The Rapid Evolution of Convection Approaching the

New York City Metropolitan Region

Brian A. Colle and Michael CharlesInstitute for Terrestrial and Planetary Sciences

SUNY at Stony Brook

False Alarm Rate (FAR) in % – Unverified warnings/total warnings (Svr tstm/torn.) ALY, BOX, OKX, and PHI Warnings from 1986 – 2003

Motivational Questions

• What is the thunderstorm distribution over the coastal Northeast?

• How do severe thunderstorms evolve as they approach the coast?

• Does the evolution of severe convection change as the sea surface temperatures warm during the summer?

• How well can mesoscale models forecast these convective events?

Sea Surface Temperatures15 June 2000 15 August 2000

Severe thunderstorm/tornado reports per 100 km2 per 1 million peopleStorm reports from 1/1/1950 to 10/31/2002

Lightning Climatology using NLDN sensor data (2000-2002)

Analysis Approach

June Density – 2000-2002 - strikes per 100 km2

August Density – 2000-2002 - strikes per 100 km2

June Lightning Hours – 2000-2002

August Lightning Hours – 2000-2002

Case Study – 18 May 2000 Squall Line

Event # of events Most severe

Lightning 5 NA

Hail 9 1”

Wind 80 126 mph

Flash Flood 2 NA

Funnel Cloud 1 NA

Tornado 2 F2

Total Storm Reports 18 May 2000 through 19 May 2000

•Total of 99 storm reports across southern New England•The only warned county that didn’t verify was Nassau county, LI.•The only damage report in Suffolk county LI was a downed tree.

18 May 2000 1200 UTC Surface Analysis

Temperature (every 2° F) Analysis 18 May 2000 18 UTC

.5° reflectivity loop – 20 UTC 18 May 2000 to 02 UTC 19 May 2000

Above: 21:36 UTC reflectivity indicating cross section.Top Right: Reflectivity cross sectionshowing vertical storm structure.Right: Velocity cross section.

Left: 23:33 UTC reflectivity indicatingcross section.Above: Cross section showing verticalstorm structure.

18 May 2000 2100 UTC Soundings (9-hr Eta model forecast)

KOKX (Brookhaven, NY)KPOU (Poughkeepsie, NY)

Eta CAPE – 9-hr forecast valid 21 UTC

MM5 Simulation

• 36, 12, and 4 km domains, centered over the Northeast U.S.

• Initial and boundary conditions from the GFS run at 0000 UTC 18 May 2000.

• Ran MM5 with the Grell convective parameterization (36/12 km), simple ice, and MRF PBL.

Model Verification – Surface Temperature 1800 UTC 18 May 2000

4km MM5 (hr-18) – MSLP and T (°C) Surface Analysis

NYC

Model Verification – Precipitation 1800 UTC 18 May 2000

Reflectivity Mosaic12km SBU MM5 – MSLP and 1-hr precip.

Model Verification – Precipitation 2200 UTC 18 May 2000

Reflectivity 4km SBU MM5 – MSLP and 1-hr precip.

Summary and Future Work• Convection weakens rapidly approaching the NYC

metropolitan region during the spring and early summer, resulting in high false alarms rates for the NWS.

• The weakening occurs as convection encounters the cool marine boundary layer as evidenced by a lightning composite and case study.

• Need to better understand this coastal marine interaction before quantifying localized urban-scale effects.

• There are model difficulties in forecasting squall lines over the NE. MM5 rarely has them correct under weak to moderate CAPE.

The End

References

• NLDN Details:– http://ghrc.msfc.nasa.gov/uso/readme/gai.html

• Wasula, Alicia C., Bosart, Lance F., LaPenta, Kenneth D. 2002: The Influence of Terrain on the Severe Weather Distribution across Interior Eastern New York and Western New England. Weather and Forecasting: Vol. 17, No. 6, pp. 1277–1289.

Probability of Detection (POD) in % – Warned events/total events(Svr tstm/torn.) ALY, BOX, OKX, and PHI Warnings from 1/1/1986 – 10/16/2003

MM5 CAPE

18 May 2000 Lightning Distribution

Grid domain

Outline

• Climatology:– Severe thunderstorm/tornado reports.– Lightning climatology.

• Results:– Results of lightning climatology.

• Case study (18 May 2000):– Squall line evolution.– NWP model simulation of case study.

Small-scale Interannual Variability

June 2000 Density June 2001 Density June 2002 Density

Small-scale Interannual Variability

August 2000 Density August 2001 Density August 2002 Density

Velocity Cross Section

CG Lightning Strike Data

• National Lightning Detection Network (NLDN)– Global Atmospherics

• CG lightning to represent convection over NE (similar to Wasula et al 2002)

• June vs. August convection (2000-2002)

• 5-7° C difference in sea-surface temp.