17.4
Stagnation points and PV generation in moist stratified flow over isolated topography
Juerg Schmidli, ETH, Zurich, Switzerland; and C. Schaer
The presence of moisture can significantly alter the properties of stratified flow past isolated topography, such as the generation, strength and breaking of gravity waves; the occurrence of upstream blocking; and the formation of lee vortices. In the present study, consideration is given to quasi-steady stratified flow over and around an isolated, Gaussian shaped mountain with an upstream profile of uniform wind speed, constant moist buoyancy frequency and constant relative humidity. Numerical experiments are undertaken with the Canadian non-hydrostatic mesoscale model MC2, using warm-rain Kessler-type microphysics and a TKE-based turbulence scheme.
The influence of moisture on typical three-dimensional flow regimes (such as "flow over" with and without gravity wave breaking, and "flow around" with low-level flow splitting) and associated stagnation points is investigated, within the convectively stable regime. The key control parameters for this experiment are the nondimensional mountain height and the relative humidity. In general, the energy input due to condensational heating tends to favour the "flow over" regime. Thus the onset of gravity wave breaking, the formation of lee vortices, and the occurrence of upstream blocking is delayed in comparison to otherwise identical dry flows. In addition, the surface pressure drag is considerably reduced - for some cases, down to 50% of its corresponding dry value. The presence of moisture does also significantly affect the nature and dynamics of the flow structure. As an example of such a new flow configuration, a moist flow with lee vortices but no upstream blocking will be presented. Usually the formation of lee vortices is associated with upstream blocking and flow around the mountain. However, it is demonstrated that moist flows may lead to vortex-formation in absence of classical flow splitting and upstream stagnation. To better understand these mesoscale flow phenomena, the generation of PV anomalies is analyzed using the generalized Bernoulli theorem.
Session 17, Lee-Side Phenomena
Friday, 21 June 2002, 8:00 AM-9:00 AM
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