comet module resources mountain weather workshop (2008...
TRANSCRIPT
6/9/2010
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Influence of Topography on Weather
John Horel
Department of Meteorology
University of Utah
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
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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