Orographic Precipitation Enhancement in Midlatitude

Baroclinic Storms: Results from MAP and IMPROVE II

Robert A. Houze and Socorro Medina

Precipitation min on crest of Alps, max on lower slopes

20-year Alpine Autumn Precipitation Climatology

(rain gauge analysis by Frei and Schaer 1998)

Major issue

Understand HOW microphysical processes are invigorated to produce quick and efficient orographic

enhancement in windward side flow

The Cascade Project(Hobbs et al. 1973, Hobbs 1975)

Low concentration (large particles)

Liquid Water ContentStreamlines

Trajectories of ice particles growing by deposition and riming

High concentration (small particles)

What microphysical processes can grow precipitation particles quickly?

Coalescence

T > 0 deg C

Aggregation Riming

T < 0 deg C

“Accretion”

How can the airflow make the accretion processes more active?

(Smith 1979)

“Cellularity”

accretion

Potentially unstable upstream flow: MAP IOPs 2b, 3, and 5

IOP2b

IOP3

IOP5

Equivalent Potential Temperature

Milan sounding

12Z 03 Oct 9900Z 26 Sep 9912Z 20 Sep 99

Stable cases: IMPROVE II Case 11

MAP IOP8

IMPROVE II Experimental Area26 November-22 December 2001

PA

CIF

IC O

CE

AN

IMPROVE II Case 11: 13-14 December 2001MM5 12 h forecast 500 mb height, wind, and temperature

Valid 00 UTC 14 Dec 01

IMPROVE II Case 11Upstream soundings

IMPROVE II Case 11Upstream Soundings of equivalent potential temperature

IMPROVE II Case 113-hour Mean Radial Velocity

Horizontal distance (km)

Hei

ght

(km

)

S-Pol radar

ESE

IMPROVE II Case 113-hour Mean Reflectivity

Horizontal distance (km)

Hei

ght

(km

)

S-Pol radar

ESE

IMPROVE II Case 11Polarimetric Particle Identification over 3 hours

P3 aircraft data

Horizontal distance (km)

Hei

ght

(km

)

S-Pol radar

large aggregates and/or graupelmelting snow

weak echo snow(high dBZ, low ZDR)

(high dBZ, high ZDR)

(low dBZ, low ZDR)

ESE

Reflectivity

IMPROVE II NOAA/ETL S-band Radar13-14 December 2001

IMPROVE II NOAA/ETL S-band Radar13-14 December 2001

Radial Velocity

Ri0.25

Time series at

McKenzie Bridge during

IMPROVE II Case 11

Shear at0.7 - 3.0 km

(profiler)

Radialvelocity

(VP S-band)

Min radialvelocity at

2-3 km(VP S-band)

Occurrence of graupel &/or aggregates

(S-Pol)

IMPROVE II Case 11Track of P3 aircraft & S-Pol reflectivity at 1.5 deg elevation

160 km

1.6 mm

9.6 mm

IMPROVE II Case 11Ice particle imagery from P3 aircraft

Stable cases:MAP IOP 8

Equivalent Potential Temperature

IOP8

Milan sounding

18Z 20 Oct 99

MAP IOP834-hour Mean radial velocity

S-Pol radar

NW

MAP IOP834-hour Mean Reflectivity

S-Pol radar

NW

MAP IOP8Polarimetric Particle Identification over 34 Hours

S-Pol radar

weak echo snow (low dBZ, low ZDR)

melting aggregates (high dBZ, high ZDR)

NW

MAP IOP8Reflectivity from vertically pointing S-band radar

at Locarno Monti

Time UTC

Hei

ght

(km

)

OPRA radar

Yuter & Houze 2003

Heavy rain

0°CTUR

BULENC

E

Microphysicalenhancement

Conceptual model for orographic precipitation enhancement in stable, sheared upstream flow

AggregationRiming

Coalescence

TURBUL

ENCE

Conclusions

Low-level growth by coalescence and/or riming is needed to make precipitation fall out quickly on lower slopes

Cellularity is required to make the coalescence and/or riming occur

Cellularity may occur by EITHER release of potential instability OR by turbulence in stable flow

In stable flow, cellularity is a manifestation of turbulence in sheared flow rising over the terrain. Cells in stable flowfavor particle growth by

accretion have updrafts >1-3 m/scontain aggregates and/or

graupelenhance precipitation

on lower slopes

Mixed case:MAP IOP 14

Equivalent Potential Temperature

IOP14

Milan sounding

00Z 4 Nov 99

MAP IOP14Mean wind shear from Lonate profiler

Mean and SD over 16 hours

MAP IOP1434-hour Mean radial velocity

S-Pol radar

NNW

MAP IOP1434-hour Mean Reflectivity

S-Pol radar

NNW

MAP IOP14Polarimetric Particle Identification over 34 Hours

S-Pol radar

weak echo snow (low dBZ, low ZDR)

melting aggregates (high dBZ, high ZDR)

NNW

MAP IOP14Reflectivity from vertically pointing S-band radar

at Locarno Monti

Time UTC

Hei

ght

(km

)

OPRA radar

IMPROVE II Case 11Newport Wind Profiler Data

Mean and SD over 8 hours

IMPROVE II Case 11McKenzie Bridge Profiler Data

Mean and SD over 8 hours

MAP IOP8Mean wind shear from Lonate profiler

Mean and SD over 34 hours

“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)

Major issue

Understand HOW microphysical processes are invigorated to produce quick and efficient orographic

enhancement in windward side flow

A Microphysical Question