Two Messages We Hope toConvey During this Conference

• Mesoscale phenomena have a significantimpact on the sensible weather at everyrange.

• The RT-FDDA (all models, for that matter)should be treated as guidance, and nottruth.

–– Weather forecasts are inescapably uncertain. And likeWeather forecasts are inescapably uncertain. And likea human forecaster, the RT-FDDA makes forecasts ina human forecaster, the RT-FDDA makes forecasts inthe face of this uncertainty. Thus, the model forecastthe face of this uncertainty. Thus, the model forecastmust be viewed as an assessment of what must be viewed as an assessment of what maymayhappen.happen.

Flow Interaction with Topography

Presented by:Presented by:Daran RifeDaran RifeNCAR RALNCAR RAL26 Jul 200526 Jul 2005

Three Factors that Affect Terrain-Forced Flows

i. Stability of the air approaching themountain,

ii. Speed of the air flow approaching themountain,

iii. Height, shape, and orientation of themountain barrier.

i. Atmospheric Stability

• Stable air–– Resists lifting.Resists lifting.–– More likely that air will flow around, beMore likely that air will flow around, be

forced through gaps in the barrier, or beforced through gaps in the barrier, or beblocked by barrier.blocked by barrier.

• Neutral or Unstable air–– Easily lifted over the top of a mountain.Easily lifted over the top of a mountain.

ii. Speed of the Air Flow

• Weak winds–– Stable flows blocked.Stable flows blocked.–– Neutral or unstable flows easily liftedNeutral or unstable flows easily lifted

over the top of mountain.over the top of mountain.• Moderate to strong winds

–– Stable flows forced over the top ofStable flows forced over the top ofbarrier or around barrier, or both.barrier or around barrier, or both.

iii. Height, Shape, and Orientationof Topography

• Isolated mountain–– Flow may either go over or aroundFlow may either go over or around

mountain, or both.mountain, or both.• Longer mountain chains

–– Flow tends to go over the barrier, or beFlow tends to go over the barrier, or beblocked by the barrier.blocked by the barrier.

Three Basic Types of FlowInteraction with Terrain

i. Mountain waves and hydraulic flows,ii. Flow blocking and lee vortices,iii. Flows through gaps, channels, and

passes.

i. Mountain Waves

• Nearly always present in thevicinity of complex terrain,

• Have a distinct signal in thevertical motion and streamline(isentrope) fields, and sometimesin the cloud patterns.

i. Mountain Waves (Cont’d)

• Most common type is thehydrostatic mountain wave

–– Do not propagate downstream.Do not propagate downstream.–– Do propagate vertically, usuallyDo propagate vertically, usually

well into the stratosphere.well into the stratosphere.

Mountain Wave Schematic

Flow follows streamlines or isentropes

Lenticular clouds

Saturated air along windward slopes

©The COMET Program

Satellite Image of Mountain Waves

Downslope Windstorms

From Whiteman (2000)

©The COMET Program

Don’t fly here!

Downslope Windstorm: AFamous Example

• A large region of high pressure upstream of themountains, and a rapidly developing lee side trough orlow pressure center.

• A strong jet core directly aloft of the mountain range.• Strong surface low pressure system centered to the north

of the mountain range.• A strong cold front moving across the windward side of

the mountain range.• Cold tropopause temperatures (less than about -60 C)

directly aloft of the mountain range.

Synoptic-scale Weather Patternsthat Produce Mountain Waves

Atmospheric Conditions Associatedwith Downslope Windstorms

• Strong winds near mountain top (>15 m s-1).• A relative absence of speed shear above mountain top.

This means that the wind component at, say, 500 This means that the wind component at, say, 500 mbmb, in the direction of the, in the direction of theflow at mountain top level should not be greater than about 1.5 times theflow at mountain top level should not be greater than about 1.5 times themountain top flow.mountain top flow.

• Flow reverses direction with height above the mountain.• A layer of enhanced stratification near or just above the

mountain.• An absence of deep cold air near the surface in the lee.

Using JViz and the FDDA ImageViewer to Diagnose Mountain

Waves29 Jan 2001: Mountain waves over WSMR

This demonstration will coverThis demonstration will cover::

1.1. The use of forecast cross sections to diagnoseThe use of forecast cross sections to diagnosestatic stability near and above the mountains.static stability near and above the mountains.

