6/9/2010

1

Influence of Topography on Weather

John Horel

Department of Meteorology

University of Utah

john.horel@utah.edu

References• Bailey, C. et al., 2003: An objective climatology, classification scheme, and assessment of sensible

weather impacts for Appalachian cold-air damming Weather and Forecasting , 18, 641-661.

• Bannon, P. R., 1992: A model of Rocky Mountain lee cyclogenesis. J. Atmos. Sci., 49, 1510–1522. • Barry, R., 1992: Mountain Weather and Climate. Rutledge

• Bell G. D., and L. F. Bosart, 1988: Appalachian cold-air damming. Mon. Wea. Rev., 116, 137–161.• Blumen, W., 1990: Atmospheric Processes Over Complex Terrain. American Meteorological Society,

Boston, MA.***

• Dickinson, M.J. and D.J. Knight. 1999: Frontal interaction with mesoscale topography. J. Atmos. Sci., 56: 3544-3559.

• Garratt, J., 1992: The Atmospheric Boundary Layer. Cambridge• Kalnay, E., 2003: Atmospheric Modeling, Data Assimilation and Predictability. Cambridge

• Kossmann, M., and A. Sturman, 2003: Pressure-driven channeling effects in bent valleys. J. Appl. Meteor., 42, 151-1158.

• Olson. J et al.,2007: A comparison of two coastal barrier jet events along the southeast Alaskan coast during the SARJET field experiment. Mon. Wea. Rev., 135, 3642-3663.

• Neiman P. J., F. M. Ralph, A. B. White, D. D. Parrish, J. S. Holloway, and D. L. Bartels, 2006: A midwinter analysis of channeled flow through a prominent gap along the northern California coast during CALJET and PACJET. Mon. Wea. Rev., 134, 1815–1841.

• Shafer, J. C., and W. J. Steenburgh, 2008: Climatology of strong Intermountain cold fronts. Mon. Wea. Rev., in press.

• Smith, R. B., 1979: The influence of mountains on the atmosphere. Adv. Geophys, 21, 87-230

• Steenburgh, W. J., and T. R. Blazek, 2001: Topographic distortion of a cold front over the Snake River Plain and central Idaho Mountains. Wea. Forecasting, 16, 301-314

• Steenburgh, W. J., 2003: One hundred inches in one hundred hours: Evolution of a Wasatch Mountain winter storm cycle. Wea. Forecasting, 18, 1018-1036.

• Stull, R. B., 1999: An Introduction to Boundary Layer Meteorology. Kluwer

• Ting, M., and H. Wang 2006: The Role of the North American Topography on the Maintenance of the Great Plains Summer Low-Level Jet. J. Atmos. Sci., 1056-1068

• Whiteman, C. D., 2000: Mountain Meteorology. Oxford

• Winstead N. S., Coauthors, 2006: Barrier jets: Combining SAR remote sensing, field observations, and models to better understand coastal flows in the Gulf of Alaska. Bull. Amer. Meteor. Soc., 87, 787–800.

COMET Module Resources

• Flow Interaction with Topography

• Thermally-forced Circulation II: Mountain/Valley Breezes

• Mountain Waves and Downslope Winds

• PBL in Complex Terrain - Part 1

• PBL in Complex Terrain - Part 2

• Gap Winds

• Cold Air Damming

• Challenges of Forecasting in the West

• Dynamics & Microphysics of Cool-Season Orographic Storm

• Real-Time Mesoscale Analysis (RTMA)

• Fire Weather Courses: S-290 and S-591

Mountain Weather Workshop (2008),

Whistler B.C. • AMS Mountain Weather and Forecasting

Monograph (2011)

• Meyers and Steenburgh (2011) Mountain Weather

Prediction: Phenomenological Challenges and Forecast

Methodology

“[mountain weather forecasting] is effective when

operational meteorologists possess in-depth

knowledge of mountain weather phenomena and the

tools and techniques used for atmospheric

observation and prediction in complex terrain”

Direct and Remote Impacts of Mountains

• Direct:

