Study Of Fitting Gamma Raindrop Size Distribution

Using the raindrop size distribution (DSD) data obtained from OTT-Parsivel II laser optical disdrometer in 4 to 23 August,2014 in Xinzhou, Shanxi Province, the moments method for estimating Gamma DSD parameters is studied in this paper. The precipitation clouds are divided into three categories:stratiform, cumuliform and stratiform mixed with cumuliform. The micro-physical characteristics values of the three categories of precipitation clouds is computed.Comparing observed and estimated DSD data and each moments in 1 min,5 min average and moving 5 min average, root mean square error (RMSE) and error distribution of DSD were computed with method of least square and moments. A new method of moments is put forward involving M0, M3 and M6 (M represents the moments of raindrop number concentration, liquid water content and radar reflectivity). The relationship between three parameters of DSD and rainfall intensity R,between μ and λ is studied. The change rule of Mo,M3 and M6 with parameter μ and the difference of obtained and estimated DSD is analyzed.The analysis shown:(1) Part of three precipitation micro physical characteristic values are as follows: the average diameter of precipitation is stratiform< 1 mm, cumuliform> 1 mm, stratiform mixed with cumuliform between 1 mm; Raindrop maximum diameter, mean-volume diameter and the effective radius is cumuliform> tratiform mixed with cumuliform> stratiform; Raindrop number concentration is cumuliform> tratiform mixed with cumuliform> stratiform.Averaged R,Z and LWC is cumuliform> tratiform mixed with cumuliform> stratiform.(2) In this paper,the M036 is the best using moments to estimate DSD, because the observed and estimated DSD data and each moments of averaged RMSE in 1 min,5 min average and moving 5 min average is minimum and the distribution of error concentrated.(3) The parameters of DSD estimated from three-kind precipitation clouds decrease with the increase of rainfall intensity R.The fitting DSD parameters (No, μ, λ) satisfied binomial distribution and their correlation coefficient is good. Using M036 estimated three-kind precipitation clouds parameter μ and λ also satisfied binomial distribution. Discussing the values distribution of Mo,M3 and M6 from obtained DSD find that (M6*M0)/M32 and parameter μ+1 has inverse correlation.Finally, difference of the obtained and estimated DSD is analyzed.