1

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

2

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

3

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.

4

Orographic lifting “invigorates” convection

4

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)

5

Orographic lifting “invigorates” convection

5

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

?

6

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.

7

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

9

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!

10

heig

ht

(

km M

SL)

time

IOP2b: inland evolution of reflectivity[dBZ]

Reflectivity SIB

SC

NRED

UP

No inland increase in echo top height!

11

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

12

heig

ht

(

km M

SL)

time

IOP2b: inland evolution of spectral width[m

s-1]

Spectral width SIB

SC

NRED

UP

13

IOP2b: Echo Tops

14

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

15

[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]

16

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

17

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

18

[% / dBZ]

SIB NRED

Freq. [%]

Freq. of dBZ>5Median & IQR

19

3 Week Holiday Break

Bulk Echo Tops Observed @ SIB and NRED

SIB

NRED

20

IOP 21: Evidence for intense low-level growth?

21

IOP 21: Evidence for intense low-level growth?

SIB [dBZ]

21

NRED [dBZ]

time

heig

ht

(

m M

SL)

22

time

heig

ht

(

m M

SL)

SIB [dBZ]

22

• 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

23

IOP 21: NREDOrographic Ratio ~1.5

Low-level increase in reflectivity

24

Comparison of IOP2b to NEXRAD Beam elevation

25

IOP2b: NEXRAD beam height

Brown et al. 2007

NEXRAD QPE estimates affected by overshooting issues? Better Coverage?

East WestEastWest

26

IOP2b: NEXRAD beam heightSIB

NRED

1.0˚

1.0˚

1.5˚

1.5˚

0.5˚

0.5˚

Beam Width

27

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

28

Extra Slides

NorthSouth

Upland

Lake

dBZ

dBZ

Vd

Vd

29

IOP2b: inland evolution seen by airborne Wyoming Cloud Radar

30

31

Variations in OR during OWLeS

IOP2IOP4

OR = Orographic Ratio = North Redfield SWE/Sandy Creek SWESWE=Snow Water Equivalent

IOP21/22{

32