– cover 25% of land surface

– Contain 26% of population

– 32% of surface runoff (Meybeck et al. 2001;

Mountain Research and Development 21, 34-

45)

• Remote:

– Modulation of general circulation and storm

tracks

– River runoff

Meyers and Steenburgh (2011)

Societal Impacts: +/-

(Meyers and Steenburgh 2011)

• Protection of lives and property from high

impact events

• Snow removal costs > $2 billion

• Closures of I-80 in WY/CO cost $1 million

per hour

• Beneficial impacts of major snowstorms on

water resources

• Outdoor recreation

– $730 billion

– 6.5 million jobs (1 in 20 in U.S)

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Phenomenological Challenges

(Meyers and Steenburgh 2011)

• Snow

• Ice storms produced by orographic

precipitation/terrain-induced cold advection and

cold-air damming

• Floods, landslides, and debris flows

• Droughts

• Wildfires

• Local windstorms from mountain waves and gap

flows

• Convective storms and severe weather

• Cold-air pools and poor air quality

Perspectives on Forecasting in Complex Terrain

• Very complicated and requires conceptual models that are physics

based

• Terrain influences signal over wide spectrum of scales with locally

unique, but recurring processes

• Free and forced mesoscale circulations influence:

– Precipitation distribution and type

– Temperature extremes

– Wind direction and speed

– Cloud characteristics

• Requires continual re-evaluation of tools and models as improvements in NWP gradually resolve the mesoscale

Colman et al. (2008) Mountain Weather Workshop

Skillful forecasting in mountainous

regions requires:

• Core understanding of synoptic scale and

orographic processes

• Careful evaluation of evolving synoptic setting

and flow interaction with terrain

• Knowledge of the advantages and limitation of

objective tools applied over complex terrain

• Subjective integration of these tools by the

forecaster

Meyers and Steenburgh (2011)

Subjective Tools In Complex Terrain

• Geographic familiarization

• Rules of thumb

• Pattern recognition and climatology

• Conceptual models

Meyers and Steenburgh (2011)

Objective Tools

• Surface-based observations

– Need high-density surface observations to

resolve fine-scale gradients due to

topography

– But recognize the limitations of the various

data assets

– Necessity for forecast verification

Meyers and Steenburgh (2011)

Objective Tools• Radars: recognize limitations in

mountainous areas

– beam blockage and ground clutter

– spacing between radars in west greater

– overshooting wintertime orographic

precipitation and other shallow precip events

• Satellite, lightning

• Numerical weather prediction

– “when guidance is needed the most, it is

generally the least useful” C. Doswell (1986)

Meyers and Steenburgh (2011)

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Challenges facing NWP in Complex Terrain

•Recent studies suggest that numerical methods

are very important for topographic flows

• Interaction of model dynamics & physics has not

been addressed adequately for prediction of

topographic flows

•Assessment of predictability of topographic flows

(including sensitivity to the initial state, boundary

conditions, and model components) just

beginning

•Are we reaching point of diminishing returns

regarding horizontal grid spacing?

Doyle et al. (2008) Mountain Weather Workshop

Mesoscale Modeling• “Mesoscale model forecasts are usually

physically realistic, but not necessarily skillful.” –B. Colman

• Topography may enhance predictability of certain flow types, but this has never been proven or refuted

• Orography can exacerbate large-scale errors, reducing forecast utility

• Expect false-alarms

Meyers and Steenburgh (2011)

What is the appropriate role of NWP

in the forecasting process?

• Produces “images” of the real world that help forecasters conceptualize processes

• NWP solutions are not as skillful (with respect to known flows, etc.) as they are physical

– significant biases

– timing and location (uncertainty in synoptic signal)

– predictability limitations (per Reinecke and Durran (2008))

• On the positive side, NWP:– Sets the synoptic-scale stage

– Allows forecasters to test hypotheses

– Alerts forecasters to potential events

– Provides excellent insight into possible future states

Colman et al. (2008) Mountain Weather Workshop

Outline

• Scales of interaction between flow and

terrain

• Dynamically-forced terrain interactions

• Terrain-precipitation processes

Analysis/Forecast Funnel

• The forecast funnel– Begin at planetary scale

– Focus attention on

progressively smaller

scales

– Build in orographic

effects

– Processes on each

scale are dependent

upon those at other

scalesSnellman (1982); Horel et al. (1988)

Atmospheric scales of motion

Whiteman (2000)

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Does Terrain Improve or Destroy Predictability?

