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R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull
University of Washington
M. SteinerPrinceton University
Mechanisms of Orographic Precipitation Enhancement:
What we’ve learned from MAP & IMPROVE II
“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
Rapid Enhancement Problem
What microphysical processes can grow precipitation particles quickly?
Coalescence
T > 0 deg C
Aggregation Riming
T < 0 deg C
“Accretion”
Liquid water content over the Cascade Mountains (Hobbs 1975)
Trajectories of ice particles growing by deposition & riming (Hobbs et al. 1973)
Small, light particles
Large, heavy particles
Similar distributionfound over theSierra Nevada(Marwitz, 1987)
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
120 90 60 30 0Distance (km) from S-Pol radar
1 2
3
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5
6
1 2
3
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6
1 2
3
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Hei
gh
t (k
m)
Up & over case: MAP IOP2b – 20 September 1999
3h MEAN S-Pol RADAR DATAREFLECTIVITY
RADIAL VELOCITY
FREQUENCY OCCURRENCE
54443424144-6-16-26
dBZ
363024181260-6-12
m/s
1614121086420
%
RADIAL VELOCITY
Dry snow (50 %)Wet snow (30 %)Graupel - Shaded
Retarded flow cases: 2D Idealized WRF simulation of cross-barrier flow MAP IOP8 & IMPROVE II CASE 11
IMPROVE CASE 11
IMPROVE CASE 11
MAP IOP8
Wind speed Wind speed
Shear
S-Pol RADIAL VELOCITYP3 RADIAL VELOCITY
120 90 60 30 0Distance (km) from S-Pol radar
1 2
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Hei
gh
t (k
m)
Retarded flow case: MAP IOP8 – 21 October 1999
3h MEAN S-Pol RADAR DATA
REFLECTIVITY
FREQUENCY OCCURRENCE
54443424144-6-16-26
dBZ
363024181260-6-12
m/s
1614121086420
%
STABILITY FROM MILAN SOUNDING
Dry snow (50 %)Wet snow (30 %)Graupel - ShadedGraupel and/or
dry aggregates –Shaded
VERTICAL POINTING RADARREFLECTIVITY
RADIAL VELOCITY
0600 0800 1000 1200 Time (UTC) 21 Oct
0
2
4
6
8
Hei
gh
t (k
m)
0
2
4
6
8
REFLECTIVITY
RADIAL VELOCITY
0 25 50 75 100Distance (km) from S-Pol radar
1
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Hei
gh
t (k
m)
Retarded flow case: IMPROVE II, Case 11, 13-14 Dec ‘01
3h MEAN S-Pol RADAR DATA
REFLECTIVITY
S-Pol RADIAL VELOCITY
FREQUENCY OCCURRENCE
54443424144-6-16-26
dBZ
484032241680-8-16
m/s
4035302520151050
%
STABILITY FROM UW SOUNDING
Dry snow (50 %)Wet snow (30 %)Graupel - ShadedGraupel and/or dry aggregates –Shaded
1
2
3 4
5
6
1
2
3 4
5
6
VERTICAL POINTING RADAR
2300 0000 0100 0200
Time (UTC) 13-14 Dec
1 2
3 4
5
Hei
gh
t (k
m)
1
2 3
4 5
RADIAL VELOCITY (m/s)
REFLECTIVITY (dBZ)
What we’ve 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
What we’ve learned about physical mechanisms of precipitation enhancement over windward slopes
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 (don’t need inst?)- Cells concentrate cloud LWC riming & increased fallout rate
RETARDED-FLOW CASES
•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?
What we’ve learned about physical mechanisms of precipitation enhancement over windward slopes
THE CASCADES
Some unanswered questions