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Mountain Weather Phenomena
John Horel
NOAA Cooperative Institute for Regional
Prediction
Department of Meteorology
University of Utah
[email protected]
With significant contributions from Dave
Whiteman, U/Utah
Cedar Fire Pyrobubble
28-29 October 2003
Mike Fromm, NRL
Rene Servranckx, CMC
Dan Lindsey, CO St.
Larry Di Girolamo, U. IL
Photos from http://www.wildlandfire.com/pics/cedar_socal/cedar.htm
Cedar Fire Hot Bubble
Hot spots just before pyrobubble
appears.
Big hot spot
is Cedar Fire.
GOES-10 Ch 2-4 29 Oct 03 02:15 UTC
The Cedar Fire Pyrobubble Sequence
GOES imagery
11 micron (Channel 4)
http://rammb.cira.colostate.edu/projects/pyrocu/29oct03/irloop.asp
3.9 micron – 11 micron (channel 2 – channel 4)
http://rammb.cira.colostate.edu/projects/pyrocu/29oct03/diffloop.asp
Comments from M. Fromm:
The pyrobubble was a singular event in the life of the So. CA fires in 2003.
Note how the smoke blows strictly offshore before the ~02 UT blob “launch.”
Then after launch, the low smoke veers from west to north to northeast to
east.
Note also a “trail” of material blowing off at different eastward
directions behind the blob. This trail no doubt reveals the wind profile
at that time.
Thanks to Dan Lindsey, CIRA, for these loops.
Reference
• Barry,
R., 1992: Mountain Weather and Climate. Rutledge (new edition soon)
• Blumen, W., 1990: Atmospheric Processes Over Complex Terrain. American
Meteorological Society, Boston, MA. (old, but still good reference material)
•Garratt, J., 1992: The Atmospheric Boundary Layer. Cambridge
•Kossmann, M., and A. Sturman, 2003: Pressure-driven channeling effects in bent
valleys. J. Appl. Meteor., 42, 151-1158.
•Stull, R. B., 1999: An Introduction to Boundary Layer Meteorology. Kluwer
•Whiteman, C. D., 2000: Mountain Meteorology. Oxford
COMET modules:
See http://meted.ucar.edu/mesoprim/flowtopo/
See http://meted.ucar.edu/mesoprim/gapwinds/
See http://meted.ucar.edu/mesoprim/mtnwave/
Outline
 Part
I
 Characteristics/impacts
 Part
II
 Basin
 Part
of complex terrain
and mountain-valley circulations
III
 ROMAN
and MesoWest: resources for observing
surface weather
Mountains in North America
Whiteman (2000)
Mountains of the western US
Whiteman (2000)
Western U.S.
Terrain
(high- dark;
low-light)
Terrain
Slope (%)
Roughness
(dark)
What are the effects of complex terrain?

Substantial modification of synoptic or mesoscale weather systems by dynamical and
thermodynamical processes through a
considerable depth of the atmosphere

Recurrent generation of distinctive weather
conditions, involving dynamically and
thermally induced wind systems, cloudiness,
and precipitation regimes

Slope and aspect variations on scales of 10100 m form mosaic of local climates
(Barry 1992)
Atmospheric scales of motion
Whiteman (2000)
Mean January 500 mb hemispheric map
Wallace & Hobbs (1977)
Winter pressure patterns
Whiteman (2000)
Summer pressure patterns
Whiteman (2000)
Areas of Cyclogenesis
Whiteman (2000)
Areas of Anticyclogenesis
Whiteman (2000)
Flow splitting around an isolated mountain
range
Convergence zones often form on the back side of
isolated barriers (Ex: Puget Sound convergence zone)
Whiteman (2000)
Frontal movement up and over a mountain
barrier
Whiteman (2000)
Terrain channeling
Steenburgh and Blazek (2001)


Terrain-parallel jet may develop in post-frontal environment
Contributes to development of frontal nose
Passage of low pressure center over
mountains
Whiteman (2000)
Planetary
boundary
layer
1 km
Energy and mass exchanges
near ground
•Canopy
•Terrain
•Heterogeneous surfaces
•Clouds/fog
•Urban environment, air
pollution
D. Lenschow
Pollutant Transport in Valleys
Nighttime
Stable Layer
in Valley
After Breakup
of Nighttime
Stable Layer
in Valley
Savov et al. (2002; JAM)
Diurnal Temperature Range
Shallow Drainage Flows – Mahrt, Vickers, Nakamura, Soler, Sun,
Burns, & Lenschow – BLM, 101, 2001.
Schematic cross-section of prevailing southerly synoptic flow, northerly surface flow down
The gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify the
Sonic anemometers on the E-W transect. E is to the right and N into the paper.
Wind flagging of trees
Justus (1985)
Introduction to terrain-forced flows


Two types of mountain winds

Terrain-forced flows: produced when large-scale winds are modified or
channeled by underlying complex terrain

Diurnal mountain winds (thermally driven circulations): produced by
temperature contrasts that form within mountains or between mountains and
surrounding plains
Terrain forcing can cause an air flow approaching a barrier to be carried
over or around the barrier, to be forced through gaps in the barrier or to be
blocked by the barrier. Use COMET modules for further background




