DOCSLIB.ORG

Some Meteorological Characteristics of Significant Tornado Events

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Some Meteorological Characteristics of Significant Tornado Events FEBRUARY 2002 NOTES AND CORRESPONDENCE 155 Some Meteorological Characteristics of Signi®cant Tornado Events Occurring in Proximity to Flash Flooding JOSEPH A. ROGASH NOAA/NWS/Weather Forecast Of®ce, El Paso, Texas JONATHAN RACY NOAA/NWS/Storm Prediction Center, Norman, Oklahoma 9 February 2001 and 5 September 2001 ABSTRACT A study was performed to determine meteorological aspects of environments in which thunderstorms produced both strong or violent tornadoes and ¯ash ¯oods within a limited temporal and spatial domain. It was found that the overwhelming majority of these episodes occurred in the spring and summer months and during the afternoon and evening hours. In most instances, at least some of the tornadoes were present when ¯ash ¯ooding was in progress. The ambient environment usually included an air mass that exhibited both relatively high convective instability and abundant lower-tropospheric moisture, including an average most-unstable CAPE of 3200 J kg21 and mean surface dewpoint and precipitable water values of 708F (218C) and 41 mm (1.6 in.), respectively. Storm-relative helicity magnitudes indicated that the vertical wind shear ranged from marginally to moderately favorable for supercell formation in all cases. Surface patterns for each episode were generally similar to patterns earlier studies determined to be frequently attendant with ¯ash ¯ooding, in which preexisting surface boundaries acted to focus deep convection. Most events also occurred east of an approaching and well- de®ned upper-tropospheric trough and in the left-front or right-rear quadrant of an upper-level jet streak in which upward vertical motion is usually present. 1. Introduction strong and violent tornadoes and excessive rain pose an exceptional hazard to both life and property, the ability Deep convection and associated weather phenomena to anticipate and identify such multiphenomena events remain a forecasting challenge for operational meteo- is critical to the mission of operational meteorologists. rologists because of both the complexity of the atmo- Thus, this paper will investigate aspects of signi®cant spheric processes involved and the hazards posed to the tornado events (de®ned here as two or more F2 torna- community. In addition to lightning, ¯ash ¯ooding and does or at least one tornado of F3 strength or stronger) tornadoes account for nearly all fatalities and a major that occur in relatively close spatial and temporal prox- proportion of property damage associated with thun- imity to ¯ash ¯ooding or excessive rainfall. The me- derstorms across the United States. Statistics show that teorological characteristics of environments favorable for the period 1955±95, an annual average of 136 fa- for such incidents, including convective instability, at- talities were attributable to ¯ood events and 73 fatalities mospheric moisture content, and surface and upper-air were attributable to tornadoes (National Climatic Data patterns, will be examined and discussed. Center 1995). Therefore, thunderstorm complexes that produce signi®cant tornadoes with ¯ash ¯oods (here- inafter referred to as STF) within a limited time and 2. Methodology and data analyses area represent an extremely dangerous situation (Cor®di Using the National Oceanic and Atmospheric Ad- et al. 1990; Rogash and Smith 2000). ministration publication Storm Data, supplemented by Because convective systems conducive for both available hydrological and cooperative observer infor- mation, convective events from 1992 through 1998 were Corresponding author address: Joseph A. Rogash, National Weath- examined for all of the continental United States east er Service, 7950 Airport Rd., Santa Teresa, NM 88008. of 1008 latitude. [For studies concerning the unique E-mail: [email protected] problems in forecasting deep convection over the west- 156 WEATHER AND FORECASTING VOLUME 17 TABLE 1. Temporal aspects of STF events including times of oc- currence and relative time of tornado development with respect to initial ¯ash ¯ood reports. Time of occurrence (LST) No. of events 0000±0600 3 0600±1200 4 1200±1800 14 1800±2400 13 Tornadoes developing and ending be- fore ¯ash ¯oods 9 (29%) Tornadoes developing before and con- tinuing during ¯ash ¯oods 11 (35%) Tornadoes developing after ¯ash ¯oods 11 (35%) FIG. 1. Monthly frequency distribution for all 31 signi®cant tor- sequently analyzed through the upper troposphere for the nadoes occurring with ¯ash ¯ood (STF) events between 1992 and area and times of interest. By combining the model in- 1998. formation with data obtained from the soundings and hour- ly surface observations, a vertical pro®le of wind velocity, temperature, and moisture was interpolated for each point ern United States, see Maddox et al. (1980) and Doswell at which F2 or stronger tornadoes ensued. In applying this (1980).] Selected cases include those with at least two methodology, it is believed a more representative tornado F2 tornadoes or at least one F3 or stronger tornado and proximity sounding is derived in comparison with that signi®cant ¯ash ¯ood reports with ¯ooding usually oc- obtained from using unmodi®ed soundings alone. In most curring over a minimum of three counties. In addition, instances, however, regularly or specially launched sound- for each event chosen, the tornadoes must exist within ings required only slight modi®cation. Each proximity 3 h (either before or after) and within a distance of 250 sounding was further analyzed in detail using an advanced km (160 mi) of the heavy rain events. For cases selected version of the SHARP workstation (Hart and Korotky with only F2 tornadoes, there must be at least two F2 1991), with parameters related to instability and wind shear tornadoes that 1) must occur within 250 km and3hof closely examined. In the calculation of instability param- the same ¯ash ¯ood reports and 2) must occur within eters, a virtual temperature correction was applied. 