Relations between precipitation rate and mass weighted mean diameter for the GPM/DPR algorithm

Abstract:

The standard precipitation retrieval algorithm for the Dual-frequency Precipitation Radar (DPR) on the core satellite of Global Precipitation Measurement (GPM) mission is revised for version 04 (V04) and will be published in early 2016. In the current version (version 03; V03), as in the standard algorithm for the Precipitation Radar (PR) on the Tropical Rainfall Measuring Mission (TRMM), relations between specific attenuation and effective radar reflectivity factor (k-Ze relations) are used to constrain the drop size distribution parameters. In V04, relations between rain rate and mass weighted mean diameter (R-Dm relations) are used instead of k-Ze relations. As both k and Ze are frequency dependent, in the DPR algorithm, k-Ze relations are prepared for KuPR (13.6GHz) and KaPR (35.5GHz). For KuPR, the same k-Ze relations with PR are used, but k-Ze relations were newly prepared for KaPR and they are not validated well. On the other hand, both R and Dm are frequency independent. R-Dm relations are calculated for PR as shown in Kozu et al. (2009), and they can be used both for KuPR and KaPR. Moreover, the same R-Dm relations are used at any height as long as “non-breakup and non-coalescence” is hold. We have measured (R, Dm) data by a disdrometer in Nagasaki, Japan and compared them with an R-Dm relation applied for stratiform rainfall (R=epsilon**4.6*0.401*Dm**6.131). As the average of epsilon is 0.97, in this comparison, it is suggested that R-Dm relation is not biased largely. Other measurement dataset under different precipitation climates should be used. Though R-Dm relations are dependent only on precipitation types in V04, we need to study the dependence of R-Dm relations on regions and seasons.