Evolution of long-axis lake-effect convection during landfall and orographic uplift Profiling radar observations during OWLeS 1 Ted Letcher & Justin Minder University at Albany Jim Steenburgh, Peter Veals & Leah Campbell University of Utah

What determines downwind evolution of LLAP bands & their snowfall? Mesocale forcings: Orography Surface fluxes (heat, moisture, momentum) Mesocale forcings: Orography Surface fluxes (heat, moisture, momentum) Cloud & precipitation structures: In-cloud ice and supercooled water Crystal habits Snowfall Cloud & precipitation structures: In-cloud ice and supercooled water Crystal habits Snowfall Convective scale dynamics: Cloud depth Updraft velocities & turbulence Horizontal scales/structures Buoyancy Convective scale dynamics: Cloud depth Updraft velocities & turbulence Horizontal scales/structures Buoyancy 2

Orographic lifting “invigorates” convection 3 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 ?

Sandy Island Beach - SIB (75 m) Sandy Creek- SC (175 m) North Redfield -NRED (385 m) Upper Plateau- 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 events Co-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. 6

7 Case-study example: IOP2b

8 IOP2b snowfall: consistent inland enhancement SC: 33.5 mm NRED: 62.5 mm SC NRED SC NRED Total snow depth 6hr. accumulated SWE Orographic ratio

Sandy Island Beach (SIB) IOP2b: time-height structure time strong updraft turbulent Heavy snow 9 height (km MSL) [dBZ] [m s -1 ] Reflectivity Doppler fall velocity Spectral width up down >6 ms -1 Updraft!

10 height (km MSL) time IOP2b: inland evolution of reflectivity [dBZ] Reflectivity SIB SC NRED UP No inland increase in echo top height!

11 height (km MSL) time IOP2b: inland evolution of Doppler fall velocity [m s -1 ] Doppler fall velocity SIB SC NRED UP up down up down up down up down

12 height (km MSL) time IOP2b: inland evolution of spectral width [m s -1 ] Spectral width SIB SC NRED UP

13 IOP2b: Echo Tops

SC NRED UP surface elev. [% / dBZ] SIB 14 IOP2b: dBZ Contoured Frequency by Altitude Diagrams (CFADs) median 75 th %-tile 25 th %-tile surface elev. Histogram of Reflectivity at each range gate 75% 25% Median

[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 NRED: Median & IQR SIB NRED SIB NRED 15 IOP2b: inland evolution of CFADs (NRED vs. SIB) [dBZ] height [km MSL]

16 IOP2b: evidence of sub-cloud sublimation at SCCS? Decrease in reflectivity at below 1km SIB SCCS UAH 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 SIB & NRED 18 [% / dBZ] SIB NRED Freq. [%] Freq. of dBZ>5 Median & IQR

3 Week Holiday Break 19 Bulk Echo Tops SIB and NRED SIB NRED

20 IOP 21: Evidence for intense low-level growth?

IOP 21: Evidence for intense low-level growth? SIB [dBZ] 21 NRED [dBZ] time height (m MSL)

time height (m MSL) 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? 2 nd MRR, with dz = 30 m

23 IOP 21: NRED Orographic Ratio ~1.5 Low-level increase in reflectivity

24 Comparison of IOP2b to NEXRAD Beam elevation

25 IOP2b: NEXRAD beam height Brown et al NEXRAD QPE estimates affected by overshooting issues? Better Coverage? EastWest East West

26 IOP2b: NEXRAD beam height SIB NRED 1.0˚ 1.5˚ 0.5˚ Beam Width

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? 27 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 Vd 29 IOP2b: inland evolution seen by airborne Wyoming Cloud Radar

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Variations in OR during OWLeS IOP2 IOP4 OR = Orographic Ratio = North Redfield SWE/Sandy Creek SWE SWE=Snow Water Equivalent IOP21/22 { 31

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