Thermally-Driven Winds Found in MountainsWhiteman(2000)

Valley Winds

Cross-section of a Mountain ValleyWhiteman(2000)

Up-Valley and Down-Valley Surface Winds(Measured in Yosemite National Park)Date and TimeObservations

Daytime:Air is warmer in the valley than over the plainPressure is lower in the valley and higher over the plain at the same elevationThe pressure gradient force is directed from the plain to the valleyA up-valley wind is produced that blows from the plain into the valley.

Nighttime:Pressure gradient force reverses directionA down-valley wind occursDown-Valley WindsValley WindsUp-Valley WindsWhiteman(2000)

Whiteman(2000)Valley Exit Jets

Valley Exit JetsWhiteman(2000)

Time-Height SODAR Wind Profiles 12 August 2003Observations

Yosemite National Park, 12 Aug. 2003Vertical Structure of Down-Valley WindsNose is location of Wind speed maximumWind minimum

Valley wind strength frequency(Whiteman 1990)

Equations for the Valley Wind SystemWhiteman(1990)

The Volume Effect of ValleysWhiteman(2000)

Examples of Valley ShapesWhiteman(2000)

Topographic Amplification Factor (TAF)

(Whiteman 1990)

Draining vs. PoolingWidth to cross sectional area for draining and pooling valley.

Draining valleys: W/A decreases with down-valley distance.

Ratio increases in pooling valleys.(Whiteman 1990)

Conceptual wind models for mountain valleys

(Whiteman 1982)

Yosemite Valley, Yosemite National Park

Mass conservation in a valley

Where h is the valley depth, is air density at height, a is the valley width, and u is the up-valley component of wind speed. Mass fluxes were computed for individual layers, i of depth z, assuming a constant valley width of a =1000 m. From the vertical wind profiles, the mass fluxes are extrapolated from the top of the profiles (~ 500 m) to the rim height of the valley (~1000 m AGL). Mass Flux in Valley Wind (Yosemite 1998)

Height (m AGL)Yosemite ValleyProfiles of mass flux show that a maximum occurs at ~ 200 m AGL with the development of the up-valley wind at 1035 PST. Up-valley winds in Yosemite Valley may produce total mass fluxes of ~ 5.0 x 10 8 kg h-1. This estimate may lead to a better approximation of pollutant transport into the valley atmosphere.

0955 PST1035 PSTMass Flux in Valley Wind (Yosemite 1998)

Tethersonde Profiles from Yosemite Valley

Potential Temperature (K)Mixing Ratio(g kg-1)Up-valley Wind Component (m s-1)Height (m AGL)Down-valleyDown-valleyUp-valleyUp-valleyBoundary-Layer Evolution: Lee Vining Canyon, 5-6 June 1998

Inversion destructionModels in mountain Valleys (Whiteman 1982):Pattern 1

Inversion destructionModels in mountain Valleys (Whiteman 1982):Pattern 2

Inversion destructionModel in mountain Valleys (Whiteman 1982):Pattern 3

Diurnal Temperature Evolution in Mountain Valleys(from Stull 1988; adapted from Whiteman 1982)

A Simplified Heat Budget of the Valley AtmosphereTerm 1: local rate of change of potential temperatureTerm 2: convergence of potential temperature flux by mean windTerm 3: convergence of radiative fluxTerm 4: convergence of turbulent sensible heat flux

The thermodynamic model developed by Whiteman and McKee (1982):

Inversion breakup according to Eq. 2 with Ao = 0.45, (a) = 0.007 K m-1 and (b) = 0.015 K m-1 TAF () values are indicated in legend. (a) (b) Modeled Inversion destruction

Diurnal Evolution of the Boundary Layer over MountainsWhiteman(2000)

Do valley winds always follow the classic understanding in all mountain areas?Of course not! One example is the Washoe Zephyr wind system.Do winds always flow up valley during the day and down valley during the night?

Surface wind roses at each site

Frequency distributions of winds from the southwest to northwest quadrant as afunction of year and hour of day for (a) all seasons.

Frequency distributions of winds from the southwest to northwest quadrant as afunction of year and hour of day for (b) the summer season.

Frequency distributions of westerly winds 5 ms1 at Galena for each hour of the day for all seasons and for 200305

(top) Difference of sea level pressure between Sacramento and Reno, (second from top) 700-mb wind speed and direction from Reno soundings at 0000 UTC on each day, and surface westerly downslope wind for each hour of the day for (third from top) Reno and (bottom) Lee Vining for summer of 2003.

RAMS simulated wind speed and (c),(d) potential temperature on an eastwest cross sectionthrough the center of the domain at (left) 1200 and (right) 2000 LST.

Findings

Washoe Zephyr is generally not caused by downward momentum transfer as the deep afternoon convective boundary layer on the lee slope of the Sierra Nevada and in the Great Basin penetrates into the layer of westerlies aloft.

Instead, the difference in elevation between the elevated, semi-arid Great Basin on the eastern side and the lower region on the western side of the Sierra Nevada provides a source of asymmetric heating across the mountain range. The asymmetric heating evolves during daytime, generating a regional pressure gradient that allows air from the west to cross over the crest and to flow down the eastern slope in the afternoon.

Although a westerly ambient wind is not necessary for the development of Washoe Zephyr, its presence leads to strong Washoe Zephyr events that start earlier in the afternoon andthat last longer.

SummaryIn mountainous regions, local wind systems develop in response to local heating of the terrain.

The winds in these areas are usually classified as:

Valley winds

Slope winds

And there are special cases when the winds blow differently thanExpected.