Terrain

Degraded

forecastsImproved

forecasts

Does Terrain Improve or Destroy Predictability?

TerrainDegraded

forecasts

Observations may not

be representative

Improved

forecasts

Inadequate model

resolution

Incomplete model

physics

Nonlinear

scale interactions

Does Terrain Improve or Destroy Predictability?

Terrain

Degraded

forecastsImproved

forecasts

Recurring phenomena

Recognizable

spatial dependencies

Physically-based

conceptual models

Does Terrain Improve or Destroy Predictability?

Terrain

Degraded

forecasts

Observations may not

be representative

Improved

forecasts

Recurring phenomena

Recognizable

spatial dependencies

Inadequate model

resolution

Incomplete model

physics

Nonlinear

scale interactions

Physically-based

conceptual models

Does forecaster have advantage over models to improve forecasts

when dealing with terrain issues compared to dealing with mesoscale instabilities?

If the earth were greatly reduced in size while maintaining its shape, it would be smoother than a billiard ball. (Earth radius = 6371 km; Everest = 8.850 km)

However, the atmosphere is also shallow (scale height ~8.5 km) so mountains are a significant fraction of atmosphere’s depth

And:

Stability gives the atmosphere a resistance to vertical displacements

The lower atmosphere can be rich in water vapor so that slight ascent brings the air to saturation

Example: flow around a 500-m mountain (<< 8.5 km) might lead to 1) broad horizontal excursions, 2) downslope windstorm on lee side, and 3) torrential orographic rain on windward side.

Smith, R. B., 1979: The influence of mountains on the atmosphere.

Adv. Geophys., 21, 87-230.

Why is Terrain So Important? 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.

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What is a mountain?

• Definition is subjective

– Roderick Peattie. Mountain Geography (1936)

Mountains are 1) impressive, 2) enter into the

imagination of people living in their shadow, and 3)

have individuality.

• Traditional definition: elevation increase above

surroundings > 300 m MSL

• Objective definitions are difficult:

– Elevation (insufficient criterion, e.g., Great Plains)

– Local relief (Grand Canyon?, incised into plateau)

– Steepness of slope

– The amount of land in slopes

Mountains of the western US

Whiteman (2000)

Western

U.S.

Terrain

(high- dark;

low-light)

Roughness

(dark)

Diurnal Temperature Range

NWS and RAWS only

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Nocturnal Inversion

All observations available in MesoWest

Planetary Scale

• Impact of terrain versus land-sea constrasts on planetary-scale circulation has been studied since Charney and Eliassen (1949) and Smagorinsky (1953)

• After 50+ years of debate, answer settled that both are important, but terrain perhaps a little more so

• Ting, M., and H. Wang 2006: The Role of the North American Topography on the Maintenance of the Great Plains Summer Low-Level Jet. J. Atmos. Sci., 1056-1068

Summer 850 hPa

with Rockies

Summer 850 hPa

without Rockies

Summer 850 hPa

Rockies – no Rockies

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Orographically modified cyclogenesis

• Vertical motion induced by orography affects the

evolution of absolute vorticity

• Low-level ascent on the windward side of a

range results in column compression and

parcels acquire anticyclonic absolute vorticity

(e.g., windward ridge)

• Low-level descent on the leeward side of a

range results in column stretching and parcels

acquire cyclonic absolute vorticity (the leeward

ridge).Core content originally developed

by Jim Steenburgh

How do cyclones strengthen?

