High percentage of positive lightning along the USA west coast



[1] Analyses of the cloud-to-ground (CG) lightning characteristics recorded by the US National Lightning Detection Network (NLDN) along the west coast reveal an average annual percentage of positive CG flashes around 40%, while the average value for the USA is approximately 10%. The primary goal of the study was to document and suggest reasons for this positive CG anomaly. Through seasonal and monthly storm analysis, it was determined that the high annual percent positive along the coast is the result of the low variability in total CG flashes throughout the year coupled with a high number of positive CG flashes during the winter season which can be directly attributed to the climate of the Pacific Coast. The topography of the region was determined to affect the areal distribution of the percent positive anomaly by restricting the inward extent of the coastal climate. The secondary goal of the study was to determine whether the following meteorological variables were related to the dominant CG polarity in a storm: 1) the variation of charge region heights using the −10°C level as a proxy and 2) the tilting of the charge regions by strong windshear. Statistical analysis showed that the height of the −10°C temperature level is related to the dominant CG polarity in a storm, while the windshear did not show a significant relationship. In addition, analyses showed that the thunderstorms that produced few CG flashes (<6 flashes) contributed most to the total number of positive CG flashes.

1. Introduction

[2] One pattern that has been observed in past CG climatology studies is a high percentage of positive lightning flashes along the west coast of the United States (Figure 1). Although this feature has been consistently seen in previous CG climatology studies of the continental U.S. [Orville and Silver, 1997; Orville and Huffines, 1999], there have been no known analyses of this specific feature. Huffines and Orville [1999] found on average, excluding the west coast, the percent positive CG flashes across the United States is approximately 5 to 10%, while the percent positive along the west coast was approximately 25%, with areas as high as 50%. It is hypothesized that the high annual percent positive is related to the typical seasonal variation of percent positive observed in other regions of the U.S. and Japan [Orville et al., 1987; Hojo et al., 1989]. In this seasonal variation, the winter storms have a high percentage of positive CG flashes and low total flash counts, while the summer storms produce a high percentage of negative CG flashes and high total flash counts. However, unlike other regions of the country, the west coast climate tends to inhibit the development of strong, electrically active storms during the summer season, which significantly reduces the number of high flash count, primarily negative CG storms. Climatological CG lightning maps on annual and seasonal time scales are used to support these hypotheses. It is also hypothesized that the local topography determines the areal extent of the percent positive anomaly. Statistical analyses are conducted to support the proposed topographical effects.

Figure 1.

Map of the percentage (color bar) of positive CG's along the west coast of the U.S. The white circles indicate the three areas used to obtain lightning and environmental data for the corresponding upper air station indicated by the white plus symbol.

[3] Previous studies have indicated that high windshear, low charge heights to ground, and weak convective updrafts tend to coincide with thunderstorms that produce predominately positive CG lightning [Takagi et al., 1986; Kitagawa and Michimoto, 1994]. A secondary goal of this study is to investigate, using available sounding data, whether the CG polarity of a storm along the coast is correlated to either of the meteorological parameters. Statistical analyses between positive CG dominant and negative CG dominant storms are conducted to determine if a relationship exists.

2. Data Source

[4] Cloud-to-ground lightning flash data from the NLDN were obtained from Vaisala, Inc., Tucson, AZ, for the period 1989–2004. The network consists of 106 sensors across the United States [Orville and Huffines, 1999]. A complete description of the upgraded NLDN is in Cummins et al. [1998].

[5] The upper air dataset was obtained from NOAA's National Climatic Data Center (NCDC) through its National Virtual Data System (NVDS). The dataset contains radiosonde information for upper air observation stations across the United States from 1990 to 2000. Data from the following west coast stations, Quillayute, Washington (UIL), Salem, Oregon (SLE), and Oakland, California (OAK), were used to analyze the atmospheric conditions related to lightning events that occurred near the stations (see Figure 1). An IDL routine is used to extract upper air observations for these stations that correspond to the daily CG lightning information. The data are passed through a filtering routine to remove storm days in which the upper air observations did not contain at least 20 temperature and wind velocity levels from the surface up to 200 mb. Storm days that pass the filter are then used to conduct individual storm day analysis to determine if any relationships exist between the vertical windshear between charge layers, height of charge layer, and storm polarity. The gross charge structure of the storms is assumed to be a normal tripole structure with the negative region around −10°C and the positive region near −30°C [Rakov and Uman, 2003].

3. Results

[6] The plot of the annual percent positive along the west coast (Figure 1) shows an area of high percentage of positive CG flashes (∼40%) that extends only a few kilometers inland. Plotting seasonal maps of percent positive (Figures 2a, 2b, 2c, and 2d) reveals a strong seasonal variation. The percent positive is highest during the winter season (∼70%) and lowest during the summer season (<10%). A similar seasonal pattern had been observed along the east coast of the U.S. and in Japan [Orville et al., 1987; Hojo et al., 1989]. However, in those regions the annual percent positive values were closer to summertime values and the winter values were not as high. This suggests that the annual high percent positive along the west coast is related to a high number of positive CG flashes during the winter seasons and a lack of predominantly negative CG activity during the warm season.

