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

2. 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?

3. 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.”

4. Data Sources • AMSR-E Sea Surface Temperature (SST) data • 0.25x0.25 degree grid • The Advanced Microwave Scanning Radiometer (AMSR) is aboard NASA’s sun-synchronous spacecraft Aqua. • Though it can see through clouds, it is limited by sun glint, high wind speeds (>20 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.

5. Methodology • The period of investigation was during a single warm season, defined as May to September 2003. • Nocturnal lightning, defined as lightning occurring between 09:00:00 pm (~0100 UTC) and 05:59:59 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:

6. Daily AMSR-E and NLDN Plot

7. 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

8. 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 Stream’s SST gradient?

9. Typical Lightning Locations

10. 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.

11. 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. • Lindzen’s 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.

12. 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, 83–98. • 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.