DOCSLIB.ORG

"Super Derecho"

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
3B.2. Environment and early evolution of the 8 May 2009 “Super Derecho” Stephen F. Corfidi*1, Michael C. Coniglio 2, and John S. Kain 2 1 NOAA/NWS/NCEP Storm Prediction Center, Norman, Oklahoma 2 NOAA/OAR National Severe Storms Laboratory, Norman, Oklahoma 1. INTRODUCTION environment over the central plains on 8 May 2009. In particular, analyses from this event are compared This study documents the complex to those from other MCSs that have occurred over environment and early evolution of the remarkable much the same region and time of year. Although the derecho-producing convective system that crossed genesis and evolution of the MCV was a significant part of the central United States on 8 May 2009 (Fig. part of this event, the focus here is on the 1a). The derecho (Johns and Hirt 1987) severely environment and early evolution of the convective damaged buildings, utility lines, and trees over a system prior to the development of the MCV. We widespread area from western Kansas to eastern emphasize the system’s early evolution since it is Kentucky as a result of multiple wind gusts > 35 m s-1 likely that the ability to accurately predict convective and isolated gusts > 45 m s-1 (Fig. 1b). The systems of this type is strongly related to a detailed associated mesoscale convective system (MCS; understanding of the processes and environmental Zipser 1982) contained bow echoes (Fujita 1978) ingredients that can create such a system. during part of its lifetime, and an intense, long-lived mesoscale convective vortex (MCV; Davis and Trier 2007) developed during the latter stage of the MCS 2. DATA AND METHODOLOGY that was associated with very severe surface winds and tornadoes (Fig. 1). In addition, multiple Our analysis of the 8 May 2009 convective tornadoes and localized swaths of intense wind system makes use of radar data from the National damage occurred in association with strong meso-g Weather Service Weather Surveillance Radar-1988 scale (Orlanski 1975) vortices (Atkins and St. Laurent Doppler (WSR-88D) network. This analysis includes 2009) along the convective line, as found for other level II data from individual WSR-88D sites obtained extreme damaging-wind MCSs (Miller and Johns from the National Climatic Data Center, base 2000; Wheatley et al. 2006). reflectivity composites generated by Unisys, and composite (column-maximum) reflectivity derived from The 8 May 2009 MCS developed in a way the National Mosaic and Multi-sensor Quantitative that is common to similar systems that affect the precipitation estimation (NMQ) project (Vasiloff et al. central United States during spring. Convective 2007). In addition, this study uses surface and upper- initiation occurred along the eastern slopes of the air observations from a variety of platforms that were Rocky Mountains, and storms subsequently moved quality controlled by the NOAA Meteorological east to consolidate in a region of lower-tropospheric Assimilation Data Ingest System (MADIS) (see Fig. 2 warm advection, convergence, and conditional for site locations of the radiosonde, wind profiler, and instability in the exit region of a nocturnal low-level jet WSR-88D data used in this study). These data are (LLJ) (Blackadar 1957; Bonner 1968; McNider and used to document the complex development and Pielke 1981; Cotton et al. 1989; Laing and Fritsch early evolution of the convective event. 2000; Tuttle and Davis 2006). The development of a large, intense MCS resulted from a complex series of The MCS environment is further examined processes and mergers of several convective lines using the hourly RUC-model analyses provided on a and clusters over a relatively short time period, which 20-km grid and on constant pressure surfaces spaced is common for warm season MCSs (McAnelly et al. 25 hPa apart. To address the question of what was 1997; Jirak and Cotton 2003). unusual about the environment, the RUC analysis of the 8 May 2009 event is compared to those from 28 Routine surface and upper air observations MCSs obtained from the dataset described in Coniglio and analyses from the Rapid Update Cycle (RUC; et al. (2010) [referred to as C2010 hereafter]. This Benjamin et al. 2004) are used in this study to comparison dataset includes MCSs that occurred determine what, if anything, was unusual about the from early May to early June in the central U.S. and __________________________________________ that had a nearly contiguous reflectivity region ≥ 35 dBZ at least 100 km in length and embedded echoes * Corresponding author address: Stephen F. Corfidi, ≥ 50 dBZ for at least five continuous hours. Although NOAA/NWS/NCEP/Storm Prediction Center, 120 D. not all of the comparison MCSs produced derechos or L. Boren Blvd., Norman, OK 73072; email: [email protected] Fig. 2. Locations