250 km of one another. Events associated with tropical With respect to the surface and upper troposphere, disturbances were not investigated. special attention was directed toward locating such fea- There was some subjectivity in selecting the ¯ash ¯ood tures as surface thermal±moisture boundaries, extra- cases because of the dif®culty in evaluating the quality tropical cyclone centers, low-level jets, and middle- and of information (Maddox et al. 1979, henceforth referred upper-tropospheric troughs and jet streaks. As discussed to as M79; Giordano and Frisch 1991). For this particular below, lower-tropospheric patterns associated with each study, signi®cant ¯ash ¯ood reports included water dam- episode were generally similar to patterns M79 found age to homes and businesses, widespread ¯ooding and attendant with ¯ash ¯ooding. Thus, patterns associated closures of roads and highways, river and stream over- with STF events in this study are categorized in a man- ¯ows that caused major disruptions, and rainfall amounts ner consistent with M79, although important variations of at least 75 mm (3 in.) in less than 6 h. In addition, to from their paradigm are also described and discussed. be selected for this study ¯ash ¯ood events had to include signi®cant reports in at least three counties. Because county size varies within different regions, another spatial 3. Environmental conditions and features requirement was at least two of the ¯ood reports must a. Temporal aspects have a distance separation of at least 75 km. For each case selected, surface, upper-air, and rawin- For the period 1992±98, there were 31 STF episodes sonde data were examined and analyzed to determine an- that meet met the time and space criteria de®ned above. tecedent conditions within3hoftheevolution of the STF The monthly distribution plot (Fig. 1) shows 25, or 81%, events occurring within the temporal and spatial proximity of these episodes evolved from April through July when of ¯ash ¯ooding that meets the requirements noted earlier. signi®cant tornado activity is normally at its peak. An For cases involving multiple tornadoes, data closest to the examination of actual time of occurrences (Table 1) in- most intense tornado(es) were considered. The construc- dicates the overwhelming number of cases occurred be- tion of proximity soundings ®rst involved an examination tween 1200 and 0000 LST, with several cases beginning of regularly scheduled or special radiosondes released during the afternoon and continuing into the evening. nearest to the tornadoes and within the air mass considered The order of occurrence of tornadoes relative to ¯ash to be most representative of the convective environment. ¯ood reports is also explored. As presented in Table 1, Six-hourly Eta and Rapid Update Cycle (RUC II) model for 11 cases (35%) strong or violent tornadoes ensued forecasts of wind, temperature, and moisture were sub- before but continued after the ®rst ¯ash ¯ood reports. FEBRUARY 2002 NOTES AND CORRESPONDENCE 157 TABLE 2. Mean environmental conditions and selected forecast conjunction with the high water vapor content in the parameters for all STF events. lower troposphere, suggests substantial lower-tropo- Parameter spheric moisture ¯ux. Also signi®cant is the mean west- 21 Wind (8/m s21) erly wind of 25 m s at 500 mb, which contributes to Surface 170/08 a composite antecedent vertical wind pro®le consisting 850 mb 210/18 of surface to midtropospheric wind speeds increasing 700 mb 240/20 and wind directions veering with height. 500 mb 240/25 From a storm-scale perspective, such a wind shear 250 mb 250/30 pro®le
Load more

Recommended publications
  • Warm-Core Formation in Tropical Storm Humberto (2001)
    APRIL 2012 D O L L I N G A N D B A R N E S 1177 Warm-Core Formation in Tropical Storm Humberto (2001) KLAUS DOLLING AND GARY M. BARNES University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 27 July 2011, in final form 18 October 2011) ABSTRACT At 0600 UTC 22 September 2001, Humberto was a tropical depression with a minimum central pressure of 1010 hPa. Twelve hours later, when the first global positioning system dropwindsondes (GPS sondes) were jettisoned, Humberto’s minimum central pressure was 1000 hPa and it had attained tropical storm strength. Thirty GPS sondes, radar from the WP-3D, and in situ aircraft measurements are utilized to observe ther- modynamic structures in Humberto and their relationship to stratiform and convective elements during the early stage of the formation of an eye. The analysis of Tropical Storm Humberto offers a new view of the pre-wind-induced surface heat exchange (pre-WISHE) stage of tropical cyclone evolution. Humberto contained a mesoscale convective vortex (MCV) similar to observations of other developing tropical systems. The MCV advects the exhaust from deep con- vection in the form of an anvil cyclonically over the low-level circulation center. On the trailing edge of the anvil an area of mesoscale descent induces dry adiabatic warming in the lower troposphere. The nascent warm core at low levels causes the initial drop in pressure at the surface and acts to cap the boundary layer (BL). As BL air flows into the nascent eye, the energy content increases until the energy is released from under the cap on the down shear side of the warm core in the form of vigorous cumulonimbi, which become the nascent eyewall.
