Warm Season Climatology of Convective
Evolution Over the Coastal Northeast U.S.
Michael Charles and Brian A. Colle
Institute for Terrestrial and Planetary Sciences
SUNY at Stony Brook
Jeffrey Tongue
NOAA/NWS Upton, NY
Forecasting Northeast coastal convection is very difficult…
False Alarm Rate (FAR) = Unverified warnings/total warnings
(Svr tstm/torn.) ALY, BOX, OKX, and PHI from 1/1/1986 – 10/16/2003
Motivational Questions
1. What is the change in thunderstorm
distribution in the Northeast from early to
late summer?
2. How do thunderstorms evolve as they
approach the coast?
3. Under what conditions does more
significant convection occur at the coast
even with cooler early summer SSTs?
NLDN sensor locations in the US
Objective Analysis Routine
Background
• Last year’s NROW talk:
– Northeast lightning climatology (2000-2002)
– Concluded that a coastal gradient existed in
June, but not in August.
– Hypothesized that the cooler June SST’s lead
to a relatively cool marine boundary layer that
weakens convection approaching the coast.
• This year’s research:
– Included two more years (2003-2004).
– Investigate the inter-annual variability in the
lightning distribution – Is SST the whole story?
June Density – 2000-2002 - strikes per 100 km2
Sea Surface Temperatures
15 June 2000 15 August 2000
Average Lightning Distributions
Terrain
50 m
100 m
200 m
400 m
June 2000-2004 600 m August 2000-2004
800 m
Strikes per 100 km2 per month Strikes per 100 km2 per month
June – Sharp coastal gradient
Terrain
50 m
100 m
200 m
400 m
June 2000 600 m June 2004
800 m
Strikes per 100 km2 Strikes per 100 km2
June – More significant coastal lightning
Terrain
50 m
100 m
200 m
400 m
June 2002 600 m June 2003
800 m
Strikes per 100 km2 Strikes per 100 km2
August – Sharp coastal gradient
Terrain
50 m
100 m
200 m
400 m
August 2002 600 m August 2004
800 m
Strikes per 100 km2 Strikes per 100 km2
August – More significant coastal lightning
Terrain
50 m
100 m
200 m
400 m
August 2000 600 m August 2001
800 m
Strikes per 100 km2 Strikes per 100 km2
Is synoptic forcing a factor in the
lightning distribution?
Synoptic climatology
• Cases:
• Lightning cases defined as days that New Jersey,
southeast New York or Connecticut received >10
lightning strikes.
• Divided convection into two categories:
– Near Front (Frontally maintained, referred to as frontal)
• Along surface cold front ( 100 km ahead of a cold front
• > 200 km ahead of a surface warm front
– Both types of convection were summed into monthly
totals for each year
Non-frontal: 11-12 June 2000 case
00 UTC 12th NCEP sfc. analysis 00 UTC 12th OKX sounding
~100 km
storms
CAPE=258 J/kg
Non-frontal: 11-12 June 2000 case
11 June 2000 Lightning Strikes (per 100 km2)
Terrain
50 m
100 m
200 m
400 m
600 m
800 m
Frontal: 27-28 August 2001
00 UTC 28th NCEP sfc. analysis 00 UTC 28th OKX sounding
~100 km
storms
CAPE=793 J/kg
Frontal: 27-28 August 2001
21 August 2004 Lightning Strikes (per 100 km2)
Terrain
50 m
100 m
200 m
400 m
600 m
800 m
June average lightning distribution
Terrain
50 m
100 m
200 m
Frontal convection Pre-frontal convection
400 m
600 m
800 m
Strikes per 100 km2 per month Strikes per 100 km2 per month
August average lightning distribution
Terrain
50 m
100 m
200 m
Frontal convection 400 m
Pre-frontal convection
600 m
800 m
Strikes per 100 km2 per month Strikes per 100 km2 per month
June - Little coastal lightning
2000 2000
2004 2004
June – Significant coastal lightning
2002 2002
2003 2003
August - Little coastal lightning
2002 2002
2004 2004
August – Significant coastal lightning
2000 2000
2001 2001
Conclusions
• There are large spatial gradients in lightning
(convection) across the Northeast U.S. because of
terrain (e.g. Wasula et al. 2002) and land-sea
contrasts.
• The relatively cool SST’s in the early summer can
lead to rapid weakening of convection near the
coast, but there is a lot of inter-annual variability.
• The rapid weakening of coastal convection is
favored for prefrontal (propagating) convection.
Convection supported by a surface cold/warm
front weakens less approaching the coast.
• Those months with significant lightning near the
coast have more anomalous upper-level troughing
around the U.S. Eastern Seaboard.
Future Work
• Convective mode:
– Convection divided into two convective modes:
• Organized/long-lived
• Disorganized/short-lived
• Determine the processes which favor more long-
lived convection near the coast during warm
season frontal passages. Is the convection more
elevated?
• Mesoscale modeling of specific cases at < 5-km
grid spacing using WRF.
The End