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

MAP and IMPROVE II Experimental Areas

“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

Smith & Barstad (2004): Particle Trajectories over Mountains

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

2D Idealized WRF simulation of cross-barrier flow

“Up & over”

“Retarded”

120 90 60 30 0Distance (km) from S-Pol radar

1 2

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t (k

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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

Enhancement in up and over flow conditions

Enhancement in up and over flow conditions

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

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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

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8

Hei

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t (k

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REFLECTIVITY

RADIAL VELOCITY

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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

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VERTICAL POINTING RADAR

2300 0000 0100 0200

Time (UTC) 13-14 Dec

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RADIAL VELOCITY (m/s)

REFLECTIVITY (dBZ)

IMPROVE II CASE 11 – 13-14 December 2001Idealization of retarded-flow case

2ndary reflectivity max

IMPROVE II CASE 11 – 13-14 December 2001

Ice particle images obtained by NOAA P3

28 Nov. 28 Nov.

30 Nov.30 Nov.

17 Dec. 17 Dec.

18 Dec.18 Dec.

13-14 Dec. 13-14 Dec.

Repeatability

28 Nov. 30 Nov.

17 Dec. 18 Dec.

14 Dec.

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

End