    [Show full text]
  • Mesoscale Convective Complexes: an Overview by Harold Reynolds a Report Submitted in Conformity with the Requirements for the De
    Mesoscale Convective Complexes: An Overview By Harold Reynolds A report submitted in conformity with the requirements for the degree of Master of Science in the University of Toronto © Harold Reynolds 1990 Table of Contents 1. What is a Mesoscale Convective Complex?........................................................................................ 3 2. Why Study Mesoscale Convective Complexes?..................................................................................3 3. The Internal Structure and Life Cycle of an MCC...............................................................................4 3.1 Introduction.................................................................................................................................... 4 3.2 Genesis........................................................................................................................................... 5 3.3 Growth............................................................................................................................................6 3.4 Maturity and Decay........................................................................................................................ 7 3.5 Heat and Moisture Budgets............................................................................................................ 8 4. Precipitation......................................................................................................................................... 8 5. Mesoscale Warm-Core Vortices.......................................................................................................
    [Show full text]
  • Chapter 3 Mesoscale Processes and Severe Convective Weather
    CHAPTER 3 JOHNSON AND MAPES Chapter 3 Mesoscale Processes and Severe Convective Weather RICHARD H. JOHNSON Department of Atmospheric Science. Colorado State University, Fort Collins, Colorado BRIAN E. MAPES CIRESICDC, University of Colorado, Boulder, Colorado REVIEW PANEL: David B. Parsons (Chair), K. Emanuel, J. M. Fritsch, M. Weisman, D.-L. Zhang 3.1. Introduction tion, mesoscale phenomena occur on horizontal scales between ten and several hundred kilometers. This Severe convective weather events-tornadoes, hail­ range generally encompasses motions for which both storms, high winds, flash floods-are inherently mesoscale ageostrophic advections and Coriolis effects are im­ phenomena. While the large-scale flow establishes envi­ portant (Emanuel 1986). In general, we apply such a ronmental conditions favorable for severe weather, pro­ definition here; however, strict application is difficult cesses on the mesoscale initiate such storms, affect their since so many mesoscale phenomena are "multiscale." evolution, and influence their environment. A rich variety For example, a -100-km-Iong gust front can be less of mesocale processes are involved in severe weather, than -1 km across. The triggering of a storm by the ranging from environmental preconditioning to storm initi­ collision of gust fronts can actually occur on a ation to feedback of convection on the environment. In the -lOO-m scale (the microscale). Nevertheless, we will space available, it is not possible to treat all of these treat this overall process (and others similar to it) as processes in detail. Rather, we will introduce s~veral mesoscale since gust fronts are generally regarded as general classifications of mesoscale processes relatmg to mesoscale phenomena.
