Study On The Tropopause Structure And Stratospheric Intrusion Processes Based On MST Radar Observations

The MST(Mesosphere-Stratosphere-Troposphere)radar operating at VHF(VeryHigh-Frequency)band is a single-station Doppler phased array coherent scatter radar with high sensitivity and high transmission power.It is mainly based on isotropic turbulent scattering and the anisotropic local specular reflection mechanism to obtain the echo power spectrum signal and to derive the atmospheric three-dimensional winds.It can perform continuous unattended detection of the mesosophere,lower stratosphere and troposphere under any weather conditions,which makes MST radar an indispensable and unique remote sensing tool for atmospheric science research.Up to now,MST radar has been widely used in the observational research of various small and medium-scale weather and meteorological processes(such as precipitation,gravity waves,tidal waves,atmospheric fronts,typhoons,stratospheric intrusions and tropopause folding,etc.).The development of Chinese Mainland MST radar technology will help us to understand the dynamics of the middle and upper atmosphere at high spatial and temporal resolution.Based on the completion of the first phase of the Chinese Meridian Project,two MST radars have been constructed and operated in China,they are the Beijing and Wuhan MST radars.Beijing MST radar is located in Xianghe,Hebei Province(116°59?24??E,39°45?14??N),Wuhan MST radar is built in Chongyang,Hubei Province(114°8?8??E,29°31? 58" N).The advantages of MST radar detection are very prominent,its detection data has the characteristics of wide coverage,high spatial and temporal resolution and high precision.This paper describes its application in common weather and meteorological research,focusing on study its potential to detect tropopause and stratospheric air intrusions.The specific work is as follows:1.Effective data acquisition rate can be used as an indicator to evaluate the overall detection performance of MST radar equipment.Based on the horizontal wind data from 5 beam detection,we analyzed the data acquisition rate of the low and middle mode of Beijing and Wuhan MST radar during 2012~2017.Due to the difference in latitude,the effective data acquisition rates of the two MST radars are significantly different.Beijing MST radar results: the effective data acquisition rate of low and meddle modes decreases with height,and the decline rate of low mode is larger than that of meddle mode.In addition,the curves of data acquisition rate changing with height show obvious inversion at 10.5km and 16 km respectively,which is mainly related to the position of the first and second tropopause.Wuhan MST radar results: the data acquisition rate curve can be divided into two segments,the lower layer of each mode basically does not change with the height,and remains around 80%;and then goes up,it decreases rapidly with the increase of altitude;The reversal of the data acquisition rate occurs at ~16 km.2.Ground clutter has great influence on vertical wind extracted from vertical beam.Considering that the radial velocity resolution is the key factor determining the influence of ground clutter on the vertical wind extraction,we change the radial resolution from the original 0.53 m/s to 0.189 m/s(2.8 times)by changing the coherent accumulation number and FFT point using the low mode of Wuhan MST radar.The results show that the ground clutter and the true turbulent echo can be separated from the original power spectrum even without any treatment.Obviously,the improvement of radial velocity resolution is helpful to extract more accurate vertical wind velocity.3.Based on several typical cases,we introduced the observational applications of MST radars on atmospheric fronts,precipitation,typhoons and atmospheric gravity waves.The results show that: 1)the approximate position of the cold front can be identified by the structural characteristics of the echo power;2)in the precipitation environment,MST radar can simultaneously detect the atmospheric turbulent echoes and precipitation echoes of the same magnitude;3)even the center of typhoon is far away from the radar station(more than 700 km),Wuhan MST radar has observed traces under the influence of typhoon Chan-hom in the upper troposphere and lower stratosphere,namely local northward wind jet..4.Based on the specular echo mechanism,the Radar Tropopause(RT)can be determined by MST radar.Using the Beijing MST radar vertical beam echo power data from November 2011 to May 2017,we conducted a detailed study of the highresolution tropopause structure and changes.By comparing with Lapse-Rate Tropopause(LRT)and Potential Vorticity Tropopause(PVT),the results show that RT and LRT in different seasons show good consistency,with correlation coefficient ?0.74.The correlation between Rt and PVT in winter and spring is relatively well,with correlation coefficients being 0.72 and 0.76 respectively.But the correlation between RT and PVT in summer is poor,with the correlation coefficient being only 0.33.The poor consistency between RT and PVT in summer is mainly related to the subtropical jet that moves to ~40°N.The monthly mean RT and LRT both show significant annual periodic changes: with highest tropopause height in early autumn(September)of ~11.6 km and lowest in early spring(March)of about 10.3 km.5.Atmospheric tides are one of the main sources of modulation of diurnal/semidiurnal periodic variations in tropopause.Through the Lomb-Scargle periodic spectrum analysis on RT height,the results show that RT has obvious diurnal variation in different seasons.But the semi-diurnal variation is not obvious,only occasionally observed in summer and late spring.The research shows the great potential of the Beijing MST radar to measure the tropopause height and observe its diurnal/semi-diurnal variations.6.The process of stratospheric intrusions is considered as an important source of tropospheric ozone.There are still several unresolved issues regarding the use of VHFMST radar to detect and identify stratospheric intrusions.We conducted a detailed study of a cut-off low(COL)case in late November 2014.Based on global ECMWF reanalysis data,AIRS satellite data,and HYSPLIT trajectory patterns,we observed significant evidence of stratospheric intrusions: dry,ozone-rich,high PV values,and low CH4 air intruded downward into the free troposphere at least 500 hPa.During the intrusions,Beijing MST radar observed two very interesting features: RT ascent and the strong downdrafts just preceding the rapid RT ascent.We initially suspect that the rapid RT ascent and strong downdrafts preceding the RT ascent can be used as a potential indicator to detect stratospheric intrusion events.The anomalous high aspect sensitivity zone observed in the troposphere after the strong downdrafts is also a potential clue of stratospheric intrusions.7.In order to verify the potential of the rapid RT ascent and strong downdrafts preceding the RT ascent in the routine detection and identification of stratospheric air intrusions,we selected another 20 typical cases under different weather conditions from March 2012 to January 2015.Combined with the trajectory results and the AIRS satellite detection data,it is found that 15 cases are related to a certain form of stratospheric intrusions.The results show that the rapid RT ascent and strong downdrafts preceding the RT ascent can be used as complementary diagnostic criteria for the observation and identification of possible stratospheric intrusion events.This is of great significance for the monitoring of atmospheric quality and the long-term estimation of tropospheric ozone.