Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds

or:The importance of the lower boundary

Qun MiaoNingbo University

Bart GeertsUniversity of Wyoming

NCAR orographic precip workshop, 13-15 March 2012

acknowledgements: Yang Yang, UWKA crew, Roy Rasmussen, Dan Breed

The advantage of a nadir view …

rad

ar+

lidar

rad

ar+

lidar

radar on

ly

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

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 number 1.4

mean Richardson number 1.3

mixing ratio at 200 m AGL (g kg-1) 3.1

mean LCL (km MSL) 2.78

mean LCL temperature (°C) -5

𝑅𝑖= 𝑁 2

(𝜕𝑈𝜕 𝑧 )2

300 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 number 5

mean Richardson number 0.4

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)

2009-03-10

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) plotsal

titud

e ab

ove

th

e g

rou

nd

reflectivity or vertical velocitybin Dz

bin

Z o

r V

incr

em

ent

nn

(Yuter & Houze 1997)

date18 Jan

0627-28 Jan 06

2 Feb 06

11 Feb 08

25 Feb 08

18 Feb 09

20 Feb 09

10 Mar 09

25 Mar 09

30 Mar 09

# along-wind flight legs

5 17 10 2 3 3 2 4 4 3

# ladder legs 0 0 0 16 16 16 12 14 14 12

Med Bow Range

crest

LCL

wind

1.upwind

below LCL

2.upwind

above LCL

3.lee

4.4 5.9 5.9 107 profiles

west east

WCR reflectivity (dBZ)

crest

LCL

1 23

123

1 2 3

crest

LCL

12 3

1. 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 displaynon-bright-band rain at CZD

Hei

ght,

MS

L (

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?

2006-01-18

(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 30-40 kts during flight

sounding from BL2:deep well-mixed layerstrong windsT<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

SW NE

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

SW NE

cloud top

WCR vertical velocity

terrain zoom-in (next slide)

Depol Ratio is low at cloud top (droplets) and higher below (ice)

terrain

terrain

WCL backscatter power

WCL depolarization ratio

500

mcloud 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

date18 Jan

0627-28 Jan 06

2 Feb 06

11 Feb 08

25 Feb 08

18 Feb 09

20 Feb 09

10 Mar 09

25 Mar 09

30 Mar 09

mean fallspeed at flight level (m s-1)

1.27 1.09 1.21 1.12 0.96 0.79 1.01 0.82 0.86 0.75

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