R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull University of Washington M. Steiner Princeton 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 AggregationRiming 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 distribution found over the Sierra 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”

Distance (km) from S-Pol radar Height (km) Up & over case: MAP IOP2b – 20 September h MEAN S-Pol RADAR DATA REFLECTIVITY RADIAL VELOCITY FREQUENCY OCCURRENCE dBZ m/s % RADIAL VELOCITY Dry snow (50 %) Wet snow (30 %) Graupel - Shaded

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 MAP IOP8 Wind speed Shear

S-Pol RADIAL VELOCITY P3 RADIAL VELOCITY Distance (km) from S-Pol radar Height (km) Retarded flow case: MAP IOP8 – 21 October h MEAN S-Pol RADAR DATA REFLECTIVITY FREQUENCY OCCURRENCE dBZ m/s % STABILITY FROM MILAN SOUNDING Dry snow (50 %) Wet snow (30 %) Graupel - Shaded Graupel and/or dry aggregates – Shaded VERTICAL POINTING RADAR REFLECTIVITY RADIAL VELOCITY Time (UTC) 21 Oct Height (km) REFLECTIVITY RADIAL VELOCITY

Distance (km) from S-Pol radar Height (km) Retarded flow case: IMPROVE II, Case 11, Dec ‘01 3h MEAN S-Pol RADAR DATA REFLECTIVITY S-Pol RADIAL VELOCITY FREQUENCY OCCURRENCE dBZ m/s % STABILITY FROM UW SOUNDING Dry snow (50 %) Wet snow (30 %) Graupel - Shaded Graupel and/or dry aggregates – Shaded VERTICAL POINTING RADAR Time (UTC) Dec Height (km) RADIAL VELOCITY (m/s) REFLECTIVITY (dBZ)

IMPROVE II CASE 11 – December 2001 Idealization of retarded-flow case 2ndary reflectivity max

IMPROVE II CASE 11 – December 2001 Ice particle images obtained by NOAA P3

28 Nov. 30 Nov. 17 Dec. 18 Dec 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

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