See http://meted.ucar.edu/mesoprim/flowtopo/
See http://meted.ucar.edu/mesoprim/gapwinds/
See http://meted.ucar.edu/mesoprim/mtnwave/
Three variables determine this behavior of an approaching flow



Stability of approaching air (Unstable or neutral stability air can be easily
forced over a barrier. The more stable, the more resistant to lifting)
Wind speed (Moderate to strong flows are necessary)
Topographic characteristics of barrier
Angle of attack
Whiteman (2000)
Landforms assoc’d with strong and weak sfc winds
Expect high winds at sites:
Located in gaps, passes or gorges in areas with strong pressure
gradients
Exposed directly to strong prevailing winds (summits, high windward
or leeward slopes, high plains, elevated plateaus
Located downwind of smooth fetches
Expect low wind speeds at sites:
Protected from prevailing winds (low elevations in basins or deep
valleys oriented perpendicular to prevailing winds)
Located upwind of mountain barriers or in intermountain basins where
air masses are blocked by barrier
Located in areas of high surface roughness (forested, hilly terrain)
Wind variations with topo characteristics







Wind increases at the crest of a mountain (more so for triangular
than for rounded or plateau-like hilltops)
Separation eddies can form over steep cliffs or slopes on either the
windward or leeward sides
Speed is affected by orientation of ridgeline relative to
approaching wind direction (concave, convex)
Winds can be channeled through passes or gaps by small
topographic features
Sites low in valleys or basins are often protected from strongest
winds, but if winds are very strong above valley, eddies can form
in the valleys or basins bringing strong winds to valley bottoms.
Wind speeds are slowed by high roughness
In complex terrain, winds respond to landforms (valleys, passes,
plateaus, ridges, and basins) and roughness elements (peaks,
terrain projections, trees, boulder, etc.)
Flow Around Mountains
A flow approaching a mountain barrier tends to go around rather than
over a barrier if:
-ridgeline is convex on windward side
-mountains are high
-barrier is an isolated peak or a short range
-cross barrier wind component is weak
-flow is very stable
-approaching low-level air mass is very shallow
Because Rockies and Appalachians are long, flow around them is
uncommon. But these types of flows are seen in the Aleutians, the
Alaska Range, the Uinta Mountains, the Olympics and around
isolated volcanoes.
Wind variations with topo characteristics
Height and length can determine whether air goes around
barrier; to carry air over a high mountain range or around
an extended ridge requires strong winds
 When stable air splits around an isolated peak, the
strongest winds are usually on the edges of the mountain
tangent to the flow

Wakes, eddies, vortices
 Wakes and eddies are common in mountains
Vertical and horizontal dimensions a function of stability
 They form behind terrain obstacles when approach flow
has sufficient speed
– Eddy: swirling current of air at variance with main current
– Wake: eddies shed off an obstacle cascading to smaller and smaller
sizes. Characterized by low wind speeds and high turbulence.
–Vortex: whirling masses of air in form of column or spiral, usually
rotate around vertical or horizontal axis.
Eddy examples: rotors; rotor clouds; drifts behind snow fences, trees and
other obstacles; cornices. Winds are slowed to distances of 15
(sometimes 60) times obstacle height.
Wakes
Large, generally
isotropic vertical-axis
eddies can be
produced by the flow
around mountains or
through gaps as eddies
are shed from the
vertical edges of terrain
obstructions.
Orgill (1981)
Separation eddies
Whiteman (2000)
(Kaimal & Finnigan, 1994).
Flow through Passes, Channels and Gaps
Gaps - major erosional openings through mountain ranges
Channels - low altitude paths between mountain ranges
Mountain passes Strong winds in a gap, channel or pass are usually pressure
driven - i.e., caused by a strong pressure gradient across the
gap, channel or pass.
Regional pressure gradients occur frequently across coastal
mountain ranges because of the differing characteristics of
marine and continental air. These pressure gradients usually
reverse seasonally.
Pressure driven channeling through Columbia Gorge
Other well-known gap winds:
Caracena Strait, CA
Strait of Juan de Fuca (Wanda
Fuca)
Fraser Valley, BC
Stikine Valley (nr Wrangell)
Taku Straits (nr juneau)
Copper River Valley (nr
Cordova)
Whiteman (2000)
Turnagain Arm (Anchorage)
Pacific High, heat low in Columbia Basin
Excellent windsurfing as wind blows counter
to the river current with high speeds.
http://www.iwindsurf.com
Flow through passes & gaps
Whiteman (2000)
Venturi or Bernoulli effect
Whiteman (2000)
Venturi effect causes a jet to form as winds pass through a terrain
constriction and strengthen.
Forced channeling
Whiteman (2000)
Pressure driven channeling
Whiteman (2000)
Dynamic Channeling
Kossman
and
Sturman
2003
Blocking
-Affect
stable air masses and occur most frequently in winter
or coastal areas in summer
-The
blocked flow upwind of a barrier is usually shallower
than the barrier depth. Air above the blocked flow layer may
have no difficulty surmounting the barrier and may respond
to the ‘effective topography’ including the blocked air mass.
-Onset
and cessation of blocking may be abrupt.
Mountains as flow barriers
Whiteman (2000)
Cloud waterfall
Whiteman (2000)
Flow Over Mountains
Approaching flows tends to go over mountains if
1) barrier is long
2) cross-barrier wind component is strong
3) flow is unstable, neutral or only weakly stable
Common in North American mountain ranges.
 Evident by presence of lenticular clouds, cap clouds,
banner clouds, rotors, foehn wall, chinook arch, and billow
clouds as well as blowing snow, cornice buildup, blowing
dust, downslope windstorms, etc.

