A Process of Hydrometeor Phase Change with Dual-polarimetric Radar

The phase of hydrometeor is one of the most important microphysics characteristics of cloud. The development of dual-polarimetric weather radar makes the retrieval of the hydrometeor phases possible theoretically, which has been one of the hottest applications of the dual-polarimetric radar. The fuzzy logic has been extensively used in the classification of hydrometeor now, and become the dominant technique in this field. Th rough continuously studying in statistics with more and more in situ measurements, the parameters in fuzzy logic algorithm have become relatively steady for individual dual-polarimetric radar in operational use. The evolution of hydrometeor phase in the cloud process is an important aspect to the research of water microphysical circular in the cloud-precipitation system, and plays great role in many meteorological fields, such as weather modification, aviation security, weather model, and so on. How ever the studies on the changing of the hydrometeor phase with time series of radar data are relatively immature, publications in which are seldom seen. A fuzzy logic system for classifying hydrometeors based on the combination of polarimetric radar measurements and conventional observation data is described, and a Beta membership function is utilized for the fuzzification, the parameters of which are also given based on the former statistics achievements for the S-band radar.The input variables include radar reflectivity, L_DR, Z_DR and the height of 0℃ and -40℃ layer, and the output types are drizzle, rain, low-density dry ice crystal, highdensity dry ice crystal, wet ice crystal, dry graupel, wet graupel, small hail, large hail, sleet, and cloud droplet. Then a case study on an evolution of the hydrometeor phase in a stratiform cloud precipitation process is analyzed based on the CAM Ra radar and RAOBs data, which takes place at Chilbolton the UK summer morning, and lasts approximate 39 minutes. The whole process is divided into three phases including the initial phase, mature phase and the dissipating phase, for each phase a analysis on the changing of the hydrometeor type is given based on the classified results of all the radar observations in it, and the results show that in the initial phase stratiform cloud has a layered structure of hydrometeor types including high-density dry ice, wet ice crystal and liquid droplet from top to bottom; the core of the large-echo region is filled by large ice crystals, and the other area in the large-echo region is filled by liquid hydrometeors in initial phase; from initial phase to mature phase liquid hydrometeors on the top of the large-echo region have a trend of freezing; in the dissipating phase the 0℃ layer bright band disappears gradually, on the top of which a wet ice crystals are wrapped by high-density dry crystal.