Weather systems & mountains

Chapter 8 Cold Air Damming in Lackmann (2011)

Chapter 8.2 Orographic effects in Wallace and Hobbs (2006)

see Elsevier companion site for additional material

This MODIS image shows smoke from forest fires in Southern California stoked by Santa Anna winds on 10/14/2008

Orographic effects: topics• lee cyclogenesis• Rossby wave propagation along sloping terrain• cold air damming• terrain-induced wind storms• orographic precipitation

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Lee cyclogenesis

Alberta low

Colorado low

regions of frequent lee cyclogenesis

Explanation: conservation of potential vorticity (PV)

slowspin

fast spin

Fig. 7.8

fast spin

q=290 K

q=330 K

zisentropesbetweendepth

vorticityabsolutePVor

zpfgPV

Dz largeDz small

Dz large

ET cyclones decay upwind of mountain barrier

ET cyclones re-emerge downwind of barrier

PV is generally conserved.

trof

ridg

e

L

trofridge

L

trof

ridge

L

trof

ridge

L

Lows and highs tend to propagate equatorward east of topographic barriers,

and poleward west of the barriers• example: examine trof-ridge movement in the Colorado low example, a

few slides back• reason: PV conservation

warm

cold

Barrier jet dynamics

H

L

H

L

cold-airmass

boundary

retardation deflection along-barrier jet

Barrier jet

• due to terrain blocking– air accelerates down the pressure gradient

(Parish 1982)

west eastxSierra

Nevada

Rocky Mtns & Plains

Cold-air damming & barrier jet east of the Rockies

• mesoscale barrier jet in Colorado20 March 2002: COMET case

radial velocity

dual-Doppler velocity

L

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Barrier jet

Barrier jet:cold-air damming east of the Appalachians

surface temperatures (F)

17-20 Z

Barrier jet:cold-air damming east of the Appalachians

HH

HH

barrier jet

scatterometer data

Alaska

scatterometer data

Gap flows

Gap winds

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acceleration through the gap strong winds downwind of the gap

Tehuantepec gap

gap flow:Tehuantepecer

gap flow: Tehuantepecer: synoptic-scale cold surge into Mexico 1993/3/13 12Z

(storm of the century)

Schultz et al 1997

isthmus of Tehuantepec

Santa Ana winds and wildfires in California

cold high

down-gradient, subsident flow channeled through the gaps

Average wind speed at 50 m AGL in 2012 in winter (DJF) (9 km WRF) source: David Siuta

10 14 182 6

Wind speed (m/s)

lee convergence zone

lee convergence zone: behind the mountain, where gap currents meet

Mountain waves and downslope wind storms

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Orographic precipitation

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Clearly mountains have a profound impact on precipitation.

The mechanism is basic: the moisture-laden marine boundary-layer is lifted over coastal ridges. These ridges are rather low.

Further downwind towards the continental divide the ascent is higher, but the remaining water vapor less.

Summary: orographic effects

• Cyclogenesis occurs in the lee of a major mountain range when strong deep-tropospheric flow crosses that range. The lee cyclone finds itself in warm air due to subsidence, and often evolves into a classic frontal disturbance.

• Low-level warm or cold anomalies tend to propagate to the left along sloping terrain, when looking towards the higher terrain.

• An example of this is cold-air damming, most prominent east of the Rockies and the Appalachians. The cold pool tends to propagate to the left (equatorward) and becomes more shallow, trapped by the terrain.

• Terrain-induced wind storms (downslope plunging flow) may occur when the upstream flow is blocked (stable) below the mountain crest, and strong cross-barrier flow is present aloft. This plunging flow may produce a hydraulic jump.

• Mountains tend to capture much water vapor crossing the barrier, as orographic precip, by forced ascent, convection, and BL turbulence.