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R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull University of Washington

Mechanisms of Orographic Precipitation Enhancement: What we’ve learned from MAP & IMPROVE II. R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull University of Washington M. Steiner Princeton University. MAP and IMPROVE II Experimental Areas. Rapid Enhancement Problem.

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R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull University of Washington

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  1. Mechanisms of Orographic Precipitation Enhancement:What we’ve learned from MAP & IMPROVE II R. A. Houze, Jr., Socorro Medina, Ellen Sukovich, B. F. Smull University of Washington M. Steiner Princeton University

  2. MAP and IMPROVE II Experimental Areas

  3. Rapid Enhancement Problem “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

  4. Smith & Barstad (2004): Particle Trajectories over Mountains

  5. What microphysical processes can grow precipitation particles quickly? Coalescence T > 0 deg C Aggregation Riming T < 0 deg C “Accretion”

  6. Liquid water content over the Cascade Mountains (Hobbs 1975) Similar distribution found over the Sierra Nevada (Marwitz, 1987) Trajectories of ice particles growing by deposition & riming (Hobbs et al. 1973) Small, light particles Large, heavy particles

  7. 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

  8. 2D Idealized WRF simulation of cross-barrier flow “Up & over” “Retarded”

  9. Up & over case: MAP IOP2b – 20 September 1999 36 30 24 18 12 6 0 -6 -12 16 14 12 10 8 6 4 2 0 54 44 34 24 14 4 -6 -16 -26 dBZ m/s % 3h MEAN S-Pol RADAR DATA 1 2 3 4 5 6 REFLECTIVITY 1 2 3 4 5 6 RADIAL VELOCITY Height (km) 1 2 3 4 5 6 FREQUENCY OCCURRENCE Dry snow (50 %) Wet snow (30 %) Graupel - Shaded RADIAL VELOCITY 120 90 60 30 0 Distance (km) from S-Pol radar

  10. Enhancement in up and over flow conditions

  11. Enhancement in up and over flow conditions

  12. Retarded flow cases: 2D Idealized WRF simulation of cross-barrier flow MAP IOP8 & IMPROVE II CASE 11 MAP IOP8 IMPROVE CASE 11 Wind speed Wind speed Shear IMPROVE CASE 11

  13. 36 30 24 18 12 6 0 -6 -12 16 14 12 10 8 6 4 2 0 54 44 34 24 14 4 -6 -16 -26 dBZ m/s % S-Pol RADIAL VELOCITY P3 RADIAL VELOCITY 0 2 4 6 8 Height (km) 0 2 4 6 8 Retarded flow case: MAP IOP8 – 21 October 1999 3h MEAN S-Pol RADAR DATA STABILITY FROM MILAN SOUNDING 1 2 3 4 5 6 REFLECTIVITY 1 2 3 4 5 6 VERTICAL POINTING RADAR Height (km) REFLECTIVITY REFLECTIVITY 1 2 3 4 5 6 FREQUENCY OCCURRENCE Dry snow (50 %) Wet snow (30 %) Graupel - Shaded RADIAL VELOCITY RADIAL VELOCITY Graupel and/or dry aggregates – Shaded 120 90 60 30 0 0600 0800 1000 1200 Distance (km) from S-Pol radar Time (UTC) 21 Oct

  14. 48 40 32 24 16 8 0 -8 -16 40 35 30 25 20 15 10 5 0 54 44 34 24 14 4 -6 -16 -26 dBZ m/s % 1 2 3 4 5 Height (km) 1 2 3 4 5 Retarded flow case: IMPROVE II, Case 11, 13-14 Dec ‘01 3h MEAN S-Pol RADAR DATA STABILITY FROM UW SOUNDING 1 2 3 4 5 6 REFLECTIVITY 1 2 3 4 5 6 S-Pol RADIAL VELOCITY VERTICAL POINTING RADAR Height (km) REFLECTIVITY (dBZ) 1 2 3 4 5 6 FREQUENCY OCCURRENCE RADIAL VELOCITY (m/s) Dry snow (50 %) Wet snow (30 %) Graupel - Shaded Graupel and/or dry aggregates – Shaded 2300 0000 0100 0200 0 25 50 75 100 Time (UTC) 13-14 Dec Distance (km) from S-Pol radar

  15. IMPROVE II CASE 11 – 13-14 December 2001 Idealization of retarded-flow case 2ndary reflectivity max

  16. IMPROVE II CASE 11 – 13-14 December 2001 Ice particle images obtained by NOAA P3

  17. Repeatability 28 Nov. 28 Nov. 30 Nov. 30 Nov. 13-14 Dec. 13-14 Dec. 17 Dec. 17 Dec. 18 Dec. 18 Dec.

  18. 28 Nov. 30 Nov. 14 Dec. 17 Dec. 18 Dec.

  19. 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

  20. What we’ve learned about physical mechanisms of precipitation enhancement over windward slopes RETARDED-FLOW CASES • 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

  21. What we’ve learned about physical mechanisms of precipitation enhancement over windward slopes THE CASCADES Some unanswered questions • 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?

  22. End

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