北太平洋阻塞個案生成之數值模擬的探討

A Numerical Study on the North Pacific's Blocking Onset

洪志誠(Chi-Cherng Hong)

《大氣科學》 32卷2期 (2004/06) Pp. 95-115

摘要

本研究以GSM探討阻生數值模擬的影響因素，研究中以1998年反聖嬰期間發生在北太洋地區之阻塞個案作為討論對象，探討初始誤差、模式解析度以及海溫對阻生數值模擬的影響。
研究顯示，阻生的預報能力與模式積分開始時間密切相關，大氣初始場主宰了阻塞的生成，模式積分開始，若初始場已存在某些利於阻生的動力條件，阻塞脊皆能被順利模擬出。海溫邊界條件雖然與阻生沒有直接關聯，但卻影響阻塞高壓之強度與位置的預報。從實驗中我們發現只有赤道東太平洋的海溫對阻生的數值模擬有顯著的影響，鄰近阻塞地區的海溫反而關係不大。在空間解析度方面，解析度較低(T42)的模式，阻生的預報能力明顯較低，且因為綜觀尺度的模擬誤差較大，造成模式無法像T62模式模擬出阻生時天氣可預報度增加的現象。
初始誤差的實驗顯示，阻塞地區對初始誤差相當敏感，且各擾動對阻生數值模擬之影響的差異性很大。研究中發現，若阻塞上游地區，擾動高度場與基本場的高度相位呈負相關，擾動對阻生數值模擬的影響最大。至於阻塞上游地區，初始誤差的影響並不顯著，積分不久誤差即呈現飽和狀態。從不同尺度的誤差演變得知，初始擾動首先影響地面旋生的預報造成綜觀尺度的模擬誤差，而此誤差在阻塞脊生成後，透過非線性交互作用傳往大尺度，降低阻塞脊的預報。

關鍵字

阻生 ； 初始誤差 ； 可預報度 ； 非線性作用

並列摘要

A numerical simulation for the Pacific blocking during the cold phase of ENSO in 1998 is performed by the NCEP's global spectral model (GSM ) to examine how the sea surface temperature (SST) anomaly , model's horizontal resolution and initial errors influence the formation of blocking . Experiments show the initial condition dominates the formation of blocking. Simulation for blocking at short led time succeeds in capturing the transition into blocked state, but which at long led time fails.
Regional SST anomaly has no significant impact on the formation of blocking. Only the SST anomaly in Tropical Easterly Pacific area has a remote forcing to change the prediction of the ridge's strength and location. The SST anomaly adjacent block area, such as North Pacific area almost has no effect on the formation of blocking.
The model’s horizontal resolution seems to play an important role in the formation of blocking. Due to the poor skill in the nonlinear interaction between synoptic scale and planetary scale, the lower resolution model (T42) model, compared with the higher resolution model (T62), is not good enough to extend the weather predictability during the formation stage.
Perturbed experiments show the simulation is very sensitive to the initial errors, and the loss of prediction skill reduced by initial errors is more significant over the block's area than over the upstream area. The imposed initial errors directly influence the prediction for the synoptic scale in the beginning of integration and therefore loss the predictability of planetary scale concerning the formation of blocking by up-scale errors transfer. Perturbation created by different lagged time also has a wildly different impact on the blocking formation. It results in a larger error if the phase of perturbation’s height field is negatively correlated with the basic flow over the upstream area .

並列關鍵字

Blocking onset ； Initial errors ； Predictability ； Nonlinear interaction