(Jason Andrew for Wall Street Journal: photo of Park Slope, Brooklyn) The Evolution of Quasi-Linear Convective Systems Encountering the Northeastern US Coastal Marine Environment Kelly Lombardo & Brian Colle Stony Brook University

31 MAY UTC – 01 JUN UTC 23 JUL UTC – 24 JUL UTC Let’s compare the evolution of 2 different QLCS events…

31 MAY UTC – 01 JUN UTC 23 JUL UTC – 24 JUL UTC Let’s compare the evolution of 2 different QLCS events… Why does one event survive over the Atlantic while the other decays upon reaching the coastline?

Data & Methods: Composites Manually examined 2-km NOWrad radar reflectivity for 6 warms seasons (May- Aug) ; Identified 73 QLCS that encountered the Atlantic coast. 65 QLCS events were classified into 4 different categories based on their evolution encountering the coastline. 32 decaying events: Decay at the coastline. 18 slowly decaying events: Show no signs of decay at the coast, but decay over the water within 100 km of the coast. 9 sustaining events: Maintain their intensity more than 100 km from coastline. 6 organize events: Organize along the coastline (not addressed in this study). Feature-based composites for decaying, slowly decaying, and sustaining events using 32-km NARR data. Centered on the point where the QLCS crosses the coast at the closest 3-hr NARR time prior to the crossing. QLCS km >100 km

MUCAPE (J kg -1 ), MSLP (hPa), 1000 theta (2 K), 10 m wnd (kts) Decaying Sustaining Decaying: MUCAPE 1250 J kg -1 ; collocated with a surface pressure trough & 1000 hPa thermal ridge. Geography for reference only: Star center point for feature based composites. Sustaining: MUCAPE 1000 J kg -1 ; surface pressure trough 300 km to west; QLCS collocated with 1000 hPa baroclinic zone.

Decaying: MUCIN 15 J kg -1 and increases rapidly offshore; RH of 68% (lowest 100 hPa); potential for evaporative cooling; Shear 15 kts. MUCIN (shaded, J kg -1 ), 1000 hPa RH (red, %), 0-3 km wind shear (kts) DecayingSustaining Geography for reference only: Star center point for feature based composites. Sustaining: MUCIN 35 J kg -1 with a weak offshore gradient; RH 77%; less of a chance for evaporative cooling; shear 25 kts

900:800 frontogenesis (10 -2 K (100 km) -1 (3 hr) -1 ), 900 tmp (black, o C), 900 tmp adv (10 -5 o C s -1 ), 900 winds (kts) Decaying: QLCS on the warm side of a fronotogenesis maximum; strengthening baroclinic zone/front. DecayingSustaining Geography for reference only: Star center point for feature based composites. Sustaining: QLCS within a region of WAA with little frontogenesis.

Motivational Questions What is the role of warm air advection during sustaining events? What is the role of the stable layer and convective inhibition? How is the enhanced vertical wind shear important to the maintenance of a QLCS? What role does low-level diabatic cooling play in the evolution of QLCSs?

2 km 500 m 2 Case Studies Better understand the processes that govern the maintenance and decay of a QLCS Simulations: WRF ARW core Initial & Boundary Conditions: 32-km NARR Explicit convection Morrison double-moment microphysics MYNN2.5 PBL Thermal LSM 2002 Decaying: 23 JUL 0600 UTC – 24 JUL 0300 UTC 2002 Sustaining: 31 MAY 1200 UTC – 01 JUN 0600 UTC

31 May 2002 Sustaining Event 0100 UTC 01 JUN 2002

NARR: 300 wnd (shaded, m s -1 ), 500 hght (solid, dam), 500 Q-vect conv (dashed, K m -2 s -1 ), 500 wnd (m s -1 ) 2100 UTC 31 May hPa jet extending into base of an upper level trough 500 hPa trough axis over eastern NY 500 hPa Q-vector convergence over the Northeast coastal region Cold front and prefrontal trough Cold front & prefrontal trough Convection ahead of cold front, consistent with composites. Thermal ridge in the Appalachian lee Coastal baroclinic zone Relatively moist air along coast (dew points o C) 2km WRF: mslp (solid, hPa), 2 m tmp (dashed, o C), 2 m dwpt (shaded, o C), 10 m winds (m s -1 )

