T-REX Boundary Layer Hypotheses

Boundary layer/mountain wave/rotor interaction hypotheses

  1. Boundary layer processes both upwind of and in the lee of the Sierras play an important role in initiating rotor events.
  2. Cold air pools in the Owens Valley can persist for extended periods of time during high wind events because turbulent erosion is an inherently slow inversion removal process.
  3. Development of adverse pressure gradients between trapped lee wave crests and troughs contribute to the flow separation and recirculations found at the surface below rotors.
  4. Surface drag is necessary for the formation of rotors.
  5. The formation of a sharp temperature jump at mountaintop level upwind of the barrier plays a major role in determining the structure and evolution of the mountain wave/rotor events.
  6. After the nocturnal inversion is removed in mountain wave episodes, the wind direction in the valley is driven primarily by strong downward momentum transport.
  7. Substantial along-valley variation in lee wave/rotor characteristics occurs because of the downwind influence of breaks in the Sierra ridgeline.

Boundary layer hypotheses

  1. Typical inversions in the Owens Valley do not extend to the ridgetops, but are confined to the lowest levels of the valley.
  2. Nighttime cooling of the valley atmosphere occurs most rapidly in the early evening and the total nighttime cooling can be closely estimated from integration of nighttime measurements of sensible heat fluxes on the valley floor and sidewalls.
  3. Strong temperature jumps are often present at the top of nocturnal inversions in the Owens Valley as warming occurs above the inversion due to compression as air subsides in the lee of the Sierra crest.
  4. Diurnal mountain wind systems (slope and valley flows) evolve normally in the deep valley during undisturbed periods. Winds over a 24-hour period turn clockwise on the west sidewall of the valley and counterclockwise on the east sidewall of the valley.
  5. Strong cross-valley flows develop in the valley during the immediate post-sunrise and pre-sunrise periods when sensible heat fluxes differ strongly across the valley between the opposing sidewalls.
  6. In undisturbed nighttime conditions, the growth of the temperature inversion in the Owens Valley can be explained by the mass convergence of drainage flows coming from the opposing sidewalls.
  7. During undisturbed daytime conditions, temperature inversions in the valley are destroyed several hours after sunrise as sensible heat released at the sidewalls drives convection and upslope flows in the convective boundary layer to remove air mass from the inversion.
  8. Representative sites for the measurement of surface sensible, latent, and ground heat fluxes can be found on the Owens Valley sidewalls.
  9. Surface energy balances can be closed for sites on the floor and sidewalls of the Owens Valley. This closure can be demonstrated by comparing convective fluxes measured using eddy correlation techniques with available energy measurements made using net radiometers and ground heat flux plates.
  10. Mass, momentum and heat budgets can be closed using measurements for an atmospheric control volume in the Owens Valley.
  11. Pressure driven channeling produces southerly up-valley flows in the Owens Valley during both day and night when flows aloft are westerly. These southerly flows overpower the typical nighttime northerly drainage flows when the pressure gradient aloft becomes strong.
  12. Forced channeling plays a minor role in the Owens Valley because the depth of the valley promotes relatively stronger thermally driven local flows.
  13. Windstorms in the Owens Valley occur more frequently in the fall and spring rather than in the winter despite the generally stronger winter winds because of the effects on lee wave development of upwind stability and the presence of subsidence inversions near mountaintop level produced by the transitory Pacific high.
  14. Mesoscale models can produce accurate simulations of inversion buildup, inversion breakup, thermally driven winds and turbulent erosion in the Owens Valley despite their failure to resolve the details of the shallow slope flows.
  15. The surface sensible heat fluxes required to produce accurate simulations of the buildup and breakup of temperature inversions are in agreement with actual measurements.