Winds

Yin (2000) JAM

Annual mean winds

Annual Cycle in Wind

Yin (2000) JAM

Annual cycle amplitude

Peak Wind Season

Yin (2000) JAM

Time of peak wind

Diurnal Mountain Winds

• Diurnal mountain winds develop from terrain of all scales

• Circulations arise as a result of differential heating between the ground in regions of complex terrain and free atmosphere at the same elevation– During day, higher terrain is an elevated heat source

– During night, higher terrain is an elevated heat sink

Sacramento Valley

Zaremba and Carroll (1999)JAM

Grand Canyon

Whiteman et al. 1999JAM

Kali Gandaki Valley

Egger et al. (2000)MWR

Mountain wind systems

• Slope winds- driven by horizontal temperature contrasts between air over valley sidewalls and air over center of valley

• Along-valley winds- driven by contrasts along valley’s axis and nearby plain

• Cross-valley winds- driven by contrasts between opposing sidewalls

• Mountain-plain winds- driven by contrasts between plateau and nearby plains

Mountain Wind Systems

Whiteman (2000)

Terminology

• Katabatic wind: cold flow of air travelling downward or down a slope

• Anabatic wind: air current or wind rising up a slope

Slope Winds

Whiteman (2000)

Slope flows• Closed circulation driven by horizontal temperature contrasts between

the air over the slope and the air at the same level over the center of the valley

• Speeds- 1-5 m/s with maximum a few meters above the ground

• Increase in speed as length of slope increases (Antarctica 14-30 m/s)

• Strongest downslope at sunset; strongest upslope in midmorning

• Depth of downslope ~5% of drop in elevation from top

• Upslope flows increase in depth as move upslope

• Stronger the stability, shallower the slope flows

• Downslope flows converge into gullies; upslope flows converge over higher ground between gullies

Slope flows

Whiteman (2000)

ColdWarmWarmCold

Du’/dt = g’ ( en- )/=g’ (T-Ten)/Ten= g’ (-en)/en

g

g’

Basin Circulations

• Enclosed terrain features develop slope flows but weak along-valley circulations

• Enhanced heating during the daytime and cooling at night as a result of absence of along-valley advection of cool/warm air

• Light winds

Night flows

Whiteman (2000)

Thermal belt

Whiteman (2000)

Slope Flows in Peter Sink Basin

• Record cold temperature in Utah: Peter Sinks –57C

• Clements (2001) conducted field program in remote basin in northern Utah to study slope flows

• Field program held 8-12 Sept. 1999

Peter Sinks

North Peter Sink

Vegetation inversion

Peter Sinks Terrain

Perimeter

Instrumentation Layout

Net Radiation and Sonic Anemometer

Surface Energy Budget- Idealized

Whiteman (2000)

Surface Energy Budget- Peter Sinks

Strong net heating during day; surface losing energy during night

Surface Temperature Variation

Coldest air in the basin- warm air on slopes

Tethersonde Operations

VerticalStructurein basin

dw/dt = -g/en(den/dz)dz

Stability increases as evening progresses

Winds weaken with time

Temperature Mast on Slope

Temperature Variation on Slope

Strong inversion below 2 m; isothermal above

Vertical Structure on Slope

Light drainage winds on slopes; nonexistent most of the time

Potential Temperature Profiles Along Slope

Observations from Peter Sinks do not agree with classical model of relatively deep cold air on slopes draining down into basin

Morning Transition

Morning Transition

dw/dt = -g/en(en/z)dz

Stability decreases as morning progresses

Winds strengthen with time

Katabatic flow

Poulos et al. 2000MWR

Simulation of Katabatic Wind

Poulos et al. (2000)MWR

Antarctica Katabatic Winds

Bromwich (1989) BAMS

Divergence Salt Lake Valley: Interaction of Slope and Valley Winds

Convergence

Divergence

October 2000. M. Splitt