Evaluation Of TRMM PR On Light Precipitation Observation

The first spaceborne precipitation radar(PR)operating at a frequency of 13.8-GHz(Ku-band),boarded on the TRMM,yields the most direct observation of the three dimensional precipitation over most of the tropics and subtropics and enables advancement in research from moderate to intense precipitation.However,its relatively low sensitivity,minimum detectable signal of～17dBZ(corresponding to～0.5mm/h of rain rate),leads to an inherent problem of limited capability for identifying and quantifying light precipitation.Evaluation of PR on light precipitation observation will provide precise information of the occurance and intensity of precipitation and facilitate our understanding on latent heat release and water cycle.In this study,the limitation in the sensitivity of the TRMM PR to detect light precipitation(both light rain and falling snow)over both ocean and land region between 35°S and 35°N is assessed by using the 4.5 years(2006-2010)CPR and PR radar reflectivity from dataset 2D-CloudSat-TRMM.The percent of the precipitation occurrence missed by PR and its contribution rate for the total precipitation volume is quantified.First,through the case study of both stratiform and convective rain,and the case study of both warm rain and tibet plateau area rainfall and snowfall,focusing on the intercomparsion of PR and CPR radar reflectivity and their near surface precipitation rate,the limitation in detecting light precipitation of PR is underlined.Results indicate that PR and CPR detection performance of precipitation are highly complementary.Because of high sensitivity,CPR has the congenital advantage of detecting the presence of the light rain and snowfall,while operating at centimeter wavelength,PR is more suitable for detecting moderate to intensive rain.In the case study,the scatterplot,cross section of reflectivity and the number of precipitation pixels for the PR and CPR coincident overpass precipitation event are analyzed in detail.The analyses show that PR has better perfomance for convective rain than stratiform rain in the near surface region.Nevertheless,PR can miss large amount of ice particles above freezing level for both stratiform and convective rain.Moreover,PR can hardly detect the warm rain because of its relative small droplet radius.At the same time,PR has poor capability for observing snowfall.The irregular form of snowfall makes its scattering character different from sphere rain droplets.Operating at higher frequency(94-GHz),CPR is more sensitive to snow particle scattering.Second,in this study,the percent of the precipitation occurrence missed by PR is estimatedby using the CPR and PR radar reflectivity data.The overall statistical result demonstrates that near the surface(2km),PR could miss about 68%precipitation occurance.In addition,the statistical characteristics including vertical,zonal and geographical distribution and the situation in the different condition are studied.The vertical distribution shows that above 3km height,the percent of missing precipitation increases with the rising of the height.At 10km height,PR can hardly detect the occurance of the precipitation,which indicates that PR has limitation in observing solid precipitation and ice particles aloft.The geographical distribution shows the regional difference.At the sametime,the research shows that PR has the poor performance for the snowfall and theprecipitation which is isolated and with small rain-intensity.Third,by using the simulated Ze-R and Liu’s Ze-S,the radar reflectivity of the precipitation missed PR can be converted to rain rate and snowfall rate,which will be finally used for quantifying the contribution rate for the total precipitation volume.The results indicate that,although PR misses about 68%occurance of precipitation,these parts contributefor less than 2%the total precipitation volume.However,in the region where PR misses the most of the precipitation occurance,the contribution rate can reach 40%,which manifests that light precipitations missed by PR still have great importance to the local precipitation volume.