1500 UTC 31 May UTC 31 May km WRF: MUCAPE (J kg -1 ), 925 hPa hght (solid, dam), 925 tmc (dashed, o C), 925 wnd( m s -1 ) 900 hPa 700 hPa 500 m JFK: 1500 UTC 31 May500 m JFK: 2100 UTC 1 Jun T 925hPa ~22 o C LI CAPE J kg -1 WAA similar to composites J kg - 1 CAPE in lee T 925hPa ~18 o C MUCAPE ~200 J kg -1 MUCIN ~25-75 J kg -1 MUCAPE ~700 J kg -1 MUCIN ~25-100J kg -1

2258 UTC 31 May 0145 UTC 1 Jun 0200 UTC 1 Jun 2215 UTC 31 May 0300 UTC 1 Jun 0145 UTC 1 Jun 2 km WRF precip mixr (shaded, g kg -1 ), 100 m omega (contour, m s -1 ), 100 m wnds Observed radar reflectivity (dBZ)

23 July 2002 Decaying Event 2200 UTC 23 JUL 2002

NARR: 300 wnd (shaded, m s -1 ), 500 hght (solid, dam), 500 Q-vect conv (dashed, K m -2 s -1 ), 500 wnd (m s -1 ) 2100 UTC 23 July hPa jet core U.S.-Canada border Broad 500 hPa trough Little 500 hPa Q-vector convergence over coastal region Limited mid- and upper-level forcing 2km WRF: mslp (solid, hPa), 2 m tmp (dashed, o C), 2 m dwpt (shaded, o C), 10 m winds (m s -1 ) Convection collocated with surface cold front, consistent with composites

1800 UTC 23 Jul UTC 23 Jul m JFK: 1800 UTC500 m JFK: 2100 UTC 700 hPa 900 hPa Still J kg -1 instability along coast and offshore Little temperature advection similar to composites J kg -1 MUCAPE over coast Inversion becoming reestablished 2km WRF: MUCAPE (J kg -1 ), 925 hPa hght (solid, dam), 925 tmc (dashed, o C), 925 wnd( m s -1 ) MUCIN ~ J kg -1

Observed radar reflectivity (dBZ) 2015 UTC 23 Jul 2115 UTC 23 Jul 0000 UTC 24 Jul 2016 UTC 23 Jul 2115 UTC 23 Jul 0005 UTC 24 Jul 2 km WRF precip mixr (shaded, g kg -1 ), 100 m omega (contour, m s -1 ), 100 m wnds

Low-level Balance Theory for Long Lived Squall Lines (Weisman & Rotunno 2004) vorticity generated by ambient low-level shear in along line direction vorticity generated by buoyancy gradients along leading edge of the cold pool = (Rotunno et al. 1988) QLCS experiences variations in low-level winds and thermodynamics

Low-level Balance Theory for Long Lived Squall Lines (Weisman & Rotunno 2004) vorticity generated by ambient low-level shear in along line direction vorticity generated by buoyancy gradients along leading edge of the cold pool = (Rotunno et al. 1988) QLCS experiences variations in low-level winds and thermodynamics

Low-level Balance Theory for Long Lived Squall Lines (Weisman & Rotunno 2004) vorticity generated by ambient low-level shear in along line direction vorticity generated by buoyancy gradients along leading edge of the cold pool = (Rotunno et al. 1988) QLCS experiences variations in low-level winds and thermodynamics

500 m 0030 UTC 1 JUN (12.5h)2130 UTC 23 JUL (15.5h) 500 m precip mixr (shaded, g kg -1 ), potential temp (solid, K), storm relative circulation vectors C C θ’ = 3.75 K h c = 1.2 km C = 19.3 m s -1 ΔU 2.5km = 15.0 m s -1 θ’ = 4 K h c = 1.3 km C = 18.3 m s -1 ΔU 2.5km = 7.5 m s -1 C/ΔU=1.3C/ΔU=2.5

0100 UTC 1 JUN (13h) 2230 UTC 23 JUL (16.5h) 500 m precip mixr (shaded, g kg -1 ), potential temp (solid, K), storm relative circulation vectors

