Local Wind Systems and Temperature Structure in Mountainous Terrain

Met 1306 May 2008

Dr. Craig Clements

Types of Thermally-Driven WindsTypes of Thermally-Driven Windsfound in Mountainous Regionsfound in Mountainous Regions

1. Plain-Mountain Winds1. Plain-Mountain Winds

2. Valley Winds2. Valley Winds

3. Slope Winds3. Slope Winds

Thermally-driven refers to the forcing Thermally-driven refers to the forcing due to temperature differences!due to temperature differences!

Thermally-Driven Winds Found in Mountains

Whiteman(2000)

Cross-section of a Mountain Valley

Whiteman(2000)

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

Down-Valley Winds

Valley Winds

Up-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

Yosemite National Park, 12 Aug. 2003

Vertical Structure of Down-Valley Winds

‘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

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)

dzudzR

cTdzudz

t p

)()ˆ(

A Simplified Heat Budget of the Valley Atmosphere

Term 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

Mass conservation in a valley

valley

plain

xy

valley

xy

plain

A h

V

A h

V

Topographic Amplification Factor (TAF)

)(tSAhcTdt

dhfo

p

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

21

12 1cos2

io

pi ttAA

cThh

Heig

ht (m

AGL

)

0

100

200

300

400

500

600

700

282 284 286 288 290 292 294 296 298

600 PST

630

700

725

955

1035

2 3 4 5 6 7 8 -6 -4 -2 0 2 4 6

Up-valley Wind Component (m s-1)Mixing Ratio(g kg-1)Potential Temperature (K)

Tethersonde Profiles from Yosemite Valley

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5 6 7 8 9 10 11Time after sunrise (Hour)

1.46

1.78

1.86

2.25

6.31

7.43

0

100

200

300

400

500

600

700

800

0 0.5 1 1.5 2 2.5 3 3.5 4Time after sunrise (hour)

1.46

1.78

1.86

2.25

6.31

7.43

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

• 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)

Slope winds

Whiteman(2000)

Consequences of downslope flows: movie

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.

Whiteman(2000)

Downslope winds are often called drainage winds

Glacier WindsA cold air layer forms over ice surface and flows downhill.

Whiteman(2000)

Diurnal Evolution of the Boundary Layer over Mountains

Whiteman(2000)

The layer of air influenced by the earth’s surface is called the planetary boundary layer (PBL).