PUBLICATIONS. Journal of Geophysical Research: Atmospheres. Variability of tornado occurrence over the continental United States since 1950

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1 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Key Points: Tornado climatology exhibits a clear spatial structure over the continental U.S. Increased tornado temporal variability occurred at only one third of U.S. area Combined tornado spatial-temporal variability remains stable on national scale Supporting Information: Supporting Information S1 Correspondence to: K. Wang, kcwang@bnu.edu.cn Citation: Guo, L., K. Wang, and H. B. Bluestein (2016), Variability of tornado occurrence over the continental United States since 1950, J. Geophys. Res. Atmos., 121, , doi:. Received 16 NOV 2015 Accepted 6 JUN 2016 Accepted article online 8 JUN 2016 Published online 27 JUN The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Variability of tornado occurrence over the continental United States since 1950 Li Guo 1,2,3, Kaicun Wang 2,3, and Howard B. Bluestein 4 1 Department of Ecosystem Science and Management, Pennsylvania State University, University Park, Pennsylvania, USA, 2 State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, China, 3 Joint Center for Global Change Studies, Beijing, China, 4 School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA Abstract The United States experiences the most tornadoes of any country in the world. Given the catastrophic impact of tornadoes, concern has arisen regarding the variation in climatology of U.S. tornadoes under the changing climate. A recent study claimed that the temporal variability of tornado occurrence over the continental U.S. has increased since the 1970s. However, that study ignored the highly regionalized climatology of U.S. tornadoes. To address this issue, we examined the long-term trend of tornado temporal variability in each continental U.S. state. Based on the 64 year tornado records ( ), we found that the trends in tornado temporal variability varied across the U.S., with only one third of the continental area or three out of 10 contiguous states (mostly from the Great Plains and Southeast, but where the frequency of occurrence of tornadoes is greater) displaying a significantly increasing trend. The other two-thirds area, where 60% of the U.S. tornadoes were reported (but the frequency of occurrence of tornadoes is less), however, showed a decreasing or a near-zero trend in tornado temporal variability. Furthermore, unlike the temporal variability alone, the combined spatial-temporal variability of U.S. tornado occurrence has remained nearly constant since Such detailed information on the climatological variability of U.S. tornadoes refines the claim of previous study and can be helpful for local mitigation efforts toward future tornado risks. 1. Introduction Tornadoes, violently rotating columns of air that make contact with the ground [e.g., Bluestein, 2013; Elsner et al., 2014a], have impacted human livelihoods throughout history. Nowadays with the increasing population and urban sprawls, more people are exposed to tornado risks [Brooks and Doswell, 2001; Ashley, 2007; Coleman and Dixon, 2014]. Due to the favorable topographic and meteorological conditions conducive to tornadoes of all types, but especially to supercell tornadoes, the United States experiences the most tornadoes of any country [Brooks et al., 1994; Brooks et al., 2003b; Elsner et al., 2014a]. Given the devastating impact of tornadoes on society, concern has arisen regarding changes in the climatology of U.S. tornadoes (including frequency, intensity, variability, seasonality, and geography) in recent decades [e.g., Tippett et al., 2012; Brooks et al., 2014; Elsner et al., 2014b; Elsner et al., 2015; Farney and Dixon, 2014; Fuhrmann et al., 2014; Long and Stoy, 2014; Trapp, 2014;Lu et al., 2015]. Despite a lack of observational evidence to support the amplification of tornado frequency (or intensity) along with the changing climate [Intergovernmental Panel on Climate Change, 2007; Long and Stoy, 2014], a recent study claimed a significant increase in tornado temporal variability over the continental U.S. [Brooks et al., 2014]. By examining the monthly national tornado frequency, Brooks et al. [2014] found that both changes in the number of tornado days and the onset timing of tornado seasons have jointly led to an elevated temporal variability of U.S. tornado occurrence since the 1970s. However, Brooks et al. [2014] did not consider the highly regionalized tornado climatology across the U.S., which has been fully recognized by a body of works [e.g., Brooks et al., 2003a, 2003b; Dixon et al., 2011, 2014; Farney and Dixon, 2014; Coleman and Dixon, 2014; Fuhrmann et al., 2014]. Thus, given that the nationwide tornado temporal variability has increased over time, one may ask (1) what geographical regions are most responsible and (2) can such a nationwide tornado variability, without regard to the spatial heterogeneity of tornado climatology, represent the actual tornado variability on a more regional scale? To answer the questions, a more comprehensive understanding of the variability of the U.S. tornadoes is required, in particular, by considering the spatially variable tornado climatology. The aim of this study is GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6943

