C波段雙偏極化雷達之雷暴觀測研究

Abstract

在2018年雙北夏季暴雨實驗期間針對午後雷暴進行密集觀測。本研究使用五分山C波段雙偏極化雷達密集式掃描，同時利用共站觀測的S波段雙偏極化雷達，針對2018年8月17日午後雷暴進行高時空解析度降雨、運動場結構，以及雙偏極化參數特徵分析。 觀測結果顯示有明顯的BWER、ZDR弧、KDP柱，以及ρHV弧等類似中緯度超級胞經常觀測到的雙偏極化參數特徵。但不論是時間或空間的規模皆和美國中西部大平原典型的超級胞無可比擬。經由自洽法及同步比較法的結果顯示，C波段在豪雨時衰減效應顯著，導致能量參數ZHH及ZDR的大值被低估。衰減效應可以透過衰減訂正法解決，雙波段Z_DR的差值並非成線性關係，而是在ZDR大於1.5 dB之後呈現另一趨勢，由此可知兩波段ZDR參數關係無法直接透過線性關係式描述之間的關係，而必須考慮共振效應帶來的非線性結果。本研究也發現共振效應在不同仰角有不同強度的反應，可能和雨滴粒子掉落時產生快速變化有關，將來必須考慮仰角之間共振效應的敏感度差異。 在水象粒子分類的結果，指出mdf-D13對於融解層附近濕雪的分辨較為敏銳，而P09則對雨雹混合物的辨識最為敏感，但分布較為零碎。透過C波段RHI掃描能觀察到接近對流層頂有大範圍霰的存在。這結果過去沒有被提及，表示雷暴低層提供充足的水汽，直到高層仍有很明顯的淞化現象。並且從淞化粒子掉落過程，可以推測這是導致後續地面量測到局部短延時強降雨的主因。另外透過水象粒子及閃電訊號的比對，發現霰和閃電的訊號有很好的相關性，且霰對閃電的躍升大約有20分鐘的領先時間。 統計對流前後期的水象粒子隨高度的變化，顯示在雷暴不同發展階段有明顯垂直結構的差異。前期的對流體積集中在對流層頂，代表受到強烈上衝流影響，除了淞化粒子之外，也有許多冰晶被傳送到對流層頂。而後期的體積則集中在中層，主要是受到前期高層冰晶落下的影響，而此時的降雨型態則偏向層狀降雨。因此，利用C波段雙偏極化觀測的優勢，結合水象粒子分類演算法，除了幫助我們更加理解雷暴的內部結構，也可大幅強化雷暴強降水的預警訊號。

Intensive observations on afternoon thunderstorms were made in 2018 TAipei Summer Storm Experiment (TASSE). This study used Wu Fen Shan C-band polarimetric radar (RCMD) with intensive scans, making contemporaneous comparisons with co-site S-band polarimetric radar (RCWF), to analyze the characteristics of precipitation and kinematic structures, and polarimetric variables of afternoon thunderstorms with fine time and space resolution on Aug. 17, 2018. The polarimetric signatures in mid-latitude supercells, usually including Bounded Weak Echo Region (BWER), ZDR arc, KDP column, and ρHV arc, have been found obvious in the case. However, the time or space scale was not comparable with the one of typical supercells in the Great Plains. By self-consistency and contemporaneous comparisons, the attenuation effect in heavy rain was significant at C band, resulting in underestimation of maxima of power parameters (ZHH and ZDR). The attenuation effect could be solved by the attenuation correction, whereas the differences of ZDR values at 2 bands have been pointed out the nonlinear relationships were notable above ZDR=1.5 dB. Therefore, the relationships between 2 bands could not be described in one linear equation, and we must consider the outcome of resonance effect. This study also suggested the strength of resonance effect could vary with elevations. It might be due to the rapid change of falling raindrops. The variations in sensitivity to heights must also be taken into account in the future. The results of Hydrometeor Classification Algorithms (HCA) indicated mdf-D13 was sensitive to “wet snow” near the melting layer. By contrast, P09 was sensitive to “a mixture of rain and hail,” but it was recognized fragmentation. A wide range of “graupel” near tropopause, having not been mentioned in the past, was observed by RHI scans at C band. It represented the significantly upper-level riming owing to sufficient water vapor from the bottom of thunderstorm. We could also speculate locally and short-term extreme precipitation on the ground mainly resulted from previously falling rimed particles. Furthermore, there was a strong positive correlation between signatures of graupel and lightning jumps, and the former had approximately 20-minute lead time. The statistics of hydrometeors in the vertical distribution revealed distinct features in the different stages of the thunderstorm. The volume of convection concentrated on upper levels with not only rimed particles but also ice crystals in the earlier stage, representing the influence of strong updrafts. In the later stage, the volume concentrated on middle levels because of the preceding falling ice crystals, and the precipitation pattern tended to stratiform component. Therefore, if we combine the advantages of C-band polarimetric observations and HCA, the structures of thunderstorms would be clearer, and earlier warning of intense rainfall on the ground could be strengthened.