Study of the structure and characteristics associated with landfalling typhoons on the southeastern coast of Zhejiang province in China using Doppler radar data

This study investigated temporal and spatial variations of the reflectivity and precipitation structure within 300 km radius of typhoon center by using the reflectivity data taken from the Doppler radars located in Zhejiang province. Three typhoons making landfall along the southeastern coast of Zhejiang province in China have been selected to examine the changes of the precipitation distribution from 6 h before landfall to 7 h after landfall. The threedimensional wind fields are retrieved from the Wenzhou Doppler radar data using the 4D-Var wind retrieval technology. The 3D structure of the mesoscale convective system producing the most severe heavy rainfall at Yunyan and Changchan is analyzed with the singleDoppler radar retrieved wind and radar reflectivity observed by the Wenzhou Doppler radar. The results show that the stronger the typhoon intensity is, the bigger the reflectivity of the middle and low levels is, and the more severe the mesoscale convective system and the precipitation rate are. The axisymmetric component of typhoon echo (rainfall), represented by the radial distribution of the azimuthal mean reflectivity, reveals that echo (rainfall) spreads outward from the typhoon eyewall before landfallling. The mean echo (rainfall rate) in the innercore region increases abruptly, accompanied with the rapid contraction of the precipitation areas toward typhoon center when typhoons are approaching the coast. The mean rainfall rate in the typhoon eyewall will intensify after landfall. The singleDoppler radar retrieved wind fields indicate that, at 1 h after typhoon landfalling, with the enhanced convergent (divergent) wind fields in the low (upper) levels, the tilted upward movement in the mesoscale convective system is enhanced and the precipitation rate is increased obviously. The typhoon intensity is proportional to the increases of the precipitation rate. The stronger the typhoon intensity is, the bigger the vertical winds shear value is. The area of the maximum vertical wind shear value is corresponding to that of the most severe heavy rainfall. The obvious increase of the vertical wind shear value plays an important role in the enhancement and sustaining of the mesoscale convective system in spiral rain bands of landfalling typhoon.