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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