Relating Lightning Frequency to SST Gradients Over the Gulf Stream

1
Relating Lightning Frequency to SST Gradients
Over the Gulf Stream

• Holly A. Anderson
• MET6480
• Satellite Oceanography
• April 21, 2008

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Reason for Study

• Although many studies have noted increased
  lightning frequency over the Gulf Stream Current,
  no study has been formally conducted to
  quantitatively research possible physical
  explanations behind the observations.
• The objective of this study is to help answer the
  question Are higher SST gradients related to
  increased lightning frequency over the Gulf
  Stream?

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Previous Observations

• From Lericos et al. (2001)
• Nocturnal lightning is found to occur mostly
  offshore related to the Gulf Stream
• During the night and early morning hours,
  lightning is more prevalent over the Atlantic
  Ocean than over any other geographical region.
• The analysis of nocturnal lightning shows no
  well-defined patterns however, flash densities
  over the Atlantic Ocean are greater than those
  over the Gulf of Mexico. This may be due to the
  influence of the Gulf Stream on convection
  associated with the eastward-moving cold fronts.

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Data Sources

• AMSR-E Sea Surface Temperature (SST) data
• 0.25x0.25 degree grid
• The Advanced Microwave Scanning Radiometer (AMSR)
  is aboard NASAs sun-synchronous spacecraft Aqua.
• Though it can see through clouds, it is limited
  by sun glint, high wind speeds (gt20 ms-1), sea
  ice, and rain.
• Swath coverage limits frequency of decent passes
  over the domain of interest.
• Side-lobe contamination limits the resolution of
  the Gulf Stream near the east coast of the United
  States.
• National Lightning Detection Network (NLDN)
  cloud-to-ground (CG) lightning data
• Lightning is detected as far east in the Atlantic
  Ocean as 60W. However, detection efficiency (DE)
  is greatest near the coastlines and decreases as
  the distance from the sensor increases.

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Methodology

• The period of investigation was during a single
  warm season, defined as May to September 2003.
• Nocturnal lightning, defined as lightning
  occurring between 090000 pm (0100 UTC) and
  055959 am (1100 UTC), was subsetted into daily
  files.
• The descending pass of AMSR-E was utilized, so
  SSTs were indicative of nighttime temperatures.
• The daily SST gradient, in Cm-1 was calculated.
• If a nocturnal CG lightning strike occurred in
  the domain and SST gradient information was
  available, it was added to the dataset.
• This led to a dataset of over 74,092 values for
  the entire northwest Atlantic region.
• A smaller subsetted domain, over the Gulf Stream,
  from 35-45N and 75-60W was taken and a second
  dataset was formed. This dataset included over
  26,000 values.
• Values were binned in 100 equal bins according to
  SST gradient value and plotted to analyze the
  relationships between the SST gradient and
  lightning frequency.

SST Gradient
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Daily AMSR-E and NLDN Plot
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Entire Atlantic Domain
Starred values above are as follows Mean
-2.861515130053e-005 Mode 0 Std. Dev
0.0001142452688411 Range 0.001688542 Min
-0.00110681 Max 0.000581732
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Smaller Gulf Stream Domain
Starred values above are as follows Mean
-3.700997284279e-005 Mode 8.53373e-005 Std.
Deviation 0.0001330702191255 Range
0.001688542 Min -0.00110681 Max 0.000581732
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Notable Findings

• Given the inherent data limitations, it appears
  from the dataset that lightning occurs
  preferentially in areas where the SST gradient is
  close to zero.
• This could indicate that lightning occurs above
  areas of constant warm temperatures or constant
  cool temperatures, not necessarily where SSTs
  change the fastest.
• This leads to the question Is lightning more
  prevalent on the warm or cool side of the Gulf
  Streams SST gradient?

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Typical Lightning Locations
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Physical Explanations

• In Lindzen et al. (1987), the relationship
  between SST gradients and increased convection
  were documented
• ? Differences in SSTs
• ? Differences in pressure due to density
  considerations
• ? Increased low-level wind convergence toward low
  pressures
• ? Increased convection over warmer areas of SST
• Since low-level air would flow to lower pressure
  areas and converge, we would assume lightning
  would be present in areas of warmer SSTs.
• The Lindzen theory does not take into account
  thermodynamic effects of changes in SST.
• After visual inspection, this is what is seen in
  the daily images.
• However, oceanic and atmospheric processes are
  nonlinear and complicated, so this is not likely
  the only mechanism.
• Weather phenomena such as fronts and other
  systems could greatly impact the patterns of
  lightning.

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Conclusions

• The distribution indicates oceanic lightning is
  indeed more frequent near the Gulf Stream, but
  that highest flash counts actually occur near
  areas of lower SST magnitude on the periphery of
  the Gulf Stream, not above areas of high
  gradient, as expected.
• Lindzens SST gradient and low-level wind field
  theory helps explain physically why this occurs.
• Visual inspection shows that lightning indeed
  occurs preferentially in areas of warmer SST,
  such as south of the Gulf Stream.
• Lightning can be associated with atmospheric
  fronts and systems, not simply due to SSTs.
  Looking at the synoptic flow for a daily pass can
  help determine whether lightning is associated
  with a weather system.
• In further research, I hope to investigate the
  low-level wind field, using AMSR-E data, to see
  areas of convergence where lightning could be
  present.

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References

• Lericos, T.P., H.E. Fuelberg, A.I. Watson, and
  R.L. Holle, 2002 Warm Season Lightning
  Distributions over the Florida Peninsula as
  Related to Synoptic Patterns. Wea. Forecasting,
  17, 8398.
• Lindzen, R.S., and S. Nigam, 1987. On the role of
  sea-surface temperature gradients in forcing
  low-level winds and convergence in the tropics.
  J. Atmos. Sci., 44, 2418-2436.
• Thanks to Henry Winterbottom for providing code
  assistance.