中尺度斜交不穩(wěn)定的波動性質(zhì)及其數(shù)值模擬

1、中尺度斜交不穩(wěn)定的波動性質(zhì)及其數(shù)值模擬沈新勇1，2 倪允琪2 丁一匯3 王 玨 1，2 1 南京信息工程大學(xué)江蘇省氣象災(zāi)害重點實驗室，南京，2100442 中國氣象科學(xué)研究院災(zāi)害天氣國家重點實驗室，北京，1000813 國家氣候中心，北京，100081摘要使用三維中小尺度擾動動力學(xué)方程組，討論了一類與基本氣流方向成任意夾角的緯向線狀擾動的斜交不穩(wěn)定問題，其主要分析結(jié)論如下：（1）在基本氣流的切變?yōu)榫€性切變的情況下，可以發(fā)生斜交不穩(wěn)定。但是發(fā)生斜交不穩(wěn)定必須要求沿著線狀擾動方向的基本氣流存在切變或者垂直于線狀擾動方向的基本氣流存在切變。此時，斜交不穩(wěn)定的波動性質(zhì)是重力慣性內(nèi)波，而不存在渦旋Ros

2、sby波。（2）對于基本氣流具有非線性二階切變的情形，此時斜交型不穩(wěn)定中的擾動除了包含重力慣性內(nèi)波之外，還包含了渦旋Rossby波。對于本文所討論的緯向線狀擾動來說，渦旋Rossby波產(chǎn)生的物理根源是基本流場的經(jīng)向風(fēng)速二次切變（），該渦旋Rossby波相對于基本氣流是單向傳播的。在中尺度斜交不穩(wěn)定中，渦旋Rossby波的產(chǎn)生主要是由于在與線狀擾動相垂直的方向上存在基本氣流的二階切變，而與線狀擾動相平行的方向上基本氣流是否存在二階切變無關(guān)。（3）對于通常討論的緯向線狀擾動的情況，在基本流場的南北風(fēng)速存在二次切變（）時，即與線狀擾動相垂直的方向上存在基本氣流的二階切變，則斜交不穩(wěn)定有可能是混合的渦

3、旋Rossby重力慣性內(nèi)波的不穩(wěn)定。最后采用WRF模式，對2006年6月的一次福建閩江暴雨過程進行了數(shù)值模擬，部分驗證了上述結(jié)論。通過分析6月6日13時降水區(qū)域平均的東西方向風(fēng)速 U 隨高度變化的曲線, 發(fā)現(xiàn)在950100 hPa 的大氣中, 緯向風(fēng)速 U 隨高度的變化具有二次切變, 在對流層低層以及高層附近緯向風(fēng)速較小, 而在對流層中層400 hPa附近的緯向風(fēng)速則較大。在這種情況下，平均緯向風(fēng)速隨高度變化的二階導(dǎo)數(shù)不為零，因此激發(fā)產(chǎn)生沿著緯向傳播的渦旋Rossby波，這對于暴雨區(qū)域中中尺度雨團的移動起到比較重要的作用。再分析6月5日23時和6月6日13時的850 hPa以及500 hPa降

4、水區(qū)域平均緯向風(fēng)隨南北方向的變化，發(fā)現(xiàn)在6月5日23時和6月6日13時的850 hPa層次上，都顯示出平均緯向風(fēng)速在南北方向具有二次切變；而在這兩個時次的500 hPa層次上，平均緯向風(fēng)速在南北方向不具有二次切變，只具有線性切變。說明在垂直方向上，對流層低層渦旋Rossby波容易被激發(fā)產(chǎn)生；而在對流層中層以上，由于不具備產(chǎn)生渦旋Rossby波的條件，因此不容易激發(fā)產(chǎn)生渦旋Rossby波，波動只是具有重力慣性內(nèi)波的特征。關(guān)鍵詞：斜交不穩(wěn)定，渦旋Rossby波，波動性質(zhì)，WRF模式，暴雨。初稿時間：2006年7月28日；修改稿時間：2007年2月12日。資助課題：國家自然科學(xué)基金項目（407750

5、33、40433007）、江蘇省氣象災(zāi)害重點實驗室基金項目（KLME050201）、中國氣象科學(xué)研究院災(zāi)害天氣國家重點實驗室基金項目（2006LASW01）、江蘇省自然科學(xué)基金項目（BK2005141）、國家重點基礎(chǔ)研究發(fā)展規(guī)劃項目（2004CB418301）以及上海臺風(fēng)研究基金課題（2003ST011）。作者簡介：沈新勇，主要從事熱帶大氣動力學(xué)、中小尺度大氣風(fēng)暴動力學(xué)及其數(shù)值模擬的研究。E-mail: shenxyWAVE PROPERTIES OF MESOSCALE OBLIQUELY CROSSING INSTABILITY AND ITS NUMERICAL SIMULATIONSh

6、en Xinyong1,2 Ni Yunqi 2 Ding Yihui 3 Wang Jue 1,2 1 Jiangsu Key Laboratory of Meteorological Disaster, Nanjing University of Information Science and Technology, Nanjing 2100442 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 1000813 National Climate Cente

7、r, Beijing 100081AbstractBy using the system of 3D dynamic equations for small-and meso-scale disturbances the series investigation is performed of obliquely crossing instability of zonal line-like disturbance moving at an arbitrary angle with basic flow, arriving at the results as follows: (1) with

8、 linear shear available, the obliquely crossing instability of disturbances will occur only when the flow shearing happens in the direction along or perpendicular to the line-like disturbance movement, with the obliquely crossing instability showing the instability of internal inertial gravity waves

9、; (2) in the presence of second order non-linear shear the disturbance of obliquely crossing instability includes internal inertial gravity and vortex Rossby waves. For the zonal line form disturbance under study, the vortex Rossby wave has its source in the second shear () of meridional wind speed

10、in the flow and propagates unidirectionally with respect to . As meso-scale obliquely crossing unstable disturbances, the vortex Rossby wave has its origin from the second shear of the flow in the direction vertical to line-form disturbance and is independent of the condition in the direction parall

11、el to the flow; (3) for general zonal line like disturbances, if the second shear happens in meridional wind speed ( (), i.e., the second shear of the flow in the direction perpendicular to the line-form disturbance, then the obliquely crossing instability of disturbances is likely to be the instabi

12、lity of mixed Vortex Rossbyinternal inertial gravity waves. Finally, we have simulated the rainstorm process in the Fujian Province in the June of 2006 by use of the WRF model to verify the foregoing theoretical results. By analysing the vertical profile of the averaged U over the precipitation area

13、 at 13:00 BST June 6, 2006, it is found that in the atmosphere between 950 and 100 hPa, the zonal wind U had the second order shear with altitude, with the small speed in lower and upper troposphere, and the larger speed in middle troposphere near the 400 hPa. In such a circumstance, the second orde

14、r derivative of the averaged zonal wind U with respect to altitude was not zero, and therefore triggered the vortex Rossby Waves, which played an important role in the moving of the meso-scale rain cluster in the heavy rainfall area. By analyzing the meridional variation of 850 hPa and 500 hPa avera

15、ged zonal winds over the precipitation area both at 23:00 BST June 5 and at 13:00 BST June 6, it is found that there existed the second-order meridional shear of the averaged zonal wind both on June 5 and June 6 at 850 hPa, but at 500 hPa there only existed the linear meridional shear instead of the second meridional shear. The analysis indicate that it's easy to produce the vortex Rossby waves in the lower troposphere, but difficult in the middle a