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Different Types of Tornadoes
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Wicker and Wilhelmson, Louis J. and Robert B. "Simulation and Analysis of Tornado Development and Decay within a Three-Dimensional Supercell Thunderstorm."
American Meteoroligical Society
Volume 52, August 1995) 1. Web.18 May 2009. <
A three-dimensional numerical simulation using a two-way interactive nested grid is to study tornado-genesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storm's mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface wind speeds exceed 60 m s−1 during both tornadoes, and horizontal pressure gradients reach 18 hPa km−1 during the second tornado. Comparison of the simulated storm evolution with Doppler and field observations of supercells and tornadoes shows many similar features.
Vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm. Tornadogenesis is initiated when mesocyclone rotation increase above cloud base. The increased rotation generates lower pressure in the mesocyclone, increasing the upward pressure gradient forces. The upward pressure gradient forces accelerate the vertical motions near cloud base, creating 20–30 m s−1 updrafts at this level. As the updraft intensifies at cloud base, the convergence in the subcloud layer also increases rapidly. The vertical vorticity is the stretched in the convergent flow, creating the tornado vortex. Tornado decay begins when the vertical pressure gradient forces decrease or even reverse at cloud base, weakening the updraft above tornado. As the updraft weakens, the low-level flow advects the occlusion downdraft completely around the tornado, surrounding the vortex with downdraft and low-level divergence. Cut off from its source of positive vertical vorticity, the tornado then dissipates, leaving a broad low-level circulation behind.
Phan and Simiu,, Long and Emil. "Tornado Aftermath: Questioning the Tools (Available Structural Engineering Special Issue Only)."
Vol. 68, No. 12,December 1998 1. Web.18 May 2009. <
In May 1997, several tornadoes hit central Texas. The strongest of these killed 27 people and destroyed about 40 single-family houses on the outskirts of Jarrell, north of Austin. A post-storm damage survey was conducted by the Office of the Federal Coordinator for Meteorological Services and Supporting Research. The results are described and the investigators conclude that the damage can be explained by wind speeds corresponding to an F3 tornado classification rather than the F5 classification assigned to the event by the National Weather Service. The authors believe the classification was due to the failure of the Fukita intensity scale to account explicitly for the dependence of wind speeds causing specified types of damage depending on quality of construction and the basic wind speed at the geographic area. The authors call for a rethinking of the Fujita intensity scale.
Marzban and Schaefer, Caren and Joseph T.. "The Correlation between U.S. Tornadoes and Pacific Sea Surface Temperatures."
American Meteoroligical Society
Volume 129, Issue (April 2001) 1. Web.18 May 2009. <
The correlation between tornadic activity in several regions of the United States and the monthly mean sea surface temperature over four zones in the tropical Pacific Ocean is examined. Tornadic activity is gauged with two mostly independent measures: the number of tornadoes per month, and the number of tornadic days per month. Within the assumptions set forth for the analysis, it is found that there appears to exist a statistically significant but very weak correlation between sea surface temperature in the Pacific Ocean and tornadic activity in the United States, with the strength and significance of the correlation depending on the coordinates at which the sea surface temperatures are assessed and the geographic region of the United States. The strongest evidence found is for the correlation between the number of days with strong and violent (F2 and greater) tornadoes in an area that runs from Illinois to the Atlantic Coast, and Kentucky to Canada and a cool sea surface temperature in the central tropical Pacific. However, there is only about a 53% chance of this relationship occurring in a specific month.
Mitchell, Vasiloff, Stumpf, Witt, Eilts, Johnson, Thomas, E. DeWayne , Steven V, Gregory J., Arthur, Michael D., J. T., Kevin W. "The National Severe Storms Laboratory Tornado Detection Algorithm."
American Meteoroligical Society
Volume 13, Issue 2June 1998) 1. Web.18 May 2009.
The National Severe Storms Laboratory (NSSL) has developed and tested a tornado detection algorithm (NSSL TDA) that has been designed to identify the locally intense vortices associated with tornadoes using the WSR-88D base velocity data. The NSSL TDA is an improvement over the current Weather Surveillance Radar-1988 Doppler (WSR-88D) Tornadic Vortex Signature Algorithm (88D TVS). The NSSL TDA has been designed to address the relatively low probability of detection (POD) of the 88D TVS algorithm without a high false alarm rate (FAR). Using an independent dataset consisting of 31 tornadoes, the NSSL TDA has a POD of 43%, FAR of 48%, critical success index (CSI) = 31%, and a Heidke skill score (HSS) of 46% compared to the 88D TVS, which has a POD of 3%, FAR of 0%, CSI of 3%, and HSS of 0%. In contrast to the 88D TVS, the NSSL TDA identifies tornadic vortices by 1) searching for strong shear between velocity gates that are azimuthally adjacent and constant in range, and 2) not requiring the presence of an algorithm-identified mesocyclone. This manuscript discusses the differences between the NSSL TDA and the 88D TVS and presents a performance comparison between the two algorithms. Strengths and weaknesses of the NSSL TDA and NSSL’s future work related to tornado identification using Doppler radar are also discussed.
