The Echo Size Distribution of Precipitating Shallow Cumuli

Abstract

S-band radar surveillance scans of precipitating shallow convection are analyzed. The scans are complied from 52 days of near-continuous measurements in the winter trades of the North Atlantic during the Rain in Cumulus over the Ocean (RICO) field campaign. After being analyzed and filtered to exclude spurious returns, the scans are segmented to identify contiguous returns, or echoes. The echo size and reflectivity statistics are then analyzed. A new normalization method is developed to account for biases associated with the nonuniformity in the native radar grid. The echo area distribution robustly exhibits power-law scaling up until sizes of about 10 km2, with a scaling exponent of about ?1.1. At larger sizes the scaling behavior breaks down and varies more markedly across subsamples of the data. Conditional sampling suggests that the scaling behavior of the larger echoes does, however, approach that of the smaller echoes as echo coverage increases, which supports the idea of a limiting distribution. Departures from this limiting distribution are argued to reflect finite size effects, modulated by the presence of a capping inversion whose height and strength varies across the samples.