Intensity estimation of tropical cyclones passing Okinawa by using GBVTD technique

Presentation Type: 
Udai Shimada (Meteorological Research Institute, Japan Meteorological Agency)
Naoko Kitabatake (Meteorological Research Institute, Japan Meteorological Agency)
Hiroyuki Yamada (University of the Ryukyus)

The intensity (i.e., central pressure and maximum surface wind) of tropical cyclones (TCs) over the ocean is estimated primarily by Dvorak technique using satellite data. However, the estimate accuracy is sometimes insufficient. In this study, the intensity of four TCs that passed Okinawa in 2012 and 2013, including Typhoons (TYs) Bolaven (1215), Sanba (1216), Jelawat (1217) and Danas (1324), is estimated by means of the GBVTD technique (Lee et al., 1999) using single Doppler radar data at 5-min intervals, and the possibility of the intensity estimation is discussed, aimed at operational use. We used Doppler velocity data of Japan Meteorological Agency (JMA) operational radar located in Okinawa Island. Because TCs frequently pass Okinawa Island without being modified by the topography, the observation of the Okinawa radar is suitable for estimating TC intensity by the GBVTD technique. The GBVTD implemented here can retrieve wind fields in TCs at altitudes of 1 and 2km. The center locations of TCs are estimated by GBVTD-simplex algorithm (Lee and Marks, 2000). Assuming the retrieved axisymmetric tangential winds are in the gradient wind balance, the central pressure is derived using surface pressure observations nearby (Lee et al., 2000).

The best track (BT) data of JMA was used for the evaluation of the estimates. The estimated central pressures were almost equal to the BT for three TCs (Bolaven, Sanba and Danas). Each of these three TCs had a distinct axisymmetric structure and had enough spatial coverage of the Doppler velocity data. On the other hand, the central pressure of TY Jelawat could not be estimated with sufficient accuracy, due to failure of correction of aliased velocities.

Future works are as follows: To develop an estimation method of surface maximum wind from the retrieved winds at altitudes of 1 and 2km, to improve the correction method for aliased velocities, and to introduce a retrieval method of environmental winds around TCs.