Lee and White, Robert R and Anderson. "Improvement of the WSR-88D Mesocyclone Algorithm."
American Meteoroligical Society
Volume 13, Issue 2June 1998) 1. Web.18 May 2009. <
The build 9 Weather Surveillance Radar-1988/Doppler mesocyclone algorithm (B9MA) is designed to locate mesocyclones [rotating thunderstorm updrafts with diameters between 1.8 and 9.2 km (1–5 n mi)]. Because there is less than a one to one correspondence between tornadoes and mesocyclones, the B9MA alerts forecasters when it detects circulations that meet its criteria but tornadoes may not be observed. On the other hand, some tornadoes are not accompanied by large-scale meoscyclonic circulations. Weather radars cannot resolve small-scale tornadic circulations, and the B9MA may fail to alert forecasters that a tornado is present. The B9MA is only one of many tools that forecasters should use to predict tornado formation.
This paper describes limitations of the B9MA and how to improve its performance. The correlation between algorithmic detections and severe weather occurrence may be optimized under the premise that storms with strong, deep rotations are more likely to be associated with severe weather. Some tornadic circulations missed by the B9MA can be detected when the value of threshold pattern vector, a B9MA adaptable parameter, is lowered. When a rotational strength filter is added to the program logic, some algorithm detections of nontornadic mesocyclones can be eliminated.
Brooks and Doswell III, Harold E and Charles A.. "Normalized Damage from Major Tornadoes in the United States: 1890–1999."
American Meteoroligical Society
Volume 16, Issue 1February 2001) 1. Web.18 May 2009. <
Historical records of damage from major tornadoes in the United States are taken and adjusted for inflation and wealth. Such adjustments provide a more reliable method to compare losses over time in the context of significant societal change. From 1890 to 1999, the costliest tornado on the record, adjusted for inflation, is the 3 May 1999 Oklahoma City tornado, with an adjusted $963 million in damage (constant 1997 dollars). Including an adjustment for growth in wealth, on the other hand, clearly shows the 27 May 1896 Saint Louis–East Saint Louis tornado to be the costliest on record. An extremely conservative adjustment for the 1896 tornado gives a value of $2.2 billion. A more realistic adjustment yields a figure of $2.9 billion. A comparison of the ratio of deaths to wealth-adjusted damage shows a clear break in 1953, at the beginning of the watch/warning/awareness program of the National Weather Service.
Brewer and Hall , Robert D. and Ferguson. "A SEQUENCE OF TORNADO DAMAGE PATTERNS."
American Meteorological Society
Volume 87, Issue 6June 1959) 1. Web.18 May 2009.
An investigation is made of the patterns of damage associated with a series of tornadoes in west-central Wisconsin on June 4, 1958. Three flow models are presented to account for the different patterns exhibited in different portions of the paths.
Doswell III, Charles A., Alan R. Moller, and Harold E. Brooks. "Storm Spotting and Public Awareness since the First Tornado Forecasts of 1948." Weather and Forecasting. 1 Oct 1998. pgs.544-547. <
The history of storm spotting and public awareness of the tornado threat is reviewed. It is shown that a downward trend in fatalities apparently began after the famous “Tri-State” tornado of 1925. Storm spotting’s history begins in World War II as an effort to protect the nation’s military installations, but became a public service with the resumption of public tornado forecasting, pioneered in 1948 by the Air Force’s Fawbush and Miller and begun in the public sector in 1952. The current spotter program, known generally as SKYWARN, is a civilian-based volunteer organization. Responsibility for spotter training has rested with the national forecasting services (originally, the Weather Bureau and now the National Weather Service). That training has evolved with (a) the proliferation of widespread film and (recently) video footage of severe storms; (b) growth in the scientific knowledge about tornadoes and tornadic storms, as well as a better understanding of how tornadoes produce damage; and (c) the inception and growth of scientific and hobbyist storm chasing.
The concept of an integrated warning system is presented in detail, and considered in light of past and present accomplishments and what needs to be done in the future to maintain the downward trend in fatalities. As the integrated warning system has evolved over its history, it has become clear that volunteer spotters and the public forecasting services need to be closely tied. Further, public information dissemination is a major factor in an integrated warning service; warnings and forecasts that do not reach the users and produce appropriate responses are not very valuable, even if they are accurate and timely. The history of the integration has been somewhat checkered, but compelling evidence of the overall efficacy of the watch–warning program can be found in the maintenance of the downward trend in annual fatalities that began in 1925.
