| 摘要: | 在進行颱風預報時，初始場的誤差會導致颱風隨時間模擬而降低路徑的準確性，為了能夠更有效地提高颱風模擬的準確性，提高初始場的準確性為首當其衝的必要條件。位渦反演法可以有效地將颱風的位渦及切向風場模擬而出，且虛擬位渦擾動可依設定而反演出颱風的擾動位渦和切向風擾動場，因此，可將位渦的擾動場以虛擬位渦擾動來取代再進行位渦反演，則可獲得新的虛擬位渦渦旋(Potential Vorticity Bogus Vortex)，並植入於所要探討的颱風初始場當中，來進行颱風初始場的修正。使用不同的背景場誤差和水平影響尺度，對於不同的颱風模擬也會有不同的結果產生。
在WRF 3DVAR的部分，對於輕度颱風凡那比而言，其初始場的颱風及駛流場強度較弱，因此在進行同化時必須同化較大的虛擬位渦渦旋，將環境流場也同化至初始場中，才能改變環境流場使颱風受駛流場的牽引讓颱風路徑產生偏移，使模擬路徑更接近於實際觀測，由於背景誤差cv3為使用NCEP GFS資料所得到的全球模式的誤差分析，會使得修正範圍較大使得環流較向外發散，能改變的環境流場也相對較多。因此，對於強度較弱的颱風而言，使用較大的同化半徑、較大的水平影響尺度和背景誤差cv3，則可以較有效地模擬出輕度颱風的行徑路徑。相對於初始場強度較強的強烈颱風梅姬而言，進行同化時虛擬位渦渦旋半徑不可同化太大，約只要同化RMW的大小，才不會讓颱風本身受到過多過強的環境流場影響而有多餘的偏移現象發生，而背景誤差cv5和cv3剛好相反，為針對實際模擬範圍網格點進行長達一個月的區域模式誤差統計結果，修正範圍較集中於植入渦旋的位置，使颱風流場較集中於颱風中心，讓颱風不受到過多的外圍流場來干涉路徑移動。因此，對於強度較強的颱風而言，使用較小的同化半徑、較小的水平影響尺度和cv5則可以較有效的模擬出強烈颱風的行徑路徑。
將凡那比颱風與梅姬颱風在WRF 3DVAR中，路徑模擬誤差較小的初始場設定改以WRF 4DVAR模擬其初始場及路徑。在輕度颱風凡那比的部分，初始場結果對於Ctrl Run而言其等壓線沒有明顯的變化，風場在同化半徑大小內為輻合，特別是眼牆的位置上最為強烈，當背景誤差為cv3時，則颱風的東北側會有明顯的增溫現象發生。和3DVAR相比，風場為輻散狀態，且最強處位於眼牆區域，等壓線向外擴散使得初始場模擬結果較為鬆散且範圍較為大，其路徑模擬結果均比3DVAR來的差、路徑誤差都較大，特別是在背景誤差為cv3時，雖然有將初期路徑特徵模擬而出，但其颱風行徑較緩慢且無登陸台灣，和實際觀測及3DVAR模擬有明顯的誤差。而強烈颱風梅姬的部分，和Ctrl Run相比，無論背景誤差為cv3或cv5，其初始場均有較強的氣旋式流場呈現，但氣壓場和溫度場均無明顯的變化產生。和3DVAR相比，在同化半徑大小內風場有些許的向外輻散，且當背景誤差為cv3時，颱風中心處有0.5度的降溫，使得颱風強度相較於3DVAR而言較微弱。而路徑模擬結果均無法勝過於3DVAR的初始場，整體路徑誤差均大於100公里，特別是在登陸呂宋島之後，其誤差更為明顯。
在此研究個案中，對於同化虛擬位渦渦旋於初始場中，使用WRF 3DVAR所模擬出的路徑不但可以修正Ctrl Run的誤差，且相對於WRF 4DVAR而言，在有限的電腦資源及有效的時間內可得到較好的模擬結果，有助於颱風預報的即時應用。
On the typhoon prediction, the error of initial field will affect the accuracy of tracks along with the time. If we want to increase the accuracy of prediction, the first necessary factor is to improve the initial field accuracy of typhoon. By using different settings of potential vorticity Inversion could retrieved the potential vorticity perturbation and tangential wind perturbation field of typhoon. Therefore, we could get the new potential vorticity bogus vortex (PV Bogus Vortex) by using the potential vorticity perturbation field which is substituted for the original perturbation field. Then insert it to correct the initial field. It will have different simulate result by using different background error and horizontal scale.
In the WRF-3DVAR, the Fanapi typhoon’s intensity and steering flow are weak, so assimilate the bigger radius of PV Bogus Vortex and environmental field can correct the typhoon track so that the steering flow could be changed and the track would be more realistic. The outward divergence and larger correct range are using background error-cv3 to correct broad environment field. For the intensity of weaker typhoon, we can simulate the track efficiently by using the bigger PV Bogus Vortex and horizontal scale on the background error-cv3. It will over correct the strong typhoon’s track, because more environmental wind field will be alter when we assimilate the PV Bogus Vortex size of RMW and smaller horizontal scale. The background error-cv5 are opposed to the background error-cv3, it’s because cv5 is a regional model statistics results of a month on real simulate domain. The cv5 is focus on the typhoon center to correct the flow, which will make environment field not to be interfered with the insert position. For the intensity of stronger typhoon, we can simulate the track efficiently by using the smaller PV Bogus Vortex and horizontal scale on the background error-cv5.
The minimum track error of WRF-3DVAR of typhoon Fanapi and Megi will be replaced by WRF-4DVAR. In the part of Fanapi typhoon, to compare with the Ctrl Run on the initial field, the isobar don’t have obvious difference, but they will have stronger convergence wind field on the eyewall, and temperature of the north-east side of typhoon will increase. To compare with WRF-3DVAR on the initial field, the simulation result is looser and wider sphere of influence that isobar and wind field are divergence to effect the out region of typhoon, so it makes the track have larger error and doesn’t make landfall in Taiwan. In the part of Megi typhoon, to compare with the initial field of WRF-3DVAR, the simulation result shows the stronger cyclonic stream flow, but don’t have temperature change no matter background error-cv3 or cv5. To compare with WRF-3DVAR on the initial field, the simulation results is weaker than WRF-3DVAR that it have divergence wind pattern on the radius of typhoon, and decrease 0.5 Celsius degree on the typhoon center. The track simulation result can’t be better than WRF-3DVAR which has over 100 km track error when it land on Philippines.
On the part of assimilate PV Bogus Vortex, It can correct the track error of the Ctrl Run by using the WRF-3DVAR which makes the result better than WRF-4DVAR. |
