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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
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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
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Does boundary-layer turbulence enhance snow growth in mixed-phase
clouds?
Med Bow Mtns
Med Bow Mtns
wind
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(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
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turbulent BL depth: ~ 1.0 km
power spectrum over this WCR section
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(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
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turbulence top = cloud top
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distance (km)
Does this turbulence really matter
brief spells of snow growth by accretion or riming in rising eddies?
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time (UTC)
2009-03-10
B
B
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surface-induced snow initiation ??
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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
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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
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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
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WCR reflectivity (dBZ)
crest
LCL
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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.
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WCR reflectivity (dBZ)
crest
LCL
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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
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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?
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low-level snow initiation?
2006-01-18
(a) Hallet-Mossop ice multiplication on rimed surfaces like trees: we have no evidence
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Does blowing snow initiate glaciation in supercooled liquid orographic clouds?
fall speed removed
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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
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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
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WCR reflectivity
WCR vertical velocity
WCL backscatter power
WCL depolarization ratio
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Another blowing snow case,with a shallow stratus cloud deck upstream of mountain, cloud top temperature -14°C
leg 3 (along-wind)
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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
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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)
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Depol Ratio is low at cloud top (droplets) and higher below (ice)
terrain
terrain
WCL backscatter power
WCL depolarization ratio
500
mcloud top
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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).
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additional slides
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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