and names of the 915 MHz wind profiler sites (★ ),WSR-88D sites (▲), and radiosonde sites (●) used in this study. Figure 1. (a) Hourly composite reflectivity images from the NMQ project plotted at 1-hr intervals (UTC) and (b) severe weather reports associated with the derecho-producing MCS (hail ≥ 0.75 inches in open 3. ANALYSIS green circles, hail ≥ 2.0 inches in filled green circles, wind damage or wind gusts ≥ 26 m s-1, or 50 kt, in a. Environmental conditions preceding open blue circles, wind gusts measured or estimated development of deep convection ≥ 33.5 m s-1, or 65 kt, in filled blue circles, and tornado reports in red), from 0300 UTC to 2300 UTC 8 May Relatively dry conditions and few clouds 2009. covered much of the central High Plains in the early afternoon of 7 May (Fig. 3a) beneath moderately strong westerly mid to upper level flow (Figs. 4 and 5). A band of cumuliform clouds developed over far severe weather, all of them eventually transitioned northeast Colorado (Fig. 3a) near the western end of into a leading line/trailing stratiform structure (Houze a surface trough that extended from a weak low over et al. 1989; Parker and Johnson 2000) that resembled far eastern Nebraska. Cloud bases were relatively the mature structure of the 8 May 2009 MCS. high, atop a well-mixed, relatively dry boundary layer (Fig. 4). Nearby WSR-88D radars detected light In the C2010 study, changes in the precipitation (mostly < 35 dBZ) with this initial environment relative to the location and movement of cloudiness, much of which did not appear to reach the the MCSs are examined by producing composite ground. The low CAPE likely limited the strength of RUC analyses at several stages in the MCS lifecycle, the convection initially. including the pre-deep-convection environment (termed the “first storms” stage in C2010) and the Vertical motion and potential temperature environment ahead of newly developed MCSs fields in vertical cross sections (not shown) suggest (termed the “genesis” stage in C2010). The that mountain waves partially were responsible for the environments at the time of the first storms' and initial clouds and precipitation. Upstream conditions genesis stages of the 8 May 2009 MCS are compared in which strong mountain waves are likely to develop to the composite environments of the first storms’ and are evident in the observed soundings at Grand genesis stages for the 28 other MCSs in the C2010 Junction and Denver, Colorado (not shown), and in data set using standardized anomalies (as), where as the nearby RUC sounding (Fig. 4). These conditions = (xa - x)σs is calculated at each grid point, xa is a consist of (i) strong wind at mountain top level, grid point value of a variable x in the RUC analysis for increasing with height and oriented perpendicular to the 8 May 2009 event, x is the mean of x from the the mountain range throughout a deep layer, and (ii) a C2010 MCS data set, and σs is the sample standard relatively stable layer near mountain top height with deviation of x from the C2010 MCS data set. The first weaker stability at higher levels (Durran 1986). storms’ and genesis stages of the 8 May 2009 MCS Furthermore, a local maximum in low-level westerly are defined to be at 0300 UTC and 0700 UTC, flow over the terrain in far southern Wyoming appears respectively. Examination of the anomalies in this to enhance the convergence and upward motion manner allows an investigation of what was unusual locally over far southeastern Wyoming (not shown), about the environment of the 8 May 2009 derecho where the shallow convection first appears. compared to other MCS environments that occurred in a similar place and time of year. The northeastern Colorado/southeastern Wyoming/Nebraska border region also was under the right entrance region of a mid-level jet streak (Fig. 5), (a) 2215 UTC 7 May (d) 50° F 60° F 70° F (b) (e) H (c) H (f) Figure 3. Manual surface analyses, valid at the indicated times (UTC), for 7 - 8 May 2009. Synoptic scale fronts in solid blue/red; troughs and axes of low level warm advection dashed black and red, respectively; outflow boundaries dotted blue. Contours of the 50, 60, and 70°F isodrosotherms included in (a); contours of sea-level pressure (grey solid lines; every 2 hPa) in (b)-(f). Surface data plots conventional, with full wind barb = 5 m s-1 in (b) - (f). NMQ composite reflectivity mosaics are included in (b) - (f), and the 2215 UTC visible data satellite image in (a). which often is collocated with MCS development (Maddox 1983; Johns 1993; Coniglio et al. 2004). The flow is significantly ageostrophic in this region (Fig. 5), which may be the result of jet-streak dynamics (Uccellini and Johnson 1979) and lee-side flow deceleration associated with terrain-induced waves (Durran 1986). Furthermore, upslope flow and an associated “Denver cyclone” (Szoke et al.