• Need to increase low-level cyclonic vorticity

• Must stretch low-level fluid columns

• To stretch the column, need mid-tropospheric ascent,

near-surface descent, or both

(z f)f > (z f)0

Column stretches

(z f)0 > 0

pp

Development of Lee Side Trough

Column stretches

(z+f) increases

Column compresses

(z+f) decreases

Column stretches

(z+f) increases

Column compresses

(z+f) decreases

z

x

y

x

Windward

Ridge

Lee

Trough

Wave

Train

Orographic effects on cyclone evolution• Column stretching contributes to acquisition of cyclonic absolute

vorticity to the lee of a mountain barrier

• Column compression contributes to acquisition of anticyclonic

absolute vorticity windward of a mountain barrier

• These effects are “superimposed” on large-scale forcing

• Best case for lee cyclogenesis is when mountain-induced column

stretching occurs with synoptic-scale conditions favorable for

cyclogenesis (e.g., 500 mb CVA, local maximum in warm advection,

condensational heating)

• Lee cyclogenesis usually associated with a pre-existing synoptic-scale

trough or cyclone

• Caveats

• Cross-barrier flow does not guarantee lee cyclogenesis/windward

cyclolysis

• Mountain-induced column stretching can be countered by

compression associated with synoptic-scale forcing (500-mb AVA or

low-level cold advection)

• Cyclone development can occur on the windward side of a mountain

range if it is favored by synoptic-scale dynamics

Passage of low pressure center over mountains

Whiteman (2000)

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Conceptual model

Primary CycloneTopographic Eddies

Bannon, P. R., 1992: A model of Rocky Mountain lee cyclogenesis.

J. Atmos. Sci., 49, 1510–1522.

• Theory suggests that cyclone evolution in

complex orography, including lee cyclogenesis,

results from the superposition of • A parent cyclone

• Topographic pressure perturbations induced by the

interaction of the parent cyclone with the orography

• This superposition results in the “amoeba-like”

movement of cyclones across the Rockies and

modification of the rate of cyclone development

• Caveat• Theory does not fully account for steep orography, diabatic

effects (and their feedback), nongeostrophic flow effects, and

nonlinear scale interations

Challenges of frontal analysis in complex terrain

• Errors arise from the reduction of surface pressure to sea level

• Difficult to determine intensity or even existence of horizontal

temperature gradients in regions due to variability in surface station

elevation

• Conventional observations (NWS/FAA/DoD) over western U.S. are of

low density and are located primarily in valleys

• Diabatic effects and boundary layer processes can obscure large-

scale airmass changes

– Surface-based inversions mask temperature changes

– Terrain-induced flows (thermally or dynamically driven) mask wind

changes

• There can be contrasts in frontal intensity and position between low

and high elevation stations

• So, even with all their faults, need to rely on mesonet observations to

fill in space/time continuity for frontal analysis

Core content originally developed by J. Steenburgh

How does orography affect fronts?

• Movement

• Low-level flow blocking and channeling may retard or

accelerate a front, resulting in a distortion of its “shape”

• Frontogenesis/frontolysis

• Terrain-induced horizontal flow field may contribute to

frontogenesis or frontolysis

• Terrain-induced vertical motion pattern (and associated

adiabatic warming and cooling) may contribute to

frontogenesis or frontolysis

• Vertical structure

• Low-level blocking may act to decouple surface-based and

upper-level portions of front

• In some cases, entire lower portion of a front may not be able

to cross a mountain ridge or range, leaving only upper-level

front

Flow splitting around an isolated

mountain range

Whiteman (2000)

Convergence zones often form on the back side of

isolated barriers (Ex: Puget Sound convergence zone)

Frontal movement up and over a

mountain barrier

Whiteman (2000)

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Discrete Frontal Propagation

Dickinson, M.J.

and D.J. Knight.

1999: Frontal

interaction with

mesoscale

topography. J.

Atmos. Sci., 56:

3544-3559.

Terrain channeling

• Terrain-parallel jet may develop in post-frontal environment

• Contributes to development of frontal nose

Steenburgh and Blazek (2001)

Figure 4. Total number of strong cold frontal passages (1979–

2003). Shafer, J. C., and W. J. Steenburgh, 2008: Climatology of

strong Intermountain cold fronts. Mon. Wea. Rev., in press.

