Polarimetric radar observation of the eyewall of Typhoon Man-yi

Typhoon Man-yi hit main Okinawa island, Japan, on 13 July 2007. The typhoon was in its mature stage with minimum sea-level pressure of 941 hPa and maximum Doppler velocity of more than 60 m s^-1. A C-band polarimetric radar, the CRL (the forerunner of NICT) Okinawa bistatic polarimetric radar (COBRA), succeeded to obtain the three-dimensional and high spatial resolution data of the eyewall from 0430 to 1000 LST with 360-degree multiple RHI scannings. In this study, we describe the characteristics of polarimetric radar variables and identified types of hydrometeors in the eyewall of the typhoon.

At low levels (from 2 to 3 km altitudes) of the eyewall, those were sufficiently below the 0 °C level (5.7 km altitude), differential reflectivity (Z_dr) increases with radar reflectivity (Z_hh). The maximum value of Z_dr is approximately 1 dB. This indicates that the long-axis direction of raindrops in the lower parts of the eyewall is averagely near horizontal even in the region of horizontal gale wind of 60 m s^-1 or more.

Around the 0 °C level, layers of low correlation coefficient between horizontal and vertical polarization signals (ρ_hv) were observed in a large area of the eyewall, which indicates the existence of melting layers. However, the levels of the low-ρ_hv layers in the eyewall are generally higher than those in surrounding areas, and rise up to 2 km. Hydrometeor-type classification was conducted using a fuzzy logic scheme. Around the highest echo-top area in the eyewall, a wet graupel region is found around the 0 °C level with the region of dry graupel overlying this. The low-ρ_hv layer in this region corresponds to the upper part of the wet graupel region. On the basis of these results, the microphysical structure of the eyewall of the mature typhoon is discussed.