Load more

Recommended publications
  • Storm Environment
    August 4th, 2008 Chicago Derecho Tim Halbach – NWS Chicago Ron Przybylinski – NWS St. Louis Advanced Spotter Training-March 14, 2009 Event Overview y Progressive Derecho developed in eastern Iowa/northwest Illinois and caused widespread damage and power outages across northern Illinois and Indiana y Dissipated over West Virginia y Worst damage occurred north of the apex of the bow where multiple mesovorticies developed y Well developed convective system featuring a long lived Book-End Vortex and Rear Inflow Jet y Strong winds persisted well after the leading edge of the derecho • very low RIJ? y 6 tornadoes were confirmed via storm surveys y Griffith, IN: EF2 y Bloomingdale, IL: EF1 (2 Tornadoes) y Bolingbrook, IL: EF1 y Orland Park, IL: EF0 y Boswell, IN: EF0 Storm Environment 250 MB 3pm CDT Surface Map Forecast NAM Bufr Soundings- 21z at ORD Severe Thunderstorm Watches Nested Storm Based Warning Sectorization y Used a nested storm based warning sectorization for this event y 2 Meteorologists issuing severe warnings y 1 Meteorologist issuing tornado warnings Radar Analysis of the 5 Main Rotations The 5 Rotations Mesovort #1 y Developed along the Ogle/Carroll County line y Strengthened near Mount Morris • straight-line wind damage w/no tornado y Grain bin blown down, phone tower knocked over, building leveled. y Rotation lasted for about 1 hour and traveled about 60 miles from Carroll County into northern De Kalb county The 5 Rotations Mesovort #1 The 5 Rotations Mesovort #1 The 5 Rotations Mesovort #1 y Strongest rotation near Mount Morris The 5 Rotations Mesovort #1 y 108.8 knots sampled at 5130ft Above Ground Level The 5 Rotations Mesovort #1 -Damage Pictures from Ogle County The 5 Rotations Mesovort #2 y Developed further to the south across north central De Kalb county.
    [Show full text]
  • An Observational Analysis Quantifying the Distance of Supercell-Boundary Interactions in the Great Plains
    Magee, K. M., and C. E. Davenport, 2020: An observational analysis quantifying the distance of supercell-boundary interactions in the Great Plains. J. Operational Meteor., 8 (2), 15-38, doi: https://doi.org/10.15191/nwajom.2020.0802 An Observational Analysis Quantifying the Distance of Supercell-Boundary Interactions in the Great Plains KATHLEEN M. MAGEE National Weather Service Huntsville, Huntsville, AL CASEY E. DAVENPORT University of North Carolina at Charlotte, Charlotte, NC (Manuscript received 11 June 2019; review completed 7 October 2019) ABSTRACT Several case studies and numerical simulations have hypothesized that baroclinic boundaries provide enhanced horizontal and vertical vorticity, wind shear, helicity, and moisture that induce stronger updrafts, higher reflectivity, and stronger low-level rotation in supercells. However, the distance at which a surface boundary will provide such enhancement is less well-defined. Previous studies have identified enhancement at distances ranging from 10 km to 200 km, and only focused on tornado production and intensity, rather than all forms of severe weather. To better aid short-term forecasts, the observed distances at which supercells produce severe weather in proximity to a boundary needs to be assessed. In this study, the distance between a large number of observed supercells and nearby surface boundaries (including warm fronts, stationary fronts, and outflow boundaries) is measured throughout the lifetime of each storm; the distance at which associated reports of large hail and tornadoes occur is also collected. Statistical analyses assess the sensitivity of report distributions to report type, boundary type, boundary strength, angle of interaction, and direction of storm motion relative to the boundary.
