Study On The Applications Of Microwave Link In Quantitative Precipitation Estimation Of Weather Radar

Quantitative precipitation estimation?QPE?has always been the focus of many research fields such as meteorology and hydrology.Improving the capability of QPE is of great significance to the study of water cycle in geo-atmospheric system,dealing with flood disaster and enhancing military capability.Weather radar has become an effective method for regional QPE because of its high spatial-temporal resolution and accuracy.However,it is still affected by attenuation,spatial-temporal variation of precipitation estimator?PE?and vertical variation of precipitation,etc.By making full use of the matching advantage between microwave link and radar,the main objective of this manuscript is to improve the capability of the radar QPE.Based on statistical analysis of local rain attenuation characteristics,the multi-level application methods of microwave link in radar QPE including attenuation correction,optimization of radar PE and calibration of radar rainfall field are studied.A novel method to correct radar reflectivity for attenuation using arbitrary oriented microwave link is proposed.The methods are also proposed to optimize the radar PE using the mean path rain rate derived by the microwave link.The radar rainfall calibration methods including the average bias calibration method,the Kriging calibration method,the Kalman filtering calibration method and the variational calibration method are analized and compared.Experiments in real rainfall cases were carried out to verify the proposed methods using the operational weather radar and microwave links.The main work and results are listed as below:a.Analysis of microwave rain attenuation characteristics.The rain attenuation model is the basis of precipitation measurement of microwave link.Because the ITU-R rain attenuation model is not suitable for all precipitations of different regions,seasons and types,the raindrop spectrum data of two consecutive summers in Nanjing area are analyzed to establish the local rain attenuation model for different types of precipitation.The results show that there is a certain difference between the statistical model and the ITU-R model.Different types of precipitation have different characteristics of rain attenuation.The application of ITU-R model will cause errors in rainrate calculation.b.Radar attenuation correction assisted by microwave link along radar radial.The impact of attenuation caused by precipitation at higher frequencies needs to be compensated for successful implementation of QPE.However,because the actual amount of attenuation is unknown,the existing methods for attenuation correction is inherently unstable and certain constraints imposing on the total attenuation are required.The path attenuation of a microwave link which is along the radar radial path is used as the total radial attenuation of the radar.The relationship between the specific attenuation and the radar reflectivity factor?A-Z?is obtained by the least residual method with maximum attenuation constraint.The reflectivity factor can also be corrected for attenuation using the A-Z relationship.The results of the proposed method in a rainstorm showed that the maximum attenuation constraint avoids the sudden increase of reflectivity factor.The corrected reflectivity factor of the X-band weather radar was enhanced effectively and was more consistent with the reflectivity factor of a nearby S-band weather radar.The rain rate estimated by the corrected reflectivity factor increased notably and was more consistent with the rain rate measured by rain gauge.c.Radar attenuation correction assisted by arbitrary oriented microwave link.The method utilizing microwave link along radar radial reduces the availability of arbitrary oriented microwave link,and the effective area of correction is too small.So a method which uses arbitrary oriented microwave link to assist the attenuation correction of weather radar is proposed.By minimize an object function which is defined as the difference between the attenuations calculated by radar and link respectively,the ratio of specific attenuation to specific differential phase,which is a key parameter in the dual-polarization?DP?radar attenuation correction method,is optimized.The proposed method was evaluated using real data of an X-band DP radar and measurements of two microwave links during two rainstorm events.The results showed that the radar reflectivity was significantly compensated after attenuation correction,especially at long distances.The corrected reflectivity was more intense than the reflectivity corrected by the“self-consistent”?SC?method and closer to the reflectivity of a nearby S-band radar.The effectiveness of the method was also verified by comparing the rain rates estimated by the X-band radar with those derived by rain gauges.The rain rates estimated by the X-band radar after attenuation correction were increased significantly and were more consistent with the measurements of rain gauges.d.Optimization of radar PE.The accuracy of radar PE impacts on the accuracy of radar QPE directly.The ideal rainfall estimation scheme should apply the PEs with temporal-spatial dynamic changes.The typical PE relationships,including Z-R relation for S-band weather radar and K_DP-R relation for X-band DP weather radar,are optimized by the optimization method,attenuation ratio method and least square fitting method,using the mean path rain rate derived by the microwave link.The experimental results showed that the microwave link can be used to optimize the PE of weather radar,and the precipitation derived using the optimized PE was closer to the rain gauges.The maximum rainrate constraint can effectively avoid the overestimation of rainrate.The optimized rainfall results of S-band radar Z-R relationship are better than those of X-band radar K_DP-R relationship.The effectiveness of the optimization method and the least square fitting method are better than the attenuation ratio methods in general.e.Calibration of radar rainfall field.In the proposed methods,the mean path rainrate of microwave link is utilized to calibrate the radar rainfall field using the average bias calibration method,Kriging calibration method,Kalman filtering calibration method and variational calibration method.The diffrences in the effectiveness and calibration factors are also analized.The S-band weather radar rainfall fields in two precipitation processes with different characteristics were calibrated.The results showed that the radar rainfall fields calibrated by the proposed methods were more consistent with rain rates measured by the rain gauges.The statistical errors of MAE,ME and RMSE were obviously reduced and the estimation accuracy of radar rainfall field was effectively improved.Among the proposed methods,the variational calibration method,Kriging calibration method and the average calibration method were relatively better than the Kalman filtering calibration method.The variational calibration method and Kriging calibration method can obtain a calibration factor field with temporal-spatial variation.Variational calibration method was most effective and stable among the proposed calibration methods.This paper discusses the multi-level applications of microwave link in radar QPE.The experiment platform with operational radar and microwave links were constructed to verify the proposed methods in precipitation cases.Since the microwave links have been widespread around the world,the proposed methods can be extended in many regions and occasions without any additional costs.So,the proposed methods are of certain scientific and application significance.