    [Show full text]
  • Synoptic Conditions Favorable for the Formation of the 15 July 1995 Southeastern Canada/Northeastern U.S
    SYNOPTIC CONDITIONS FAVORABLE FOR THE FORMATION OF THE 15 JULY 1995 SOUTHEASTERN CANADA/NORTHEASTERN U.S. DERECHO EVENT Mace L. Bentley Climatology Research Laboratory Department of Geography The University of Georgia Athens, Georgia Abstract On 15 July 1995, a derecho-producing mesoscale convective system inflicted considerable damage through southeastern Canada and the northeastern U.S. The synoptic-scale environ­ ment that precluded and persisted during this event is examined \ using swface and upper-air observations, satellite imagery v""--- and numerical model data. Evidence suggests that low-level ~~- - . -,.~ .... moisture inflow and forcing were major factors in initiating \ .-..... ~".... ". and sustaining this progressive warm season derecho event. : "?-.,. ~.:"... Favorable upper-level dynamics produced by jet streak induced Kingston. Ontario circulations were also found over the region. Products from ..------------.:t i the Eta model run initialized 12 hours prior to the event were ". used in the study to fill in between the 0000 UTC and 1200 UTC upper-air sounding times. Manipulation of these data sets was accomplished using GEMPAK 5.2.1. Calculation of 850 hPa moisture transport vectors andfrontogenesis were found to be particularly useful in determining the derecho producing mesoscale convective system's genesis and propagation regions. Future investiga­ tions of these systems should employ these techniques in order to assess their forecast applications. Fig. 1. Approximate track of the DMCS cloud shield on 15 July 1995. 1. Introduction In the early morning hours of 15 July 1995, a derecho­ producing mesoscale convective system (hereafter, DMCS) moved from southern Canada through the northeastern United States (Fig. 1). Widespread wind damage was reported through­ out the Northeast.
    [Show full text]
  • Some Basic Elements of Thunderstorm Forecasting
    (] NOAA TECHNICAL MEMORANDUM NWS CR-69 SOME BASIC ELEMENTS OF THUNDERSTORM FORECASTING Richard P. McNulty National Weather Service Forecast Office Topeka, Kansas May 1983 UNITED STATES I Nalion•l Oceanic and I National Weather DEPARTMENT OF COMMERCE Almospheric Adminislralion Service Malcolm Baldrige. Secretary John V. Byrne, Adm1mstrator Richard E. Hallgren. Director SOME BASIC ELEMENTS OF THUNDERSTORM FORECASTING 1. INTRODUCTION () Convective weather phenomena occupy a very important part of a Weather Service office's forecast and warning responsibilities. Severe thunderstorms require a critical response in the form of watches and warnings. (A severe thunderstorm by National Weather Service definition is one that produces wind gusts of 50 knots or greater, hail of 3/4 inch diameter or larger, and/or tornadoes.) Nevertheless, heavy thunderstorms, those just below severe in­ ·tensity or those producing copious rainfall, also require a certain degree of response in the form of statements and often staffing. (These include thunder­ storms producing hail of any size and/or wind gusts of 35 knots or greater, or storms producing sufficiently intense rainfall to possess a potential for flash flooding.) It must be realized that heavy thunderstorms can have as much or more impact on the public, and require as much action by a forecast office, as do severe thunderstorms. For the purpose of this paper the term "signi­ ficant convection" will refer to a combination of these two thunderstorm classes. · It is the premise of this paper that significant convection, whether heavy or severe, develops from similar atmospheric situations. For effective opera­ tions, forecasters at Weather Service offices should be familiar with those factors which produce significant convection.
    [Show full text]
  • Presentation
    14.4 INVESTIGATION OF MESOSCALE PRESSURE AND TEMPERATURE TRANSIENTS ASSOCIATED WITH BOW ECHOES Rebecca D. Adams∗ and Richard H. Johnson Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 1. INTRODUCTION over the entire state, available over the past 13 years. For the purposes of this study, only the years 2002 Bow echoes have been an established part of the through 2005 were examined, to correspond with literature since a 1978 technical report by Fujita– available radar data. The diurnal cycle was then who not only coined the term, but also created a removed from the pressure observations; they were conceptual model showcasing the lifecycle of these also corrected to a constant height (356.5 m, av- systems. The bow echo’s association with severe erage height of all stations) using virtual tempera- winds has also been both well-studied and well- ture. In addition, a high-pass Lanczos filter (Duchon documented, from Fujita’s original work to the 1979) was run on the data to remove all the longer, present. However, very little work has focused on synoptic timescales. Following the definition pro- surface features, such as pressure and temperature, vided in the American Meteorological Society Glos- associated with these storms. While the arrange- sary (2000), the upper (longer) end of the meso- ment of these parameters as associated with squall timescale is the pendulum day, which for average lines has become highly recognizable (including the Oklahoma latitude is 41.2 hours in length. Thus, mesohigh, cold pool, and wake low; Johnson & the Lanczos filter was run to retain all features with Hamilton 1988, Loehrer & Johnson 1995), it is still a period shorter than this.