2.2. How to recognize the mountain wave signatureHow to recognize the mountain wave signaturein forecast cross sections.in forecast cross sections.

3.3. What the spatial patterns of surface wind speedWhat the spatial patterns of surface wind speedlook like during a look like during a downslopedownslope windstorm. windstorm.

W E

WSMR grid 3 forecast of potential temperature,RH, and winds valid 1500 UTC 29 Jan 2001.

Narrow bandof high windsalong lee flanksof mountain

WSMR grid 3 forecast of surface wind speed and streamlines valid 1500 UTC 29 Jan 2001.

ii. Flow Blocking and Lee Vortices

• Flow blocking occurs when theatmosphere is highly stratifiedand/or the flow is weak.

• How can we tell whether flow will goover or around a mountain?

Fr = Fr = FroudeFroude number number = kinetic energy / potential energy = kinetic energy / potential energy = wind velocity / mountain height x atmospheric stability = wind velocity / mountain height x atmospheric stability = U/ = U/hhmmNN

ii. Flow Blocking (Cont’d)

• Fr < 1 : air blockedby mountain andmust go around orbe turned back.

• Fr > 1 : air goes overmountain.

Blocked flow

Flow over mountain

ii. Lee Vortices: A Schematic

From Whiteman (2000)

• Vortex pairs and awake a generatedby flow around amountain.

• 01 Dec 2000: Vortices form in the lee of theSacramento Mountains at WSMR.

This demonstration will coverThis demonstration will cover::

1.1. How to recognize the synoptic scale conditionsHow to recognize the synoptic scale conditionsthat lead to the formation of lee vortices.that lead to the formation of lee vortices.

2.2. What the spatial wind patterns look like in a leeWhat the spatial wind patterns look like in a leevortex, using both the observations and forecastvortex, using both the observations and forecastgrids.grids.

Using JViz and the FDDA ImageViewer to Diagnose Lee Vortices

WSMR grid 1 forecastValid 21 UTC 01 Dec 2000

WSMR grid 2 forecastValid 21 UTC 01 Dec 2000

WSMR grid 3 forecastValid 21 UTC 01 Dec 2000

iii. Gap Flows

• What are gap winds?–– Winds that rush through a gap in theWinds that rush through a gap in the

topography.topography.–– Result from the accumulation of aResult from the accumulation of a

pressure gradient along the gap.pressure gradient along the gap.–– Normally shallow in their verticalNormally shallow in their vertical

extent.extent.

iii. Gap Flows: A Schematic

iii. Gap Flows: A Real Example

Winds are uniform andstrong north of Peninsula.South of Peninsula, windvaries greatly, with verystrong winds downwind ofgaps in terrain.

Cool colors = Weak winds Warm colors = Strong Winds

Radar-observed windsover Alaska Peninsula

iii. Gap Flows: What they LookLike within a Forecast Grid

1993 “Superstorm” developing over Gulf of Mexico

1800 UTC 13 March 1993

From Steenburgh et al. (1998)

Gap Flow over Gulfof Tehauntepec

iii. Gap Flows: What they LookLike within a Forecast Grid

Close to resolutionused for RT-FDDAgrid 3 (3.33 km)

Further ReadingCOMET Program cited. , 2001: Flow Interaction With Topography. [Available online athttp://meted.ucar.edu/mesoprim/flowtopo/.].

COMET Program cited. , 2003: Gap Winds. [Available online athttp://meted.ucar.edu/mesoprim/gapwinds/.].

COMET Program cited. , 2004: Mountain Waves and Downslope Winds. [Availableonline at http://meted.ucar.edu/mesoprim/mtnwave/.].

Stewart, J. Q., Whiteman C. D., Steenburgh W. J., and Bian X., 2002: A climatologicalstudy of thermally driven wind systems of the U.S. Intermountain West. Bull. Amer.Meteor. Soc., 83, 699-708.

Whiteman, C. D., 2000: Mountain Meteorology: Fundamentals and Applications. OxfordUniversity Press, 355 pp.