Relating Lightning Frequency to SST Gradients over the Gulf Stream HOLLY A. ANDERSON The Florida State University, Tallahassee, Florida 1. INTRODUCTION Lericos et al. (2002) linked increased nocturnal lightning frequency over the Atlantic Ocean to the warmer oceanic temperatures of the Atlantic’s Gulf Stream Current. Warmer sea surface temperatures (SSTs) presumably would increase convection over the ocean and cause lightning associated with the convective storms. Lindzen et al. documented the connection between SST gradients and low-level wind convergence in the tropics. Despite many observations and assumptions, no previous study has sought to quantitatively determine the connection between the Gulf Stream and lightning frequency. Following the study of Lericos et al., the patterns of nocturnal lightning during the warm season are analyzed. For this study, a single warm season, specifically May through September 2003, will be analyzed. The objective of this study is to research the relationship between the gradient of the SST over the Gulf Stream and lightning frequency. 2. DATA AND METHODOLOGY a. Data Sources Sea surface temperature data on a 0.25 by 0.25 degree grid from the Advanced Microwave Scanning Radiometer, AMSR-E, is used for this study. The AMSR-E is aboard NASA’s sun-synchronous spacecraft Aqua. Due to its sun-synchronous nature, two daily passes are available; one ascending at 10:30 am local time and one descending pass at 1:30 am local time. Because this study concentrates on nocturnal lightning, the descending pass will be used in order to provide the best look at nocturnal oceanic temperatures before solar heating affects the SSTs. The microwave sensor is affected by rain, sun glint, high wind speeds (>20 ms-1), and sea ice. For this study, data limitation due to rain will be the greatest concern, as most lightning strikes are associated with areas of heavy rain. Side-lobe contamination near the coastlines also affects the retrieval of SST information near the eastern coast of the United States, so the Gulf Stream Current near the coastline is not resolved well. In addition, the satellite’s swath coverage makes daily passes of the Gulf Stream inconsistent. In addition to AMSR-E SST data, cloud-to-ground (CG) lightning data from the U.S. National Lightning Detection Network (NLDN) is used. The NLDN consists of over 100 ground sensors that can detect a ground flash over 400 km from the sensor. The NLDN can detect lightning as far east in the Atlantic Ocean as 60W, but the detection efficiency of the sensor decreases as the distance from the sensor increases. b. Methodology 2 A single warm season, defined as May through September 2003, has been analyzed for this study. Nocturnal lightning, defined as lightning occurring between 09:00:00 pm and 05:59:59 am, was subset into daily files. The daily SST gradient in °Cm-1 was calculated from the AMSR-E SST data, by: T T SST x y where SST is the horizontal SST gradient in the x and y directions. Calculations were made using centered finite differencing. Any missing values due to rain, sun glint, or other data limitations were not included in the gradient calculations because they would adversely affect the gradient values. If a nocturnal CG lightning strike occurred in the Atlantic Ocean basin domain of 20N to 50N latitude and 80W to 60W longitude, and SST gradient information was available, it was added to the dataset. This led to a dataset of over 74092 values for the entire northwest Atlantic region. An example of a daily plot of the SST gradient with CG lightning overlaid is shown in Figure 1. In order to prevent large areas of non-Gulf Stream SST gradients from affecting the sample, a smaller subset domain, centered over the Gulf Stream region, henceforth referred to as the Gulf Stream domain, from 35N to 45N and 75W to 60W was taken and a second dataset was formed. This dataset included 27139 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 distribution. Data was statistically analyzed and displayed using the statistics package SPSS and Microsoft Excel. 3. RESULTS Since the AMSR-E did not gather SST data from areas of heavy rain, any statistical analysis using the datasets generated for this study must acknowledge that the datasets created lack an all inclusive look of all oceanic lightning occurring in this domain. Because of the inherent limitations of microwave sensing, interpretations must be carefully analyzed. Figure 2 and Table 1 show the histogram of SST gradient and the related statistics for the entire northwest Atlantic Ocean domain. From the histogram it is apparent that the majority of lightning strikes occurred over low SST gradient areas. According to the results from the larger dataset, lightning most frequently occurs over areas of SST gradients of 0 °Cm-1. The distribution is skewed towards negative values with a slightly negative mean value of -2.861 x10-5 °Cm-1. It is noted that lightning is more prevalent over areas of lower SST gradients, not higher SST gradients as expected. With this result, the decision to analyze the isolated area of the Gulf Stream was made in order to exclude large areas of low gradient in non-Gulf Stream areas of the Atlantic. These areas of low SSTs could be skewing the sample to low gradient values, since the majority of the Atlantic basin is associated with low temperature gradients. The new domain yielded a dataset of roughly 26000 values. Figure 3 shows the SST gradient for the Gulf Stream domain. Even with the domain restrictions to the Gulf Stream region, a similar frequency distribution is noted. Once again, lightning 3 frequencies are elevated near lower SST gradients, as the most frequent value is 8.53373 x10-5 °Cm-1, as noted in Table 2. 