Raindrop Shape-Size Relation Retrieval from Polarimetric Radar Measurements

This paper presents a technique by which it is possible to retrieve the drop shape-size relation that governs the polarimetric radar observations of reflectivity, Z_h, differential reflectivity Z_dr, and specific differential propagation phase K_dp. An observation domain to study the shape –size relation is introduced. In this space, called Radar Drops Shape Size Domain, (RDSSD), the DSD variability is almost eliminated and any variation is essentially due to the drop shape variability allowing the same, to be observed.

This RDSSD domain can be mapped to a corresponding domain in which the relation between the drop axis ratio and its equivolumetric diameter is expressed. This drop shape-size domain, is parameterized with a fourth-order polynomial. A minimization procedure of the error between the measured values of observables in RDSSD domain and those from a priori model is developed by changing elements of the above mentioned polynomial. As a result, a relation between axis ratio and equivolumetric diameter that approaches the underlying unknown relation governing the prevailing radar measurements is found.

The procedure is applied to three different radar data sets collected by the NCAR S-POL radar during campaigns conducted in Florida (Teflun B), Brazil (LBA) and Italy (MAP). The drop shape-size relations obtained for each campaign are compared with the relations the literature namely, Pruppacher and Beard (1970) and Beard and Chuang (1987) as well as with the two recent ones of Brandes et al. (2002) and Thurai et al. (2007). The mean drop shape-size relation retrieved is analyzed to explore whether the natural raindrop shape-size relation can be described by a unique model. Specifically, for the cases considered in this study, the variability of the drop shape model is found to be between the Beard and Chuang (1987) and Brandes et al. (2002) models.