Maximum Temperature: Monday. April 15. 2002

Tax Day Storm:

April 15, 2002

Tax-Day Storm (15 April 2002):

• Extensive damage ($4M+) from high winds >

35 m / s

• Record lowest SLP (982mb) at Salt Lake City

(SLC)

• Ushered in an extended period of cold/wet

weather

• 5-10 year event

• Max temperature change with cold front 16 C /

hr

• Prefrontal blowing dust visibility < 1 km, closed

roads,

• Rained mud, brownish/orange-colored snow

(J. Shafer)

Todd Foisy. April 15, 2002

Bagley. Salt Lake Tribune

6/9/2010

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April 14-15, 2002 WBBSummary

• Topography can distort the structure of a low-level cold front in several ways

• Fronts can be retarded by pre-frontal downslope and blocking of the post-frontal airmass windward of the topography

• Along-valley or gap winds may accelerate fronts through lowland regions

• Low-level and upper-level portions of a front may become decoupled

• Mountain-induced horizontal winds and vertical motion can result in frontogenesis or frontolysis

• Analyze using:– pressure on constant height surfaces other than sea level

– potential temperature

– temporal and spatial continuity provided by mesonets

Terrain-forced flows• Two types of mountain winds

– Diurnal mountain winds (thermally driven circulations): produced by temperature contrasts that form within mountains or between mountains and surrounding plains

– Terrain-forced flows: produced when large-scale winds are modified or channeled by underlying complex terrain

• 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

Terrain is not the only factor:

Horizontal heterogeneity in other surface properties

• Changes in surface roughness

– Rough to smooth

– Smooth to rough

• Changes in surface energy fluxes

– Sensible heat flux

– Latent heat flux

• Changes in incoming solar radiation

– Cloudiness

Diurnal mountain wind systems

Whiteman (2000)Whiteman (2000)

Atmospheric structure in valley

Whiteman (2000)Whiteman (2000)

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Wind and Terrain• 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)

• 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.)

Angle of attack

Whiteman (2000)

Over or Around?

• Potential energy: energy required to lift parcel over obstacle in statically stable environment– PE proportional to stability (N2) * obstacle height (h2)

• Kinetic energy: energy available due to air‟s motion– KE proportional to wind speed (U2)

• Froude number squared: ratio of kinetic energy to potential energy– Fr = U/(Nh)

– Fr >> 1 plenty of kinetic energy to lift air over obstacle

– Fr << 1 not enough kinetic energy and flow blocked by terrain

Channeling of synoptic/mesoscale winds

Whiteman (2000)Whiteman (2000)

Pressure driven channeling

Whiteman (2000)

Blocking

-Affects stable air masses and occurs 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

-Predicting onset often easier than predicting demise

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Mountains as flow barriers

Whiteman (2000)

Barrier Jet

Bell and Bosart

1988

Barrier Jet

Winsted et al. (2006)

Barrier Jet

Olson et

al. 2007

Gap Flow Barrier/Gap Flow Interactions

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Gap/Barrier Flow Interactions

Nieman

et al.

2006

Building blocks for orographic storms

• Large-scale weather (e.g., cyclones and fronts)

– Determines the airmass characteristics, including wind speed, wind direction, stability, and humidity

• Dynamics of air motion over and around the mountains

– Determines depth and intensity of the orographic ascent

• Cloud and precipitation microphysics

– Determines if condensation will lead to precipitation

Core content originally developed by Jim Steenburgh

See COMET module Dynamics & Microphysics of Cool-Season Orographic Storms

Recent Progress in Orographic

Precipitation Research• New tools to observe and measure over terrain (aircraft Doppler,

S-Pol, vertical S-Band, profilers, etc..…)

• Microphysical context to the precipitation.

• Better understanding of relationship between moist dynamics and

orographic precipitation (flow blocking, gravity waves, etc…).

• New theoretical models (e.g. R. Smith Linear theory to orographic precipitation).

• Common features between numerous field studies (MAP,

IMPROVE-2, CALJET, IPEX, PACJET, …).

