A Spatial Climatology of Convection in the Northeast U.S.

John Murray and Brian A. ColleNational Weather Service, WFO New York NY Stony Brook University, NY2nd Tri-State Weather Conference, Danbury CT4/25/09

Motivation• There is little understanding of the spatial distribution

of convection over the Northeast U.S. and the relationship with different geographical features.

• How do the preferred regions for convective activity over the Northeast vary diurnally and during the warm season?

• How does convection evolve around terrain, coastal geometry, and urban areas?

• What are the different flow patterns associated with the convective maximum and minimum areas across the Northeast?

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)

Figure from thunder.msfc.noaa.govWSR 88D coverage at 3km above MSL, from Maddox et al 2002

Percentage of Time Composite Reflectivity ≥ 45 dBZ

Radar Composites compared with cloud to ground lightning data

For 2001-2007:

• 30 warm season months included from May 2001 through August 2007

• May missing for 2001, 2003, June missing in 2003, September missing for 2006, 2007

1996-2007 April-September

Diurnal tendency variation between 1st (April-June) and 2nd (July-August) half of warm season

Increase in daytime convection along coastal plain and Hudson Valley and greater nighttime convection over the ocean during the July-September period.

54

1

2

3

Frequency of 45+dBZ

0

5

10

15

20

25

30

35

4012

Z-1

3Z

13Z

-14Z

14Z

-15Z

15Z

-16Z

16Z

-17Z

17Z

-18Z

18Z

-19Z

19Z

-20Z

20Z

-21Z

21Z

-22Z

22Z

-23Z

23Z

-00Z

00Z

-01Z

01Z

-02Z

02Z

-03Z

03Z

-04Z

04Z

-05Z

05Z

-06Z

06Z

-07Z

07Z

-08Z

08Z

-09Z

09Z

-10Z

10Z

-11Z

11Z

-12Z

Time (UTC)

Fre

qu

ency

Box 1

Box 2

Box 3

Box 4

Box 5

3 2

14

5

54

1

2

3

Topography (m) and Echo Frequency % of Time Composite Reflectivity ≥ 45dBZ averaged across 38.5 through 41.5 N

Favored Genesis and Decay Locations for Organized Convection

• Count all convective events of at least 3° longitude in width with ≥ 30 % echo frequency.

• For example,

Total Amount of streaks = 219

Origin and Decay Frequency

0

10

20

30

40

50

60

(>-8

6.4

)

(-86.4

,-85)

(-85,-

84)

(-84,-

83)

(-83,-

82)

(-82,-

81)

(-81,-

80)

(-80,-

79)

(-79,-

78)

(-78,-

77)

(-77,-

76)

(-76,-

75)

(-75,-

74)

(-74,-

73)

(-73,-

72)

Longitude (degrees)

Fre

qu

en

cy

Origin Frequency Decay Frequency

Genesis

Dec

ay

Convective Frequency over the Northeast vs. Flow Regimes and Synoptic Conditions

• Using NARR (North American Regional Reanalysis) Available every 3 hours at 32 km resolution

• Convective hours related to nearest NARR time or convective hours plotted for certain conditions in an area as denoted by NARR

• Goal is to better understand the evolution of convection and convective flow regimes

For 925 hPa

XXX X

For 500 hPa

X X

XX

Convective Frequency vs. Wind Direction at 500 hPa SW->W (226-270), W->NW (271-315)

Convective Frequency vs. Wind Direction at 925 hPa SE->SW (145-225), SW->NW (225-315) for western Long

Island

Evolution of Convection• What synoptic conditions favor deep convection over the

coastal waters?• Approach: Take two adjacent boxes and compute areal

means of convective frequency (or 45+dBZ frequency), and list out days in which there was > 1 standard deviation of the mean.

• Then list out the times in which convection decreased by at least 50 % from one box to another.

543

≥ 50 % Decay from North NJ to Western Long Island

≥ 50 % Decay from Western to Eastern Long Island

≥ 50 % Increase from Western to Eastern Long Island

Summary• Convection (≥ 45dBZ) varies across the region, with localized

maxima across western PA, Hudson Valley, and along the coastal plain from VA to NJ.

• Convection is a maximum over inland locations during the day (18-00 UTC), and this maximum moves to the coastal ocean by the late night (03-09 UTC).

• The favored genesis regions of organized convection are from the Midwest and central and eastern Appalachians. Decay occurs just west of the Appalachians and in the coastal areas.

• Convection is most frequent over inland locations with SW-W flow at 500mb. Southerly flow at 925mb results in a greater influx of marine air into southern New England resulting in a larger gradient of convective frequency west to east.

• Generally, for enhanced coastal convection, there is a little low-level CAPE, but there is a well defined surface trough, with convection developing along a west-east baroclinic zone.

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