本研究使用加拿大麥基爾大學(McGill University)所發展之雷達回波外延預報系統(McGill Algorithm for Precipitation nowcasting using Lagrangian Extrapolation, MAPLE)結合中央氣象局(Central Weather Bureau, CWB)的整合最大回波資料，檢視及評估台灣地區的極短期(0~6小時)降水預報情形。其中MAPLE系統包含兩個程序，依序為：1.雷達回波變分追蹤法(Variational Echo Tracking, VET)決定降水系統的移動場；2.由半拉格朗日後推平流法(semi-Lagrangian backward advection)決定降水系統的預報場。 本研究選取兩種不同天氣型態的三個個案進行分析，分別為：1.南修(NAMTHEUN)颱風(2010年08月30日~31日)、2.鋒面系統(2012年06月11日~12日)與3.麥德姆(MATMO)颱風(2014年07月21日~23日)。首先對VET過程所使用的兩種參數──移動向量密度及回波資料時間間隔進行敏感度測試。接著，利用最佳化設定進行定量降水即時預報(Quantitative Precipitation Nowcasting, QPN)評估。整體而言，除系統生成或消散迅速之區域MAPLE預報掌握程度較為不足之外，1~3小時的累積降水量值評估和空間分佈情況，MAPLE即時預報系統具有相當程度的水準與能力。另外，藉由VET移動場與環境風場(ECMWF再分析風場)進行相關性分析。結果顯示，移動場會與不同天氣型態的環境風場產生不同程度的關聯，颱風系統相較於垂直結構複雜之鋒面系統，其移動場與環境風場有較高的關聯性。最後討論VET移動場與複雜地形之作用。當系統進入臺灣陸地時，移動速度大致都有下降情況，確實能夠反映出受到地形阻擋影響，減緩預報過程系統外延速率，接近短時間內的觀測資訊。但是主要影響外延預報能力隨時間下降的主因，依然為缺乏系統與地形交互作用下的生成和消散機制。 ;In this study, the McGill Algorithm for Precipitation nowcasting using Lagrangian Extrapolation (MAPLE) system developed by McGill University is used to evaluate the very short-term forecast of precipitation in Taiwan. There are two procedures in MAPLE system. First, the motion field of precipitation is determined by Variational Echo Tracking (VET) technique. Second, a radar map is advected by means of a semi-Lagrangian backward advection scheme to be a precipitation forecast. By using composite radar data from Central Weather Bureau (CWB), three real cases are selected for this study: a typhoon event occurred during 30-31 August 2010 (Namtheun typhoon); a Mei-Yu front case occurred during 11-12 June 2012; and a typhoon event occurred during 21-23 July 2014 (Matmo typhoon). Firstly, sensitivity tests between two parameters--vector density of motion field and time difference between each radar map in VET procedure is to determine the best setting of VET parameters for forecast. Secondly, the Quantitative Precipitation Nowcasting (QPN) would be evaluated. In general, forecasts by MAPLE system provide a certain degree capability about values and spatial patterns of the accumulated rainfall except for some rapidly growth and decay regions. Thirdly, the correlation of the VET motion field and the ECMWF reanalysis wind field is examined. It is found that most motion fields have a good relation with wind fields at each level in typhoon events, because the horizontal wind structure at one level is similar to the others. Finally, we focus on those VET motion fields that rainfall echoes are moving into Taiwan. The decrease of motion speed over Taiwan area is a good sign for extrapolation forecast, because it captures the blocking effect by orography to a certain extent. Must pay attention that main reason for rapid decrease in forecast ability is lack of growth and decay mechanism.