    [Show full text]
  • Downloaded 10/01/21 08:47 AM UTC 2062 JOURNAL of the ATMOSPHERIC SCIENCES VOLUME 57 Et Al
    1JULY 2000 BRYAN AND FRITSCH 2061 Diabatically Driven Discrete Propagation of Surface Fronts: A Numerical Analysis GEORGE H. BRYAN AND J. MICHAEL FRITSCH Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania (Manuscript received 2 July 1998, in ®nal form 30 August 1999) ABSTRACT Discrete frontal propagation has been identi®ed as a process whereby a surface front discontinuously moves forward, without evidence of frontal passage across a mesoscale region. Numerical simulations are employed to examine the upper-level evolution of a discrete frontal propagation event and to explore the processes that were responsible for the discrete movement. Model results indicate that a frontal pressure trough was not able to penetrate through a deep surface-based layer of cool air created by a precipitating convective system several hundred kilometers in advance of the front. Meanwhile, a new low-level baroclinic zone formed well ahead of the front along the southern side of the cool layer. As the midlevel front moved continuously over the cool layer, a new low-level front developed in the new baroclinic zone and the original low-level front dissipated. At the surface, the simulated front did not pass through the cool layer. Frontogenesis terms reveal that the prefrontal circulation that becomes the new frontal circulation initially forms directly from diabatic frontogenesis. Daytime heating in the prefrontal boundary layer and cooling from thunderstorms combine to create a thermal gradient and a mesoscale pressure perturbation. Winds turn in response to the altered pressure ®eld and form a convergent boundary, resulting in kinematic frontogenesis. The boundary subsequently undergoes rapid intensi®cation.
    [Show full text]
  • Manual on Low-Level Wind Shear
    Doc 9817 AN/449 Manual on Low-level Wind Shear Approved by the Secretary General and published under his authority First Edition — 2005 International Civil Aviation Organization Suzanne Doc 9817 AN/449 Manual on Low-level Wind Shear Approved by the Secretary General and published under his authority First Edition — 2005 International Civil Aviation Organization AMENDMENTS Amendments are announced in the supplements to the Catalogue of ICAO Publications; the Catalogue and its supplements are available on the ICAO website at www.icao.int. The space below is provided to keep a record of such amendments. RECORD OF AMENDMENTS AND CORRIGENDA AMENDMENTS CORRIGENDA No. Date Entered by No. Date Entered by 1 26/9/08 ICAO 1 17/11/05 ICAO 2 21/2/11 ICAO (iii) FOREWORD Since 1943, low-level wind shear has been cited in a number of aircraft accidents/incidents that together have contributed to over 1 400 fatalities worldwide. Increased awareness within the aviation community of the hazardous and insidious nature of low-level wind shear has been reflected in the fact that the ICAO Council has considered it to be one of the major technical problems facing aviation. Until the 1980s, lack of adequate operational remote-sensing equipment, the complexity of the subject, the wide range of scale of wind shear and its inherent unpredictability all conspired to hinder a complete solution to the problem which, in turn, limited the development of the necessary international Standards and Recommended Practices for the observing, reporting and forecasting of wind shear. In 1975 there were five jet transport aircraft accidents/incidents in which wind shear was cited, one of which resulted in major loss of life.1 The latter accident, which occurred at John F.
    [Show full text]
  • Storm Evolution and Transition Into a Bow Echo
    VOL. 58, NO.13 JOURNAL OF THE ATMOSPHERIC SCIENCES 1JULY 2001 Numerical Simulation of Tornadogenesis in a High-Precipitation Supercell. Part I: Storm Evolution and Transition into a Bow Echo CATHERINE A. FINLEY,* W. R. COTTON, AND R. A. PIELKE SR. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado (Manuscript received 27 August 1998, in ®nal form 16 November 2000) ABSTRACT A nested grid primitive equation model (RAMS version 3b) was used to simulate a high-precipitation (HP) supercell, which produced two weak tornadoes. Six telescoping nested grids allowed atmospheric ¯ows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation. All convection in the simulation was initiated with resolved vertical motion and subsequent condensation±latent heating from the model microphysics; no warm bubbles or cumulus parameterizations were used. Part I of this study focuses on the simulated storm evolution and its transition into a bow echo. The simulation initially produced a classic supercell that developed at the intersection between a stationary front and an out¯ow boundary. As the simulation progressed, additional storms developed and interacted with the main storm to produce a single supercell. This storm had many characteristics of an HP supercell and eventually evolved into a bow echo with a rotating comma-head structure. An analysis of the storm's transition into a bow echo revealed that the interaction between convective cells triggered a series of events that played a crucial role in the transition. The simulated storm structure and evolution differed signi®cantly from that of classic supercells produced by idealized simulations.
