| 摘要: | 利用雙偏極化都卜勒雷達，能反演空間中的三維風場和DSD。暖雨區內雨滴的成核、凝結、蒸發、結合和分裂都會改變DSD。除了微物理過程外，雨滴在空氣中的平流與沉降作用也是DSD的變因。若已知兩時間點的DSD與其間的三維風場，便可透過收支計算分離雨滴運動對DSD變化的影響，探討空間中的微物理機制。 本研究分析個案為西南氣流實驗IOP-8期間，NCAR之SPOL雙偏極化都卜勒雷達朝高、屏外海執行密集扇形掃描時所觀測到的對流胞。資料處理流程包括雷達資料的內插、計算系統平均移速、修正觀測時間差、反演三維風場、反演DSD及計算對流胞各時期之收支方程。反演三維風場方面，使用Liou（2007）之單雷達風場反演方法。反演DSD方面，使用Brandes et al.（2003）之constrained gamma method。 該對流胞演化過程分為三個時期，每個時期回波核心區域雨水含量之收支分析如下：在增強期時，全導數項所有粒徑之雨水含量都增加，推論結合與凝結為主要過程。在成熟期I時，全導數項小雨滴和大雨滴減少，中型雨滴增加，推論除凝結外，小雨滴以結合為主，大雨滴以分裂為主。在成熟期II時，全導數項所有粒徑之雨水含量維持穩定，推論各種微物理過程旗鼓相當。在衰減期時，全導數項所有粒徑之雨水含量都減少，懷疑為較乾空氣逸入所造成的蒸發作用。 3-D wind and DSD can be retrieved using dual polarization Doppler radar data. In addition to warm-rain microphysical processes such as nucleation, condensation, evaporation, coalescence and breakup, advection and sedimentation also lead to variation of DSD. The DSD at two time steps and the 3-D wind between allow a budget analysis of the drop number concentration, separating microphysical and kinematic effects. This article analyzes a convective cell observed by NCAR’s SPOL radar when it performed intensive sector scans toward its south during IOP-8 of SoWMEX/TiMREX. The data processing steps include interpolating radar data, calculating the average system speed, correcting the observation time lag, retrieving 3-D wind, retrieving DSD and calculating the budget equation at different stages of the convective cell. The 3-D wind is recovered by the single-Doppler velocity retrieval method of Liou (2007). The DSD is retrieved by the constrained gamma method of Brandes et al. (2003). The evolution of the convective cell is divided into 3 stages, during which the budget analysis of rain water content in the reflectivity core is as follows. During the intensifying stage, the total derivative of rain water content is positive for all drop sizes, which infers coalescence and condensation are the dominant microphysical processes. During mature stage I, the total derivative is negative for small and big drops but positive for median ones, which infers, besides condensation, coalescence and breakup dominate for small and big drops respectively. During mature stage II, the total derivative is nearly zero for all sizes, which infers opposite microphysical processes are well-matched. During the dissipating stage, the total derivative is negative for all sizes, which results, in doubt, from evaporation due to entrainment of drier air. |
