Analysis On Cloud Top Physical Characteristics Of Convection Initiation Based On FY-2F Satellite Data

Based on the high resolution data from FY-2F stationary meteorological satellite during May to September in 2015,the cloud top physical characteristics of the Convective Initiation(CI)stage were studied and analyzed by identifying and tracking the cloud clusters.The index and threshold of SATSAST method are selected and modified as well as the cloud pixel filtering method when identifing CI.The displacement between the location of CI at adjacent time is calculated from continuous satellite images to track the development of CI.By using temperature threshold method to identify the deep and shallow convection,the variations and similarities of cloud top physical quantity characteristics such as the cloud top height,the fast cooling rate CTC(Cloud Top Cooling rate)and the multichannel difference in the deep and shallow convection in the CI to development stage are analyzed and compared.Based on the difference of minimum values of CTC between deep and shallow convections in CI stage,we conducted early warning experiments for deep and shallow convections.On the basis of selecting the appropriate threshold of the convective area,the characteristics of the area change of the convective cloud cluster are studied and analyzed.Thus the fitting function of the convective area is obtained and the meaning of the parameters of the function is analyzed.The conclusions are as follows.(1)By using modified SATCAS method and cloud pixel fitering method,most of the misjudged CIs arising from horizontal movement can be removed.The development of CI is traced according to the displacement distance of CI in adjacent satellite cloud images.The experiment shows that the tracking method is simple and effective.(2)The characteristics of the cloud top physical quantities are similar for deep and shallow convections during CI stage.The results show that,the cloud top height increases rapidly in a short time,and the CTC value decreases first and then increases.The deep and shallow convective difference shows that the rising height of the deep(shallow)convective cloud top can(cannot)exceed the height of the water vapor layer;the minimum value of the deep convective CTC is relatively smaller than shallow convective.(3)Through the case study,whether a CI can develop into a deep connection or not can be judged from the difference of minimum values between deep and shallow convections.Therefore,early warning can be made by analyzing the minimum value of convective CTC.(4)The area of CI increases slowly at the beginning,then grow rapidly at mid-term and finally grow slowly to the maximum value.The fitting function of area development is E = a*exp[-(b-t/c)2],a represents the maximum value of the area in the process of convection development,b is the time when the convection development reaches the largest area,c is related to the maximum value of area change rate.The parameters in the fitting function can be determined by the area values at previous times in the development of the cloud cluster,thus the area values at subsequent times can be predicted.