An Investigation of Tornado Production from Atlantic Basin Hurricanes

This topic submitted by Karen Kosiba, Jessica Downing on 4/27/02 .

Hurricane Studies in the Northern Atlantic Basin

Atlantic Basin Hurricanes: Policy and Threats

This is Hurricane Andrew from 1992

I. Abstract With the increase in the United States coastal population as well as the concern for the affects of global warming, it is necessary to understand the parameters that affect hurricane (also known as a tropical cyclone) development and intensification. Research has shown a slight increase in hurricane activity has occurred in the last 111 years. Positive relationships or correlations,although tentative, have been established between an increase the in the mean surface sea temperatures (SST), increased levels of atmospheric carbon dioxide, non-El Nino Years, increased monsoonal activity and increased hurricane activity in the Northern Atlantic Basin. Although global warming will increase the amount of energy in the atmosphere/ocean system, scientists are unsure as to how this energy change will manifest (Elsner and Kara, Hurricane of the North Atlantic, Chapter 10, 1999).

A diagram of the number of hurricane systems per year.

Through extensive literature review, it is apparent that a relationship between hurricane intensity and hurricane-spawned tornadoes has not yet been established. We will investigate if a relationship exists between intense hurricanes and hurricane-spawned tornado frequency. Furthermore we will investigate the correlation between hurricane-spawned tornadoes and El Nino years as well as the correlation between hurricane-spawned tornadoes and coastal SST. We will look at data fromhurricane/tornado 1990-2001 in the Northern Atlantic Basin.

A nice diagram of hurricane frequency per year.

It is also of aramount interest to investigate the policies that have been implemented to address storm preparedness as well asdisaster relief in those coastal communities. This will be accomplished by extensive review of hurricanes that occurred in the1990-2001 time frame.

Ia. Research Question This study of hurricanes will be two fold, consisting of a social aspect as well as hurricane dynamics. First, the question ofhurricane preparedness and mitigation will be assessed by making reference to how it has changed over the years, as a result ofincreasing hurricane destruction. We will also touch on what sort of policies have been put forth by those mitigation entities inorder to ensure that safety remains top priority in times of such natural disasters. Second we are going to investigate if correlations exist between the following variable: -Intensity (maximum wind speed/minimum pressure) and the number and intensity of hurricane-spawned tornadoes;

Hurricane Bertha from 1996

II. Introduction Hurricanes are perhaps natures most destructive storms. For people living along the coastal regions of the Gulf of Mexico aswell as the Atlantic Coast, hurricanes have the possibility of destroying homes, taking lives, and even stopping daily activity.Hurricanes are not a storm to be taken lightly.

As the coastal population in the United States increases, hurricanes become an increasing concern. With the potential to causemillions of dollars in property damage and a large death toll, it is important to understand policies and programs that have beenimplemented to protect those areas. Furthermore, it is necessary to understand the influence of climatological factors that affecthurricane development and intensification.

Hurricanes contain damaging winds, dangerous storm surges, torrential rains, inland flooding, and the potential to producetornadoes. Hurricane-spawned tornadoes, although less violent than their frontal-spawned counterparts, account for up to 10%of all hurricane fatalities and 50% of all property damage. Therefore it is of paramount interest to investigate the frequency ofhurricane-spawned tornadoes.

Due to the potentiality to disrupt economic functions both on a local and federal level, governmental and humane programs havebeen implemented to minimize these [possibly] detrimental effects that hurricanes and all the threats associated with them, mighthave on the immediate environment. These programs could be aimed at evacuating the people to ensure that everyone remainssafe, or they could be programs in which the local community effectively analyzes its resources to put a price on them so that itis easier to collect damages after a destructive storm. Whether it is a storm preparation program or a disaster relief program,these packages, in light of the costly damages associated with such recent storms, have been re-vamped in order to handlemodern worries.

III. Definitions A tropical cyclone is a non-frontal vortex that develops in the tropical latitudes. The pressure gradient force acting inward andthe Coriolis and centrifugal forces acting outwards balance air circulating around a low-pressure center.

There are four stages of tropical cyclone development (listed in order of increasing intensity and organization): tropicaldisturbances, tropical depressions, tropical storms and hurricanes.

A hurricane is defined as a tropical cyclone with near surface (below 10 meters) winds in excess of 73 mph and a centralpressure deficit of at least 60 mb. Hurricanes are categorized according to the Saffir-Simpson intensity scale , with a Category 1hurricane as the weakest (central pressure > 980 mb, sustained wind speeds between 74-95 mph and a storm surge of 4-5feet) and a Category 5 hurricane as the strongest (central pressure < 920 mb, sustained wind speeds in excess of 155 mph anda storm surge greater than 18 feet).