2 twofold: (1) to stratify the trends of tornado temporal variability in each U.S. state besides the overall continental-scale dynamics and (2) to reveal the combined spatial-temporal variability of U.S. tornadoes after taking into account the spatial structure of tornado climatology. Our results indicate that the trend of temporal variability is not uniform across the contiguous U.S. The Great Plains (including the states of Nebraska and Texas) and Southeast (including the states of Kentucky, Virginia, Tennessee, North Carolina, South Carolina, Alabama, and Georgia) regions, with a greater frequency of tornado occurrence, primarily led to the increased nationwide tornado temporal variability. For all of the 48 contiguous states, only 15 states showed an increased tornado temporal variability, which cover approximately one third of the continental area. However, the other two thirds of the continental area, where 60% of the U.S. tornadoes were reported, although with a lower frequency of tornado occurrence, showed a decreasing or a flat trend in tornado temporal variability. Furthermore, unlike the temporal variability alone, the combined interstate-temporal variability of tornado occurrence has remained constant since Such understanding of tornado climatology refines previous findings on U.S. tornado variability [e.g., Brooks et al., 2014] and could be used to assist local preparation for mitigating the impacts from tornado risks. 2. Data Reports of tornadoes in the U.S. are available from the Severe Weather Database maintained by the Storm Prediction Center (SPC) of the NOAA s National Weather Service (NWS) ( in which tornado records are documented from 1950 to the present [Schaefer and Edwards, 1999]. This tornado database is updated on a yearly basis with reports compiled from local NWS storm data publications and is reviewed by the U.S. National Climatic Data Center [Elsner et al., 2014a; Tippett, 2014]. Each tornado report includes detailed information on the time of occurrence, location of touchdown and liftoff, path width, damage, and intensity rate [Schaefer and Edwards, 1999]. The tornado intensity is rated on the Fujita scale (F scale; introduced in 1973) and its successor, the Enhanced Fujita scale (EF scale; adopted by the NWS since 2007), with 0 being the weakest and 5 the strongest [Doswell et al., 2009; Tippett, 2014]. The intensities of tornadoes that occurred before 1973 were rated retrospectively from archived newspaper accounts and photographs [Brooks and Doswell, 2001; Doswell et al., 2009]. According to previous studies [e.g., Doswell et al., 2009; Brooks et al., 2014], the EF scale was developed, in part, to maintain the climatological consistency over time. Thus, this study considered the F and EF ratings to be equivalent. It has been acknowledged that weak tornadoes (i.e., (E)F0) are reported more frequently over the tornado database [Brooks and Doswell, 2001; Verboutetal., 2006]; however, the reporting of (E)F1 and stronger tornadoes remains constant and is thus more representative of the actual tornado activity [Fuhrmann et al., 2014]. Therefore, only (E)F1+ tornadoes were retained from the SPC tornado database to offset the general inflation in the reporting of weak tornadoes in this study, which was consistent with the study of Brooks et al. [2014]. In total, 30,747 (E)F1+ tornadoes were identified from the 48 states in the contiguous U.S. over Methodology 3.1. Temporal Variability of Tornado Occurrence Although the SPC tornado database represents the official archive of tornado activity in the U.S. [Fuhrmann et al., 2014], its quality is impaired by the inconsistencies in the observation frequency, data verification, introduction of new monitoring technologies, implementation of storm spotter networks, and the increased population and public awareness [Verbout et al., 2006; Farney and Dixon, 2014]. To mitigate the impacts from the inconsistency of tornado record on the quantification of tornado variability, Brooks et al. [2014] employed the standard deviation of the ranks of monthly tornado counts (i.e., a measure of the relative abundance of tornadoes) to indicate tornado temporal variability. The use of relative ranks has the desirable effect that the years with an anomalous number of tornadoes distributed in proportion to the climatological tornado frequency do not affect this measure of tornado variability, since the monthly tornado rank is always in the same range, regardless of how the actual values of tornado count are fluctuating. By using tornado ranks, tornado variability (indicated by the standard deviation of tornado rank) is comparable throughout the study period. In this way, potential impacts of observation bias on the analysis of tornado variability could be further GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6944

