| 摘要: | 融化層(ML, Melting Layer)是層狀降水的重要特徵，融化層的頂部接近0°C等溫線的高度，與雪花或冰晶的融化有關，並且融化層的底部指示了液體降水的垂直範圍。偵測融化層其中一項目的是為了改善定量降水估計(QPE, Quantitative Precipitation Estimation)。本研究探討2022年TAHOPE IOP3及IOP10中，高或低可降水量(Precipitable Water, PW)影響融化層厚度與否，進而分析融化層之上或之下雲微物理過程差異，並比較不同可降水量條件下的雷達回波、降雨量與降雨特性。
依據地基GPS大氣觀測站北臺灣區域的每小時平均可降水量資料分類高、低可降水量時間區間，並根據Giangrande等人在2008提出計算融化層厚度的方法，找出雷達回波(radar reflectivity, ZH)、差異反射率(differential reflectivity, ZDR)的最大值和降水相關係數(cross-correlation coefficient, ρhv)的最小值區間，分析在水氣多寡情況下S-Pol雷達RHI掃描垂直剖面中融化層厚度差異以及雙偏極化參數垂直剖面雲微物理過程。
融化層厚度和ZH最大值(ZH peak)以及ZH最大值高度到ρhv最小值(ρhv peak)高度的距離成正比，和探空溫度於 ±0.2°C區間內的環境遞減率以及探空0°C層的平均相對溼度呈負相關。高可降水量個案中，ZH、ZDR與ρhv所對應之融化層厚度中位數與低可降水量個案相當；然而，高可降水量的ZH、ZDR、ρhv與KDP垂直剖面中，可以在ML之上的冰相區域觀察到枝狀冰晶成長層(Dendritic Growth Layer, DGL)，形成較大的雷達回波與較多的地面降水量；相較之下，低可降水量個案呈現發展高度較低的層狀降水及ML之下的蒸發現象，導致地面降水量較少。;The melting layer (ML) provides valuable information into the vertical structure of precipitation. The base of the ML indicates how far liquid precipitation extends downward, while its top is typically near the altitude of the 0°C isotherm. Detecting the ML is essential for quantitative precipitation estimation (QPE), since mixed-phase hydrometeors may contaminate rainfall estimates. This study investigates how high and low precipitable water (PW) and strong and weak radar echoes influence the melting layer thickness during TAHOPE IOP3 and IOP10 in 2022. Furthermore, it examines how these environmental conditions modify the microphysical processes above and below the melting layer and compares the rainfall and characteristics under different PW conditions.
Hourly mean PW observations from ground-based GPS atmospheric stations over northern Taiwan were used to classify high- and low-PW periods. Following the algorithm proposed by Giangrande et al. (2008), the ML is identified on polarimetric RHI (Range Height Indicator) scans as the vertical interval where the ZH and ZDR maximum and the minimum of ρhv. The vertical structure of the ML and its associated microphysical processes are then analyzed using RHI scans from the S-Pol radar.
The ML thickness is positively correlated with the maximum ZH and the vertical distance between the height of the ZH peak and the minimum ρhv, while it is negatively correlated with the environmental lapse rate within the ±0.2°C temperature interval and the mean relative humidity at the 0°C level derived from soundings. In high PW cases, the median ML thicknesses identified using ZH, ZDR, and ρhv are comparable to those in low PW cases. However, vertical profiles of ZH, ZDR, ρhv, and KDP in high PW cases reveal the presence of a dendritic growth layer (DGL) above the ML, which favors the development of stronger radar echoes and results in larger surface precipitation amounts. In contrast, low PW cases are characterized by shallower stratiform precipitation and evaporation below the ML, leading to reduced surface rainfall. |