Colle et al. (2008) Mountain Weather Workshop

Key Elements for Orographic Precipitation: water

vapor flux (Fw), large-scale ascent, terrain-forced

ascent, microphysics and fallout

,

,

0 oC

qe

Fw

Large

scale

ascent

Microphysics

Terrain-

forced

ascent

Colle et al. (2008) Mountain Weather Workshop

Smith and Barstad (2004)

Growth

Fallout

Evaporation

Model uses:

- linear mountain wave theory

- adjustable time scales for precipitation growth, fallout, and

evaporation

Precipitation efficiency increases with:

More moist inflow

Higher barrier

Wider barrier

Stoelinga and Stewart (2008) Mountain Weather Workshop

“Pineapple Express” or “Atmospheric River”

6-8 Feb 1996 (Colle and Mass 2000)

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Orographic precipitation mechanisms• Stable upslope

• “Seeder-Feeder”

• Potential instability release

• Sub-cloud evaporation

contrasts

• Terrain-driven convergence

• Terrain-induced thunderstorm

initiation

• Usually more than one

mechanism is operating

University of Utah

Stable upslope

• Stable (laminar) ascent is forced by flow over a

mountain

• If air forced over mountain is sufficiently moist

through a deep layer, precipitation develops

• If not, shallow, non-precipitating clouds develop

• Not very efficient if operating alone

www.capetownskies.com

Seeder-Feeder

• Snow or rain generated in “seeder” clouds aloft falls through low-

level orographic “feeder” clouds

– Feeder cloud might not otherwise produce precipitation

– Precipitation enhanced by collision-coalescence and accretion

in feeder cloud

• Seeder cloud can be frontal, or orographically generated/enhanced

• Common over Cascades, Sierra, and coastal ranges, particularly in

pre-frontal environment

Seeder Cloud

Feeder Cloud

Jay Shafer

Upslope release of “potential instability”

• Potential instability – Special situation where orographic

lift triggers convection

• Convection may be deep or shallow

– both can result in substantial precipitation

enhancement

• Important for postfrontal snow, or precipitation just ahead

of cold front if there‟s a pre-frontal surge of cold air aloft

Upslope release of potential instability (dqe/dz < 0)

Very effective over relatively small hills, particularly if a small amount of lift is needed to release instability

Favorable synoptic setting/geographic locations

– Warm sector (particularly within 300 km of cold front): British Isles, CA coastal Mts.

– Post-cold-frontal: Most ranges of western U.S. including Cascades & Wasatch

Colle et al. (2008)

Courtesy: Jim Steenburgh

Terrain-driven convergence

• Terrain-induced flow produces

convergence, lift, and precipitation

• Examples

– Windward convergence in

Wasatch, San Juans, Front

Range, Park Range

• Ascent shifted upstream of initial

mountain slope

• Slight reduction in crest-level

precip (Petersen et al. 1991)

– Lee-side convergence zones

• e.g., Puget Sound Convergence

Zone

• Flow converges to lee of

Olympics

Mass (1981)

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Medina & Houze (2003)

Conceptual model of orographic precipitation

Mesoscale Alpine Program (MAP)

Willamette Valley

Coast

Longitude (Deg)

Latitude (Deg)

S

N

W

E

Height (m)

Conceptual Model of Impact of

Gravity Waves on Precipitation over

the Oregon Terrain

Garvert et al. (2007)

Conceptual Model of An

Orographic StormCold, Dry

Upper

Cold FrontCold Front

Steenburgh, W. J., 2003: One hundred

inches in one hundred hours: Evolution of

a Wasatch Mountain winter storm cycle.

Wea. Forecasting, 18, 1018-1036.

Some Unresolved Issues Related to

Orographic Precipitation:

• Categorizing precipitation structures over terrain

• Relative importance of intermittent versus quasi-steady precipitation

• Changes in water budget over terrain from the Pacific to the western interior U.S.

• Need for improvements in model PBL and microphysics over terrain

• Understanding microphysical processes within 1-2 km of the ground

• Impact of small-scale terrain ridges and/or shear turbulence over windward slopes

Colle et al. (2008) Mountain Weather Workshop