全球二維傳送模式在長生命期大氣追蹤劑分佈的研究

A Study on the Distribution of Long-Lived Atmospheric Tracer by Using a Global 2-D Transport Model

周仲島(Ben Jong-Dao Jou) ； 鄧秀明(Shiu-Ming Deng)

《大氣科學》 15卷2期 (1987/10) Pp. 199-214

摘要

本研究使用長期大氣環流資料及質量守恆原理，建立一全球二度空間傳送模式，以計算長生命期大氣化學物質設大尺度運動重新分佈情形。本模式最大的特徵是採用轉換尤拉緯向平均質量連續方程式。使用轉換尤拉方程式，在數值方法上，可避免傳統尤拉方程中平均環流與渦流通量輻合兩大項相減變成一較小項所造成的巨大誤差，同時能正確地描述空氣質點在經向剖面上的運動軌跡，較具有物理意義。再者，在此系統下，渦流運動對平均場隨時間變化的淨效應，可以單一項(ρ cosψ)^(-1)∇∙(ρ cosψF)來表示。其中F的大小，可以渦流擴散係數張量K與追蹤劑緯向平均場的空間梯度相乘積予以參數化計算之。本模式初步測試結果顯示，渦流擴散效應基本上在消除初始追蹤劑原有存在的空間梯度。但是經由長時間積分，在擴散速率較慢的地區如冬季低平流層，將會形成物質累積的情形，造成較大梯度值的結果。平均環流的平流過程，基本上代表了空氣質點沿氣流方向移動情形，對於促進追蹤劑平均場的均勻性並無作用。由於使用上游差分法具有較強衰減作用，而且網格間距過大，使得模式的保守性顯得並不太理想。兩種過程合併的結果顯示剩餘平均環流的平流過程仍與淨渦流運動的擴散過程存有相互抵消的情形。擴散的效應在長期積分後期愈來重注明顯，使得追蹤劑的分佈在360天積分結果已幾乎完全散失對初始場的記憶了。此點顯示本模式所採用的渦流擴散係數Kyy及Kzz的數值可能有過大的情形。

關鍵字

長生命期追蹤劑 ； 二維傳送模式 ； 非絕熱平均環流

並列摘要

A 2-D transport model was constructed based on the Transformed Eulerian-Mean (TEM) mass continuity equation. The advantage of using TEM continuity equation, on the one hand, can avoid unnecessary numerical in accuracy due to large cancellation between the wave and the mean circulation transports under traditional Eulerian-mean formulation: on the other hand, the mean circulation described by TEM system represents the true air particle trajectory on a meridional plane under certain restricted conditions. Furthermore, the net eddy transport effect can be described by a single term (ρcosφ)^(-1)∇∙(ρcosφF), where F consists of the correlations of eddy displacement vectors and can be parameterized by flux-gradient relationship.Some preliminary results by using an Ozone-like tracer distribution as initial condition were discussed. It is shown that the eddy diffusion process was to diminish the initial space gradient of the tracer. In the area of low eddy diffusivity, however, large gradient could be formed after long time integration. This is especially true over the lower stratosphere area.The advective processes of the mean circulation had no apparent effect in demolishing the initial tracer gradient. However, unexpected strong damping was found over the absolute maximum value area. This is possible due to the strong damping effect of the upstream difference scheme.The combination effect of these two transport processes still results in some degree of cancellation. Nevertheless, the magnitudes are comparable. After 360 days integration, the diffusive process dominates the net transport. It is possible that the eddy diffusion has a faster rate than other similar studies.

並列關鍵字

Long-Lived tracer ； 2-D transport model ； diabatic mean circulation