Analysis Of Heavy Rainfall In Hanzhong Basin Under The Influence Of Southwest Vortex Based On Radar Data

The basin Hanzhong is the main area influenced by precipitation of Southwest Vortex moving along the northeast direction.The topography of this area is complex,the difference of altitude is large,the distribution about precipitation is extremely uneven,and the coefficient of disaster is high.Under the influence of the southwest vortex moving along the northeast direction,the heavy precipitation occurred from 28 to29 June 2015.The maximum hourly precipitation was 70.9mm/h.The heavy rainfall caused disasters in seven counties,including Foping,Yangxian and Zhenba.According to the municipal civil affairs bureau,the precipitation caused 4 deaths and 10 missing people,with direct economic losses as high as 683 million yuan.In this paper,the data as NECP reanalysis,Doppler radar,FY-2G satellite TBB,Micaps precipitation are used to study the circulation situation,precipitation area,the movement of heavy precipitation center and the meso-scale structure of southwest vortex in Hanzhong Basin under the influence of southwest vortex.Secondly,the timing scale of precipitation in meso-scale system is about 5-30 minutes,which its precipitation is concentrated and the intensity of rainfall is large.At present,the threshold of short-time heavy precipitation usually takes 1 hour precipitation as 20 mm,which can not reflect the order of short-time heavy precipitation recorded by automatic station minute observation,nor can it reflect the characteristics of precipitation time of disastrous weather in Hanzhong Basin.Therefore,based on the radar 6 minute scan,the threshold of the every six minute very short time heavy precipitation in Hanzhong Basin is studied and divided by using the minute order precipitation data for this short time heavy precipitation process.In addition,a criterion for the determination of precipitation size in meso-scale systems is proposed.Finally,in order to effectively forecast precipitation in the middle and small scale of Hanzhong basin,radar data and minute precipitation are used to establish radar reflectivity factor-precipitation data pair,and the time relationship between radar reflectivity and minute precipitation is explored.The localized Z-R equation is established to provide a reference for rainfall prediction in mesoscale systems using radar echo.The main conclusions are as following:?1?Through the analysis of the situation on circulation of the process of heavy precipitation,it is found that the this is a process with heavy precipitation under the influence of 850hPa Southwest Vortex,which is influenced by the 700hPa low level jet stream and the divergence of the upper air,resulting in the process with strong rainfall in the left front of the southwest Vortex's moving direction.The research on the moving path of the strongly precipitating center shows that it moves gradually along the southwest to northeast,which is consistent with the direction of the southwest Vortex.By analyzing the characteristics of radar's echo in heavy precipitating process,it is shown that the echo is presented as stable layer resembling cloud before the occurrence of precipitation;When precipitation occurs,it gradually develops into mixed-cloud precipitation echo field with strong echo center.On the velocity image,there is a headwind zone or meso-cyclone formation.In the later stage of precipitation,the echo is weakened and converted from mixed-cloud echo to stratiform-cloud echo.The transforming process of radar's echo,"stratiform-cloud—echo-mixed cloud echo—stratiform-cloud echo"is the reflection of the various characteristics of southwest Vortex on radar's echo.?2?The Doppler Radar in Hanzhong Basic has completed tracking and monitoring of the very short term heavy precipitation process.According to the characteristics of radar's echo of two centers with strong precipitation,we find that the system with mesoscale in MCC caused by the southwest Vortex is the direct cause of the process of heavy precipitation.The multi monomer structure mosaicked with super monomer storm is its specific form.The strength of the supercell center in the two regions is maintained above 53dBZ and accompanied by a deep mesocyclone structure,in which Zhenba has a zonal headwind zone.The intensity of precipitation is directly affected by the structure of supercell.?3?Based on the analysis of the precipitation in Chenggu and Yangxian,the precipitation time is about 10-40 minutes,which is a very short time heavy precipitation process,but there is a lack of the basis to judge the size of the very short term heavy precipitation from the minute precipitation.In this paper,the precipitation threshold of 6minutes is divided by using the precipitation data of the whole city and corresponding to the radar scanning in 6 minutes,and the results are verified.This paper is based on the probability that the frequency of each order of precipitation accounting for the total number of effective precipitation.According to the times including 95%,97.5%and99%of cumulative probability of totally effective precipitation of the rainfall,critical value of 1.3 2.0mm/6min were defined as general heavy rainfall,and 2.1 3.0mm/6min as a heavy rainfall in short time,more than 3.1mm/6min in extremely short strong precipitation.The results show that the criterion of heavy precipitation in extremely short term is reasonable after bringing into the verification of center with strong precipitation and comparing with the order of strong precipitation in short time.?4?Radar reflectivity factor is an important information to reflect the magnitude of heavy precipitation.The average reflectivity factor intensity of the peripheral area of Chenggu and Yangxian stations is selected.The relationship between reflectivity factor and 6 minute precipitation at the corresponding station is analyzed.By analyzing the relationship between the reflectivity factor of different elevation angle and the precipitation of 6 minutes in the measuring station,it is found that compared with the radar strong reflectivity factor,The strong precipitation corresponds to the strong reflectivity factor of the low elevation angle?0.5°and 1.5°?70 minutes ago,indicating that the heavy precipitation lags behind the strong reflectivity factor for 70 minutes.?5?Based on the factor of reflectivity of center with heavy rainfall and precipitating data of regression,a localized Z-R relationship likeZ?28?5051.R.²¹⁶⁴was built by using a linear regression least-squares fitting function to estimate minute precipitation,the test and the actual value shows that the fitting formula can very good at valuing precipitating inversion.This provides a reliable basis for the prediction of precipitation by radars echo and reduces the occurrence of weather with severe precipitation.