    [Show full text]
  • Mechanisms for Organization and Echo Training in a Flash-Flood-Producing Mesoscale Convective System
    1058 MONTHLY WEATHER REVIEW VOLUME 143 Mechanisms for Organization and Echo Training in a Flash-Flood-Producing Mesoscale Convective System JOHN M. PETERS AND RUSS S. SCHUMACHER Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado (Manuscript received 19 February 2014, in final form 18 December 2014) ABSTRACT In this research, a numerical simulation of an observed training line/adjoining stratiform (TL/AS)-type mesoscale convective system (MCS) was used to investigate processes leading to upwind propagation of convection and quasi-stationary behavior. The studied event produced damaging flash flooding near Dubuque, Iowa, on the morning of 28 July 2011. The simulated convective system well emulated characteristics of the observed system and produced comparable rainfall totals. In the simulation, there were two cold pool–driven convective surges that exited the region where heavy rainfall was produced. Low-level unstable flow, which was initially convectively in- hibited, overrode the surface cold pool subsequent to these convective surges, was gradually lifted to the point of saturation, and reignited deep convection. Mechanisms for upstream lifting included persistent large-scale warm air advection, displacement of parcels over the surface cold pool, and an upstream mesolow that formed between 0500 and 1000 UTC. Convection tended to propagate with the movement of the southeast portion of the outflow boundary, but did not propagate with the southwest outflow boundary. This was explained by the vertical wind shear profile over the depth of the cold pool being favorable (unfavorable) for initiation of new convection along the southeast (southwest) cold pool flank. A combination of a southward-oriented pressure gradient force in the cold pool and upward transport of opposing southerly flow away from the boundary layer moved the outflow boundary southward.
    [Show full text]
  • ESSENTIALS of METEOROLOGY (7Th Ed.) GLOSSARY
    ESSENTIALS OF METEOROLOGY (7th ed.) GLOSSARY Chapter 1 Aerosols Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropogenic) sources, such as the burning of fossil fuels. Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols. Air density The ratio of the mass of a substance to the volume occupied by it. Air density is usually expressed as g/cm3 or kg/m3. Also See Density. Air pressure The pressure exerted by the mass of air above a given point, usually expressed in millibars (mb), inches of (atmospheric mercury (Hg) or in hectopascals (hPa). pressure) Atmosphere The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen and oxygen. Carbon dioxide (CO2) A colorless, odorless gas whose concentration is about 0.039 percent (390 ppm) in a volume of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is important in the earth's atmospheric greenhouse effect. Solid CO2 is called dry ice. Climate The accumulation of daily and seasonal weather events over a long period of time. Front The transition zone between two distinct air masses. Hurricane A tropical cyclone having winds in excess of 64 knots (74 mi/hr). Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist. Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.) Mesosphere The atmospheric layer between the stratosphere and the thermosphere.
