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Documenting Orographic Enhancement of Precipitation in Olympic Mountains Region

This study analyzes orographic enhancement of precipitation in the Olympic Mountains region through radar and ground network data. It identifies physical processes responsible for the observed enhancement and factors influencing leeside rain shadow formation.

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Documenting Orographic Enhancement of Precipitation in Olympic Mountains Region

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  1. Precipitation Processes and their Modulation by Synoptic Conditions Observed during the GPM Ground Validation Olympic Mounts Experiment (OLYMPEX) Lynn McMurdie, Robert A. Houze, Jr., Joseph Zagrodnik, Angela Rowe, Jennifer DeHart, and Hannah Barnes University of Washington AGU, 12 Dec 2016

  2. Documenting Orographic Enhancement of PrecipitationOutline • Orographic enhancement for the entire 2015-2016 season via Radar and ground network • Physical processes accounting for observed enhancement • Factors controlling leeside rainshadow

  3. Documenting Orographic Enhancement from NPOL east west

  4. Documenting Orographic Enhancement from NPOL East - Terrain West - Ocean

  5. Documenting Orographic Enhancement from NPOL East – West difference Orographic Enhancement

  6. Documenting Orographic Enhancement from NPOL No Sensitivity NPOL GPM-Ku Had to include regions over ocean well outside of NPOL range

  7. Documenting Orographic Enhancement from Ground network Prairie Creek – 540m CRN – 115 m Beach Houses – 3 m Orographic enhancement factor ~ 1.5 – 2 from coast to inland

  8. Focus on a subset of full season data: Steady rainy periods, during mostly prefrontal/warm sector • All storms Oct 7, 2015 – April 30 2016 • Postfrontal and periods with < 1 mm/h rain rate at Prairie Creek removed. • 3-hr periods of Parsivel, Rain Gauge, and NARR data for environmental parameters • North America Regional Reanalysis (NARR) data from the 32 km grid box closest to NPOL

  9. Documenting Orographic Enhancement from Ground network Frequency Rainrate Rain Rate – Prairie Creek Log of Nw – number of drops Log of Nw – number of drops Do Median Diameter (mm) Do Median Diameter (mm) These are for Prairie Creek (540 m). Low elevation stations similar plots, but lower rainrates and fewer instances of the extremes

  10. IVT Melting Level Prairie Creek Prairie Creek Log of Nw – number of drops Median Diameter (mm) Median Diameter (mm) 925 hPa Wind Speed 925 hPa Wind Direction Prairie Creek Prairie Creek Log of Nw – number of drops

  11. The high rain rate, high melting level, high IVT and strong winds at 925 hPa periods tend to be associated with atmospheric river type storms. Now will look at an example storm.

  12. 12 – 13 November 2015850 hPa Heights + Temp 0600 UTC 13 Nov Strong low-level onshore flow. Very warm airmass

  13. 12 – 13 November 2015 Integrated Vapor Transport Strong vapor transport

  14. 12 – 13 November 2015Observed Precipitation Totals – Nov 13 High Terrain less than forward slopes Factor 4 -5 Enhancement Next plot of DSD from CRN site located here – elevation 115m Different than seasonal norm

  15. 12 – 13 November 2015 Warm Sector RHI Locations near coast do not have this increase of small drops in warm sector Increase in rain rate due to increase of small drops

  16. 12 – 13 November 2015 Reflectivity There is substantial ice layer (to 8-10km depth) also being lifted by terrain and producing precipitation NPOL CRN Velocity Lifting low-level jet produces drop formation + growth occurs below melting levelat initial slopes of high terrain. Brightband CRN

  17. Summary • NPOL documented orographic enhancement in ice layer • Ice processes  large drops • High melting level height  small drops • AR events have both ice-phase and warm process (due to lifting below brightband) PSDs • Leeside rain shadow affected by frontal waves and low-level flow (not shown ….)

  18. Southern Leg NE SW • December 8 – AR event. APR3 from the DC-8 Aircraft • Largest reflectivity on windward slopes and not high terrain • Echo top height decrease in lee • Melting level variability

  19. Southern Leg Windward SW NE • Largest reflectivity on windward slopes and initial high terrain • Frontal band fills in leeside echo tops

  20. Acknowledgments • Work supported by: • NASA grants NNX15AL38G, NNX16AD75G and NNX16AK05G • NSF grant AGS-1503155

  21. GPM Kavs Ku Band GPM-Ku GPM-Ka

  22. GPM Ka Minus Ku • Ka has slightly higher sensitivity at all heights • Ku generally less attenuated • No significant difference between the two wavelengths • Toyoshima et al (2015)

  23. Nov 12-13 Rain rates ~18 hour period of orographic enhancement More rain at low-elevation CRN site compared with interior/higher elevation sites Maximum Orographic Enhancement

  24. Summary • NPOL documented orographic enhancement with higher reflectivity aloft ~4km and near brightband • Dropsize distribution changes with melting level height, water vapor transport and low level winds • During periods with strong onshore low-level moist jet – flow is lifted below brightband and large production of small drops contribute strongly to orographic enhancement of precip • Leeside rain shadow affected by frontal waves and changes in low-level flow

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