Mountain Weather Phenomena

John HorelNOAA Cooperative Institute for Regional

PredictionDepartment of Meteorology

University of Utahjhorel@met.utah.edu

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.aspComments 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 of complex terrain

Part IIBasin and mountain-valley circulations

Part IIIROMAN 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)

TerrainSlope (%)

Roughness (dark)

What are the effects of complex terrain?

Substantial modification of synoptic or meso- scale 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 10-100 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

Whiteman (2000)

Convergence zones often form on the back side ofisolated barriers (Ex: Puget Sound convergence zone)

Frontal movement up and over a mountain barrier

Whiteman (2000)

Terrain channeling

Terrain-parallel jet may develop in post-frontal environment Contributes to development of frontal nose

Steenburgh and Blazek (2001)

Passage of low pressure center over mountains

Whiteman (2000)

Energy and mass exchanges near ground•Canopy •Terrain•Heterogeneous surfaces•Clouds/fog•Urban environment, air pollution

Heig

ht

(m)

Planetary boundary layer

1 km

D. Lenschow

Pollutant Transport in Valleys

Savov et al. (2002; JAM)

Nighttime Stable Layer in Valley

After Breakup of Nighttime Stable Layer in Valley

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 downThe gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify theSonic 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 MountainsA 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 mountainsVertical 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

Orgill (1981)

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.

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

Whiteman (2000)

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)

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 long2) cross-barrier wind component is strong3) 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

Bérenger &Gerbier (1956)

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.

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

Carney et al. (1996)

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.

Sierra wave photo

Kuettner/Klieforth 1952

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).

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. Local topography often plays an important role (Ex: Boulder, CO and Livingston, MT).

Steep leeside slopes, canyons, concave ridgeline. Can bring cold (Bora) or warm (Foehn, Chinook) air to leeward foothills.

Foehn winds of the intermountain west

Schroeder & Buck (1970)

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

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

Math (1934)

Foehn (Chinook) pause: abrupt cessation of downslope winds.

Alternating strong wind break-ins and foehn pauses can cause temperatures to oscillate wildly.

Chinook wall cloud

Ronald L. Holle photo

Whiteman (2000)

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