Hurricane intensity is defined by either the minimum sea level pressure (MSLP) or the maximum sustained low-level winds.Hurricane intensification is said to occur when either of the aforementioned parameters increases. Hurricane frequency is defined as the number of hurricanes that occur within a given time interval. A hurricane makes landfallwhen part or its entire eye is over land.

Where Hurricanes have made Landfall in the Past.

IV. Hurricane Structure

Hurricanes are two-celled structures (contain both upward and downward air movement) that derive their energy from therelease of latent heat due to condensation. Hurricanes contain a low-pressure, warm-core center known as the eye. The eye is a16-70 km diameter column of subsiding air that is being warmed by compression. As the warm air sinks, it spreads out, risesand feeds the cumulonimbus towers surrounding the eye. The surrounding annular area of cumulonimbus towers is a regionknown as the eye wall. It is in the eye wall that the most intense vertical motions, the heaviest rain and the largest wind speedsoccur. Surrounding the eye wall are spiral-like arms known as the feeder or spiral bands. These bands contain heavy rains andstrong, gusty winds.

Hurricane Mitch from 1998.

Va. Literature Review of Climate Factors Hurricanes are complex dynamic structures influenced by a variety of atmospheric and oceanic factors. Gray (1981) has identified six conditions that support the development of tropical cyclone development. They are:1. Large values of low-level vorticity,2. Coriolis parameter (development is favored several degrees poleward from the equator since the Coriolis parameter at theequator is zero),3. Week vertical shear of horizontal winds,4. High SSTs (in excess of 26 C) and a deep thermocline, 5. Conditional instability through a deep atmospheric layer, and6. Large values of relative humidity in the lower and middle tropopause.Although these conditions may contribute to hurricane development, they are not sufficient to predict hurricane development.

With the concern of global warming and its affects on the atmosphere and oceans, the scientists are interested on how thepredicted effects will influence tropical cyclone activity. It is though with an increase in global warming, that the oceantemperatures will increase. Furthermore, it is thought that warm ocean water will migrate northward from their previouslytropical locations. It has been shown that sea temperatures 26 C and upwards are favorable for the development of tropicalcyclones. With an increase in sea temperatures, will hurricanes become more frequent or more intense? Research has indicatedthat although there is a correlation between SSTs and hurricane intensity, SSTs alone cannot predict whether or not a hurricanewill intensify (DeMaria and Kaplan, 1994).

It has been found that there is a correlation between SST and hurricane intensity. SST alone, though, cannot predict whether aparticular hurricane will intensify.

Studies have shown there to be a modest increase in hurricane development in the North Atlantic Basin from 1886-1996.

For the same time frame, there has also been a positive trend in baroclinic hurricanes and the mean latitude of occurrence.

As hurricanes make landfall, frictional dissipation of surface winds occur more readily then at higher elevations. This, along withother favorable environmental conditions, combine to produce tornadoes. Hurricane-spawned tornadoes are most frequent inthe front right quadrant of the hurricane system. Most of the tornadoes produced are F2 or below.

An example of some damage caused by hurricanes.

Vb. Literature Review of Societal Aspects For this reason, there have been countless governmental and insurance programs designed to keep those people, who live in thepotential path of a hurricane, safe. As far as records go, there have been massive hurricanes dating back to the 1920s. In fact,during those times, there were two extremely destructive hurricanes, which some analysts say were more economicallydestructive (with economic inflation taken into account) than the 1992 hurricane Andrew, because of lack of programs to keeppeople safe. The USGS offers a great comparison, of the last century, between property damage and loss of life, and how theyhave both oscillated over time. Nowadays, there is the renowned FEMA, and Red Cross institutions, to name a couple, that arethe primary players in hurricane preparedness and mitigation.

In recent times, FEMA was forced to re-evaluate its mitigation plans when the ordered evacuation of thousands of people led tomass amounts of traffic which put peoples lives in danger by drastically slowing down the evacuation process. As the directorof FEMA, James Lee Witt appropriately said "..the increase in infrastructure has not kept up with the increase in population.."and publicly called for the re-evaluationof the programs current policies relating to hurricane preparation and mitigation.

