Tuesday, 24 January 2017
One important source of aviation turbulence is gravity wave breaking induced by airflow over mesoscale mountains, which is responsible for many severe turbulence encounters. The purpose of this study is to investigate gravity wave breaking over terrain in different parameter space. In order to gain a more in-depth understanding of the dynamics, we perform a series of systematic idealized simulations with idealized bell-shaped mountains by adopting the Cloud Model version 1 (CM1) to avoid some unnecessary complications from the real atmosphere and test some hypotheses. For a uniform, but structured flow over a mesoscale mountain, two distinguished flow regimes are found: (I) upward propagating moist gravity waves (GW) regime and (II) evanescent fluid flow (EV), depending upon the hydrostatic control parameter (Na/U), similar to a uniform, unstructured flow. The flow associated with the GW regime tends to steepen, overturn, create a wave-induced critical level, trap the energy below it, and then generate a “dead” region with strong turbulence with well-mixed regions, and severe downslope winds. In addition to the turbulent region over the lee slope, it is found that in Regime I, strong turbulence regions may also occur in upper troposphere and lower stratosphere, depending upon the basic flow Froude number (Fr=U/Nh) and the hydrostatic control parameter (Na/U). In this study, we are particularly interest in flow response to orographic effects in the typical commercial aircraft cruise levels in the upper troposphere and lower stratosphere (i.e., approximately 8–14 km above the earth surface). In addition, we will investigate the effects of moisture and the PBL. Both two- and three-dimensional idealized simulations with PBL (coarse resolution) or Large Eddy Simulations (LES) with finer resolutions are also conducted.
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