Convective Storm Evolution and Frequency Around Coastal Southern New England

1. Convective Storm Evolution and Frequency Around Coastal Southern New England Dr. Brian A. Colle, Kelly Lombardo, and John Murray School of Marine and Atmospheric Sciences Stony Brook University – SUNY Sponsor: NSF

2. Questions • What is spatial and temporal distribution of convective storms over the Northeast U.S.? • What factors control the evolution of convective linear systems around the southern New England coastal zone? • What are the convective structures (linear, cellular, non-linear, etc…) and large-scale flow patterns associated with severe storms over the Northeast U.S. – See Lombardo and Colle (next presentation)?

3. Observational Datasets and Approach • Cloud to Ground Lightning Data National Lightning Detection Network (NLDN) • Available 2000-2007 • Interpolated to a 10x10 km grid centered over the Northeast U.S. • NOWrad (WSI NCAR): WSR-88D • Use NOWrad every 2x2 km2 at 15 minute interval (largest reflectivity at a point from surrounding radars) • Composite from April through September 1996-2007 by counting frequency of reflectivity exceeding a threshold (45 dBZ for this study unless otherwise mentioned) WSR 88D coverage at 3km above MSL, from Maddox et al 2002 Figure from thunder.msfc.noaa.gov

4. Spatial Distribution of Northeast U.S. Convective Storms During the Warm Season(also see: Murray and Colle MWR 2011) Radar Frequency > 45 dBZ (in percent) Lightning Frequency (per km2) -- maxima: West of the Appalachians, SE PA, and around Chesapeake Bay. -- sharp decrease east of the mid-Atlantic coast and across southern New England.

6. Interannual Variability of Warm Season Convective Storm Frequency for 6 boxesacross Northeast U.S.

7. Reflectivity frequency (> 45 dBZ) and NARR composites for the two stndev days (1998-2001 vs 2002-2005) 1998-2001 2002-2005 500z, slp and anom (shaded hPa) 500z, slp and anom (shaded hPa)

8. Diurnal Variation of > 45dBZ Frequency for 5 Boxes Across Northeast U.S. 1 2 3 4 5 3 2 1 4 5

9. Topography (m) and Hovmoller Plot of Reflectivity ≥ 45dBZ averaged from 38.5 to 41.5 N versus time of day (UTC).

10. Hour (UTC) of Maximum Reflectivity (> 45 dBZ) Frequency

11. Evolution of Linear Convective Systems Near the Coast • Warm Season Linear Events (2002-2007) • Categories: • DECAY (32 cases): Decays at the coast • SLOW DECAY (19 cases): Decay from coast to 100 km offshore. • MAINTAIN (9 cases): Little change in linear system at least 100 km offshore. • definition for decay: • Clear weakening trend in the line for at least 30 minutes. For most cases, the continuous line became < 50 dBz, although some cells were still > 50 dBZ. • Use coastal point at ~0.75o ahead of the center of the convective line to construct NARR composite (closest 3-h period before decay time).` MAINTAIN 6/1/02 DECAY 7/23/02

12. Comparisons of MUCAPE (for parcel in lowest 180 hPa) and 0-3 km Shear max 75% 25% min SLOW DECAY SLOW DECAY MAINTAIN DECAY DECAY MAINTAIN

13. 900-800 hPa Frontogenesis and 900 hPa Temp Adv DECAY SLOW DECAY SLOW DECAY MAINTAIN MAINTAIN DECAY

14. NARR Composite (900 hPa Temp, 900-800 hPa Frontogenesis –shaded, and 900 hPa temp advection) SLOW DECAY MAINTAIN DECAY

15. Summary • Convective storm frequency (≥ 45dBZ) varies across the region, with localized maxima across western and SE PA, and around Chesapeake. There is a sharp gradient at coast. • There is a maximum in convective storms over inland locations during the day (18-00 UTC), and this maximum is more over the coastal ocean by late night (03-09 UTC). • There is large interannual variability of storm frequency. More active periods (e.g. 1998-2001) near the coast have more anomalous surface troughing over the Northeast. • Linear convective systems maintain their intensity longer offshore when there is moderate shear as well as relatively large low-level frontogenetical forcing and warm advection. Linear storms with little decay are associated more with low-level warm advection.