The T-TREC technique for retrieving the winds of landfalling typhoons in China

Abstract

In this study, an extension of the TREC (Tracking Radar Echo by Correlations) technique, named Tropical Cyclone (TC) circulation TREC (T-TREC), is developed to retrieve the winds of landfalling typhoons in China. The T-TREC analysis is performed on a polar grid centered at the TC center, using arc-shaped correlation cells and an arc-shaped search area. The search for the best correlation match is confined along the cyclonic direction with a limited search distance in the radial direction based on the cyclonic circulation characteristics of TCs in the Northern Hemisphere. The TC center is determined objectively using reflectivity data while the Doppler radar radial velocities are incorporated to estimate the search range and create a velocity correlation matrix as auxiliary constraints.

The T-TREC was applied to the landfalling Typhoon Saomai (0608) observed by Chinese next generation Weather Surveillance Radar 1998 Doppler (CINRAD WSR-98D) on the southeast coast of China. The results show that the T-TREC has the ability to estimate the typhoon circulation with an average bias of < 4 m s⁻¹. The incorporation of radial velocity data could distinctively improve wind retrievals for intense typhoons, especially by reducing the underestimation caused by fairly uniform reflectivity patterns in the vicinity of the eyewall and the outer rainband. A quantitative evaluation of the influence of typhoon center and cell size on the wind estimation demonstrates that the quality of the T-TREC retrieved wind circulation depends on the estimation of the typhoon center. A 4-km deviation of the TC center may result in a 10% increase in the retrieved wind error. The effect of cell size depends on the typhoon scale: better wind retrieval results can be obtained for a smaller typhoon.