Characteristics of polarimetric radar variables in three types of rainfalls in a baiu front event over the East China Sea

Abstract

Characteristics of polarimetric radar variables in three rainfall types in a Baiu front event over the East China Sea observed on 1 June 2004 were studied and compared using a C-band polarimetric radar, the CRL Okinawa bistatic polarimetric radar (COBRA). The selected rainfalls are common types in the Baiu season in this area:(1) stratiform type (ST), (2) isolated convective type (ICT) and (3) embedded convective type (ECT). ST was characterized by an obvious bright band in the field of radar reflectivity (Zhh). ICT and ECT had almost the same 30-dBZ echo-top height of about 5.5 km, and their strong echo region (Zhh > 40 dBZ) did not exceed the 0 Clevel(4.4 kmaltitude)evenintheirmaturestages.Around the 0 C level, overall decrease in correlation coeffcient between horizontal and vertical polarization signals (rhv) and increase in differential reflectivity (Zdr) were observed in ST and ECT, which indicated the presence of a layer of mixed-phase precipitation. By contrast, significant decrease in rhv and increase in Zdr were not found in ICT. At lower levels, Zdr ranged from 0 to 1.5 dB and most of rhv were higher than 0.98 in ST and ECT. The values of Zdr and rhv had wider variations in ICT. The characteristics of the vertical profiles of Zdr and rhv in ECT are consequently more similar to those in ST rather than to those in ICT, although their echo-top heights of 30 dBZ and maximum Zhh near the surface were almost equal.Near the surface, suffciently below the 0 C level, ICT had larger Zdr and lower rhv than ECT in the region with Zhh stronger than 45 dBZ. This indicates that larger raindrops were more dominant in ICT than in ECT at the same Zhh in the intense rainfall regions. This difference of dominant raindrop sizes appears to reflect the difference in precipitation growth processes between ICT and ECT. © 2009, Meteorological Society of Japan.