Detailed vertical structure of orographic precipitation
development in cold clouds An illustration of high-resolution airborne mm-wave
radar observations and flight-level cloud data
Bart Geerts, Heather McIntyreUniversity of Wyoming
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.
generating cells?
low-level echo intensification across the crest
low-level snow outflow
u215552-220402 UTC
Synoptic situation at this time (20060118, 20 Z)
prefrontal, SW flow aloft(UL trof evident to the NW)
flight level temperature: -16°Csurface wind speed near crest: 11 ms-1
The increase in reflectivity sometimes coincides with a sudden drop in LWC.
wedge of growing reflectivity in upslope PBL, disconnect from snow aloft
Upstream Downstream
LWC100 0.20 g/m3 0.06 g/m3
PVMLWC 0.27 g/m3 0.10 g/m3
Vertical Velocity 0.93 m/s -0.33 m/s
Relative Humidity 88 % 78 %
WCR reflectivity (lowest 500m AGL)
-4.6 dBZ +11.8 dbZ
January 18,2006 213935-215050 UTC
mean values within 10 km from the ridgeflt level 4,400 m MSL, T=-15°C
flight level temperature: -17°Csurface wind speed near crest: 13 ms-1
Is wind blowing over a snow-covered
surface a possible nucleation source?
We need to estimate snow particle trajectories to
distinguish between fall-streaks and lofted
surface snow
t=0
t=14 min
t=27 min
t=40 min
Barrett Ridge Med Bow peak
Natural seeding by snow-covered surfaces
• “surface-induced snowfall” (SIS): snow seems to appear from the surface, and is mixed into the PBL
• 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
Natural seeding by snow-covered surfaces
• Examination of data collected last winter suggests the following most-likely mechanisms– Lofting of snow from surface– Hallet-Mossop ice splintering when a supercooled drop hits an
ice surface
• Conditions under which this appears to be most likely are:– Surface covered by fresh snow– Windy (>10 m/s ?) and cold (T<-5°C?)– Possibly: cloud present and tree surfaces are rimed
Post-frontal cumuliform orographic
snowfall(2 Feb, 20 UTC)
upwind (SRT) soundingupwind (SRT) sounding
GOES VIS
GOES IR
20060202 1900-1912 UTC
flight level temperature: -19°Csurface wind near crest: 12 ms-1 from NW
Post-frontal cumuliform orographic snowfall (2 Feb)
conclusions• High-resolution vertical-plane reflectivity and vertical velocity
transects reveal a range of orographic precipitation structures.
• Pre-frontal: deep precipitation may be distinguished from shallow orographic component.
• Post-frontal orographic precip is far more cumuliform, with locally large LWC.
• Natural glaciation may be rapid, and can occur both upstream and just downstream of the crest.
• Natural seeding may occur by blowing snow or cloud contact with rimed surfaces (SIS).
Further work using winter 06 data
• objectives:1. Describe snow growth
relative to mountain ridge.
2. Gain clues about snow growth processes (deposition, riming, aggregation)
3. Examine differences between select cases, in terms of Fr and presence of upstream clouds
• methods:1. Estimate snow crystal
trajectories from VPDD and an assumed fall speed.
2. Examine LWC data and 2D particle imagery, in the context of WCR vertical velocity and echo structure
3. Plot upstream soundings (from WKA and model) and construct summary table
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