Consistency Analysis Of Dual-frequency Radar Detection And The Preliminary Research On Microphysical Parameter Retrieval Of Clouds And Precipitation

It is of great significance to accurately retrieve the microphysical parameters of cloud and precipitation to study the microphysical process of the formation and development of precipitation system.It can be used to analyze the distribution and evolution of precipitation,and then improve the accuracy of precipitation estimation based on radar.By combining the return signals of the two frequency channels of the dual-frequency radar,more accurate estimation of the microphysical parameters of precipitation can be provided.The Ka-band millimeter radar,which was located in the meteorological observation field in Longmen,Guangdong,has been upgraded to a dual-frequency radar?Ka/Ku?by the State Key Laboratory of Severe Weather of the Chinese Academy of Meteorological Sciences.The study on retrieving the microphysical parameters of cloud and precipitation using dual-frequency technique was carried out based on this dual-frequency radar and other meteorological equipments in this observation field.Firstly,detection consistency of the dual-frequency radar was analyzed based on the situ data.A stable and uniform 3-hour observation of stratiform cloud precipitation event occurred on April 27,2019 was selected to calculate fluctuates of reflectivity factor difference with time,swings of difference frequency ratio?DFR?with height and the systematic error between two frequency channels of the dual-frequency radar.The results are given as follows:1)After smoothing average,the reflectivity factor difference decreased significantly with time?e.g.from 8d B down to 4d B?;2)The systematic error between Ku-and Ka-band is very small and could be ignored;3)In this process,DFR basically increased steadily with height.Secondly,the empirical relationship between the attenuation coefficient and the liquid water content was fitted by using the raindrop spectrometer observations collected from Longmen meteorological observation field,and the change of the total liquid water with time was preliminarily retrieved by using the measurements of dual-frequency radar based on two precipitation durations of 16:00?19:00 on April 27,2019 and 10:00?14:00 on May 4,2019.The DFR difference caused by Mie scattering was corrected using raindrop spectrum data from disdrometer.The retrieval results were compared with the measurements of microwave radiometer.The following conclusions were given:1)Both the variation trends of the total liquid water and rainfall rate retrieved by dual-frequency radar were in good agreement with those of microwave radiometer;2)Compared with attenuation,the contribution of Mie scattering to DFR is small.Moreover,the total amount of liquid water were found to be very similar,with little differences between before and after DFR-corrected using raindrop spectrum.Finally,the summer raindrop size distribution?DSD?measurements collected from Longmen,Guangdong in 2014?2017 and Naqu,Tibet in 2014 and 2015 were used to analyze the effects of DSD parameters on precipitation estimation using dual-frequency technique.Compared with the Doppler radar,which uses the Z-I relationship to estimate rain rate,dual-frequency radar estimate more accurate rain rate by retrieving the DSD parameters.However,natural DSDs variety in time and space and are hard to be described.To evaluate the retrieval precipitation errors of the dual-frequency radar caused by the difference between the Gamma DSD parameterizations and natural raindrop size distribution,the DSD data from PARSIVEL disdrometer in Longmen and Naqu was used to calculate the reflectivity factor at Ka/Ku-band for simulating dual-frequency technique.Then,the rain rate,median mass-weighted diameter and attenuation coefficient were simultaneously retrieved using dual-frequency algorithm,and evaluated by those directly computed from DSD spectra measurements.The relative bias of the retrieved mass-weighted diameter?D_m?,the Ka-band attenuation coefficient(k _Ka),the Ku-band attenuation coefficient(k _Ku)and rain rate?R?indicates that the impacts of DSD parameters on the dual-frequency radar technique are different in Naqu and Longmen.In general,more stable and accurate retrievals of the four parameters can be gotten in the two regions and the relative bias is generally within 10%,when the shape factor of the Gamma distribution???is chosen as 2,3 or 4.In terms of k _Ka and rain rate of 5?40 mm·h^-1,the estimates from dual-frequnecy technique yield the best accuracy and the relative bias fluctuates near 0 in the Longmen area if?=3.While in the Nagqu area,when?=4,6,the relative bias of k _Ka and rain rate of 5?40 mm·h^-1can attain the smallest value.