Dowell, David C.. "Abstract Views." AMS Journals Online. 8 June 1995. American Meteorological Society. 21 Apr 2009 <
During the first stage of development of each storm-scale circulation, interaction of the updraft with the environmental low-level horizontal vorticity produced a vorticity column that increased in intensity with height. As the vortex matured, vorticity increased greatly at low levels (i.e., below 2 km AGL) and exceeded that aloft. Each tornadic vortex was located near the rear side of the updraft, where the surrounding low-level horizontal vorticity was modified locally, most likely by weak baroclinity within the storm. Tilting of low-level horizontal vorticity into the vertical, followed by stretching of the vertical vorticity, occurred in the air parcels that entered the rear portion of the main storm updraft from its left (as viewed in the direction of storm motion). Although the region of tilting was near the interface of the main updraft and that portion of the downdraft to the left of the updraft, there is no direct evidence in the observations (above 500 m AGL) of generation of cyclonic vertical vorticity by tilting in the downdraft itself.
For this storm, the cyclic tornadogenesis process was associated with a mismatch between the horizontal motion of successive tornadoes and the horizontal velocity of the main storm-scale updraft and downdraft. Low-level updraft-relative flow seemed to be the most important factor in determining tornado motion.
Diffenbaugh, Noah S., Robert J. Trapp, and Harold Brooks. "Does Global Warming Influence Tornado Activity." Eos, Transactions American Geophysical Union. 30 Dec 2008. Volume 89, Issue 53. P. 553-554. 25 Apr 2009. <
Tornadoes and other severe thunderstorm phenomena frequently cause as much annual property damage in the United States as do hurricanes, and often cause more fatalities (see
). In 2008, there were 2176 preliminary tornado reports logged through mid-December, with 1600 ``actual counts'' (duplicate reports removed) through September, the highest total in the past half century (Figure 1). The mass media have covered these events extensively, and experts have been deluged with requests for explanations, including possible links to anthropogenic global warming. Although recent research has yielded insight into the connections between global warming and tornado and severe thunderstorm forcing, these relationships remain mostly unexplored, largely because of the challenges in observing and numerically simulating tornadoes. Indeed, a number of questions that have been answered for other climate-related phenomena remain particularly difficult for climate and severe weather scientists, including whether there are detectable trends in tornado occurrence and if so, what causes them. This article explores the challenges and opportunities in pursuing these areas of research.
Holle, Ronald L., Maier, Michael W. "Tornado Formation from Downdraft Interaction in the FACE Mesonetwork." AMS Journals Online. March 10 1980. American Meteorological Society . 25 Apr 2009. Monthly Weather Review. Article: pp. 1010–1028 <
A tornado observed on 15 June 1973 in the FACE surface mesonetwork was studied on the mesoscale and cloud scale. Downdrafts from two preexisting cumulonimbi, initially 80 km apart, met along a north-south line in the center of the mesonet 30 min before tornado formation. Fed by the convergence of flow from the outflows of the predecessor cumulonimbi, a line of deep cumuli formed and developed rapidly. A tornado was observed as it dropped from this cumulus line. When the tornado dissipated 10 min later, heavy precipitation was reaching the surface and new outflow began to spread from the now vigorous cumulonimbus that had spawned the tornado. The life cycle of the tornado and a period of 90 rain surrounding its occurrence studied in detail from observed surface winds, radar reflectivity and surface rain gage data. The evolution of the parent cloud and tornado in a tropical thermodynamic environment with local forcing, weak shear and winds, and a potentially unstable sounding contrasts with the conditions that accompany large-scale forcing of the parent clouds in which extratropical tornadoes are found. The 850–200 mb wind shear of <2 m s−1 was the weakest found over many summers of FACE at Miami and was the only unique environmental parameter detectable on the day when the tornado formed. The similarity of the 15 June FACE tornado to Florida waterspout life cycles is noted.
Novlan , David J. and William M. Gray. "Hurricane-Spawned Tornadoes." AMS Journals Online. Monthly Weather Reviews. 9 May 1974. pp. 476-478. Web. <
Hurricane-spawned tornadoes are most frequent at the time when hurricanes initially cross land and undergo rapid filling. This paper presents data composite information on all available rawinsonde and pibal reports surrounding this type of tornado genesis in the United States and Japan. Information has also been gathered on tropical storms entering land which did not produce tornadoes.
The most important difference between storms which produce tornadoes and those which do not is a very large increase of the vertical shear of the horizontal wind between the surface and 4–5 thousand feet. This averages about 40 knots for the tornado cases, but is much less in the cases which do not produce tornadoes. Differences in vertical stability are observed to be small.
An overland hurricane dissipation model is proposed whereby the boundary layer frictional inflow towards the hurricane center occurs without the usual ocean sensible heat gain and is not, as over the ocean, isothermal. Over land the inward spiraling air parcels cool. This reverses the usual hurricane boundary layer baroclinicity and allows for large observed low-level positive vertical wind shear during the short period of rapid filling. This large magnitude vertical wind shear appears to be required for tornado formation. It should be used as a forecast tool in hurricane tornado prediction.
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