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