Another major player in aiding in hurricane preparation and mitigation for the United States is the American MeteorologicalSociety and their Atmospheric Policy Program, and they also recognized the importance of changing their policies afterHurricane Floyd, which resulted in the largest peacetime evacuation, resulting in completely chaotic traffic and numerous deathsto flooding and other events which could have been avoided. The director of the APP was quoted saying that our currentsystems-for forecasting these major storms, getting the word to those in harms way, and getting them out of danger-areinadequate to handle such a disaster.

Because of this, FEMA has came up with their Disasters Mitigation Act of 2000, which they claimed will aid in the process ofpreparing the affected regions for possible hurricane onslaught, as well as instigating some community-based preparation tobetter assess values of local resources (which can then be filed for damages in case of being destroyed).

Along with this new FEMA program, there are countless websites, insurance programs, and other humanitarian/governmentalresources, which the public can access, in order to get up to the minute readings of the impending storm, as well as pamphletson hurricane readiness, and even a Hurricane Disaster Supply Kit. There is no question that in current times, the vast majority ofthe coastal and inland communities (as even those far inland are eventually affected by the dying tropical storm, or depression),are at the very basic level, adequately prepared for any sort of tropical storm situation, and that there will be a very resourcefulclean-up crew (and process) following the storm.

On this note, one area that I find worthy of investigating is the way that other nations in the Gulf are affected by these monstrousstorms, and what sort of aid, if any, the US provides. One issue that I found off the CNN website, which peaked my interest, isthat when Hurricane Michelle ravaged the nation of Cuba, the United States was very reluctant to offer any sort of humanitarianaid. The reply of the US interest section to this issue was that the US would provide aid, through an intermediary nation, if itwas requested, only for the Cuban people, not for Castros regime. Hmm. Interesting how political factions run so deep...

VI. Statement of the Problem As is evident from the aforementioned discussion, hurricanes making landfall pose great threat to the population and economy.Therefore, it is our intent to analyze the policies adapted to address these concerns. As mentioned earlier, there is a significantamount of damage generated from landfall hurricanes that spawn tornadoes. It is also our intent, then, to look at the number andintensity of hurricane-spawned tornadoes for a given length of time. We have chosen the span from 1951-2001 due to thefluctuation in hurricane intensity throughout that period.

VII. Methods Hurricane data was downloaded from the National Hurricane Center. Tornado data was downloaded fromthe Storm Prediction Center. We also used the National Climate Database to obtain both tornado and hurricane data. We will categorize hurricane intensities using the Saffir-Simpson Scale (Maximum WindSpeed/Minimum Pressure) and tabulate the number of tornadoes and their F-Scale Rating that occurred for a given hurricane ofa particular intensity. We will perform a correlation analysis on the data to see if there is a statistically significant effect on hurricane intensity andtornado production and intensity.

After extensive review of this data, we will look at the correlation between increased storm activity and new and existinggovernment programs aimed at minimizing potential damage.

While compiling our data, we came across many problems. The greatest hardship we encountered was that there were a great number of uncertainties associated with actual reportings of tornadoes. The National Hurricane Center would have one report for the number of tornadoes produced for a given hurricane, while the Tornado Project and National Climate Database would have varying reports of tornadic activity, mainly at the discretion of the weather reporter. Also, within the tornado reports from the NHC, the tornadoes would not always be assigned an intensity, so it led to ambigious data, thus creating the 'unclassified tornado' category. As a result, subjectivity on the part of the researchers could not be avoided. Therefore, our results have shown that all catagories of TC have the capability to produce numerous tornadoes. For example, in our findings, tropical storm Alison produced 23 tornadoes, while hurricane Fran, an category 3, produced only one [officially reported] tornado.

VIII. ResultsAfter conducting our research of tornado production as a function of hurricane intensity, we came to the conclusion that, based on the narrow range of years investigated, there were no definitive relationships uncovered. We did find some statistically significant results, but interpertation is speculative. Specifically, we found a coorelation between the number of tornaodes produced in tropical storms and in catorgory 4 hurricanes. Also, we found significant differences between the number of tornadoes produced and their respective intensities, as evidenced between the F2 and F0 tornadoes, as well as the unclassified grouping and F0 tornadoes. As shown in the graphed data, the first half of the decade was catagorized by low hurricane frequency, whereas the latter half of the decade had a higher hurricane frequency.