Response of QLCS to Low Level (Nocturnal) Cooling (Parker 2008) Surface-based phase: Lifting by the surface cold pool. Stalling phase: Mechanism for surface lifting disappears as the relative strength of the cold pool approaches zero. Elevated phase: Convection forced by a bore atop the stable layer. Limited cooling: t=6h30m Unimited cooling: t=6h30m Unimited cooling: t=8h30m

precip mixr (shaded, g kg -1 ), potential temp (solid, K), storm relative circulation vectors Sustaining Event Forcing similar to a bore, though not purely bore driven. Stronger, deeper (up to 925 hPa; 750 m) temperature inversion (WAA) Moist Brunt-Vaisala Frequency 0.04 s -1 Decaying Event More dominantly forced by a surface based density current. More shallow inversion (975 hPa; 300 m) Moist Brunt-Vaisala Frequency 0.36 s -1

Sensitivity Experiments

t = 16 h t = 17 h t = 24 h 23 JULY 2002: Decrease Diabatic Cooling At t = 13h, reduced the evaporative cooling to 15% of the original value Convection more intense and moves over the coastal waters 2 km 15%EVAP CTRL t=15h15%EVAP t=17h θ’4 K3 K hchc 1.5 km0.9 km C13.9 m s m s -1 ΔU 2.5km 4.0 m s m s -1 C/ΔU km CTRL t=15h

23 JULY 2002: ‘Remove’ Atlantic Ocean Ocean replaced with land surface representative of the northeastern U.S. A few stronger convective cores, but decay similar to CTRL t=16h CTRL-LAND RH (%) at t=16hLAND precip mixr (g kg -1 ) and CTRL-LAND line-perpendicular wind (m s -1 ) at t=16h t=17h Drier, warmer, deeper boundary layer in LAND run. Increase ‘offshore’ CAPE for LAND run. Increase chance for evaporative cooling. Reduced vertical wind shear for LAND run.

Summary In the mean, QLCSs that decay upon encountering the northeastern U.S. coastline are collocated with frontal boundaries and regions of 900:800 hPa frontogenesis, with little temperature advection over the QLCS. Events that survive over the ocean waters are associated with warm air advection (destabilize atmosphere and strengthen low level temperature inversion: 31 May 2002 case) with little 900:800 hPa frontogenesis associated with the QLCS. 31 May 2002 event – Stronger vertical wind shear helps to balance the cold pool, extending the longevity of the QLCS. – Forcing transitions from a surface-based cold pool to more of a bore type feature (perhaps due to a stronger stable layer compared to 23 July?). 23 July 2002 event – Reducing the diabatic cooling to 15% of the CTRL simulation extended the longevity of the QLCS. – Shows that diabatic processes can be as important as the marine layer in influencing the evolution of QLCS (though this may not always be the case).

extra slides

2km WRF: mslp (solid, hPa), 2 m tmp (dashed, o C), 2 m dwpt (shaded, o C), 10 m winds (m s -1 ) Surface observations, mslp (black, dam), surface temp (blue, o C) 2100 UTC 31 May 2002 Cold front and prefrontal trough Convection ahead of cold front, consistent with composites. Thermal ridge in Appalachian lee Coastal baroclinic zone Relatively moist air along coast WRF ~1 o C cooler compared to surface obs within thermal ridge WRF and obs same at buoy WRF 0.5 o C too cool at Ambrose Light Tower

1500 UTC 31 May UTC 31 May km WRF: MUCAPE (J kg -1 ), 925 hPa hght (dam), 925 tmc ( o C), 925 wnd( m s -1 ) 900 hPa 700 hPa 500 m OKX: 0000 UTC 31 MayKOKX: 0000 UTC 1 Jun 1200 UTC WRF 2-3 o C cooler than obs T 925hPa increases ~5 o C NARR MUCAPE is ~400 J kg -1 greater WAA similar to composites

2100 UTC 23 Jul km WRF: mslp (solid, hPa), 2 m tmp (dashed, o C), 2 m dwpt (shaded, o C), 10 m winds (m s -1 ) Surface observations, mslp (black, dam), surface temp (blue, o C) Convection collocated with surface cold front, consistent with composites WRF does not capture mesoscale details of surface pressure features WRF ~2 o C too warm in cold sector and ~2 o C too cool in warm sector WRF 1 o C too cool at buoy WRF 100 m winds 2.5 m s -1 too weak at Ambrose Light Tower H L

Let’s compare the evolution of 2 different QLCS events… 31 MAY UTC – 01 JUN UTC 23 JUL UTC – 24 JUL UTC