3 suppressed. Therefore, we adopted this method of measuring tornado variability proposed by Brooks et al. [2014] in this study. On the continental scale, each calendar month was ranked from 1 (with the fewest monthly tornado count) to 64 (with the most monthly tornado count) against the same month in all other years of the study period ( , i.e., a total of 64 years), with ties assigned the mean value of the ranks of the ties (namely, the Type A rank; see Figure S1a in the supporting information). A running standard deviation of the monthly ranks among all of the calendar months was then calculated over a 15 year (or 180 month) period. Variation of the running standard deviation can indicate the dynamics of tornado temporal variability over time [Brooks et al., 2014]. According to Brooks et al. [2014], the 95% confidence interval about the standard deviation over the 64 year period was determined from 1000 samples, each with 180 random integers from 1 to 64 inclusive to mimic the 15 year sampling process. Next, we calculated the long-term trend of tornado temporal variability state by state. In doing so, we examined the consistency of tornado temporal variability across the continental U.S. We then computed the ratio of the areas of states with a significantly increasing/decreasing trend or no significant trend to the total area of the continental U.S. In this way, we determined the contributions from regional variations in tornado temporal variability to the overall continental pattern. Because the SPC tornado database has removed the tornadoes moving across state boundaries, geopolitical boundaries of each state have limited impact on this analysis Spatial Variability of Tornado Occurrence We first mapped the average annual tornado totals ( ) per state to depict the spatially variable climatology of tornadoes over the continental U.S. Next, we examined the temporal evolution of the distribution of interstate tornado rank among states throughout the tornado record. Thus, we could quantitatively evaluate the historical consistency of tornado spatial climatology. Moreover, in the spirit of using relative tornado ranks to measure tornado temporal variability [Brooks et al., 2014], we developed a new method to quantify continental-scale tornado spatial variability by using the interstate ranks of tornado counts. Unlike the temporal variability of tornado occurrence, which is studied on both continental and state scales, the spatial variability is studied only on the continental scale by considering the interstate difference of tornado frequency. Specifically, in a studied year, we ranked the tornado counts from 1 (the fewest) to 48 (the most) across the contiguous 48 states (including all of the states in the continental U.S.), with ties assigned the mean value of the ranks of the ties (namely, the Type B rank; see Figure S1b). Next, for each state, a running standard deviation of its interstate ranks was calculated over a 15 year period, which indicates the relative variation in tornado numbers of the studied state with respect to the remaining contiguous U.S. Then, the average standard deviation across the 48 states was calculated for each year. The higher the average standard deviation of interstate tornado ranks, the greater the spatial variability of tornado occurrence over the continent. The 95% confidence interval about the standard deviation over the 64 year period was determined from 1000 samples, each with 720 random integers from 1 to 48 to mimic the 15 year 48-state sampling process. In general, Type B rank quantifies the irregularity of the distribution of tornado occurrences among states Spatial-Temporal Variability of Tornado Occurrence Besides the temporal variability and interstate variability, we proposed a new integrated method to capture the combined interstate-temporal variability of tornado occurrence on the continental scale. For each state, we first calculated the annual rank of tornado counts from 1 (the fewest) to 64 (the most) over the period of (i.e., a total of 64 years), with ties assigned the mean value of the ranks of the ties (namely, the Type C rank; see Figure S1b). We then calculated a 15 year running standard deviation of the annual ranks across the 48 contiguous states (15 years 48 states) so that both the temporal and spatial scales were taken into account. The larger the running standard deviation of the Type C ranks, the greater the combined interstate-temporal variability of tornado occurrence on the continental scale. A high or low rank of this type could result from either an anomalous geographic distribution of tornadoes in a given year or/and an anomalous number of tornadoes across states in that year. The 95% confidence interval about the standard deviation over the 64 year period was determined from 1000 samples, each with 720 random integers from 1 to 64 to mimic the 15 year 48-state sample. As for the interstate variability, the combined spatial-temporal variability is studied only on the continental scale. GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6945

