Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo University Bart Geerts University of Wyoming NCAR orographic precip workshop, March 2012 acknowledgements: Yang Yang, UWKA crew, Roy Rasmussen, Dan Breed

The advantage of a nadir view … radar+lidar radar only vertical plane dual-Doppler below flight level Wyoming Cloud Radar Wyoming Cloud Lidar

Does boundary-layer turbulence enhance snow growth in mixed-phase clouds? Med Bow Mtns wind

(equivalent) potential temperature (K) mean echo top height (km MSL)6.2 mean wind speed (m s -1 )20 mean Brunt-Väisälä frequency (10 -2 s -1 )1.02 mean shear (10 -3 s -1 )9.2 mean Froude number1.4 mean Richardson number1.3 mixing ratio at 200 m AGL (g kg -1 )3.1 mean LCL (km MSL)2.78 mean LCL temperature (°C) mb height, 800 mb T & wind barbs

turbulent BL depth: ~ 1.0 km  power spectrum over this WCR section

(equivalent) potential temperature (K) mean echo top height (km MSL)4.4 mean wind speed (m s -1 )12 mean Brunt-Väisälä frequency (10 -2 s -1 )0.2 mean shear (10 -3 s -1 )3.8 mean Froude number5 mean Richardson number0.4 mixing ratio at 200 m AGL (g kg -1 )2.6 mean LCL (km MSL)2.6 mean LCL temperature (°C)-8

turbulence top = cloud top

distance (km) Does this turbulence really matter brief spells of snow growth by accretion or riming in rising eddies?

time (UTC) B B

surface-induced snow initiation ??

Composite analysis of snow growth, based on 10 flights over the Med Bow Range in SE Wyoming, using CFADs black lines: along-wind legs red lines: ladder pattern

Frequency by altitude (FAD) plots altitude above the ground reflectivity or vertical velocity bin  z bin Z or V increment nn (Yuter & Houze 1997) date 18 Jan Jan 06 2 Feb Feb Feb Feb Feb Mar Mar Mar 09 # along-wind flight legs # ladder legs

Med Bow Range crest LCL wind 1. upwind below LCL 2. upwind above LCL 3. lee profiles west east

WCR reflectivity (dBZ) crest LCL

crest LCL rapid snow growth across the LCL … 2.yet very little change in MEAN vertical velocity across the LCL. conclusion: snow growth must jump-start when the turbulent BL enters into cloud.

WCR reflectivity (dBZ) crest LCL

scatterplot where LWC > 0.05 g m -3, and the aircraft is within the BL Liquid water in turbulent eddies within the BL there is some positive correlation …  snow must consume some of the droplets in the updrafts

frequency-by-altitude display non-bright-band rain at CZD Height, MSL (km) (Neiman et al., 2005, Mon. Wea. Rev.) profiling S-band radar data, time resolution 6 min (~4 km) Is BL turbulence important also for the low-level growth by collision-coalescence in non-brightband rain?

low-level snow initiation? (a) Hallet-Mossop ice multiplication on rimed surfaces like trees: we have no evidence

Does blowing snow initiate glaciation in supercooled liquid orographic clouds?  fall speed removed

Blowing snow flights in ASCII (Jan-Feb 2012) leg 5 along the Sierra Madre crest KRWL * winds kts during flight sounding from BL2: deep well-mixed layer strong winds T<0°C

WCR reflectivity WCR vertical velocity WCL backscatter power WCL depolarization ratio blowing snow plumes?? high depol ratio suggests this is ice, not water terrain outline, seen by radar & lidar NW SE

WCR reflectivity WCR vertical velocity WCL backscatter power WCL depolarization ratio

Another blowing snow case, with a shallow stratus cloud deck upstream of mountain, cloud top temperature -14°C leg 3 (along-wind)

wind WCR reflectivity WCR vertical velocity WCL backscatter power WCL depolarization ratio SWNE cloud top (T~-12°C) cloud must be thin because terrain can often be seen DR is low at cloud top (droplets) and higher below (ice) first snow (very light) deep, turbulent BL no seeding from aloft

deep, turbulent BL, smooth wave motion aloft wind WCR reflectivity WCL backscatter power WCL depolarization ratio SWNE cloud top WCR vertical velocity terrain zoom-in (next slide)

Depol Ratio is low at cloud top (droplets) and higher below (ice) terrain WCL backscatter power WCL depolarization ratio 500 m cloud top

conclusions A turbulent BL drapes complex terrain. –readily distinguishable from stratiform flow aloft FADs indicate rapid snow growth within the BL as the BL air rises through the cloud base. Shallow orographic clouds may be glaciated by the surface below. BL turbulence can strong (~convective updrafts) –  may increase the fraction of accretional growth (riming).

additional slides

date 18 Jan Jan 06 2 Feb Feb Feb Feb Feb Mar Mar Mar 09 mean fallspeed at flight level (m s -1 ) estimating hydrometeor terminal velocity The FADs show the particle vertical motion. The fallspeed of snow is NOT removed. gust probe: air vertical motion WCR (mean close-gate below & above): particle vertical motion