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

target mountain range

Wyoming

Snow

y RangeS

ierra Madre

elevation range 2000-3500 m

flight legs roughly parallel with wind

50 km

View from the south

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.

PBL turbulence (~1 km deep)

u

rising motion sinking motion

215552-220402 UTC

Houze and Medina (2005)

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

Battle

MountainSaratoga

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

(Rogers and Vali 1987)

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)

Post-frontal cumuliform orographic snowfall (2 Feb)

upstream views downstream view

21:38 UTC

21:24 UTC

20060202

21:25 UTC

Lee waves (5 Feb, 15 Z)

Lee waves (5 Feb)

KennadayPeak Med Bow Peak

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