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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/66746


    Title: 利用系集卡曼同化和四維變分同化系統探討GPS掩星觀測對颱風梅姬(2010)模擬之影響;Use Ensemble Kaman filter and 4DVAR systems to compare the effect by assimilation GPSRO data in Megi
    Authors: 陳冠翰;Chen,Guan-han
    Contributors: 大氣物理研究所
    Keywords: 資料同化;WRF_4DVAR;WRF_EAKF
    Date: 2014-11-14
    Issue Date: 2015-03-16 15:08:58 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 由於3DVAR的背景誤差協方差是不具有flow-dependent的特性,但一般認為,如果背景誤差協方差具有流場相依的特性,可能會比較符合大氣變數的實際狀況,於是本篇論文研究挑選了兩個具有flow-dependent特性的同化系統,EAKF是屬於系集卡曼濾波器的一種,因為其背景誤差協方差是利用完全非線性模式預報出來的背景場計算得到的,所以具備了明顯的flow-dependent的特性,4DVAR則是四維變分方法的一種,由於其cost function加入了時間的維度(所以稱為四維),所以相較於3DVAR來說,具備了隱性flow-dependent的特性。本篇論文於是想要比較這兩種同化系統的表現。
    主要研究實驗計劃是在2010101900開始進行同化,由於NCEP全球分析場資料已經包含了GPSRO和GTS觀測資料在其中,為了避免分析場本身已有的觀測資訊干擾實驗,也為了讓初始場跟模式進行一定的平衡,所以在EAKF方面,先在2010101818進行預報六小時,把此預報場當作初始猜測場進行同化,再把每同化完的 Ensemble Mean進行72小時的模擬預報,另外把每個member進行六小時預報當作下個階段的背景場以及初始猜測場,再進行同化得到分析場。依序在10/19/00、10/19/06、10/19/12/、10/19/18、10/20/00,進行五次的同化,4DVAR則是先在2010101818進行預報三小時,把此時的預報場當作是初始猜測場,進行六小時窗區的資料同化,得到分析場,把分析場預報3小時得到10/19/00的初始場,利用此初始場進行72小時的模擬預報,再把同化後的分析場進行六小時的預報得到下個階段的初始猜測場,在進行資料同化,依序在10/18/21、10/19/03、10/19/06/、10/19/15、10/19/21,進行五次的同化。
    研究發現,EAKF、4DVAR經過cycle run的同化策略,颱風路徑的表現上EAKF在經過多個cycle窗區之後,有慢慢把資料同化的效果表現出來,4DVAR則較不一定,在颱風強度的部分,4DVAR在五個cycle窗區裡面,颱風強度整體來說是偏低的,在觀測資料部分發現EAKF因為是系集初始場的關係,較可以表現出GPSRO掩星觀測的效益,4DVAR則是比較跟觀測資料的遠近有關,在環境場的表現,EAKF整體來說都要比4DVAR還要來的優異。
    針對EAKF進行了同化策略差異的實驗,以及系集數的敏感度測試,發現經過spin up 72小時的EAKF在颱風路徑會比spin up 6小時的EAKF有較好的模擬,但颱風強度有低估的情形。在系集數的敏感度測試中,發現當系集數增加不夠明顯時,對背景誤差斜方差的估計改善程度並不會有多大的幫助。
    ;The background error covariance (COV) in 3DVAR does not have the flow-dependent quality. It will have a higher possibility to fit better to the realistic atmosphere data if the COV in other systems is flow-dependent. This study selects two data assimilation systems with the flow-dependent quality. EAKF is a kind of ensemble Kalman filter with the background error based on a calculation of various member spreads predicted by the perfect nonlinear model, therefore contributing to the flow-dependent quality. On the other hand, 4DVAR is a kind of four-dimensional variational method based on the model adjoint. Compared to 3DVAR, 4DVAR contains an implicit flow-dependent quality. In this study, we attempt to compare the performances of these two assimilation systems with the WRF model.
    The first assimilation starts at 0000 UTC 19 Oct. 2010 as the Typhoon Megi is west of Philippines. Due to that fact that the NCEP global analysis has already contained the observations of GPSRO and GTS, EAKF has been conducted for six hours in order to avoid the influences of GPSRO and GTS, which also allows the initial field to have a better balance with the model, and then this forecast field is used as the initial field to proceed the next assimilation. After each cycle, the model simulation is integrated for 72 hours using the ensemble mean for the EAKF experiments.
    For the 4DVAR experiments, the WRF model with the NCEP analysis at 1800 UTC 18 Oct. 2010 is integrated for three hours to obtain the initial field as the background with assimilation of observation data in next six hours to obtain the analysis field which is then used as the initial field for a simulation of three hours. The simulated field is then integrated for 72 hours for comparison with EAKF experiments. In total, there are five cycles for both EAKF and 4DVAR, separated by six hours.
    On verification of the analyses, EAKF is found to be better than 4DVAR. The model results after cycle run show that the simulated typhoon track from EAKF is improving as cycling increases, however the 4DVAR performances are less improved at later cycles. EAKF also gives a weaker intensity of Megi than 4DVAR. For impact of observation data, EAKF shows bigger benefits of GPSRO in typhoon predictions than 4DVAR.
    For the sensitivity of spin-up period in EAKF experiments, it was found that the simulated typhoon track is improved as the spin-up of 6 h for the EAKF experiments is increased to 72 h. However, an increase in the ensemble members doesn’t lead to significant improvement on both model analysis and typhoon forecasts.
    Appears in Collections:[大氣物理研究所 ] 博碩士論文

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