Figure 2.

Same as (Figure 1) but for the (a) winter (DJF), (b) spring (MAM), (c) summer (JJA), and (d) autumn (SON) seasons.

[7] Utilizing a program to give daily CG flash counts for the three west coast upper air stations and the three locations 400 km inland (see Figure 1), it is possible to further investigate the seasonal variation in the percent positive. Table 1 lists the total number of flashes, positive flashes, negative flashes, and the percent positive for each month for the west coast stations and the inland stations. Examining this table, the total flashes per month along the west coast tend not to fluctuate much throughout the year compared to locations inland. September does stand out as an exception, which had three storm days during the sixteen year period that produced over 2/3 of the CG lightning flashes. The table shows that along the coast, the positive flash counts were lowest during the summer while the negative flash counts showed a slight peak. This is very different from the inland region, which had the highest positive and negative flash counts during the summer. Unlike inland, it is evident that the west coast climate tends to inhibit the development of thunderstorms during the summer that produce high CG flash counts, which tend to be predominately negative [Orville and Silver, 1997]. The data also shows that the storms throughout the year go through a large variation in positive CG flashes, but little to no variation in negative CG flashes. These two things combined, lead to the presence of a high annual percentage of positive CG flashes.

Table 1. CG Flash Data for A Fifteen Year Period (1990–2004) From Three Representative Circles for the West Coast and Inland Regionsa
 TotalPositiveNegative% Positive
  • a

    Total flashes, positive flashes, negative flashes, and percent positive for each month.

West Coast Region
Inland Region

[8] Individual storm analysis [not presented] shows that 81% of the storms along the coast produced between 1–6 CG flashes per storm. Focusing on the storms with 1–6 flashes that produced exclusively positive or negative CG's, a simple comparison of the average windshear and charge height for these storms was made. It was found that the average windshear values were very similar with the windshear value of 3.7 m s−1 km−1 for all positive CG storms and a value of 3.5 m s−1 km−1 for all negative CG storms. Using a simple t-test to determine if the means are significantly different produced a p-value of 0.69, which indicates that we can not say that the means of the two populations are significantly different. The positive polarity storms had an average −10°C isotherm height of 3.4 km, while the negative polarity storms had an average isotherm height of 4.0 km. The t-test results for the isotherm height, returned a p-value of 0.0002, which means that the there is a 99.98% confidence that the means are significantly different.

4. Discussion and Conclusions

[9] Although no detailed study has been conducted, the annual high percentage of positive CG flashes (Figure 1) along the west coast has been seen consistently for many years [Orville and Silver, 1997; Orville and Huffines, 1999]. From seasonal CG flash analysis (Figures 2a, 2b, 2c, and 2d), this feature is due to the strong influence of the high percentage of positive flashes during the winter season. Table 1 shows that the total number of CG flashes remain fairly constant throughout the year, thus the annual percent positive values along the west coast are fairly equally influenced by the percent positive values for each month. In addition, the winter season shows an increase in positive CG flashes, while the negative remain nearly constant. This is very different from inland regions, which show a large increase in storms during the summer that produce predominantly negative flashes. This shows that the west coast climate, which is highly regulated by the cool sea surface temperatures (SSTs), tends to stabilize the lower atmosphere in the region. This stabilizing effect tends to inhibit electrically active thunderstorm development during the summer months. This lack of significant storm active during the summer coupled with the tendency towards a high ratio of positive CG flashes during the winter months leads to the high annual percentage of positive CG flashes along the coast. Examining the annual percent positive map (Figure 1), a rapid decrease in the percent positive value from around 50% down to 10% occurs a few hundred kilometers inland, which is directly attributable to the mountain barrier restricting the inland extent of the coastal climate.

[10] Analysis of individual storms showed that the least active thunderstorms (1–6 CG flashes) produced a majority of the CG lightning along the coast. Dividing the storms into positive CG dominant and negative CG dominant storms, statistical analysis showed that the height of the −10°C temperature level had a strong relationship to the dominant polarity of CG flashes in a storm. Using the height of the −10°C isotherm as a proxy for the height of charge layers in a normal tripole thunderstorm charge structure, the results suggest that the lower the charge layers are to the Earth's surface, the greater likelihood of storms producing positive polarity CG flashes. Unlike the analysis of the height of the charge layers, the analysis of windshear shows no significant relationship to a storms dominant CG polarity.


[11] The lightning data were obtained from Vaisala (Global Atmospherics) Inc., Tucson, Arizona. Data handling at Texas A&M University is under the direction of Jerry Guynes and we thank him for his help. Our research is part of a lightning program supported by the National Science Foundation (ATM-0119476 and ATM-0442011) and the National Oceanic and Atmospheric Administration (cooperative agreement NA17WA1011).