4 Figure 1. Spatially variable tornado frequency across the continental United States. (a) Topography of the contiguous United States, as depicted by a ~30 m resolution digital elevation model from the Global Multiresolution Terrain Elevation Data 2010 (GMTED2010), distributed by the United States Geological Survey and the National Geospatial-Intelligence Agency ( Solid black lines divide the continental U.S. into five geographic regions: West, Great Plains, Midwest, Southeast, and Northeast, adopting the regional divisions from the National Climate Assessment (NCA) ( (b) Average annual tornado frequency per state over the period of (c) Temporal evolution of annual interstate tornado rank (ranging from 1, the fewest tornado frequency to 48, the greatest tornado frequency) per state from 1950 to Red dashed lines divide the 48 contiguous states into the above geographic regions. Furthermore, we established a linear regression model between the temporal (or spatial) variability and the combined interstate-temporal variability of tornado occurrence. By checking the coefficient of determination (R 2 ) of regression, we quantified the relative contribution of temporal variability (or interstate variability) to the combined interstate-temporal variability. 4. Results 4.1. Climatological Distribution of U.S. Tornado Occurrence The annual average frequency of (E)F1 and stronger tornadoes per state is presented for the period in Figure 1. Annual tornado frequency depicts a clear spatial structure across the continental U.S. (Figure 1). Regionally, the Great Plains (including the states of Montana, North Dakota, South Dakota, Wyoming, Nebraska, Colorado (Colorado is included in the Plains region even though there are sometimes tornadoes over the mountainous west [e.g., Bluestein, 2000], where the frequency of the occurrence of tornadoes is much less than it is to the east, over the Plains.), Kansas, Oklahoma, and Texas), Midwest (including the states of Minnesota, Wisconsin, Michigan, Iowa, Illinois, Indiana, Ohio, and Missouri), and Southeast (Kentucky, Virginia, Tennessee, North Carolina, South Carolina, Arkansas, Louisiana, Mississippi, Alabama, Georgia, and Florida) generally show more frequent annual tornadoes than the West (including the states of Washington, Idaho, Oregon, California, Nevada, Utah, Arizona, and New Mexico) and Northeast (including the states of West Virginia, Pennsylvania, New York, Maryland, Delaware, New Jersey, Connecticut, Rode Island, Vermont, Massachusetts, New Hampshire, and Maine) (Figure 1). Characterized by a regional higher tornado frequency, the Tornado Alley running north-south in the Great Plains and covering Nebraska (NE), Kansas (KS), Oklahoma (OK), and Texas (TX) [Long and Stoy, 2014] is evident (Figure 1b), which is consistent with previous findings for a shorter period of time [e.g., Tippett et al., 2012; Farney and Dixon, 2014]. Moreover, when looking at the temporal evolution of the annual tornado frequency by state as indicated by interstate tornado ranks, this spatial structure is relatively consistent throughout the 64 year period (Figure 1c). In general, the Great Plains, Midwest, and Southeast are subject to more frequent tornadoes than the West and Northeast (Figure 1c). Therefore, due to the spatially variable tornado climatology (Figure 1), the GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6946

5 Figure 2. Continental-scale variability of tornado occurrence in the United States from the 1950s to 2010s. (a) Temporal variability. Linear regression shows the overall increasing trend. (b) Spatial (or interstate) variability. Error bar indicates the standard deviation of changes in interstate tornado ranks across states. (c) Spatial-temporal (or interstate-temporal) variability. Red lines indicate the average values of three types of tornado variabilities throughout the length of the tornado database. Red dashed lines are the 95% confidence intervals determined by random numbers. Grey-shaded areas indicate the time periods when the spatial or the combined spatialtemporal variability of continental tornado occurrence consistently exceeds the long-term average. continental-scale trend of tornado temporal variability (i.e., that was calculated in Brooks et al. [2014]) might not be representative of the actual tornado variability on the regional or state scale Temporal Variability of U.S. Tornado Occurrence Continental-Scale Dynamics As indicated by the 15 year running standard deviation of the ranks of monthly tornado counts (i.e., Type A rank), the temporal variability has shownanincreasingtrend(p < 0.001) since the 1950s (Figure 2a). The average temporal variability is 17.82, with a 95% confidence interval for a 15 year period between and (Figure 2a). The temporal variability has traversed the upper 95% confidence interval after 2000, which suggests an extremely high-level variability (which might be caused by strong volatility of both tornado day and tornado season) [Brooks et al., 2014; Elsner et al., 2015] State- and Regional-Scale Dynamics In addition to the steady increasing trend on the continental scale (Figure 2a), we mapped the secular trends in tornado temporal variability over the past six decades across the continental U.S. states (Figure 3). We found that the changing trends in temporal variability are spatially variable (Figure 3a). Then, we classified the contiguous 48 states into three groups according to their trends: states showing a significant decreasing trend, states showing a significant increasing trend, and states showing no significant trend. For all of the 48 contiguous states, 15 states show significantly increasing trends, indicating that only three out of 10 states have a greater tornado temporal variability. To account for the different sizes of U.S. states, we analyzed state area instead of summing up the number of states that belong to each group (Figure 3b). The ratios of the areas of the states with significant decreasing or increasing trends or no significant trend to the entire continental area are approximately equal (39%, 33%, and 28%, respectively) (Figure 3b). This result signifies that only one third of the continental area (where 40% of the U.S. tornadoes were reported throughout the tornado record) demonstrates an increasing trend; however, the temporal variability over the other two-thirds GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6947

