Wind fields of typhoon Songda (2004) observed by the Okinawa Doppler radar (COBRA)

Typhoon Songda passed over Okinawa island on September 5, 2004, and caused extensive damage to several places in the main island of Japan. The typhoon center passed over the C-band Okinawa Bistatic Polarimetric Radar (COBRA) developed by the National Institute of Information and Communications Technology. The COBRA observed the typhoon in its mature stage within the range of 280 km for 18 hours. Although the COBRA measured many kinds of data including dual-polarization meteorological quantities (ZDR, RhoHV, PhiDP, etc.) in three-dimensional scans (14-PPIs and 4-RHIs) every 10 minute, reflectivity and Doppler velocity data in a low elevation (0.9 degree) are used in this study.

Firstly, ground-based meteorological observation data at the COBRA radar site are investigated. The minimum pressure dip of 917.6 hPa was recorded on 0835Z, though the minimum wind speed was measured on 0900Z when the typhoon center was passed over the COBRA site. The maximum mean wind speed of 30 m/s and maximum instantaneous wind speed of 56 m/s were measured at 20 km from the typhoon center. A gradient wind, which is calculated from the horizontal pressure gradient estimated from a regression curve of the measured pressure change and the typhoon motion, shows that the theoretical maximum wind speed was 42 m/s.

Secondly, the radial distribution of tangential wind speed of the typhoon is retrieved from the single Doppler radar data. Measured Doppler velocity Vd is expressed by Vd = Vt*sin(T-P) + Vr*cos(T-P) + Vm*cos(P-M), where Vt and Vr are tangential and radial velocity components respectively. T and P show the directions of the measurement point from the typhoon center and from the radar site, respectively. Vm and M show the typhoon motion and its direction. In the regions where the contribution of the tangential velocity component is dominant in the measured Doppler velocity (sin(T-P)/{sin(T-P)+cos(T-P)} > 0.7), the radial velocity component may be ignored, because the radial velocity must be much smaller than the tangential velocity in a typhoon. Since the observed typhoon has a symmetrical structure, the estimated tangential wind speed is averaged around the typhoon center. The mean tangential wind speed has the maximum of 40 m/s at 20 km from the typhoon center. Since the radius of the eye-wall echo is 20 km, it means that the maximum wind speed occurs in the eye-wall. The radial distribution of the mean tangential wind speed that is estimated from radar observations at different times, which represents the wind speed between 2 km and 4 km in height, is consistent with the theoretically estimated gradient wind distribution.