莫拉克颱風路徑與降雨作業預報校驗

Verification on the Typhoon Morakot Track and Rainfall Forecast Issued by the Central Weather Bureau

葉天降(Tien-Chiang Yeh) ； 郭鴻基(Hung-Chi Kuo) ； 呂國臣(Kuo-Chen Lu) ； 王世堅(Shih-Chien Wang) ； 陳怡良(Yi-Liang Chen)

《大氣科學》 38卷2期 (2010/07) Pp. 85-98

摘要

98年8月莫拉克颱風侵臺期間降雨劇烈且持續，在臺灣中南部與臺東造成歷年來最嚴重之氣象災害，本文針對中央氣象局對此颱風之路徑與降雨量預報進行校驗。結果顯示中央氣象局對莫拉克颱風24小時颱風路徑預報平均誤差值為87公里，較過去預報誤差平均值為小，也較日本氣象廳、美國聯合颱風警報中心對該颱風相同時間所發布預報之誤差為小。對莫拉克颱風48小時與72小時路徑預報之平均誤差，中央氣象局分別為192公里與276公里，誤差則略較日本氣象廳與美國聯合颱風警報中心所發布預報之誤差為大。在警報發布初期，中央氣象局則對颱風移速有較明顯之高估，以致對莫拉克颱風影響臺灣整體時間之估計上低估。
在雨量預報上，中央氣象局對莫拉克颱風之總雨量預報基本上能掌握降雨較大區域之定性分布，第一份發布的預報單之雨量估計值以臺灣北部與實際較為相符，而中南部與臺東之總雨量預報偏少，尤以臺東預報誤差最大。臺東雨量預報誤差主要因為其處於颱風環流過山之背風面，依據過去之統計為少雨區。由校驗也顯示，中央氣象局根據新增資訊更新總雨量預報，而此些預報對降雨尚有相當長之預警時間：如屏東山區降雨量預報達上限之預警時間大部分在18小時左右，最少有11小時；高雄山區預警時間大部分在23小時左右，最少亦有8小時；嘉義山區在前期之預警時間在20小時以上，而後則為7、8小時。
改進颱風作業預報之方向，建議可由改善48小時與72小時之路徑預報著手，並持續進行颱風降雨預報技術之研發，而為減少颱風災害，國內民眾與防救災單位必須體認颱風路徑與雨量預報之不確定性，尤其是較長期間之預報。因此需避免僅重視第一次警報發布時之資訊，而須隨時注意作業單位更新後之資訊，充分掌握預報預警時間，採取防範措施，以降低災害損失。

關鍵字

颱風 ； 颱風路徑預報 ； 颱風雨量預報 ； 預報校驗

並列摘要

Typhoon Morakot (2009) made a record breaking torrential rainfall and caused massive damage over Taiwan area. In this study, verification on the operational forecast of the track and rainfall of Morakot issued by the Central Weather Bureau (CWB) was performed and discussed. Results show that the averaged distance error of the CWB 24-hour track forecasts of Typhoon Morakot is 87 km which is smaller than the mean track forecast error of the CWB in the last few years, and is also smaller than the averaged forecast errors of Japan Meteorology Agency of Japan (JMA) and Joint Typhoon Warning Center of USA (JTWC) in homogenous comparisons. For the 48-hour and 72-hour forecast errors, the CWB errors are 192 km and 276 km, respectively, and are larger than those of JMA and JTWC. The overestimate of the moving speed of Typhoon Morakot nearby Taiwan results in the underestimate of the influence of the typhoon on the island.
About the Morakot rainfall forecast, CWB predicted 800 to 1,000 mm of rainfall over western Taiwan at their first warning issued at 10:30 local time on 6 August. The official rainfall forecast was able to predict the general distribution of the typhoon rainfall pattern over Taiwan. Major areas with torrential rainfall were accurately predicted by CWB. However, the amount of the total rainfall was much underestimated. Particularly, over the southeastern Taiwan where lee side dry area was expected as the cyclonic flow over the Central Mountain Range from the west, but the rainfall system was able to maintain the intensity when it moved over the mountain and resulted the largest rainfall forecast error. Furthermore, the successive rainfall forecasts delineated that the CWB was able to update and issue more accurately rainfall forecast as more observations available. In most of the time, the forecasts were provided more than 10 hours prior to the rainfall occurring.
Typhoon rainfall forecast is still a big challenge in typhoon operational forecast. With steep mountains modifying the typhoon motion and rainfall distribution, to accurately forecast the typhoon rainfall over Taiwan is even more difficult. Timely use of the update information provided by CWB in disaster mitigation operation may be able to reduce the damage and loss. To improve the typhoon track forecast, particularly in the 48-hour and 72-hour, is also desired to upgrade CWB typhoon rainfall forecast.

並列關鍵字

Typhoon ； Typhoon track prediction ； Typhoon rainfall prediction ； forecast ； Verification