Rainfall-type classification by polarimetric radar

Several previous studies presented that shallow convective rainfalls, which were characterized by low altitude of radar reflectivity peaks and large decrease of reflectivity with height above the 0oC level, are one of the main structures and have large contribution to the rainfall amount during the rainy season over East Asia (e. g., Chen et al. 2003; Zhang et al. 2006). Spatial distribution of hydrometeors in such rainfall systems is essential information to understand rain formation processes and also to upgrade rain retrieval algorithms from spaceborne radars and microwave radiometers (e.g., TRMM and GPM). However, little is known about such cloud microphysical properties in the shallow convective rainfall, mainly due to the inadequacy of those measurements. Polarimetric radar measurements have been exploited to examine microphysical properties within precipitation systems. Some of the measurements are sensitive to the presence of large wet particles. For example, the cross-correlation coefficient (ρhv) and linear depolarization ratio (LDR) show the robust signatures for the melting-layer in the stratiform rainfall (e.g., Herzegh and Jameson 1992; Brandes and Ikeda. 2004). These polarimetric measurements can give the signature of mixed-phase hydrometeors also in convective rainfall. Many previous studies, however, have focused on the presence of large, tumbling nonspherical particles such as irregular hail or graupel in wet growth in severe thunderstorm (e. g., Herzegh and Jameson 1992; Bringi et al. 1997). Only a few studies examined the polarimetric measurements in shallow convective rainfalls. Shusse et al. (2006) compared the polarimetric measurements between two shallow convective rainfalls with almost the same echo-top height, one was isolated and the other was embedded in the preexisting stratiform rainfall. The melting layer signature of low ρhv(0) was observed in the embedded-type convective rainfall, which was closely related to the preexisting stratiform rainfalls accompanied with abundant snow aggregates above 0oC level. On the other hand, the melting signature was not observed in the isolated-type convective rainfall. This difference indicates that the conventional convective-stratiform classification based on the radar reflectivity field is insufficient for cloud microphysical characterization of the shallow convective rainfall systems. The spatial distribution of melting layer should be one factor to characterize these rainfall systems. In this study, the rainfall-type classification method which gives a judgment of the presence of the melting layer in addition to a conventional convective-stratiform classification is developed using the C-band polarimetric radar, CRL Okinawa bistatic polarimetric radar (COBRA) (Nakagawa et al. 2003). This method is applied to a rainfall system consisting of shallow convective rainfalls and broad stratiform rainfall region during the rainy season over East China Sea, which is called Baiu season in Japan. Using this rainfall-type classification, we estimate the appearance ratio of the melting layer and the level of melting layer in the convective rainfall region.