Tornado Production as a Function of TC Intensity Expressed as a Percent of Total



Number of Tornadoes Produced for a Given TC Intensity Expressed as a Percent of Total

Tornado Production Stats

In conclusion, TC frequency is relatively cyclical in nature, having a 23-28 year flucuation (Landsea, 2001). During the 70s, 80s, and early 90s, the United States experienced a relative minimum in TC activity. Also during this time, there was a significant increase in coastal development and population. Since the mid 1990s, TC frequency has been increasing. This growth in storm activity, coupled with the rapid development of US coastlines, are prime ingredients for disaster. Therefore, the US Gulf and Atlantic coastlines are under a much greater threat now than in previous years.

Suggestions for future research would be to explore a much larger range of data, and also to use the TC category at landfall, as opposed to the maximum category of its lifecycle.

Our data can be viewed by following these links:
Excel Raw Data
Tornado Production as a Function of TC Intensity.

For a more indepth exploration of our studies, please feel free to peruse our powerpoint presentation.

VIII. Bibliography

1. Henderson-Sellers, H. et. al., 1998:
Tropical cyclones and global climate change: a post-IPCC assessment, 79, Bulletin of the American Meteorological Society.

2. Merrill, R. T., 1988: Environmental influences on hurricane intensification, 45, Journal of the Atmospheric Sciences.

3. Evans, J. L., 1993: Sensitivity of tropical cyclone intensity to sea surface temperature, 6, Journal of Climate.
----, B. F. Ryan, and J. L. McGregor, 1994: A numerical exploration of the sensitivity of tropical cyclone rainfall intensity to sea
surface temperature.
J Climate., 7, 616-623.

4. DeMaria, M., J. Kaplan, 1994: Sea surface temperature and the maximum intensity of Atlantic tropical
, 7, Journal of Climate.

5. Emanual, K. A., 1987: The dependence of hurricane intensity on climate, 326, Nature.
----, 1991: The theory of hurricanes. Annu. Rev. Fluid. Mech., 23, 179-196.
----, 1997: Some aspects of hurricane inner-core dynamics and energetics. J. Atmos. Sci.,52, 3969-3976.

6. Knutson, T. R., et. al, 1998: Simulated increase of hurricane intensities in a CO2-warmed climate, 279, Science.

7. Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Nino/Southern Oscillation
Mon. Weather Rev., 113, 599-606.

8. Gray, W. M., 1984a: Atlantic seasonal hurricane frequency. Part I: El Nino and 30 mb quasi-biennial oscillation influences.
Mon. Wea. Rev., 112, 1649-1668.

9. Lander, M. A., 1994: An exploratory analysis of the relationship between tropical storm formation in the western North
Pacific ESNO.
Mon. Wea. Rev., 122, 636-651.

10. Landsea, C. W., 1993: A climatology of intense (or major) Altantic hurricanes. Mon. Wea Rev., 121, 636-651.
----, N. Nicholls, W. M. Gray, and L. A. Avila, 1996: Downward trends in the frequency of intense Atlantic hurricanes during
the past five decades. Geophys. Res. Lett., 23, 1697-1700.

11. Lighthill, J., et. al., 1994: Global Climate Change and tropical cyclones. Bull. Amer. Meteor. Soc., 75, 2147-2157.

12. Revell, C. G., and S. W. Goulter, 1986: South Pacific cyclones and the Southern Oscillation. Mon. Wea. Rev., 114,

13. Mayo, N., 1994: A Hurricane Model for physics students. The Physics Teacher, 34.

14. Goldenberg, S. B., et. al., 2001: The recent increase in hurricane activity: causes and implications Science.

15. Lugo, A. E., 2000: Effects and outcomes of Caribbean hurricanes in a climate change scenario.The science of the environment, 262, 243-251.

1.FEMA, Mitigation Policies.
2.Hurricane Threats.
3.Hurricane Facts.
4.Some great basic hurricane information.
5.Here are the guys who know all about how mean hurricanes can be, the Hurricane Hunters!
6.FEMAs newest Disaster Mitigation Act of 2000.
7.FAQ: Typhoons, Tropical Cyclones and Hurricanes Contains much general information as well as relevant links on hurricanes.
8.The Tropical Meteorology Project
9.The National Hurricane Prediction Center
10.Atlantic Oceanographic and Meteorological Laboratory
11.CISMM Tropical Cyclones
12.GFDL Hurricane Dynamics
13.Storm Prediction Center Climatological Data
14.Need for Change in Hurricane Mitigation and Preparation Policies
15.A nice USGS analysis of current hurricane trends
18.A WSR-88D Assessment of Tropical Cyclone Outer Rainband Tornadoes

1. Elsner, J. B. and Kara A. B. Hurricanes of the North Atlantic. Oxford University Press, New York. (c) 1999.

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