    [Show full text]
  • Chapter 15, Part 1 What Are Thunderstorms? Ordinary
    Chapter 15, Part 1 Thunderstorms What are thunderstorms? • Thunderstorms are simply storms containing thunder and lightning. • Sometimes also gusty winds, heavy rain, or hail. • Categorized as – Ordinary – short lived, rarely produce strong winds or hail – Severe – high winds, flash floods, damaging hail, and even tornadoes Ordinary Thunderstorms • There are three stages of an ordinary thunderstorm: cumulus, mature, dissipating. 1 Cumulus Stage • Warm humid air rises, cools, and condenses into cumulus cloud(s) • Sometimes cloud evaporates in dry air, but then a new cloud grows higher. • Condensing water releases latent heat making cloud warmer than air around it. • Insufficient time for precipitation to form. • Updrafts keep water droplets and ice crystals suspended. Mature Thunderstorm • Entrainment – dryer colder air Anvil shape at drawn in. Sometimes rain drops stable region. evaporate further cooling air. • Creates downdraft. • Cell = updraft + downdraft. • Most storms several cells (~1hr). • Storm most intense with heavy rain and sometimes small hail. • Cold air at onset of precipitation. • Top reaching stable region leads to anvil shape. Picture of Mature Thunderstorm • Note anvil top. 2 Overshoot • Updrafts can be so strong than the cloud rises into the stable region of the atmosphere. Dissipating Stage • Updrafts weaken and downdrafts dominate. • Light precipitation, weaker winds. • Down drafts can feed other updrafts, creating multicell storms (common). • At end of storm surface temperature can be much cooler (10-20oF), but rain evaporating increases moisture. Dissipating Thunderstorm • Most of the lower half of cloud has evaporated leaving only cirrus anvil top. 3 A Multicell Storm • Middle cell is mature stage, right cell in cumulus stage, and left cell is almost mature.
    [Show full text]
  • Information to Users
    INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitled. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6* x 9* black arxf white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. Bell & Howell Information and Learning 300 North Zfteb Road, Ann Arbor, Ml 48106-1346 USA UMI 800-521-0600 UNIVERSITY OF OKLAHOMA GRADUATE COLLEGE A PSEUDO-DUAL-DOPPLER ANALYSIS OF CYCLIC TORNADOGENESIS A Dissertation SUBMITTED TO THE GRADUATE FACULTY in partial fulfillment of the requirements for the degree of Doctor of Philosophy By DAVID C. DOWELL Norman, Oklahoma 2000 UMI Number 9956999 UMI UMI Microform9956999 Copyright 2000 by Bell & Howell Information and Learning Company.
    [Show full text]
  • The Impact of Hail Size on Simulated Supercell Storms
    1596 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 61 The Impact of Hail Size on Simulated Supercell Storms SUSAN C. VAN DEN HEEVER AND WILLIAM R. COTTON Colorado State University, Fort Collins, Colorado (Manuscript received 14 November 2002, in ®nal form 15 January 2004) ABSTRACT Variations in storm microstructure due to updraft strength, liquid water content, and the presence of dry layers, wind shear, and cloud nucleating aerosol concentrations are likely to lead to changes in hail sizes within deep convective storms. The focus of this paper is to determine how the overall dynamics and microphysical structure of deep convective storms are affected if hail sizes are somehow altered in a storm environment that is otherwise the same. The sensitivity of simulated supercell storms to hail size distributions is investigated by systematically varying the mean hail diameter from 3 mm to 1 cm using the Regional Atmospheric Modeling System (RAMS) model. Increasing the mean hail diameter results in a hail size distribution in which the number concentration of smaller hailstones is decreased, while that of the larger hailstones is increased. This shift in the hail size distribution as a result of increasing the mean hail diameter leads to an increase in the mean terminal fall speed of the hail species and to reduced melting and evaporation rates. The sensitivity simulations demonstrate that the low-level downdrafts are stronger, the cold pools are deeper and more intense, the left-moving updraft is shorter-lived, the right-moving storm is stronger but not as steady, and the low-level vertical vorticity is greater in the cases with smaller hail stones.
    [Show full text]