Local Wind Systems and Temperature Structure in Mountainous Terrain Met 130 6 May 2008 Dr. Craig Clements

Types of Thermally-Driven Winds found in Mountainous Regions 1. Plain-Mountain Winds 2. Valley Winds 3. Slope Winds Thermally-driven refers to the forcing due to temperature differences!

Thermally-Driven Winds Found in Mountains Whiteman(2000)

Cross-section of a Mountain Valley Whiteman(2000)

Valley Winds Daytime: Air is warmer in the valley than over the plain Pressure is lower in the valley and higher over the plain at the same elevation The pressure gradient force is directed from the plain to the valley A up-valley wind is produced that blows from the plain into the valley. Nighttime: Pressure gradient force reverses direction A down-valley wind occurs Up-Valley Winds Down-Valley Winds Whiteman(2000)

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

A SODAR (sound-detection-ranging) is similar to RADAR

Vertical Structure of Down-Valley Winds Yosemite National Park, 12 Aug. 2003 ‘Nose’ of Down-valley wind Nose is location of Wind speed maximum

Conceptual wind models for mountain valleys

(Whiteman 1982)

The Volume Effect of Valleys Whiteman(2000)

Examples of Valley Shapes Whiteman(2000)

Yosemite Valley, Yosemite National Park

Pattern 1 Inversion destruction Models in mountain Valleys (Whiteman 1982): Pattern 1

Pattern 2 Inversion destruction Models in mountain Valleys (Whiteman 1982): Pattern 2

Pattern 3 Inversion destruction Model 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 Atmosphere Term 1: local rate of change of potential temperature Term 2: convergence of potential temperature flux by mean wind Term 3: convergence of radiative flux Term 4: convergence of turbulent sensible heat flux

Mass conservation in a valley

Topographic Amplification Factor (TAF)

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

Tethersonde Profiles from Yosemite Valley Height (m AGL) 100 200 300 400 500 600 700 282 284 286 288 290 292 294 296 298 600 PST 630 725 955 1035 2 3 4 5 6 7 8 -6 -4 -2 Up-valley Wind Component (m s-1) Mixing Ratio(g kg-1) Potential Temperature (K)

(a) (b) Modeled Inversion destruction 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. (b)

Slope winds Whiteman(2000) Slopes winds are usually in the range of 1-4 m/s (2-8 mph); are weaker and more gentle than valley winds. Peak wind speed occurs a few meters above the the slope surface. Daytime upslope winds are typically stronger and deeper than nighttime downslope winds. A transition period occurs between the upslope and downslope winds in evening and morning. (See Time-Lapse Video)

Consequences of downslope flows: movie Whiteman(2000) Downslope winds are often called drainage winds Downslope winds can produce a cold air pool in a valley or basin. Some of these cold air pools can last several days to a week, trapping pollutants in the valley/basin. Cold air pools are often associated with dense fog, which is hazardous to aviation.

Glacier Winds A cold air layer forms over ice surface and flows downhill. Whiteman(2000)

Diurnal Evolution of the Boundary Layer over Mountains The layer of air influenced by the earth’s surface is called the planetary boundary layer (PBL). Whiteman(2000)