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 McIntyre University of Wyoming

target mountain range Wyoming

Snowy Range Sierra Madre elevation range 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: ns max range: 3-10 km volume 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 UTC

Houze and Medina (2005)

generating cells? low-level echo intensification across the crest low-level snow outflow u UTC

Synoptic situation at this time ( , 20 Z) prefrontal, SW flow aloft (UL trof evident to the NW)

flight level temperature: -16°C surface 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

UpstreamDownstream LWC g/m g/m 3 PVMLWC0.27 g/m g/m 3 Vertical Velocity0.93 m/s-0.33 m/s Relative Humidity88 %78 % WCR reflectivity (lowest 500m AGL) -4.6 dBZ+11.8 dbZ January 18, UTC mean values within 10 km from the ridge flt level 4,400 m MSL, T=-15°C

flight level temperature: -17°C surface 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 Mountain Saratoga

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 ,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) sounding GOES VIS GOES IR

UTC flight level temperature: -19°C surface wind near crest: 12 ms -1 from NW Post-frontal cumuliform orographic snowfall (2 Feb)

upstream views downstream view 21:38 UTC 21:24 UTC :25 UTC

Lee waves (5 Feb, 15 Z)

Lee waves (5 Feb) Kennaday Peak 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