impact of surface interaction and cloud seeding on orographic snowfall a downlooking airborne cloud...
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Impact of surface interaction and cloud seeding on orographic snowfall
A downlooking airborne cloud radar view
Bart GeertsUniversity of Wyoming
Gabor Vali, Jeff French, Yang Yang
two types of surface interaction
• PBL turbulence– mainly mechanical, in post-frontal situations this may be
convective
• ice nucleation near the surface
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.
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
-1
fallspeed of unrimed snow
implications of BL turbulence
• ground-generated seeding agent mixes effectively
• natural enhancement of precipitation
-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
mountaincrest
18 Jan 2006, 21:20-21:51 UTC
impact of ground-based AgI seeding? no seeding seeding
supercooled liquid water
ice crystal concentration
vertical air velocity
Turpin
flow
AgIseeding
surface-induced nucleation
reflectivity
vertical velocity
wave cloud
mountaincrest
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
surface-induced snow growth
mountaincrest
18 Jan 2006, 22:42-22:55 UTC
flight level
flight level
GLEES
view from cockpit
upstreamwind speed
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 cloudb) 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 windyb) 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)
impact of ground-based AgI seeding? no seeding seeding
Barret Turpin
supercooled liquid water
ice crystal concentration
vertical air velocity
flow into page
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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