Rainfall-type classification by polarimetric radar

The improvement of precipitation physical models related to raindrop size distribution (DSD) and hydrometeor information around and above an altitude of 0°C level is desired to upgrade rain retrieval algorithms for spaceborne radars and microwave radiometers. In addition, an appropriate rainfall-type classification method to select the DSD and melting layer models is very important for practical use. In the TRMM PR 2A25 algorithm, for example, two DSD models are assumed corresponding to stratiform and convective types of rainfall based mainly on the bright band information (Iguchi et al. 2000). However, there are still difficulties and uncertainties in rainfall-type classification based on the cloud microphysical information.

Polarimetric radar observation is one of the useful methods to characterize the microphysical information in clouds. In the present study, polarimetric radar data are utilized to classify the rainfall type by using a C-band polarimetric radar, COBRA (CRL Okinawa bistatic polarimetric radar) (Nakagawa et al. 2003). As a first step, characteristics of polarimetric radar variables in (1) stratiform type (ST), (2) isolated convective type (ICT), and (3) embedded convective type (ECT) were examined to develop a new classification index. These three types are often observed during rainy season over the East China Sea. The ST was accompanied with an obvious bright band. The ICT and ECT were relatively shallow convections, whose echo-top heights of 30 dBZ were about equal. The ECT appeared and developed in the preexisted substantial stratiform rainfall.

One important result is that the values of ρhv(0) significantly decreased around the 0°C level in the ST and the ECT, but did not decreased in the ICT. These features indicate that the melting aggregated snow existed around the 0°C level in the ST and the ECT, but did not exist in the ICT. It follows from this that conventional convective-stratiform classification methods based on radar reflectivity data are not always effective to characterize the microphysical information in the precipitation. On the bases of these facts, the rainfall-type classification method was developed, which gives a judgment of the presence of the melting layer in addition to a conventional convective-stratiform classification, and early results of the application study will be shown.