6 Figure 3. Inconsistent variation trends in the temporal variability of tornado occurrence across the continental United States. (a) Secular trend in tornado temporal variability per state for the past six decades. Shaded areas indicate that no significant trend was detected. (b) Ratio of the areas of the states with a significant decrease (or increase) or no significant trend in tornado temporal variability to the entire area of the continent. The different colors indicate the five geographic regions. area (where 60% of the U.S. tornadoes were reported) shows a decrease or a flat trend (Figure 3b). The elevated continental-scale tornado temporal variability is mainly driven by changes occurring in the Great Plains and Southeast regions (Figures 3a and 3b). Moreover, the two rapid increases in continental-scale tornado temporal variability, i.e., and , are associated with tornado variability in the West, Great Plains, and Southeast ( ) and in the Midwest and Southeast ( ), respectively (Figure 4). Throughout the length of the tornado database, the Southeast exhibits the most similar pattern of tornado temporal variability to the overall continental-scale pattern (r = 0.97), the Great Plains shows the second highest correlation with the continental-scale pattern (r = 0.83), the West shows the least correlation (r = 0.42), and the Midwest (where tornadoes also frequently occurred) and the Northeast show moderate correlations (r = 0.53 and 0.32, respectively) (Figure 4). In the West, Great Plains, and Midwest, most states exhibit no trend or show a significant decreasing trend in tornado temporal variability; only California (CA), Texas (TX), Nebraska (NE), and Minnesota (MN) show a significant increase (Figures 3a and 3b). Tornado Alley also fails to show a consistent change in tornado temporal variability. The southern and northern boundaries of Tornado Alley (i.e., Texas (TX) and Nebraska (NE)) manifest an increasing temporal variability, while the central Tornado Alley (i.e., Kansas (KS) and Oklahoma (OK)) depicts a declining trend (Figure 3a). Such a difference in the changing scenarios in tornado temporal variability is jointly determined by both the monthly tornado frequency (i.e., tornado seasonality) and its interannual variation (see examples for Texas and Oklahoma in Figure 5). GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6948

7 Figure 4. Dynamics of the temporal variability of tornado occurrence over the United States from 1957 to 2006 on the regional scale. Grey shaded areas indicate two separate periods with rapid increases on the continental scale temporal variability (i.e., and ; Figure 2a). The different colors indicate the five geographic regions. The Southeast displays a more consistent increase in tornado temporal variability, except for Florida (FL) (Figures 3a and 3b). The Northeast shows a bimodal pattern in tornado temporal variability. Tornado temporal variability decreases significantly in the northeastern corner of the U.S., but increases significantly from north to south along the Appalachian Mountains, from New York (NY) to Virginia (VA) (Figures 3a and 3b). Such highly variable spatial pattern of tornado temporal variability could not be fully represented by the linearly increasing trend detected on the continental scale (Figures 2 and 3) Spatial Variability of U.S. Tornado Occurrence The interstate variability has been very restrained throughout the past six decades (Figure 2b). Only between 1985 and 2000 was the interstate variability constant over its multiyear average, indicating a higher spatial variability of continental tornado occurrence in that period (Figure 2b). The overall interstate variability is 6.63, suggesting that the annual interstate tornado rank of a state (i.e., Type B rank) averagely changes within seven ranks during a 15 year period (Figure 2b). The error bars in Figure 2b indicate the deviation of the changes in interstate tornado ranks across 48 states. Even considering the interstate deviations, the overall interstate variability (6.63 ± 2.04) varies at a much lower level, when compared to the 95% confidence interval obtained for a 15 year period (10.32, 17.24) (Figure 2a). Such a limited interstate variability indicates a fairly stable structure of tornado occurrence across the contiguous U.S., which agrees with the historical consistency in tornado spatial climatology (Figure 1c). When calculating the running standard deviation of the tornado ranks for other periods of time (e.g., 1, 5, and 10 years), the interstate variability still exhibits a flat trend since 1950 (Figure S2). Moreover, to examine the impact from low-tornado states on the calculation of tornado interstate variability, we exclude the states with an annual average tornado occurrence of less than five from the analysis. The restrained tornado interstate variability remains throughout the entire tornado record (Figure 