雙偏極化雷達觀測參數反求雨滴粒徑分佈以及降水估計

Estimation of Drop Size Distribution and Rainfall from Dual Polarimetric Radar Measurements

紀博庭(Bo-Ting Ji) ； 陳台琦(Tai-Chi Chen Wang) ； 呂崇華(Chung-Hua Lu)

《大氣科學》 36卷1期 (2008/03) Pp. 21-41

摘要

雙偏極化雷達的觀測資料會受雨滴大小、形狀、傾斜角度等情形所影響，並能藉此特性來提供雨滴粒徑分布(Drop Size Distribution, DSD)以及降雨率等資訊。在Gamma DSD假設下(N(D)=N0D(上標 μ)e(上標 -ΛD))，DSD的控制參數μ和Λ之間會有一約束關係存在，Zhang et al.(2001)利用此約束關係搭配S-Band雙偏極化雷達所觀測到之回波(reflectivity, Z(下標 H))、差異反射率(differential reflectivity, Z(下標 DR))來反演DSD進而計算降雨率，藉此來提升降水估計之準確性。本研究利用中央大學C-Band 偏極化雷達資料對冬颱南瑪都(NANMADOL)做降水估計，除了使用Zhang et al.(2001)之方法外，並另行嘗試加入比差異相位差(Specific differential phase shift; K(下標 DP))此資訊來做反演，結果顯示反演過程中使用K(下標 DP)在強降水時的表現優於使用Z(下標 H)。相較於利用經驗公式求降雨率，使用此法的優點除了能改善降水估計外，還能獲得Gamma DSD的三個控制參數供雲物理探討及預報模式使用。

關鍵字

雙偏極化雷達 ； 雨滴粒徑分佈 ； Gamma約束關係 ； 降雨率

並列摘要

High time resolution C-Band polarimetric Doppler radar data from the measurements of NANMADOL typhoon rain were analyzed to study the algorithms for retrieving rain rate.
A constrained relation between the shape (μ) and slope (Λ) parameters will appear when the drop size distribution (DSD) is assumed to be a Gamma distribution. In this study, the constrained relation and two of the three radar parameters: reflectivity (Z(subscript H))、differential reflectivity (Z(subscript DR)) and specific differential phase shift (K(subscript Dp)) were used to retrieve the DSD and rain rate. The result showed that, using of K(subscript Dp) is much better than Z(subscript H) in heavy rain, and the retrieved accumulative rain agree with the measurements of raingauges very well. The method shows improvement over the existing method using to estimate rain rate in heavy rain and it can provide Gamma distribution for research of cloud physics.

並列關鍵字

Dual polarization radar ； Drop size distribution DSD ； Constrained-gamma ； Rain rate