evolution of long-axis lake-effect convection during landfall and orographic uplift profiling radar...
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Evolution of long-axis lake-effect convection during landfall and orographic uplift
Profiling radar observations during OWLeS
Ted Letcher & Justin Minder University at Albany
Jim Steenburgh, Peter Veals & Leah Campbell
University of Utah
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What determines downwind evolution of LLAP bands & their snowfall?
Mesocale forcings:• Orography• Surface fluxes (heat, moisture, momentum)
Cloud & precipitation structures:• In-cloud ice and supercooled water• Crystal habits• Snowfall
Convective scale dynamics:• Cloud depth• Updraft velocities & turbulence• Horizontal scales/structures• Buoyancy
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Orographic lifting “invigorates” convection
Plausible hypotheses
Orographic lifting produces more “populous” (or wider) convective cells
Orographic lifting enhances low-level growth … or suppresses low-level sublimation
Inland transition leads to clouds that are more “efficient” at producing snowfall.
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Orographic lifting “invigorates” convection
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Plausible hypotheses
Orographic lifting produces more “populous” (or wider) convective cells
Orographic lifting enhances low-level growth … or suppresses low-level sublimation
Inland transition leads to clouds that are more “efficient” at producing snowfall.
Lackman (2011)
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Orographic lifting “invigorates” convection
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Orographic lifting produces more “populous” (or wider) convective cells
Orographic lifting enhances low-level growth … or suppresses low-level sublimation
Inland transition leads to clouds that are more “efficient” at producing snowfall.
Plausible hypotheses
?
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Sandy IslandBeach - SIB (75 m)
SandyCreek- SC(175 m)
NorthRedfield -NRED
(385 m)
UpperPlateau- UP
(530 m)
4 Micro Rain Radars (MMR2’s)• 24 GHz, FM-CW, profiling, Doppler• Δz= 200 m• max. height = 6km• Δt = 60 s
Deployment• IOP-phase: Dec-Jan (All sites)• Extended : Oct-Feb (SIB & NRED)
• Observed 17 LLAP eventsCo-located radars for inter-comparison before and during the field campaign
Profiling Radar Observations Goals• Characterize along-band variations in convective
structure with high temporal & vertical resolution
• Improve operational forecasting through a better understanding of involved physical processes.
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Case-study example:IOP2b
IOP2b snowfall: consistent inland enhancement
SC: 33.5 mm
NRED: 62.5 mm
SC
NRE
D
SC
NREDTotal snow depth
6hr. accumulated SWEOrographic ratio
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Sandy IslandBeach (SIB) IOP2b: time-height structure
time
strong updraft
turbulent
Heavy snow
heig
ht
(
km M
SL)
[dBZ][m
s-1]
[m s
-1]
Reflectivity
Doppler fall velocity
Spectral width
updow
n
>6 ms-1 Updraft!
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heig
ht
(
km M
SL)
time
IOP2b: inland evolution of reflectivity[dBZ]
Reflectivity SIB
SC
NRED
UP
No inland increase in echo top height!
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heig
ht
(
km M
SL)
time
IOP2b: inland evolution of Doppler fall velocity[m
s-1]
Doppler fall velocity SIB
SC
NRED
UP
updow
nup
down
updow
nup
down
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heig
ht
(
km M
SL)
time
IOP2b: inland evolution of spectral width[m
s-1]
Spectral width SIB
SC
NRED
UP
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IOP2b: Echo Tops
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SC NRED UP
surface elev.surface elev.
surface elev.
[% / dBZ]
SIB
IOP2b: dBZ Contoured Frequency by Altitude Diagrams (CFADs)
median
75 th %-tile
25th %
-tile
surface elev.
Histogram of Reflectivity at each range gate
75%25%
Median
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[dBZ] Freq. [%]
Freq. of dBZ>5
• Larger vertical gradient in dBZ• Narrower distribution of dBZ • Less frequent echoes aloft • More frequent low-level echoes• No evidence of sub-cloud sublimation
@ NRED:
Median & IQR
SIB
NRED
SIB NRED
IOP2b: inland evolution of CFADs (NRED vs. SIB)
[dBZ][dBZ]
heig
ht
[
km M
SL]
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IOP2b: evidence of sub-cloud sublimation at SCCS?
Decrease in reflectivity at below 1km
SIB SCCSUAH MIPS:XPR SCCS
Continuation of decreasing trend below 1km MSL
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Multi-storm perspective:statistics from 17 LLAP storms
17 LLAP events (Nov 2013-Feb 2014)
Same inland evolution seen in IOP 2:• Reduced variability• Reduced dBZ aloft• Increased low-level echo frequency• Loss of sublimation signature
Bulk CFADS for all LLAP events observed @ SIB & NRED
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[% / dBZ]
SIB NRED
Freq. [%]
Freq. of dBZ>5Median & IQR
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3 Week Holiday Break
Bulk Echo Tops Observed @ SIB and NRED
SIB
NRED
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IOP 21: Evidence for intense low-level growth?
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IOP 21: Evidence for intense low-level growth?
SIB [dBZ]
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NRED [dBZ]
time
heig
ht
(
m M
SL)
22
time
heig
ht
(
m M
SL)
SIB [dBZ]
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• Not riming (low density aggregates observed)• Not blowing snow (winds are weak)
NRED [dBZ]
IOP 21: Evidence for intense low-level growth?
2nd MRR, with dz = 30 m
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IOP 21: NREDOrographic Ratio ~1.5
Low-level increase in reflectivity
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Comparison of IOP2b to NEXRAD Beam elevation
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IOP2b: NEXRAD beam height
Brown et al. 2007
NEXRAD QPE estimates affected by overshooting issues? Better Coverage?
East WestEastWest
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IOP2b: NEXRAD beam heightSIB
NRED
1.0˚
1.0˚
1.5˚
1.5˚
0.5˚
0.5˚
Beam Width
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Conclusions (thusfar)
Orographic “invigoration” of convection is not responsible for Tug Hill precip maximum
Compared to upwind, echoes over the Tug are often:• weaker aloft• more-frequent near the ground• Less convective ?
? Hints of important low-level processes over Tug:• Suppressed sublimation?• Enhanced Growth?
Time-height structure of convection typically exhibits a common change in structure between shore and Tug Hill
NEXRAD QPE estimates of LE precipitation may be altered due to overshooting
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Extra Slides
NorthSouth
Upland
Lake
dBZ
dBZ
Vd
Vd
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IOP2b: inland evolution seen by airborne Wyoming Cloud Radar
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Variations in OR during OWLeS
IOP2IOP4
OR = Orographic Ratio = North Redfield SWE/Sandy Creek SWESWE=Snow Water Equivalent
IOP21/22{
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