4. PHYSICAL EXPLANATIONS Given the inherent data limitations, the results appear to indicate that a positive correlation between the SST gradient and lightning frequency is not observed in the large Atlantic Ocean basin nor the smaller Gulf Stream domain. Instead of seeing higher frequencies of lightning over larger SST gradients, it is apparent that lightning occurs preferentially in areas of just slightly negative SST gradients. This could imply that lightning occurs over oceanic areas with near constant warm or near constant cool temperatures, not where SSTs fluctuate the most as expected. This led to the question of whether lightning occurred most frequently on the warmer or the cooler side of the Gulf Stream Current. Given this unexpected result, the search for a physical explanation led to the location of a paper by Lindzen et al. (1897), which linked SST gradients to increased convection. In Lindzen et al., the authors state that differences in SSTs lead to different pressures over the water due to differences in density. Therefore, warmer SSTs are linked to lower densities and low pressure, whereas cooler SSTs are linked to higher densities and higher pressure. Because low-level wind would flow from higher to lower pressures, low-level convergence and convection would be expected over the warmer SSTs. Therefore, a logical continuation of this theory would lead to the extension of increased lighting frequency over the warmer SSTs. A visual inspection of the daily SST gradient and lightning files shows the majority of nocturnal lightning strikes occur south of the Gulf Stream, or in areas of warmer SSTs. Therefore, the theory proposed by Lindzen et al. (1987) supports this study’s findings that lightning occurs preferentially in areas of low SST gradient. Despite the applicability of Lindzen’s theory to this study’s findings, it would be incorrect to assume that lightning frequency is related to the SST gradients alone, as the oceanic and atmospheric interactions are highly complicated. Further research would benefit from looking at the synoptic patterns over and near the domain of study to see the influence of frontal systems, troughs, highs, and other weather systems on the frequency of lightning and the results of their interactions with changes in SSTs. It is possible that there is a time delay between a weather system moving over areas of increased SST gradients and the actual occurrence of lightning after the weather system’s modification. In addition, future research hopes to associate the low-level wind patterns to increases in vertical convection using AMSR-E wind speed data. 5. CONCLUSION In conclusion, it appears that the SST gradient does indeed affect the frequency of cloud-to-ground lightning strikes. It appears that lightning occurs preferentially in areas of low to no-gradient, where SSTs are at near constant values. These findings provide a natural extension to a theory by Lindzen et al. (1987) in which increased convection occurs over warmer SSTs due to density effects. Lightning therefore occurs near SST gradients, but not directly above those areas, as was expected. Future research hopes to 4 link low-level wind field patterns to the increased convection, and study the influence of synoptic systems to the lightning frequency as well. 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. 5 Figure 1. Plot of SST gradient for 9 May 2003. Gradients are in units of °C/m. Pink dots of CG lightning strikes are few over the oceanic area of interest for this particular day. Increased SST gradients associated with the Gulf Stream are apparent. 6 Figure 2. Lightning frequency distribution for the entire Atlantic domain. 7 Statistics V2 N V alid 74092 Mis sing 0 Mean ******** Std. Dev iation ******** V arianc e .000 Skew nes s -.437 Std. Error of Skew ness .009 Range ******** Minimum ******** Max imum ******** Starred values above are as follows: Mean: -2.861515130053e-5 Mode: 0 Std. Dev: 0.0001142452688411 Range: 0.001688542 Min: -0.00110681 Max: 0.000581732 Table 1. SST gradient statistics for the entire Atlantic domain. 8 Figure 3. Lightning frequency distribution for the smaller Gulf Stream domain. 9 Statistics V2 N V alid 27139 Mis sing 0 Mean ******** Std. Dev iation ******** V arianc e .000 Skew nes s -.641 Std. Error of Skew ness .015 Range ******** Minimum ******** Max imum ******** Starred values above are as follows: Mean: -3.700997284279e-5 Mode: 8.53373e-5 Std. Deviation: 0.0001330702191255 Range: 0.001688542 Min: -0.00110681 Max: 0.000581732 Table 2. SST gradient statistics for the smaller Gulf Stream domain.