Thermally-Driven Winds Found in Mountains
Whiteman(2000)
Valley Winds
Cross-section of a Mountain Valley
Whiteman(2000)
Observations
Up-Valley and Down-Valley Surface Winds
(Measured in Yosemite National Park)
Date and Time
Valley Winds
Daytime:
Air is warmer in the valley
than over the plain
Up-Valley Winds
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.
Down-Valley Winds
Nighttime:
Pressure gradient force
reverses direction
A down-valley wind occurs
Whiteman(2000)
Valley Exit Jets
Whiteman(2000)
Valley Exit Jets
Whiteman(2000)
Observations
Time-Height SODAR Wind Profiles 12 August 2003
Vertical Structure of Down-Valley Winds
Yosemite National Park, 12 Aug. 2003
Wind
minimum
‘Nose’ of
Down-valley
wind
Nose is location of
Wind speed
maximum
Valley wind strength frequency
(Whiteman 1990)
Equations for the Valley Wind System







Q *
u
v
w
  (u )  (v )  ( w ) 
t
x
y
z
x
y
z
z
u
u
u
u
p
u
v w
 
F
t
x
y
z
x
dp
gdz

p
RT
u v w
 
0
x y z
p  RT
Whiteman(1990)
The Volume Effect of Valleys
Whiteman(2000)
Examples of Valley Shapes
Whiteman(2000)
Topographic Amplification Factor (TAF)
 Axy valley  h  


 Vvalley 
 
 Axy plain  h  


 V plain 
 W 


 Ayzvalley 

 W 


 Ayz plain 
(Whiteman 1990)
Draining vs. Pooling
Width to cross sectional
area for draining and
pooling valley.
Draining valleys: W/A
decreases with downvalley distance.
Ratio increases in pooling
valleys.
(Whiteman 1990)
Conceptual wind models for mountain valleys
(Whiteman 1982)
Yosemite Valley, Yosemite National Park
Mass conservation in a valley
Mass Flux in Valley Wind (Yosemite 1998)
h
M    ( z )a( z )u ( z )dz   i aiui zi
0
i
Where h is the valley depth,  is air density at height, a is the valley
width, and u is the up-valley component of wind speed. Mass fluxes
were computed for individual layers, i of depth z, assuming a constant
valley width of a =1000 m. From the vertical wind profiles, the mass
fluxes are extrapolated from the top of the profiles (~ 500 m) to the rim
height of the valley (~1000 m AGL).
Mass Flux in Valley
Wind
Yosemite
Valley (Yosemite 1998)
1000
900
0955 PST
800
Height (m AGL)
Profiles of mass flux show
that a maximum occurs at ~
200 m AGL with the
development of the up-valley
wind at 1035 PST. Up-valley
winds in Yosemite Valley
may produce total mass
fluxes of ~ 5.0 x 10 8 kg h-1.
This estimate may lead to a
better approximation of
pollutant transport into the
valley atmosphere.
700
600
500
1035 PST
400
300
200
100
0
0x100
1x104
2x104
3x104
4x104
Mass Flux (kg s -1 )
5x104
Tethersonde Profiles from Yosemite Valley
700
600
600 PST
630
Height (m AGL)
500
700
400
300
200
725
955
1035
100
0
282 284 286 288 290 292 294 296 298
Potential Temperature (K)
2
3
4
5
6
7
8
Mixing Ratio(g kg-1)
-6
-4
-2
0
2
4
6
Up-valley Wind Component (m s-1)
Boundary-Layer Evolution: Lee Vining Canyon, 5-6 June 1998
600
Downvalley
2050 PST
500
2116
Upvalley
045
400
117
Height (m AGL)
300
200
100
0
298
300
302
304
306
308
5
5.5
6
6.5
-5 -4 -3 -2 -1
0
1
2
3
4
5
4
5
600
405 PST
500
Downvalley
435
Upvalley
502
400
702
300
755
200
100
0
298
300
302
304
306
Potential Temperature (K)
308
5
5.5
6
Mixing Ratio(g
6.5
kg-1)
-5 -4 -3 -2 -1
0
1
2
3
Up-valley Wind Component (m s-1)
Inversion destruction
Models in mountain
Valleys (Whiteman 1982):
Pattern 1
Inversion destruction
Models in mountain
Valleys (Whiteman 1982):
Pattern 2
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


ˆ )dz     (u )dz


dz




u

dz


(


R
 t 
 Tc p

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
The thermodynamic model developed by
Whiteman and McKee (1982):
dh
 

Ao S f (t )
dt
T c ph
 2
 

h  hi  2
Ao A1
T c p


 

cos t  ti   1
 

1
2
Modeled Inversion destruction
800
1.46
700
(a)
600
1.78
1.86
500
2.25
6.31
400
7.43
300
200
100
0
0
1
2
3
4
5
6
7
Time after sunrise (Hour)
8
9
10
11
800
(b)
700
600
500
1.46
400
1.78
300
1.86
2.25
200
6.31
100
7.43
0
0
0.5
1
1.5
2
2.5
Time after sunrise (hour)
3
3.5
4
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.
Diurnal Evolution of the Boundary Layer over Mountains
Whiteman(2000)
Do valley winds always follow the classic
understanding in all mountain areas?
Do winds always flow up valley during the day and down
valley during the night?
Of course not! One example is the Washoe Zephyr wind
system.
Surface wind roses at each site
Frequency distributions of
winds from the southwest to
northwest quadrant as a
function of year and hour of
day for (a) all seasons.
Frequency distributions of
winds from the southwest to
northwest quadrant as a
function of year and hour of
day for (b) the summer
season.
Frequency distributions of westerly winds 5 ms1 at Galena for
each hour of the day for all seasons and for 2003–05
(top) Difference of sea level pressure between Sacramento and Reno, (second from top) 700-mb
wind speed and direction from Reno soundings at 0000 UTC on each day, and surface westerly
downslope wind for each hour of the day for (third from top) Reno and (bottom) Lee Vining for
summer of 2003.
RAMS simulated wind speed and (c),(d) potential temperature on an east–west cross section
through the center of the domain at (left) 1200 and (right) 2000 LST.
Findings
Washoe Zephyr is generally not caused by downward momentum
transfer as the deep afternoon convective boundary layer on the lee
slope of the Sierra Nevada and in the Great Basin penetrates into the
layer of westerlies aloft.
Instead, the difference in elevation between the elevated, semi-arid
Great Basin on the eastern side and the lower region on the western
side of the Sierra Nevada provides a source of asymmetric heating
across the mountain range. The asymmetric heating evolves during
daytime, generating a regional pressure gradient that allows air from
the west to cross over the crest and to flow down the eastern slope in
the afternoon.
Although a westerly ambient wind is not necessary for the
development of Washoe Zephyr, its presence leads to strong
Washoe Zephyr events that start earlier in the afternoon and
that last longer.
Summary
In mountainous regions, local wind systems develop in response
to local heating of the terrain.
The winds in these areas are ‘usually’ classified as:
Valley winds
Slope winds
And there are special cases when the winds blow differently than
Expected.