1. 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

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 refraction range vs height diagram r  h  Earth radius R’

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

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

7. Houze and Medina (2005) flow

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

11. unrimed rimed

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

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

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

15. Natural seeding by the surfaces snow seems to appear from the surface, and is mixed into the PBL mechanisms: a)growth of blowing snow in cloud b)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: a)surface covered by fresh snow, cold, and windy b)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)

16. wind speed ~ 18 m/s AgI generator AgI generator cloud seeding

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

18. 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