rain-rate map based on this hybrid appro ach captures many of the details of the cloud model rain field but sti ll slightly underestimates the rain-rate maximum.