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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