BIASES OF RAIN RETRIEVAL ALGORITHMS FOR SPACEBORNE RADAR CAUSED BY NONUNIFORMITY OF RAIN

The biases of various rain retrieval algorithms for a spaceborne rain radar due to nonuniformity of rain are studied using simple models and an actual time sequence of rainfall rate. A conventional rain retrieval algorithm in which measured radar reflectivity factors are simply converted to rainfall rates always overestimates the rainfall rate, but the bias is moderate. The surface reference method, which is one of the algorithms using rain attenuation, always underestimates the rainfall rate. The bias is larger for larger rain depth and has a possibility to give a significant error in estimating mean rainfall rate. The dual-frequency method, which is another algorithm using rain attenuation, has a bias that depends on the rain depth and the distance between two range gates where rain attenuation is estimated and shows large variations from overestimation to underestimation. The biases are also studied using actual rainfall rates.