Lee Vining: LVHF
Lee Vining, CA (eastern Sierras)
Lee waves
Stull (1995)
Lee waves are gravity waves produced as stable air is lifted over a
mountain. The lifted air cools and becomes denser than the air around
it. Under gravity’s influence, it sinks again on the lee side to its
equilibrium level, overshooting and oscillating about this level.
Amplification and cancellation of lee waves
If the flow crosses more
than one ridge crest, the
waves generated by the
first ridge can be amplified
(a process called
resonance) or canceled
by the second barrier,
depending on its height
and distance downwind
from the first barrier.
Orographic waves form
most readily in the lee of
steep, high barriers that
are perpendicular to the
approaching flow.
Bérenger &Gerbier (1956)
Formation of waves


The basic form of a wave (trapped or vertically propagating) and its
wavelength depend on variations of speed and stability of the
approach flow. One of 3 flow patterns results depending on the
characteristics of the vertical profile of the horizontal wind.
 If winds are weak and change little with height, shallow waves
form downwind of the barrier.
 When winds become stronger and show a moderate increase
with height, air overturns on the lee side of the barrier, forming a
standing (i.e., non-propagating) lee eddy with its axis parallel to
the ridgeline.
 When winds become stronger still and show a greater increase in
speed with height, deeper waves form and propagate farther
downwind
The wavelengths of orographic waves increase when wind
velocities increase or stability decreases.
Trapped and vertically propagating lee waves
Carney et al. (1996)
Lenticular with rotor
Whiteman photo
 Campbell
Scientific
Hydraulic flow
Under certain stability,
flow and topography
conditions, the entire
mountain wave can
undergo a sudden
transition to a hydraulic
flow involving a hydraulic
jump and a turbulent
rotor. This exposes the lee
side of the barrier to
sweeping, high speed
turbulent winds that can
cause forest blowdowns
and structural damage.
Carney et al. (1996)
Sierra wave photo
View is toward south
from 11 km height.
Airflow is from right to
left. The cloud mass on
the right is plunging
down the lee slope of
the Sierra Nevada; the
near-vertical ascending
cloud wall of the
mountain wave is on the
left. The turbulent lower
part of the cloud wall is a
"rotor”; the smooth
upper part is the
"lenticular" or "wave
cloud". The cloud mass
to the right is a "cap
cloud" (= Föhn-Mauer);
the cloud-free gap
(middle) is the "Foehn
gap" (= Föhn-Lücke).
Kuettner/
Klieforth 1952
Downslope windstorms - Bora, Foehn,
Chinook






Form on the lee side of high-relief mountain barriers when a stable
air mass is carried across the mountains by strong cross-barrier
winds that increase in strength with height.
Strong winds are caused by intense surface pressure gradients (high
upwind, low pressure trough downwind). Pressure difference is
intensified by lee subsidence which produces warming and lower
pressure.
Elevated inversion layers near and just above mountaintop levels
play a role that is now under investigation.
Occur primarily in winter
Are associated with large amplitude lee waves
May be associated with wave trapping, or wave breaking regions
aloft.
Foehn winds of the intermountain
west
Chinook winds usually occur on the
east side of N American mountain
ranges since winds aloft are usually
westerly. But, they can occur on the
west sides when upper-level winds
are from the east (Ex: Santa Ana and
Wasatch winds).
Santa Ana winds - late Fall and
Winter, cause horrendous wildfires.
Wasatch downslope winds - affect a
more or less contiguous zone
immediately adjacent to the foothills.
These are produced by hydraulic
jumps and interaction with flows in
vicinity of canyon mouths
Schroeder & Buck (1970)
Synoptic conditions for Santa Ana winds
Ahrens (1994)
Santa Ana winds (e.g., 02/11/02)
Rosenthal (1972)
Foehn pauses & rapid T changes, Havre, MT
Foehn (Chinook) pause: abrupt
cessation of downslope winds.
Alternating strong wind break-ins
and foehn pauses can cause
temperatures to oscillate wildly.
Math (1934)
Chinook wall cloud
Whiteman (2000)
Ronald L. Holle photo
Summary- Impacts of Complex Terrain
 Terrain
affects atmospheric
circulation on local to
planetary scales
 Terrain induced eddies
modify and contribute
strongly to the vertical and
horizontal exchange of
mass, temperature, and
moisture
Photo: J. Horel