(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 2002 1700 UTC – 01 JUN 2002 1000 UTC

23 JUL 2002 1600 UTC – 24 JUL 2002 0400 UTC

Let’s compare the evolution of 2 different QLCS events…

31 MAY 2002 1700 UTC – 01 JUN 2002 1000 UTC

23 JUL 2002 1600 UTC – 24 JUL 2002 0400 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) 2002-2007; 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

0-100 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)

Decaying Sustaining

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.

70 70

80

80

900:800 frontogenesis (10-2 K (100 km)-1 (3 hr)-1), 900 tmp (black, oC), 900 tmp adv (10-5 oC s-1), 900 winds (kts)

Decaying: QLCS on the warm side of a fronotogenesis maximum; strengthening baroclinic zone/front.

Decaying Sustaining

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 2002Sustaining Event

0100 UTC 01 JUN 2002

NARR: 300 wnd (shaded, m s-1), 500 hght (solid, dam), 500 Q-vect conv (dashed, 10-15 K m-2 s-1), 500

wnd (m s-1)

2100 UTC 31 May 2002

• 300 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 17-18oC)

2km WRF: mslp (solid, hPa), 2 m tmp (dashed, oC), 2 m dwpt (shaded, oC), 10 m

winds (m s-1)

1500 UTC 31 May 2002 2100 UTC 31 May 2002

2km WRF: MUCAPE (J kg-1), 925 hPa hght (solid, dam), 925 tmc (dashed, oC), 925 wnd( m s-1)

900 hPa 900 hPa

700 hPa 700 hPa

500 m JFK: 1500 UTC 31 May 500 m JFK: 2100 UTC 1 Jun

T925hPa ~22oC

LI CAPE 400-1600 J kg-1

WAA similar to composites

1600-2000 J kg-

1 CAPE in lee

T925hPa ~18oC

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, 10-2 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, 10-15 K m-2 s-1), 500

wnd (m s-1)

2100 UTC 23 July 2002

• 300 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, oC), 2 m dwpt (shaded, oC), 10 m

winds (m s-1)

• Convection collocated with surface cold front, consistent with composites

1800 UTC 23 Jul 2002 2100 UTC 23 Jul 2002

500 m JFK: 1800 UTC 500 m JFK: 2100 UTC

700 hPa 700 hPa

900 hPa 900 hPa

Still 1200-1600 J kg-1 instability along coast and offshore

Little temperature advection similar to composites

1600-2000 J kg-1 MUCAPE over coast

Inversion becoming reestablished

2km WRF: MUCAPE (J kg-1), 925 hPa hght (solid, dam), 925 tmc (dashed, oC), 925 wnd( m s-1)

MUCIN ~25-150 J kg-1 MUCIN ~25-150 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 Jul2 km WRF precip mixr (shaded, g kg-1), 100 m omega (contour, 10-2 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

=

˜ U 2 ˜ b o ˜ h c CU shear perpendicular to line

hc height of cold pool

bo buoyancy of cold pool = g 'o

(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

=

˜ U 2 ˜ b o ˜ h c CU shear perpendicular to line

hc height of cold pool

bo buoyancy of cold pool = g 'o

(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

=

˜ U 2 ˜ b o ˜ h c CU shear perpendicular to line

hc height of cold pool

bo buoyancy of cold pool = g 'o

(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

CC

θ’ = 3.75 Khc = 1.2 km C = 19.3 m s-1

ΔU2.5km= 15.0 m s-1

θ’ = 4 Khc = 1.3 km C = 18.3 m s-1

ΔU2.5km= 7.5 m s-1

C/ΔU=1.3 C/ΔU=2.5

0100 UTC 1 JUN (13h) 2230 UTC 23 JUL (16.5h)

500 m500 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=15h 15%EVAP t=17hθ’ 4 K 3 K

hc 1.5 km 0.9 km

C 13.9 m s-1 13.2 m s-1

ΔU2.5km 4.0 m s-1 6.0 m s-1

C/ΔU 3.5 2.2 2km 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=16h LAND 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, oC), 2 m dwpt (shaded, oC), 10 m winds (m s-1)

Surface observations, mslp (black, dam), surface temp (blue, oC)

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 ~1oC cooler compared to surface obswithin thermal ridge

WRF and obs same at buoy 44025

WRF 0.5oC too cool at Ambrose Light Tower

1500 UTC 31 May 2002 2100 UTC 31 May 2002

2km WRF: MUCAPE (J kg-1), 925 hPa hght (dam), 925 tmc (oC), 925 wnd( m s-1)

900 hPa 900 hPa

700 hPa 700 hPa

500 m OKX: 0000 UTC 31 May KOKX: 0000 UTC 1 Jun

1200 UTC

WRF 2-3oC cooler than obs

T925hPa increases ~5oC

NARR MUCAPE is ~400 J kg-1

greater

WAA similar to composites

2100 UTC 23 Jul 2002

2km WRF: mslp (solid, hPa), 2 m tmp (dashed, oC), 2 m dwpt (shaded, oC), 10 m winds (m s-1)

Surface observations, mslp (black, dam), surface temp (blue, oC)

• Convection collocated with surface cold front, consistent with composites

• WRF does not capture mesoscale details of surface pressure features

WRF ~2 oC too warm in cold sector and ~2 oC too cool in warm sector

WRF 1oC too cool at buoy 44025

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 2002 1700 UTC – 01 JUN 2002 1000 UTC

23 JUL 2002 1600 UTC – 24 JUL 2002 0400 UTC