weather systems & mountains chapter 8 cold air damming in lackmann (2011) chapter 8.2 orographic...
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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.