A numerical study of the interaction of the convective boundary layer and orographic
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![Page 1: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/1.jpg)
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
![Page 2: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/2.jpg)
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
![Page 3: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/3.jpg)
Does boundary-layer turbulence enhance snow growth in mixed-phase
clouds?
Med Bow Mtns
Med Bow Mtns
wind
![Page 4: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/4.jpg)
![Page 5: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/5.jpg)
(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
![Page 6: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/6.jpg)
turbulent BL depth: ~ 1.0 km
power spectrum over this WCR section
![Page 7: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/7.jpg)
(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
![Page 8: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/8.jpg)
turbulence top = cloud top
![Page 9: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/9.jpg)
distance (km)
Does this turbulence really matter
brief spells of snow growth by accretion or riming in rising eddies?
![Page 10: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/10.jpg)
time (UTC)
2009-03-10
B
B
![Page 11: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/11.jpg)
surface-induced snow initiation ??
![Page 12: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/12.jpg)
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
![Page 13: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/13.jpg)
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
![Page 16: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/16.jpg)
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.
![Page 17: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/17.jpg)
WCR reflectivity (dBZ)
crest
LCL
![Page 18: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/18.jpg)
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
![Page 19: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/19.jpg)
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?
![Page 20: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/20.jpg)
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
![Page 23: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/23.jpg)
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
![Page 24: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/24.jpg)
WCR reflectivity
WCR vertical velocity
WCL backscatter power
WCL depolarization ratio
![Page 25: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/25.jpg)
Another blowing snow case,with a shallow stratus cloud deck upstream of mountain, cloud top temperature -14°C
leg 3 (along-wind)
![Page 26: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/26.jpg)
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
![Page 27: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/27.jpg)
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)
![Page 28: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/28.jpg)
Depol Ratio is low at cloud top (droplets) and higher below (ice)
terrain
terrain
WCL backscatter power
WCL depolarization ratio
500
mcloud top
![Page 29: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/29.jpg)
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).
![Page 30: Boundary-layer turbulence, surface processes, and orographic precipitation growth in cold clouds or: The importance of the lower boundary Qun Miao Ningbo.](https://reader033.fdocuments.net/reader033/viewer/2022051316/5697bfe01a28abf838cb2f61/html5/thumbnails/30.jpg)
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