Analysis and Retrieval of Drop Size Distribution in 2016 Taipei Summer Storm Experiment

Abstract

<p>雨滴粒徑分布(DSD)可呈現降雨過程中碰撞、結合及分裂的微物理特徵，近地表的DSD可由雨滴譜儀觀測獲得，空間中搭配雙偏極化雷達高時空解析度的觀測，使用相關公式即可反演三維的DSD參數。DSD參數反演可使用由形狀參數(μ)、斜率參數(λ)及截距參數(N_0 )組成的Gamma DSD；雙偏極化雷達參數亦可由撞擊式雨滴譜儀(JWD)觀測的Gamma DSD模擬，兩者需由μ-λ約束關係做連結。以往使用長期統計之DSD，但DSD的特徵因時、空、降雨類型不同而改變，測試不同DSD觀測有助於改善雷達反演DSD參數的成效。</p> <p>本研究使用2016雙北都會區夏季暴雨觀測預報實驗(TASSE)期間，佈署於五股的移動式X波段雙偏極化都卜勒雷達(TEAM-R)及新店、翡翠水庫的JWD。反演所需的μ-λ約束關係使用本個案於翡翠水庫JWD的觀測資料及Brandes (2003)的統計；另測試直接擬合以JWD計算之DSD參數、雙偏極化雷達參數的關係式，討論三種方法反演對流胞DSD三維結構的成效，結果以μ-λ約束關係的反演結果較佳，直接擬合式無法表現合理約束關係。反演結果在淺對流有較窄的DSD及較小的均值粒徑(D_m )、較低的雨滴濃度(N_w )；深對流有較寬的DSD、較淺對流高數百至數千倍的D_m 、 N_w；層狀降雨有偏窄的DSD、介於深對流及淺對流間的N_w 、 D_m。使用μ-λ約束關係得到雲物理特徵一致，且使用實驗期間的地面DSD觀測能得到更好的結果。</p> <p> </p><p>The microphysical process in precipitation contains collision, coalescence, and break. The Polarimetric Radars can obtain the 3-D structure of the precipitation system, and the disdrometers obtain the drop size distribution (DSD) characteristic on the ground. The 3-D DSD structure could be retrieved through polarimetric variables and DSD statistics data. The constrained relation between the shape (μ) and slope (λ) is necessary to retrieve the Gamma DSD from Z_dr and K_dp. The DSD varies from case to case. Therefore, understanding the variety of DSD is helpful to improve the accuracy of DSD retrieved by radar.</p> <p>The TEAM-R radar and Impact Disdrometer (JWD) data were collected in 2016 TASSE. Two constrain relations were applied: Brandes in 2003, and the JWD statistics during the TASSE. A third approach is directly fitting the Gamma DSD with the polarimetric variables to retrieve the DSD. Because the lack of constraint, the fitting method is unable to retrieve reasonable DSD at large Z_dr. The relatively smaller concentration and drop diameter were found in the shallow convections, whereas higher concentrations were found in deep convection through the convective core.</p> <p> </p>