SPATIAL CORRELATION OF BEAM-FILLING ERROR IN MICROWAVE RAIN-RATE RETRIEVALS

In this paper the authors consider the possibility of correlations bet ween the random part of the so-called beam-filling error between neigh boring fields of view in the microwave retrieval of rain rate over oce ans. The study is based upon the GARP(Global Atmospheric Research Prog ram) Atlantic Tropical Experiment(GATE) rain-rate dataset, and it is f ound that there is a correlation of between 0.35 and 0.50 between the errors in adjacent rainy fields of view. The net effect of this correl ation is reducing the number of statistically independent terms accumu lated in forming area and time averages of rain-rate estimates. In GAT E-like rain areas, this reduction can be of the order of a factor of 3 , making accumulated standard error percentages increase by a factor o f the order of root 3. For the Tropical Rainfall Measuring Mission usi ng the microwave radiometer alone, this could increase the accumulated random part of the beam-filling error for month-long 5 degrees x 5 de grees boxes from about 1.2% to 2%. The effect is larger for less rainy areas away from the equatorial zone.