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 AggregationRiming 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 99 00Z 26 Sep 99 12Z 20 Sep 99

Stable cases: IMPROVE II Case 11 MAP IOP8

IMPROVE II Experimental Area 26 November-22 December 2001 PACIFIC OCEAN

IMPROVE II Case 11: December 2001 MM5 12 h forecast 500 mb height, wind, and temperature Valid 00 UTC 14 Dec 01

IMPROVE II Case 11 Upstream soundings IMPROVE II Case 11 Upstream Soundings of equivalent potential temperature

IMPROVE II Case 11 3-hour Mean Radial Velocity Horizontal distance (km) Height (km) S-Pol radar ESE

IMPROVE II Case 11 3-hour Mean Reflectivity Horizontal distance (km) Height (km) S-Pol radar ESE

IMPROVE II Case 11 Polarimetric Particle Identification over 3 hours P3 aircraft data Horizontal distance (km) Height (km) S-Pol radar large aggregates and/or graupel melting snow weak echo snow (high dBZ, low ZDR) (high dBZ, high ZDR) (low dBZ, low ZDR) ESE

Reflectivity IMPROVE II NOAA/ETL S-band Radar December 2001

IMPROVE II NOAA/ETL S-band Radar December 2001 Radial Velocity Ri  0.25

Time series at McKenzie Bridge during IMPROVE II Case 11 Shear at km (profiler) Radial velocity (VP S-band) Min radial velocity at 2-3 km (VP S-band) Occurrence of graupel &/or aggregates (S-Pol)

IMPROVE II Case 11 Track of P3 aircraft & S-Pol reflectivity at 1.5 deg elevation 160 km

1.6 mm 9.6 mm IMPROVE II Case 11 Ice particle imagery from P3 aircraft

Stable cases: MAP IOP 8

Equivalent Potential Temperature IOP8 Milan sounding 18Z 20 Oct 99

MAP IOP8 34-hour Mean radial velocity S-Pol radar NW

MAP IOP8 34-hour Mean Reflectivity S-Pol radar NW

MAP IOP8 Polarimetric Particle Identification over 34 Hours S-Pol radar weak echo snow (low dBZ, low ZDR) melting aggregates (high dBZ, high ZDR) NW

MAP IOP8 Reflectivity from vertically pointing S-band radar at Locarno Monti Time UTC Height (km) OPRA radar Yuter & Houze 2003

Heavy rain 0°C TURBULENCE Microphysical enhancement Conceptual model for orographic precipitation enhancement in stable, sheared upstream flow Aggregation Riming Coalescence

TURBULENCE 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 flow  favor particle growth by accretion  have updrafts >1-3 m/s  contain aggregates and/or graupel  enhance precipitation on lower slopes

Mixed case: MAP IOP 14

Equivalent Potential Temperature IOP14 Milan sounding 00Z 4 Nov 99

MAP IOP14 Mean wind shear from Lonate profiler Mean and SD over 16 hours

MAP IOP14 34-hour Mean radial velocity S-Pol radar NNW

MAP IOP14 34-hour Mean Reflectivity S-Pol radar NNW

MAP IOP14 Polarimetric Particle Identification over 34 Hours S-Pol radar weak echo snow (low dBZ, low ZDR) melting aggregates (high dBZ, high ZDR) NNW

MAP IOP14 Reflectivity from vertically pointing S-band radar at Locarno Monti Time UTC Height (km) OPRA radar

IMPROVE II Case 11 Newport Wind Profiler Data Mean and SD over 8 hours

IMPROVE II Case 11 McKenzie Bridge Profiler Data Mean and SD over 8 hours

MAP IOP8 Mean 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