Study On Wind Asymmetry Of Tropical Cyclones Over The Northwest Pacific Ocean By Spaceborne Microwave Radiometer

The Northwest Pacific Ocean is one of the most cyclone-prone regions in the world.As tropical cyclones(TCs)can cause natural disasters such as storm surges and heavy rainfall during their landfall,they will cause severe damage to the coastal areas in southeast China and affect the safety of life and property of local residents.Therefore,it is very important to study the wind field structure of tropical cyclones and obtain accurate key parameters of TCs to improve the disaster risk assessment level of TCs.Based on the observed wind field data obtained over the Northwest Pacific Ocean from 2015-2017 by the L-band passive microwave radiometer on the Soil Moisture Active Passive(SMAP)satellite of NASA,this paper studied the asymmetry of wind speed in the surface wind field of tropical cyclones.The L-band microwave radiometer is not susceptible to rainfall attenuation and it can provide accurate measurements of strong wind speeds under extreme weather conditions,up to 70 m/s,so the wind speed asymmetry in tropical cyclones can be studied within a larger wind speed range.The azimuthal asymmetry of a tropical cyclone is defined as the degree factor by which the maximum wind speed(MWS)is larger than the mean azimuthal wind speed at the radius of the maximum wind speed(RMW).In this paper,the effects of storm motion and environmental vertical wind shear on the asymmetric degree of surface wind speed of tropical cyclones under different TC intensities were investigated by the quantitative tropical cyclone asymmetry factor.In addition,this article also determined the functional expression of the key coefficient of the parameterized tropical cyclone surface wind field in the Northwest Pacific Ocean through the best track dataset of the Joint Typhoon Warning Center(JTWC)between 2001-2014 over the Northwest Pacific ocean,verified and applied the results.The results show that with the increasing intensity of tropical cyclone,the asymmetry degree decreases obviously,indicating that the significant asymmetry of tropical cyclone wind speed mainly occurs in the weak cyclone systems.At the same time,the asymmetry degree of wind speed of tropical storms and super typhoons increase significantly with the increase of storm translation speed.However,typhoons and severe typhoons do not show similar trends for storm motion.For tropical cyclones of all intensities,the degree of asymmetry increases with the increase of the environmental vertical wind shear magnitude.Through a composite wind field analysis method,it is found that the maximum wind speed of each tropical cyclone intensity group tends to appear on the left side of the wind shear direction and the right side of the storm motion direction.Meanwhile,the relative strength between the storm motion velocity and the environmental vertical wind shear magnitude may also affect the location of the maximum wind speed.When the storm motion velocity is greater than the wind shear magnitude,the position of the maximum wind speed is more on the right side of the storm motion direction.In contrast,when the storm motion velocity is less than the wind shear magnitude,the maximum wind speed position is more on the left side of the wind shear direction.It is also found that when the storm motion direction is close to the wind shear direction,the maximum asymmetry degree exists.However,when the direction of the wind shear is opposite to the storm motion direction,there is a minimum degree of asymmetry.In addition,the expression fitting result of key coefficient of the parameter wind field is ideal and can be applied to the corresponding wind pressure and radial wind profile model.The results show that the maximum sea surface wind speed of the tropical cyclone simulated by the wind pressure model combined with the expression of the key coefficient of parameter wind field in this paper has a relatively small deviation and root-mean-square error compared with the maximum wind speed of JTWC,while the wind profile model can better simulate the wind speed near the peak of the radial wind profile observed by the radiometer which passes the maximum wind speed of the tropical cyclone.