利用雙偏極化雷達分析颱風雨帶之運動場與雲物理特性

Abstract

本論文利用地面雷達觀測資料對蘇迪勒颱風雨帶的運動場和雙偏極化參數特性進行分析，主要目的為對登陸颱風螺旋雨帶的雲物理過程有更近一步的了解。我們利用了對流與層狀降雨分解演算法來定義位於颱風移動方向第一象限的兩個次雨帶，分別稱之為「北雨帶」和「南雨帶」。 由雙都卜勒合成風場分析結果得知，北雨帶高層為輻合，底層為輻散，伴隨弱的下沉氣流；而南雨帶底層為輻合，高層為輻散，伴隨增強的上升運動。此結果顯示，兩個雨帶可能處於不同的生命期：北雨帶處於消散期，而南雨帶處於發展期。雨帶的雙偏極化參數垂直剖面的變化顯示，在登陸之前，南雨帶的在溶解層之上有較多的冰相雲物理過程，可能和南雨帶增強的上升氣流有關，較強的上升氣流可將較多的水氣傳送到高層，而有較高的過飽和度讓冰相粒子生長；在溶解層以下，雨滴可能經由碰撞凝結的過程成長或是藉由雲水凝聚成雨滴所成長；北雨帶雖然有類似特性，但雙偏極化參數數值都較南雨帶為小，且變化不明顯，推測和北雨帶處於消散期有關係。因此，推測雨帶在登陸前，冰相雲物理過程和暖雨過程都對降雨有貢獻。 當雨帶登陸山區之後，雨帶的底層回波明顯增強，且回波垂直上向上發展，雙偏極化參數也顯示，在溶解層以下，雨滴的平均粒徑變大且液態水含量增加，這樣的特徵可能和雨帶受到地形抬升，導致降雨增強有密切的關係。然而，雨帶登陸後大約30分鐘之後，從回波場幾乎沒辦法辨別北雨帶，表示北雨帶已經消散，其原因可能和原本所伴隨的下沉運動受到背風坡的下沉氣流加強所導致。同時，南雨帶的底層回波減弱，且所伴隨的上升運動減弱，顯示南雨帶也減弱。另外，在溶解層高度以上，雙偏極化參數的垂直剖面變化並不大，推測在雨帶登陸之後，雨帶所伴隨的強降雨的雲物理過程主要是透過暖雨過程。

Typhoon Soudelor (201513) made landfall and moved across the topography in northern Taiwan. The Convective-Stratiform Separation Algorithm were used to identify two secondary rainbands (“rainband N” and “rainband S”) located in the right front quadrant during Soudelor’s landfall. The evolution, kinematic and microphysical characteristics of these rainbands are examined in this study. The rainband kinematic structures were examined using dual-Doppler retrieved winds. The reflectivity and band-relative horizontal winds of two rainbands seem to suggest they were in different developing stages: rainband S was intensifying with convergence below and divergence above while rainband N was dissipating with divergence below and convergence above. The line-averaged vertical airflow and precipitation structure associated with the offshore segments of two rainbands exhibit some similar features to inner rainbands while some resemble outer rainbands. Consistent with the kinematic structures, the polarimetric vertical profiles reveal corresponding microphysics in two rainbands. The vertical profiles of polarimetric variables in rainband S indicate more moisture content was transported to the higher level through the intensifying updraft, which caused greater supersaturation for ice particles to grow above the melting layer. The features below the melting layer suggest collision-coalescence process and/or accretion of cloud water by raindrops. Although the polarimetric vertical profiles of rainband N have similar distributions, the values are all smaller than that in rainband S. This might be related to the dissipating of rainband N. The microphysics suggest both ice-phased and warm rain process play important roles in the major pathway to heavy rainfall in rainbands when they were offshore. After two rainbands made landfall, the reflectivity structures at near-ground level became stronger. The polarimetric features also suggest the mean raindrop size and liquid water content are the largest, which relates closely to heavier rainfall within the rainbands due to the terrain lifting. However, at later time spans, rainband N was observed mostly dissipated. This might be related to the intensification that original downward motion combines with the downslope flow in the lee. In contrast, the reflectivity structure and airflow pattern in rainband S associated with the landfalling segment are similar to that with the offshore segment, but with weakening updrafts. The features of polarimetric variable above the melting layer presented less curved profiles, indicating the less moisture content transported for ice-phased processes. Therefore, the dominated process to the precipitation after two rainbands made landfall is mainly via warm-rain processes like collision and coalescence.