Flow Interaction with Topography Fundamental concepts: Mountain waves and down slope winds: Overview: Hazards Waves Features Climatology (pg 1 only) Downslope winds: Concepts Factors Interactions Uniform flow Complex flow Exercise Rotors (pgs 1-2)

Flow Interaction with Topography Notes:

SUMMARY: Mountain waves form above and downwind of topographic barriers when strong winds blow with a significant vector component perpendicular to the barrier in a stable environment. Mountain waves frequently pose a serious hazard to mountain aviation because of strong-to-extreme turbulence, which sometimes occurs without visual indicators. Under some circumstances, mountain wave activity can lead to strong and/or damaging downslope windstorms in the lee of a mountain barrier. A series of lenticular clouds downstream of the barrier often indicates the existence of trapped lee waves. Cap clouds indicate likely wave activity downstream. Clear air turbulence often occurs near the tropopause due to vertically-propagating waves. Rotors are part of a low-level turbulent zone that is another region of potentially significant turbulence. Foehn winds are warm, dry, gusty downslope winds found along mountain ranges throughout the world. Their warmth results from adiabatic compression as winds descend lee slopes. Bora winds are cold downslope winds that result from a deep and very cold upstream air mass that spills over the barrier and displaces a warmer air mass. Downslope winds occur most often during cold months and at night when the atmosphere is most stable. The Froude number expresses a ratio between the kinetic energy (wind speed) and the potential energy (stability times mountain height). If the Froude number is equal to or slightly greater than 1, then there is the likelihood of mountain wave activity If the Froude number is less than one, then the airflow is insufficient to carry the flow over the mountain and the flow is blocked If Froude number is much more than 1, airflow proceeds right over the mountain and down the other side, with no significant oscillations A critical level results when the cross-barrier flow goes to zero above mountaintop level. Critical levels do not allow the vertically-propagating energy associated with mountain waves to continue upwards. Instead, that energy is deflected off the critical layer back toward the surface. Consequently, critical levels can contribute to the development of and/or the strengthening of downslope windstorms. The speed of those winds can be 2-3 times the upwind speed at mountaintop height. Critical levels may be self-induced by wave breaking or result from a mean state condition in the overall flow. NWP models can provide valuable information on the conditions that lead to mountain wave activity and downslope winds. However, these NWP models require a horizontal grid spacing of 10 km or less to provide details of a mountain wave. Eta coordinate models tend to have difficulties generating vertically-propagating gravity waves and downslope wind forecasts. Observational platforms are valuable tools for determining the current existence of and predicting mountain wave activity within 12 hours. Observations of particular interest include satellite imagery, rawinsondes, and pilot reports.