Characteristics of DSD Bulk Parameters: Implication for Radar Rain Retrieval

With the use of 213,456 one-minute measured data of droplet-size distribution (DSD) of rain collected during several National Aeronautics and Space Administration (NASA)-sponsored field campaigns, the relationships between rainfall rate R, mass-weighted diameter D-m and normalized intercept parameter N-w of the gamma DSD are studied. It is found, based on the simulations of the gamma DSD model, that R, D-m and N-w are closely interrelated, and that the ratio of R to N-w is solely a function of D-m, independent of the shape factor mu of the gamma distribution. Furthermore, the model-produced ratio agrees well with those from the DSD data. When a power-law equation is applied to fit the model data, we have: R = aN(w)D(m)(b), where a = 1.588 x 10(-4), b = 4.706. Analysis of two-parameter relationships such as R-D-m, N-w-R and N-w-D-m reveals that R and D-m are moderately correlated while N-w and D-m are negatively correlated. N-w and R, however, are uncorrelated. The gamma DSD model also reveals that variation of R-D-m relation is caused primarily by N-w. For the application of the Ku- and Ka-band dual-frequency radar for the retrieval of the DSD bulk parameters as well as the specific radar attenuations, the study is carried out to relate the dual-frequency radar reflectivity factors to the DSD and attenuation parameters.