Characteristics of DSD Bulk Parameters: Implication for Radar Rain Retrieval

Abstract

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 Dm and normalized intercept parameter Nw of the gamma DSD are studied. It is found, based on the simulations of the gamma DSD model, that R, Dm and Nw are closely interrelated, and that the ratio of R to Nw is solely a function of Dm, independent of the shape factor μ 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 × 10 − 4 , b = 4.706 . Analysis of two-parameter relationships such as R–Dm, Nw–R and Nw–Dm reveals that R and Dm are moderately correlated while Nw and Dm are negatively correlated. Nw and R, however, are uncorrelated. The gamma DSD model also reveals that variation of R–Dm relation is caused primarily by Nw. 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.