Download Diurnal Mountain Winds

Winds
Annual mean winds
Yin (2000) JAM
Annual Cycle in Wind
Annual cycle amplitude
Yin (2000) JAM
Peak Wind Season
Time of peak wind
Yin (2000) JAM
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. 1999
JAM
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
Whiteman (2000
Mountain Wind Systems
Terminology
• Katabatic wind: cold flow of air travelling
downward or down a slope
• Anabatic wind: air current or wind rising up
a slope
Whiteman (2000)
Slope Winds
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
Whiteman (2000)
g’
Slope flows
g
Warm
Cold
Cold
Warm
Du’/dt = g’ (r en- r)/r=g’ (T-Ten)/Ten= g’ (Q-Qen)/Qen
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
Whiteman (2000)
Night flows
Whiteman (2000)
Thermal belt
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
Vertical
Structure
in basin
dw/dt = -g/Qen(dQen/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/Qen(dQen/dz)dz
Stability decreases as
morning progresses
Winds strengthen with
time
Katabatic flow
Poulos et al. 2000
MWR
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
Divergence
Convergence
October 2000. M. Splitt