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Bart Geerts University of Wyoming Gabor Vali, Jeff French, Yang Yang

Impact of surface interaction and cloud seeding on orographic snowfall A downlooking airborne cloud radar view. Bart Geerts University of Wyoming Gabor Vali, Jeff French, Yang Yang. two types of surface interaction. PBL turbulence

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Bart Geerts University of Wyoming Gabor Vali, Jeff French, Yang Yang

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  1. Impact of surface interaction and cloud seeding on orographic snowfallA downlooking airborne cloud radar view Bart Geerts University of Wyoming Gabor Vali, Jeff French, Yang Yang

  2. two types of surface interaction • PBL turbulence • mainly mechanical, in post-frontal situations this may be convective • ice nucleation near the surface

  3. Radar beam refractionrange vs height diagram a R’ Earth radius h r fo

  4. Wyoming Cloud Radar • 3 mm (95 GHz, W-band), dual-polarization • pulse width: 250-500 ns • max range: 3-10 km • volume resolution @ 3 km range: < 40 m • minimum detectable signal (@ 1 km): ~-30 dBZ • Cloud droplets are much smaller than ice crystals, thus in a mixed-phase cloud, reflectivity is dominated by ice crystals.

  5. WCR observations of orographic precipitation under unseeded conditions fallspeed of unrimed snow sinking rising -1 flow 215552-220402 UTC flight level mountain crest 1:1 aspect ratio intense turbulence in the lowest ~ 1 km AGL mountain crest

  6. implications of BL turbulence • ground-generated seeding agent mixes effectively • natural enhancement of precipitation

  7. flow Houze and Medina (2005)

  8. flight-level glaciation as snow generated in the upslope PBL mixes up to flight level near the crest LWC flow -16°C; 19 ms-1 -11°C; 11 ms-1 mountain crest snow cloud base wedge of growing reflectivity in upslope PBL, disconnect from snow aloft 18 Jan 2006, 21:20-21:51 UTC

  9. 18 Jan 2006, 21:20-21:51 UTC

  10. unrimed rimed

  11. impact of ground-based AgI seeding? no seedingseeding flow AgI seeding Turpin supercooled liquid water ice crystal concentration vertical air velocity

  12. surface-induced nucleation 2D-C image 0.8 mm ~200 mm size rimed particles wave cloud reflectivity flight level GLEES mountain crest view from cockpit vertical velocity sinking rising 27 Jan 2006, 22:22-22:31 UTC

  13. surface-induced snow growth flight level flight level GLEES mountain crest view from cockpit upstream wind speed 18 Jan 2006, 22:42-22:55 UTC

  14. Natural seeding by the surfaces • snow seems to appear from the surface, and is mixed into the PBL • mechanisms: • growth of blowing snow in cloud • secondary ice nucleation, by splintering when a supercooled drop hits an ice surface (Hallet-Mossop) • Conditions under which this appears to be most likely are: • surface covered by fresh snow, cold, and windy • cloud base below ridge level, right temperature range (-3 to -8°C, Mossop 1976), trees or other rimable surfaces • Rogers and Vali (1987, “Ice Crystal Production by Mountain Surfaces”) found that the air sampled on Elk Mountain contained 10 - 1,000 more ice crystals than the free atmosphere upstream (Rogers and Vali 1987)

  15. wind speed ~ 18 m/s cloud seeding AgI generator AgI generator Snowy Range

  16. impact of ground-based AgI seeding? no seeding seeding flow into page Barret Turpin supercooled liquid water ice crystal concentration vertical air velocity

  17. conclusions • High-resolution vertical-plane reflectivity and vertical velocity transects reveal the importance of surface processes: • PBL turbulence • ice nucleation near the surface • Deep tropospheric precipitation is distinct from from shallow orographic component. • PBL turbulence • effectively mixes seed material in cloud • appears to be an important precip enhancement mechanism • It remains unclear • how common these conditions are • how useful additional seeding is under these conditions

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