Low-level vertical wind shear effects on the gravity wave breaking over an isolated two-dimensional orography

• Main Authors: Xu-Wei Bao, Zhe-Min Tan
• Format: Article
• Language: English
• Published: Taylor & Francis Group 2012-02-01
• Series: Tellus: Series A, Dynamic Meteorology and Oceanography
• Subjects: low-level vertical wind shear; gravity wave breaking; flow regime; orography
• Online Access: http://www.tellusa.net/index.php/tellusa/article/view/17265/pdf_2
• ISSN: 0280-6495; 1600-0870

Flow regimes of dry, stratified flow passing over an isolated two-dimensional (2-D) orography mainly concentrate at two stagnation points. One occurs on the upslope of the orography owing to flow blocking; another is related to gravity wave breaking (GWB) over the leeside. Smith (1979) put forward a hypothesis that the occurring of GWB is suppressed when the low-level vertical wind shear (VWS) exceeds some value. In the present study, a theoretical solution in a two-layer linear model of orographic flow with a VWS over a bell-shaped 2-D orography is developed to investigate the effect of VWS on GWB's occurring over a range of surface Froude number Fr0=U0/Nh (U0 is surface wind speed, h is orography height and N is stability parameter), over which the GWB occurs first and the upstream flow blocking is excluded. Based on previous simulations and experiments, the range of surface Froude number selected is 0.6 ≤ Fr0≤2.0. Based on this solution, the conditions of surface wind speed (U0) and one-to-one matching critical VWS (Δuc) for GWB's occurring are discussed. Over the selected range of Fr0, GWB's occurring will be suppressed if the VWS (Δu) is larger than Δuc at given U0. Moreover, there is a maximum value of Δuc over the selected range of Fr0, which is labelled as Δumax, and its matching surface wind speed by U0m. Once the Δu is larger than Δumax, the flow will pass over the orography without GWB's occurring. That means, over the selected range of Fr0, the flow regime of 2-D orographic flow related to GWB occurring primarily will be absent when Δu > Δumax, regardless of the value for U0. In addition, the vertical profile of atmospheric stability and height of VWS could result in different features of mountain wave, which leads to different Δuc and Δumax for the GWB's occurring. The possible inaccuracy of estimated Δuc in the present linear model is also discussed.