6) Combined Spatial-Temporal Variability of U.S. Tornado Occurrence Like the interstate variability (Figure 2b), the combined interstate-temporal variability of the continental-scale tornado occurrence has shown no significant trend since the 1950s (Figure 2c). Only between the 1980s and 1990s did the combined interstate-temporal variability show constantly greater values than its multiyear average (Figure 2c). The average variability (17.40) is lower than the 95% confidence interval for a 15 year period (17.88, 19.06) (Figure 2c). Namely, in a long-term perspective, tornado occurrence over the continental U.S. exhibits a certain spatial-temporal structure rather than random outbreaks. However, during the 10 year period around 1990, the combined interstate-temporal variability fluctuated within the 95% confidence interval, implying that continental-scale tornado occurrence had no clear spatial-temporal pattern from the 1980s to the 1990s (Figure 2c). This is probably due to the rapid increase in both temporal variability and spatial variability during that period (Figures 2a and 2b). GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6949

8 Figure 5. Understanding the changes in tornado temporal variability. The long-term dynamics of tornado temporal variability are shown for (a) Texas and (c) Oklahoma. Vertical dashed lines divide the tornado data set into three periods: before 1970, 1970 to 1990, and after Horizontal dashed lines indicate the decadal average of tornado temporal variability. (b, d) The average monthly tornado frequency over three periods, , , and in (b) Texas and in (d) Oklahoma. Error bars indicate the interannual variations of monthly tornado frequency within each 20 year period. The temporal variability is determined by the monthly tornado frequency and its interannual variations together. For example, the occurrence of a second tornado season (from September to December, indicating an increased volatility in tornado season) in Texas in recent two decades (black lines in Figure 5b) might explain the recent relatively high temporal variability of tornado occurrence in Texas (Figure 5a); and the restrained interannual variations in monthly tornado frequency in Oklahoma (black lines in Figure 5d) might explain the recent relatively low temporal variability of tornado occurrence in Oklahoma (Figure 5c). When calculating the running standard deviation of the tornado ranks for other periods of time (e.g., 1, 5, and 10 years), the combined interstate-temporal variability also shows no secular trend since the 1950s (Figure S2). In addition, when the low-tornado states (i.e., state with annual average tornado occurrence less than five) are excluded, still no significant trend can be detected for the combined interstate-temporal variability over the entire record length (Figure 6). Therefore, in contrast with the increasing trend in temporal variability alone, both the interstate and interstate-temporal variabilities of continental-scale tornado occurrence remained relatively stationary for the past six decades (Figures 2 and 6). To separate the independent contribution of the spatial (or temporal) variability from the combined interstatetemporal variability, we regressed the combined interstate-temporal variability against the temporal variability and interstate variability, respectively (Figure S3). Significant correlations were detected between the combined interstate-temporal variability and temporal variability (r =0.61,p < 0.001) and between the combined interstatetemporal variability and interstate variability (r = 0.80, p < 0.001). This analysis justifies the effectiveness of the combined interstate-temporal variability obtained by our method for representing the information on both the temporal and spatial scales (Figure S3). Moreover, the temporal variability explains 37% of the total yearto-year variations in the combined interstate-temporal variability, and the interstate variability accounts for 63% of the variations (Figure S3). In other words, the relative contribution of interstate variability is nearly twice that of the temporal variability to the combined interstate-temporal variability of continental-scale tornado occurrence. This finding would explain the synchronicity between the interstate variability and the combined interstate-temporal variability since the 1950s (Figures 2b and 2c) 5. Discussion This study extended the method for quantifying (E)F1+ tornado variability introduced by Brooks et al. [2014], from temporal scale alone to both spatial and temporal scales and from continental scale to regional and GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6950

