The scattering mechanism of squall lines with C-Band dual polarization radar.Part Ⅱ:the mechanism of an abnormal ZDR echo in clear air based on the parameterization of turbulence deformation

In part I,the clear air echo in front of the squall line is caused by turbulence diffraction,which makes the Z_(DR) echo characteristics different from particle scattering.To study the turbulence deformation phenomenon that is affected by environmental wind,the turbulence-related method is used to analyze the characteristics of three-dimensional turbulence energy spectrum density,and the parametric model of turbulence integral length scale and environmental wind speed is established.The results show that the horizontal scale of turbulence is generally larger than the vertical scale.The turbulence is nearly isotropic in the horizontal direction,presenting a flat ellipsoid with the vertical orientation of the rotation axis when there is no horizontal wind or the horizontal velocity is small.When horizontal wind exists,the turbulence scale increases along the dominant wind direction.The turbulence scale is positively correlated with the wind speed.The power function is used to fit the relationships of turbulence integral length scale and horizontal wind speed,which obtains the best fitting effect,and the goodness of fit(GF)is above 0.99 in each direction.The deforming turbulence can cause 8-9 dB Z_(DR) anomalies in the echo of dual polarization radar,which the ratio of scales in the dominant wind and the vertical direction of deforming turbulence(L_(u)/L_(w))is around 4.3.The variation in Z_(DR) depends on the turbulence shape,orientation and the relative position between turbulence and radar.The shape of turbulence derived from radar detection results is consistent with that of the parametric model,which can provide a parametric scheme for turbulence research.The results reveal the mechanism of abnormal Z_(DR) echo caused by deforming turbulence.

The scattering mechanism of squall lines with C-Band dual polarization radar.Part Ⅰ:echo characteristics and particles phase recognition

Squall line is a kind of common mesoscale disaster weather.At present,there are few studies on the elaborate detection of squall line by dual polarization radar.With the dual polarization upgrade of weather radar network,we need to study the relationship between squall line echoes of base data and polarization data to reveal new echo phenomena and formation mechanisms.The relationship between radar parameters and atmospheric physical processes also need to be examined.Based on the NUIST CDP radar,a squall line in the Yangtze and Huaihe River basin that occurred from July 30 to 31,2014 is analyzed.The results show that polarization parameters have obvious advantages in the characteristics analysis of size,phase state,shape and orientation of the water condensate particles.The phase states of water condensate particles in convection cell can be distinguished through comparative discussion.Several phase states exist in the squall line,including small,medium and large raindrops,melting hails,dry hails and ice crystal particles and the Z_(DR)column can be used to identify the location of the main updraft.In addition,the polarization parameters are more sensitive to the melting layer.The gust front is presented as a narrow linear echo in Z affected by strong turbulence.It is an obvious velocity convergence line in V and approximately 0.70 inρ_(HV).The Z_(DR)can be used as a criterion to distinguish the horizontal and vertical scale of turbulence.The deforming turbulence,which is affected by environmental airflow,will cause an abnormally high Z_(DR)in the gust front and a negative Z_(DR)before and after the gust front.The variation of Z_(DR)depends on the turbulence arrangement,orientation and relative position between turbulence and radar.These dual polarization parameter characteristics offer insights into understanding the structure and evolution of the squall line.

Statistics-based Optimization of the Polarimetric Radar Hydrometeor Classification Algorithm and Its Application for a Squall Line in South China

