梅雨期水氣效應對低層噴流影響性之探討

A Study of the Moisture Effect on a Low-Level Jet during Mei-Yu Season

宋偉國(W. K. Soong) ； 陳泰然(G. T. J. Chen)

《大氣科學》 28卷1期 (2000/03) Pp. 1-15

摘要

本文利用MM5中尺度模式進行1987年TAMEX期間5月15-16日之個案模擬研究，探討去除潛熱效應時，水氣存在與LLJ的相關性。模式物理過程完整的CNR實驗模擬結果顯示，850hPa LLJ隨時間往東南移動並發展為近乎2000公里長的強風帶，此均和觀測接近。降水模擬，在初期落後於衛星觀測的MCS，此種現象主要是在模式預報初期需一段物理調整時間，唯大致上強降水系統分佈和衛星顯示的MCS接近。敏感度測試實驗顯示，去除潛熱效應但保留水氣之FAK實驗，LLJ強度比完全無水氣之實驗DRY實驗強許多，此意味著MCS潛熱釋放對於LLJ的加強有顯著貢獻外，LLJ強度之所以被維持，另有其它物理過程。LLJ可平流水氣提供水氣來源，同時於LLJ在其下游提供舉升運動，使水氣形成雲區；雲區擋住白天太陽短波輻射，使地面溫度無法升高。LLJ東南側則在副熱帶太平洋高壓影響下，不易形成雲區，因此地面溫度隨時間上升。此種LLJ區域南北兩側雲區有無的差別，使得LLJ兩側地面具顯著溫度梯度，透過靜力平衡反應，850hPa之LLJ區域南北兩側具顯著重力位高度梯度，經由地轉調整過程，因而維持LLJ強度。此外，實驗結果亦顯示水氣被西南氣流帶到LLJ區域時，由於南北兩側水氣造成的虛溫差距很小，反映到850hPa時，並無明顯之重力位高度梯度的存在，因此本個案水氣的虛溫效應對LLJ強度並無明顯貢獻。

關鍵字

低層噴流 ； 虛溫效應 ； 短波輻射

並列摘要

The purpose of this paper is to study the moisture effect on a low-level jet (LLJ) using the Penn/NCAR MM5 mesoscale model. The case used in this study occurred on 15-16 May 1987 during TAMEX period. Results of the control run (CNR) show that the LLJ with a horizontal length of 2000-kmmoves southeastward and intensifies similar to the observations. At very early phase of the model run, the occurrence of precipitation lags behind the formation of MCSs revealed by GMS observations. This is probably due to the model adjustment process in the moisture field. Nevertheless overall precipitation distribution is similar to that of the MCS observed from satellite.
Besides the contribution of latent heating of the mesoscale convective system(MCS),LLJ was also intensified by the local rate of change of geostrophic gradient through geostrophic adjustment process. The moisture advection provided by the LLJ forms clouds and blocks the solar radiation which reduces the surface temperature during the day. In contrast, at the southeastern side of the LLJ, clouds does not develop due to the subsidence of subtropical Pacific high, and results in the increasing of surface temperature with increasing solar radiation in the day time. The difference in cloud cover between the northern and southern sides of the LLJ produces stronger north-south surface temperature gradient and thus increases geopotential height gradient which leads to a greater intensity of the LLJ. It is also found that the virtual temperature effect due to the moisture difference on both sides of the LLJ is too small to produce significant geopotential hight gradient at 850hPa.Hence the virtual temperature has no significant contribution to the intensification of the LLJ.

並列關鍵字

Low-level jet(LLJ) ； Virtual temperature effect ； Solar radition