9 Figure 6. Spatial (interstate) and spatial-temporal (interstate-temporal) combined variabilities of tornado occurrence over the continental United States from the 1950s to the 2010s, calculated after the low-tornado states (i.e., a state with average annual tornado occurrence less than five) are excluded. A total of 21 states are removed, leaving 27 continental U.S. states for analysis. (a) Spatial (interstate) variability calculated with 15 year running standard deviations (SD) of Type B ranks. Error bar indicates the standard deviation of changes in interstate tornado ranks across states. (b) Spatialtemporal (interstate-temporal) combined variability calculated with 15 year running SD of Type C ranks across states. Red lines indicate the average values of the two types of tornado variabilities throughout the study period. Red dashed lines are the 95% confidence intervals determined by random numbers. state scales. Additionally, the use of the entire continental U.S. as the study area expanded the understanding of local tornado variability in areas that are commonly ignored in tornado studies [Farney and Dixon, 2014]. Based on the monthly continental tornado count ranks, Brooks et al. [2014] indicated that the temporal variability of U.S. tornadoes significantly increased in recent decades (Figure 2a). However, such a claim on the continental scale failed to consider the spatially varied tornado climatology over the U.S. Our results suggest that tornado occurrence is regionalized across the contiguous U.S. and that Tornado Alley has the highest tornado frequency (Figure 1), most likely owing to its access to moisture from the Gulf of Mexico; its location east of higher terrain is conducive for the production of a lee trough when there are westerly winds aloft, so southerly winds at low levels are produced, which advect the Gulf moisture poleward. The Great Plains lie within the latitude belt of the baroclinic westerlies, which when combined with the low-level southerly winds result in strong vertical shear necessary for supercells. Disturbances propagate eastward in the baroclinic westerlies, which can trigger the development of convective storms even when there is a capping inversion restraining convection initiation [Bluestein, 2013]. To account for the spatially varied tornado frequency, we examined tornado temporal variability in each contiguous U.S. state (Figure 3) and in different geographic regions (Figure 4). We divided the contiguous states into five regional divisions following the National Climate Assessment (NCA), which is especially designed for climate assessment and adaption, only except that we merged Northwest and Southwest into one region, the West, owing to the relatively low numbers of tornado reports. However, while there is a lot of documentation in the literature on the types of tornadoes in California [e.g., Bluestein, 1979; Carbone, 1983; Blier and Batten, 1994; Monteverdi et al., 2003], there is relatively less documentation on tornadoes in the rest of the West, so it is not known how similar the types of tornadoes are in the Northwest and Southwest. Our results indicate that (1) temporal variability of tornado climatology varied across the contiguous U.S. (Figures 3 and 4) and (2) the Great Plains and Southeast regions mainly resulted in the increased nationwide tornado temporal variability, which might be related to temporal variability in the environmental conditions needed to produce supercells or the conditions necessary for landfalling tropical cyclones or quasi-linear convective systems (Figures 3 and 4). Therefore, the claim from Brooks et al. [2014] should be interpreted with great caution, especially when applying it on a more regional scale. Moreover, locating the geographical regions that were most responsible for the nationwide increase in tornado temporal variability could help direct the search for an explanation for these changes. Our results also suggested that adjacent states or states with the same geographic regions tend to show similar trends in tornado temporal variability (Figure 3). Therefore, compared with the national scale, regional scale and state scale are better to study tornado variability in the U.S. GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6951

10 Likely, one of the reasons that Brooks et al. [2014] did not pursue the state-scale contribution to the overall nationwide tornado temporal variability is because the atmosphere conditions and the occurrence of tornadoes should be insensitive to geopolitical boundaries. To account for this issue, the SPC tornado record has removed almost all of the tornadoes that moved across state boundaries. In our case, for the total of 30,747 tornadoes studied, only 415 tornadoes (1.35%) initiated and ended in different states, which would have very limited effect on our results. Therefore, our analysis avoided the possible impact of vagaries of political boundaries on measuring tornado count and on studying tornado variability on the state scale. Moreover, if the tornadoes that moved across state boundaries are taken into consideration, they would probably increase the uncertainty of tornado occurrence in each state and even amplify the heterogeneity of interstate tornado distribution. In addition to the temporal variability, we introduced two new ways of ranking tornado counts and studying tornado variability (Figure S1). The first new index (Type B) measured tornado spatial variability (or interstate variability) by, for each year, ranking the 48 states by their tornado count and then averaging (across states) the standard deviations of the time series of these interstate ranks. This quantified the irregularity of the distribution of tornadoes among states over time. The second (Type C) measured the combined spatialtemporal variability (or interstate-temporal variability) by ranking the annual tornado counts for each state over the 64 year period. Even when calculated on the continental scale, both