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

Tellus A
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  • Low-Level Vertical Wind Shear
  • Gravity Wave Breaking
  • Flow Regime
  • Orography


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 bellshaped 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 (△uc) for GWB’s occurring are discussed. Over the selected range of Fr0, GWB’s occurring will be suppressed if the VWS (△u) is larger than △uc at given U0. Moreover, there is a maximum value of △uc over the selected range of Fr0, which is labelled as △umax, and its matching surface wind speed by U0m. Once the Du is larger than △umax, 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 △u > △umax, 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 △uc and △umax for the GWB’s occurring. The possible inaccuracy of estimated △uc in the present linear model is also discussed. Keywords: low-level vertical wind shear, gravity wave breaking, flow regime, orography(Published: 20 February 2012)Citation: Tellus A 2012, 64, 17265, DOI: 10.3402/tellusa.v64i0.17265

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