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

Radar beam refractionrange vs height diagram

r

h

'sin'2' 21

22 RrRRrh o

Earth radius

R’

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

Houze and Medina (2005)

flow

-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

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

unrimed

rimed

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)

wind speed ~ 18 m/s

AgIgenerator

AgIgenerator

cloud seeding

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