the spatial variability and the combined spatial-temporal variability have included information on state-specific tornado climatology (Figure S1). Our results indicated that these two new measures of spatial and spatial-temporal tornado variability, unlike the temporal variability alone, did not increase over the period of record (Figure 2). Even if the deviation of interstate ranks were calculated over different time periods or after excluding the low-tornado states from the analysis, these results were robust (Figures 6 and S2). For the combined spatial-temporal variability, temporal variability explains one third of the total year-to-year variations, and interstate variability accounts for the other two thirds (Figure S3). From these results it may be inferred that the main factors that influence U.S. tornado geospatial climatology have remained relatively constant for the past several decades. However, both Brooks et al. [2014] and this study considered the (E)F1 and stronger tornadoes as a whole, without differentiating the possible discrepancies in the tornado variability at different tornado intensities. Future efforts can be made to look at the typical variability of tornado events with regard to varying intensities. In addition, even for the (E)F1+ tornadoes, the increase in population and public awareness as well as the spreading of new observation technology can still impact data homogeneity. Thus, the results reported in Brooks et al. [2014] and this study might contain information of nonmeteorological effects. More efforts are required to enhance our ability to separate nonmeteorological effects from actual meteorological effects in the SPC tornado database [Brooks et al., 2014]. Moreover, both Brooks et al. [2014] and this study are statistics based. More physical interpretation or explanation should be given for the observed pattern in U.S. tornado variability in future studies. For example, one could estimate the variability in boundary layer moisture, vertical shear, and a measure of thermal buoyancy, which might be a measure of the variability of the occurrence of supercells, which can produce tornadoes. Results of this study signify the importance of recognition of regionality in assessing tornado variability in the U.S. Although we have downscaled the nationwide pattern of tornado variability into regional and state levels, we admit that regionality would still exist at the state scale, in particular, for the states with relatively large area, which could partially explain the sharp gradients in the trend of tornado variability in neighboring states (e.g., Texas versus Oklahoma; Figure 3). In addition to state-scale analysis, we suggest future study to grid tornado data at different resolutions to further evaluate the spatially regionalized tornado variability in the U.S. 6. Conclusions Trends of tornado temporal variability are regionalized across the contiguous U.S., with only one third of the continental area or three out of 10 contiguous states displaying a significant increase in tornado temporal variability over the 64 year period of record ( ). However, the other two thirds of the continental area, where 60% of the U.S. tornadoes were reported, show a decreasing or a near-zero trend. The continental-scale increase in tornado temporal variability can be attributed mainly to variability in the Great Plains and Southeast regions, which might be related to temporal variability in the environmental GUO ET AL. U.S. TORNADO OCCURRENCE VARIABILITY 6952

11 conditions needed to produce supercells or the conditions necessary for landfalling tropical cyclones or quasi-linear convective systems. Unlike the steady increasing trend of tornado temporal variability, both the interstate variability and the combined interstate-temporal variability have shown a flat trend on the continental scale since Our results signify that the combined interstate-temporal variability of continental tornado occurrence has remained relatively stable over the past six decades, and the increasing trend of continental-scale tornado temporal variability cannot represent the regionally dependent tornado variability across the continental U.S. Such detailed information on tornado variability could be helpful for local warning dissemination and mitigation efforts toward site-specific tornado hazards. Acknowledgments This study was funded by the National Natural Science Foundation of China ( , , and ), the National Basic Research Program of China (2012CB955302), and the Fundamental Research Funds for the Central Universities by Beijing Normal University. The U.S. tornado data set was downloaded from the Storm Prediction Center of NOAA ( gov/wcm/#data). H.B.B. is supported by NSF grant AGS ; he is indebted to the National Center for Atmospheric Research (NCAR) in the Mesoscale and Microscale Meteorology (MMM) Laboratory for support during his sabbatical leave from the University of Oklahoma, when he contributed to this study. The authors would like to thank the Editor and two anonymous reviewers for their comments in improving the manuscript. References Ashley, W. S. (2007), Spatial and temporal analysis of tornado fatalities in the United States: , Weather Forecasting, 22(6), , doi: /2007waf Blier, W., and K. A. 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