弱綜觀環境下台灣午後對流特徵及其客觀預報

Warm Season Afternoon Thunderstorm Characteristics under Weak Synoptic-Scale Forcing over Taiwan Island and Its Objective Prediction

林品芳(Pin-Fang Lin) ； 張保亮(Pao-Liang Chang) ； 周仲島(Ben Jong-Dao Jou)

《大氣科學》 40卷1期 (2012/03) Pp. 77-107

摘要

本研究分析2005－2008年暖季（5－10月）弱綜觀環境下之午後對流（afternoon thunderstorm, TS（下標 A））時空分佈特徵及有利北台灣TSA發展的對流前環境（preconvective environment），並發展模糊邏輯客觀潛勢預報午後對流方法（Afternoon Thunderstorm Occurrence Potential forecast using Fuzzy LOgic, ATOPFLO）結合有利TS（下標 A）發展的環境因子，進行北台灣TS（下標 A）的客觀預報。研究結果發現，在弱綜觀環境下的暖季，其平均小時雨量最大值發生於1500－1600 LST間，且降雨頻率最大值主要分布於沿著平行山脈走向的斜坡帶上。此外，北台灣發生對流的時間較台灣中南部為早，但中南部的對流活動明顯較北台灣活躍，持續時間較長，對流強度也較強。地面測站與探空觀測分析顯示，在相對較暖且濕的大尺度環境下，沿著淡水河與基隆河河谷富有水氣的海風在台北盆地輻合，提供了激發對流的有利環境。經ATOPFLO的整合結果，進一步確認且量化了上述環境特性因子的重要性，同時也顯示內陸的弱風速條件對於TS（下標 A）的發生也扮演著關鍵的角色。由臨界成功指數（Critical Success Index, CSI）評估ATOPFLO方法與目前中央氣象局（Central Weather Bureau, CWB）預報TS（下標 A）的表現，顯示現有CWB主觀預報有過度預報的趨勢，而ATOPFLO則可有效地整合對流前環境特徵，因此具有較準確的預報度。ATOPFLO目前已完成作業設計並進行線上平行測試，未來希望可提供弱綜觀環境下北台灣地區逐時的TS（下標 A）客觀機率預報資訊，並進而整合至實際作業預報中。

關鍵字

午後對流特徵 ； 對流前環境因子 ； 模糊邏輯 ； ATOPFLO客觀潛勢預報方法

並列摘要

The spatial and temporal characteristics and distributions of afternoon thunderstorms (TS(subscript A)s) in Taiwan during the warm season (May-October) from 2005-2008 and under weak synoptic-scale forcing are documented, and a approach, Afternoon Thunderstorm Occurrence Potential forecast using Fuzzy LOgic (ATOPFLO), is developed to provide objective guidance for the prediction of afternoon thunderstorm (TS(subscript A)) in northern Taiwan using preconvective predictors.Under weak synoptic-scale forcing, average hourly rainfall amounts peaked in mid-afternoon (1500-1600 LST). The maximum frequency of rain was located in a narrow strip, parallel to the orientation of the mountains, along the lower western slopes of the mountains. Although TS(subscript A)s occurred earlier in northern Taiwan, the duration of thunderstorm activity and its intensity in central to southern Taiwan was longer and stronger respectively than in the north. The Analyses from surface stations and sounding observations show that a local trigger for initiating thunderstorms under a favorable large scale environment appears to be the convergence of moist sea breeze air into the Taipei Basin from two separate valleys that open towards the sea. Relationships between observations from three surface stations and a sounding established the following scenario favorable for TS(subscript A)s in northern Taiwan. The most important predictors revealed from ATOPFLO approach illustrate that under relatively warm and moist synoptic conditions, sea breeze transport of moisture into the Taipei Basin along with weak winds inland provide favorable conditions for the occurrence of afternoon convective storms.Skill score CSI (Critical Success Index) comparison between the ATOPFLO approach and forecasters from the Taiwan Central Weather Bureau showed that for forecasting TS(subscript A), the ATOPFLO approach outperformed the operational forecasters. This was the case for both the calibration and independent datasets. There was a tendency for the forecasters to over-forecast the number of TS(subscript A) days. The ATOPFLO approach of objective prediction is able to integrate the preconvective predictors and provide probability guidance for the prediction of TS(subscript A) under weak synoptic-scale conditions, and therefore there is the forecasting ability with more accurate predictand. Currently, the ATOPFLO approach has been implemented and put into a real-time test-bed's operation. It is expected that the ATOPFLO can provide the hourly information of objectively potential prediction of TS(subscript A) in northern Taiwan and a practical application as a forecaster aid.

並列關鍵字

Afternoon Thunderstorm Characteristics ； Preconvective Predictors ； Fuzzy Logic ； ATOPFLO Approach