雙北盆地夏季午後短延時劇烈降水預報探討

Abstract

針對無擾動環境下，雙北盆地區域內之夏季午後短延時降水，本研究以對流熱力機制為出發點，分析包含地面測站所觀測之各氣象參數(Lin et al., 2012；周等, 2016)，以及包含探空資料等其他有助於改善預報之因子，探討午後雷雨預報的可能性。 本研究著重在2012至2017年夏季（7、8月份）共372天中，排除鋒面、颱風、熱帶低壓、以及外部移入系統、或持續性雲系影響之個案，以當天當地降水時間12至18時為篩選條件，共得223個無擾動個案。其中極端降水（Ex-PR；雙北區域河谷及盆地內測站時雨量大於等於40毫米）個案有33個；一般降水（PR；雙北區域河谷及盆地內測站時雨量超過0毫米且不超過40毫米）有81個；無降水（No-PR；符合篩選條件但雙北區域河谷或盆地內任一測站皆未下雨）個案有109個。針對以上無擾動個案，計算地面水氣輻合通量(Moisture Flux Convergence)及當地時間8時無線電探空資料之熱力參數，結合雷雨檢查表進行分析及校驗。 上述223個無擾動個案分析結果比對實際降水情況顯示，除考量雷雨檢查表之外，探空資料等熱力參數亦提供良好的預報因子，包含：K-Index、總可降水量(Total Precipitable Water, TPW)、700 hPa溫度露點差。考量探空預報因子後，午後降水預兆得分(Threat Score, TSc.)明顯提升。水氣通量輻合(Moisture Flux Convergence, MFC)合成分析則未顯示極端降水、一般降水與無降水之明顯差異，故難以水氣通量輻合作為午後對流降水之預報因子。

This study aims for a better objective forecast for the Summer afternoon short-duration thunderstorm over Taipei basin. Based on the thermodynamics of convection, this study utilizes factors from surface observations (Lin et al., 2012; 周等, 2016) and information from upper-air radiosondes. This study focuses on the afternoon thunderstorm cases occurring in July and August from 2012 to 2017. Among the total 372 days, 223 days are considered as undisturbed cases including 33 days of extreme precipitating cases (Ex-PR, the station at Taipei basin and valley which has hourly rainfall over 40 mm); 81 days of precipitating cases (PR, the station at Taipei basin and valley which has hourly rainfall over 0 mm but less than 40 mm), and 109 of non-precipitating cases (No-PR, the stations at Taipei basin and valley are all without rainfall). Those days affected by fronts, tropical cyclones (TCs), tropical depressions (TDs), and external and/or large-scale weather systems are not considered. Surface moisture flux convergence (MFC) and other observational factors for weather forecasting are calculated and analyzed in these 223 days. Comparing the forecasting factors and the occurrences of afternoon thunderstorms, K-Index, Total Precipitable Water and moisture in mid-level (700 hPa) show better Threat Scores (TSc.) than surface observations. However, insignificant differences were found among MFC in Ex-PR, PR, and No-PR cases.