A modified hydrometeor classification algorithm （HCA） is developed in this study for Chinese polarimetric radars. This algorithm is based on the U.S. operational HCA. Meanwhile, the methodology of statistics-based optimization is proposed including calibration checking, datasets selection, membership functions modification, computation thresholds modification, and effect verification. Zhuhai radar, the first operational polarimetric radar in South China, applies these procedures. The systematic bias of calibration is corrected, the reliability of radar measurements deteriorates when the signal-to-noise ratio is low, and correlation coefficient within the melting layer is usually lower than that of the U.S. WSR-88D radar. Through modification based on statistical analysis of polarimetric variables, the localized HCA especially for Zhuhai is obtained, and it performs well over a one-month test through comparison with sounding and surface observations. The algorithm is then utilized for analysis of a squall line process on 11 May 2014 and is found to provide reasonable details with respect to horizontal and vertical structures, and the HCA results---especially in the mixed rain-hail region--can reflect the life cycle of the squall line. In addition, the kinematic and microphysical processes of cloud evolution and the differences between radar- detected hail and surface observations are also analyzed. The results of this study provide evidence for the improvement of this HCA developed specifically for China.

Multiple interferences suppression with space-polarization null-decoupling for polarimetric array

The adaptive digital beamforming technique in the space-polarization domain suppresses the interference with forming the coupling nulls of space and polarization domain.When there is the interference in mainlobe,it will cause serious mainlobe distortion,that the target detection suffers from.To overcome this problem and make radar cope with the complex multiple interferences scenarios,we propose a multiple mainlobe and/or sidelobe interferences suppression method for dual polarization array radar.Specifically,the proposed method consists of a signal preprocessing based on the proposed angle estimation with degree of polarization(DoP),and a filtering criterion based on the proposed linear constraint.The signal preprocessing provides the accurate estimated parameters of the interference,which contributes to the criterion for null-decoupling in the space-polarization domain of mainlobe.The proposed method can reduce the mainlobe distortion in the space-polarization domain while suppressing the multiple mainlobe and/or sidelobe interferences.The effectiveness of the proposed method is verified by simulations.

STUDY ON APPLICATIONS OF C-BAND DUAL LINEAR POLARIZATION RADAR IN METEOROLOGY

Theoretical study and development of a dual linear polarization weather radar in China are briefly presented. Also discussed are the potential uses of the new radar system in improving the accuracy of areal rainfall measure- ments and analysing the spacial structure of storms and distribution of hydrometecrs in clouds based on the radar observational data from the field experimcnts during the summers of 1987—1989. The results indicate that a C-band dual polarization weather radar, after considering the microwave attenuation correction, may be employed to quantitatively measure rainfall and to monitor heavy rain and flood events and becomes an impor- rant means to study storm structure.

Utilization of Specific Attenuation for Rainfall Estimation in Southern China

This study uses rain gauge observations to assess the performance of different radar estimators R(ZH),R(KDP)and R(A)in estimating precipitation based on the observations of an S-band polarimetric radar over southern China during a typical convective storm and an extremely severe typhoon,i.e.,Typhoon Manghkut.These radar estimators were derived from observations of a local autonomous particle size and velocity(Parsivel)unit(APU)disdrometer.A key parameter,alpha(α),which is the ratio of specific attenuation A to specific differential phase KDP with three fixed values(α=0.015 dB deg^(-1),α=0.0185 dB deg^(-1)andα=0.03 dB deg^(-1))was examined to test the sensitivity of the R(A)rain retrievals.The results show that:(1)All radar estimators can capture the spatio-temporal patterns of two precipitation events,R(A)withα=0.0185 dB deg^(-1)is well correlated with gauge measurement via higher Pearson’s correlation coefficient(CC)of 0.87,lower relative bias(RB)of 16%,and lower root mean square error(RMSE)of 17.09 mm in the convective storm while it underestimates the typhoon event with RB of 35%;(2)R(A)withα=0.03 dB deg^(-1)shows the best statistical scores with the highest CC(0.92),lowest RB(7%)and RMSE(25.74 mm)corresponding to Typhoon Manghkut;(3)R(A)estimates are more efficient in mitigating the impact of partial beam blockage.The results indicate thatαis remarkably influenced by the variation of drop size distribution.Thus,more work is needed to establish an automated and optimizedαfor the R(A)relation during different rainfall events over different regions.