R' A' Houze, Jr', Socorro Medina, Ellen Sukovich, B' F' Smull

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Mechanisms of Orographic Precipitation
EnhancementWhat weve learned from MAP
IMPROVE II
R. A. Houze, Jr., Socorro Medina, Ellen
Sukovich, B. F. Smull University of Washington
M. Steiner Princeton University
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MAP and IMPROVE II Experimental Areas
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Rapid Enhancement Problem
Even if we accept the idea that large-scale
orographic lifting can cause some release, it is
surprising in light of the difficulties in
forming precipitation-size particles, to find
release efficiencies of 70 to 100, Is it
possible to convert such a high fraction of the
condensed water into precipitation? Ron
Smith (1979)
Physical understanding of orographic
precipitation enhancement reduces to
understanding the physical mechanisms by which
the orographic enhancement process can occur so
quickly and efficiently in windward side flow
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Smith Barstad (2004) Particle Trajectories
over Mountains
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What microphysical processes can grow
precipitation particles quickly?
Coalescence T gt 0 deg C
Aggregation
Riming
T lt 0 deg C
Accretion
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Liquid water content over the Cascade Mountains
(Hobbs 1975)
Similar distribution found over the Sierra
Nevada (Marwitz, 1987)
Trajectories of ice particles growing by
deposition riming (Hobbs et al. 1973)
Small, light particles
Large, heavy particles
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How can the airflow make the accretion processes
more active?
Smith 79 Cellularity
Cells of embedded convection or turbulence in
upslope cloud can accelerate particle growth by
coalescence, riming, aggregation
Adapted from Smith 1979
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2D Idealized WRF simulation of cross-barrier flow
Up over
Retarded
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Up over case MAP IOP2b 20 September 1999
3h MEAN S-Pol RADAR DATA
1 2 3 4 5 6
REFLECTIVITY
1 2 3 4 5 6
RADIAL VELOCITY
Height (km)
1 2 3 4 5 6
FREQUENCY OCCURRENCE
Dry snow (50 ) Wet snow (30 ) Graupel - Shaded
RADIAL VELOCITY
120 90 60 30 0
Distance (km) from S-Pol radar
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Enhancement in up and over flow conditions

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Enhancement in up and over flow conditions

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Retarded flow cases 2D Idealized WRF simulation
of cross-barrier flow MAP IOP8 IMPROVE II CASE
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IMPROVE CASE 11
MAP IOP8
Wind speed
Wind speed
Shear
IMPROVE CASE 11
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Retarded flow case MAP IOP8 21 October 1999
3h MEAN S-Pol RADAR DATA
STABILITY FROM MILAN SOUNDING
1 2 3 4 5 6
REFLECTIVITY
1 2 3 4 5 6
VERTICAL POINTING RADAR
Height (km)
REFLECTIVITY
REFLECTIVITY
1 2 3 4 5 6
FREQUENCY OCCURRENCE
Dry snow (50 ) Wet snow (30 ) Graupel - Shaded
RADIAL VELOCITY
RADIAL VELOCITY
Graupel and/or dry aggregates Shaded
120 90 60 30 0
0600 0800 1000 1200
Distance (km) from S-Pol radar
Time (UTC) 21 Oct
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Retarded flow case IMPROVE II, Case 11, 13-14
Dec 01
3h MEAN S-Pol RADAR DATA
STABILITY FROM UW SOUNDING
1 2 3 4 5 6
REFLECTIVITY
1 2 3 4 5 6
S-Pol RADIAL VELOCITY
VERTICAL POINTING RADAR
Height (km)
REFLECTIVITY (dBZ)
1 2 3 4 5 6
FREQUENCY OCCURRENCE
RADIAL VELOCITY (m/s)
Dry snow (50 ) Wet snow (30 ) Graupel - Shaded
Graupel and/or dry aggregates Shaded
2300 0000 0100 0200
0 25 50 75 100
Time (UTC) 13-14 Dec
Distance (km) from S-Pol radar
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IMPROVE II CASE 11 13-14 December
2001 Idealization of retarded-flow case
2ndary reflectivity max
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IMPROVE II CASE 11 13-14 December 2001
Ice particle images obtained by NOAA P3
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Repeatability
28 Nov.
28 Nov.
30 Nov.
30 Nov.
13-14 Dec.
13-14 Dec.
17 Dec.
17 Dec.
18 Dec.
18 Dec.
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What weve learned about physical mechanisms of
precipitation enhancement over windward slopes
FLOW-OVER CASES

• Direct up and over lifting of high Fr upstream
  flow
• Produces cellularity by concentrating lifting of
  near surface flow over each small-scale rise in
  the terrain
• Stable lifting of high Fr flow, release of
  instability, or both
• Pockets of high LWC over each local windward
  slope ? riming increased fallout rate
• Applies to Alps warm-sector flows
• May apply to Cascades post-frontal flows

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What weve learned about physical mechanisms of
precipitation enhancement over windward slopes
RETARDED-FLOW CASES

• Two-layered orographic enhancement
• Upper levels
• - Precipitation growth enhanced in a layer aloft
  (2ndary refl max)
• - Could be gravity wave enhancement?
• Low levels
• - Shear layer produced by flow retardation
• - Cellular overturning in shear layer
• - Seen in both Alps and Cascades
• - Overturning may be buoyant or mechanical (dont
  need inst?)
• - Cells concentrate cloud LWC ? riming
  increased fallout rate

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What weve learned about physical mechanisms of
precipitation enhancement over windward slopes
THE CASCADES
Some unanswered questions

• This two-layered enhancement occurs in middle
  part of frontal system
• To what extent does the 2-layered enhancement
  overwhelm frontal mechanisms?
• Can they be distinguished from precipitation
  processes unaffected by orography?

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End