區域波浪系集預報系統建立及其應用之研究

Abstract

系集預報的目的是為了彌補單一模式預報的不足且能包含模式的不確定性，同時能夠提供機率性預報。發展系集預報系統，需提供有效且合理的系集預報成員，以產生足夠的系集分歧，據以最大程度涵蓋可能的發生機率，同時需透過量化系集預報指標，來檢視系集預報系統之預報能力和可信度。本研究引用氣象系集的概念，建置2層的多重網格NWW3波浪模式，使用中央氣象局WRF系集模式之20組系集風場來建置作業化系集預報系統，並利用氣象局設置之波浪浮標觀測資料當作真值，驗證其系集成員的分歧度是否足夠及可否最大程度涵蓋可能的發生機率，並檢視系統是否具備預報能力與區分事件發生及未發生的能力。 研究結果顯示，在系集成員的組成部分，使用風驅動公式Tolman之設定參數對於深海測站而言，在季風期有較佳的預報能力，但在颱風時期則呈現偏大的趨勢；而使用WAM4公式則剛好相反，顯示無法以一個公式同時滿足二種風況。對此，本研究首創提出之二種風場輸入公式組合方式以涵蓋季風及颱風的波浪特性(10個系集使用Tolman公式，10個系集使用WAM4公式)，結果顯示此種方式可保留各個風場輸入公式在不同風場狀況下的優點，同時提高SPRD，拉近RMSE及SPRD的差距，使得系集系統已較使用單一公式能掌握模式的不確定性。在系集預報系統的執行力分析部分，透過Reliability diagram、BSS及ROC分析，顯示系統針對深海測站已具備預報能力及區別事件發生及未發生的預報能力，同時優於氣候平均及單一決定性預報，亦堪與NCEP的系集系統相較之，換言之，本研究所研發之區域性作業化波浪系集預報系統已具有預報能力，可以提供波浪的機率預報。然而對於近域部分，受限於網格太大導致無法解析近岸複雜地形而導致差異較大的現象，將先以提高網格解析度在下一步進行改善之。 系集預報的應用研究包括作業化系集預報系統的建置及機率預報用於海上施工的決策。系集預報系統已作業化運作，每天四次、每次預報72小時產出波浪機率預報，輸出包括點輸出及面輸出，點輸出使用盒鬚圖，並於三天後另產出即時驗證盒鬚圖，面輸出包括系集成員圖、系集平均、系集平均及系集分歧、機率分布圖、Spaghetti圖、10%超越機率圖等，可供不同的使用需求使用。而機率預報結合蒙地卡羅法，可分析某一施工序在波高、風速及施工所需時間等限制條件下之機率預報，提供作為施工決策之參考，而使用介面可以更改接收不同的系集預報來源，提供快速的施工延時資訊，便利施工決策參考。

A wave ensemble forecast system is being developed based on the NOAA WAVEWATCH III (NWW3) two nesting multi-grid model over Taiwan area. The ensemble system consisted of 20 ensemble members and was set with spatial resolutions of 0.25 and 0.1. The wind forcing is coming from the WRF-based ensemble forecast system (WEPS) 10m wind fields of Central Weather Bureau (CWB) with spatial resolutions of 45km and 15km. The cycle initial condition of each wave ensemble member from the previous run of the same ensemble member is applied to generate a history perturbation of swell. The objectives of this work are to verify the impact of different wind forcing formulas, to find the better composition of ensemble members, and to evaluate the forecast capacity of resulting ensemble forecast system. We first proposed the combination of using two built-in wind forcing formulas to form twenty ensemble members (each for ten members), which can reserve the advantages of different formulas under various wind fields (monsoon and typhoon period), increase the average ensemble spread and decrease the difference between the root mean square error and average ensemble spread based on the truth value at open seas. With Reliability diagram, Brier Skill Score and Relative Operating Characteristic analyses of assessing the quality of forecasts, the ensemble system has good forecast capacity and discriminate between the events and non-events. It also has better forecast skill than the operational deterministic forecast, and can be comparable with NCEP global ensemble system. Consequently, the wave ensemble forecast system is approved to have the skill in terms of probability forecast at open sea and some coast areas around Taiwan. Nevertheless the overestimation near some coast areas could be improved by increasing the grid resolution and resolving nearshore wave simulation to reduce RMSE. For the underestimation of SPRD we intend to add perturbation at low frequency swell as initial condition to increase SPRD in the near future. Application of ensemble forecast includes the establistment of operational wave ensemble forecast system and probability forecast on decision making of marine installation. The operational wave ensemble forecast system performs 4 times daily and 72 hours forecast for each time. Products of ensemble forecast system involve point output and gridded output. The point output utilizes boxplot to show ensembles. The gridded output contains ensemble members, ensemble means and spread, 10% exceeding probabilities, probability and spaghetti diagrams at different thresholds every three hours. Ensemble forecast combined with monte-carlo method could provide the probability of operation under the thresholds of wave height, wind speed and duration of operation for decision making.