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Precipitation min on crest of Alps, max on lower slopes

Orographic Precipitation Enhancement in Midlatitude Baroclinic Storms: Results from MAP and IMPROVE II Robert A. Houze and Socorro Medina. 20-year Alpine Autumn Precipitation Climatology (rain gauge analysis by Frei and Schaer 1998). Precipitation min on crest of Alps, max on lower slopes.

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Precipitation min on crest of Alps, max on lower slopes

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  1. Orographic Precipitation Enhancement in Midlatitude Baroclinic Storms: Results from MAP and IMPROVE II Robert A. Houze and Socorro Medina

  2. 20-year Alpine Autumn Precipitation Climatology(rain gauge analysis by Frei and Schaer 1998) Precipitation min on crest of Alps, max on lower slopes

  3. Major issue Understand HOW microphysical processes are invigorated to produce quick and efficient orographic enhancement in windward side flow

  4. High concentration (small particles) Low concentration (large particles) The Cascade Project (Hobbs et al. 1973, Hobbs 1975) Streamlines Liquid Water Content Trajectories of ice particles growing by deposition and riming

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

  6. accretion How can the airflow make the accretion processes more active? “Cellularity” (Smith 1979)

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

  8. Equivalent Potential Temperature IOP2b IOP3 IOP5 12Z 20 Sep 99 00Z 26 Sep 99 12Z 03 Oct 99 Milan sounding

  9. Stable cases: IMPROVE II Case 11 MAP IOP8

  10. IMPROVE II Experimental Area26 November-22 December 2001 WASHINGTON OREGON Salem PACIFIC OCEAN Newport NOAA S-Band NCAR S-Pol UW MM5 Medford

  11. IMPROVE II Case 11: 13-14 December 2001 MM5 12 h forecast 500 mb height, wind, and temperature Valid 00 UTC 14 Dec 01

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

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

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

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

  16. IMPROVE II NOAA/ETL S-band Radar 13-14 December 2001 Reflectivity

  17. IMPROVE II NOAA/ETL S-band Radar 13-14 December 2001 Radial Velocity Ri»0.25

  18. Time series at McKenzie Bridge during IMPROVE II Case 11 Shear at 0.7 - 3.0 km (profiler) Radialvelocity (VP S-band) Min radialvelocity at2-3 km(VP S-band) Occurrence of graupel &/or aggregates(S-Pol)

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

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

  21. Stable cases:MAP IOP 8

  22. Equivalent Potential Temperature IOP8 18Z 20 Oct 99 Milan sounding

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

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

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

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

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

  28. 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 byaccretion • have updrafts >1-3 m/s • contain aggregates and/or graupel • enhance precipitationon lower slopes TURBULENCE

  29. Mixed case:MAP IOP 14

  30. Equivalent Potential Temperature IOP14 00Z 4 Nov 99 Milan sounding

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

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

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

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

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

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

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

  38. MAP IOP8 Mean wind shear from Lonate profiler Mean and SD over 34 hours

  39. A Microphysical Question “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

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