    [Show full text]
  • The Initiation and Evolution of a Mini-Bow Echo Occurring Behind a Prefrontal Squall Line
    THE INITIATION AND EVOLUTION OF A MINI-BOW ECHO OCCURRING BEHIND A PREFRONTAL SQUALL LINE Alan E. Gerard NOAA/National Weather Service Forecast Office Jackson, Mississippi Corey M. Mead NOAA/National Weather Service Forecast Office OmahalValley, Nebraska Abstract tern, as well as shear zones and circulations along the leading edge of the convection, were observed by the Early on 13 April 1996, a mini-bow echo moved Jackson, Mississippi (KJAN) WSR-88D. through parts of south central Mississippi. This bow echo produced one report of severe weather, namely $10K in 2. Synoptic Environment wind damage near the town of Bassfield in Jefferson Davis county. The evolution of this bow echo was some­ During the early evening hours of 12 April 1996, syn­ what unusual, in that the storm developed within the optic features and parameters came together for an active southern part of the enhanced stratiform rain region of a severe weather event over the western Gulf Coast region well developed prefrontal squall line. This paper will (Fig. 2). At 0000 UTC 13 April 1996, an upper-level short­ examine the development of this small bow echo, begin­ wave trough had become negatively-tilted over north ning with the synoptic and mesoscale environments. This Texas as a 30 m s'! 500 mb wind maximum lifted out ofthe will be followed by the storm evolution as observed by the base of the trough and into the mid-Mississippi valley WSR-88D Doppler radar in Jackson, Mississippi. region. A 300 mb polar jet streak in excess of 50 m s'! was located downstream from the trough axis from eastern 1.
    [Show full text]
  • A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary
    University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Dissertations & Theses in Earth and Earth and Atmospheric Sciences, Department Atmospheric Sciences of 5-2010 A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary Jennifer M. Laflin University of Nebraska at Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/geoscidiss Part of the Earth Sciences Commons Laflin, Jennifer M., "A Targeted Modeling Study of the Interaction Between a Supercell and a Preexisting Airmass Boundary" (2010). Dissertations & Theses in Earth and Atmospheric Sciences. 6. https://digitalcommons.unl.edu/geoscidiss/6 This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Dissertations & Theses in Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. A TARGETED MODELING STUDY OF THE INTERACTION BETWEEN A SUPERCELL AND A PREEXISTING AIRMASS BOUNDARY by Jennifer M. Laflin A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree Master of Science Major: Geosciences Under the Supervision of Professor Adam L. Houston Lincoln, Nebraska May, 2010 A TARGETED MODELING STUDY OF THE INTERACTION BETWEEN A SUPERCELL AND A PREEXISTING AIRMASS BOUNDARY Jennifer Meghan Laflin, M. S. University of Nebraska, 2010 Adviser: Adam L. Houston It is theorized that supercell thunderstorms account for the majority of significantly severe convective weather which occurs in the United States, and as a result, it is necessary that the mechanisms which tend to produce supercells are recognized and investigated.
    [Show full text]
  • PRELUDE at 4:30 A.M. on June 29, I Began My Day with a Quick Look at the Weather Charts and Prepared to Make a Forecast. It
    PRELUDE At 4:30 a.m. on June 29, I began my day with a quick look at the weather charts and prepared to make a forecast. It was to be yet another hot day across the region; you did not need a meteorologist to tell you that it would be hot because, you could feel already it before the sun ever rose. The morning air was thick, heavy and the winds offered no relief, they merely stirred the soupy air in the kettle of the Great Lakes. I knew that thunderstorms were expected that day, I had been eyeing a system on the charts for the past three days, and it was to arrive during the late afternoon across the region I forecasted for (Cleveland, Ohio; Erie, Pennsylvania; Fort Wayne, Indiana; and Toledo, Ohio). I remember vividly making the comment “You’re going to need a big [weather] event to push this heat back” on my website’s morning forecast update. Little was I prepared that the event would be a derecho – an event that squall-line lovers, like myself, adore. Lucille was returning from Chicago, Illinois on her way back to Lima, Ohio, so I had to watch the weather of northern Indiana since I was expecting thunderstorms. I was located in Bowling Green, Ohio – a town which is no stranger to straight line winds from the squall lines that inhabit our world in the summer time. To our south was the city of Findlay, Ohio, where Kelly and her church were located near the downtown region, a city where their biggest weather event was the semi-decadal floods that occur along the banks of the Blanchard River.
    [Show full text]
  • Glossary of Severe Weather Terms
    Glossary of Severe Weather Terms -A- Anvil The flat, spreading top of a cloud, often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind. Anvil Dome A large overshooting top or penetrating top. -B- Back-building Thunderstorm A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction. Back-sheared Anvil [Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential. Beaver ('s) Tail [Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud).
    [Show full text]
  • Midlevel Ventilation's Constraint on Tropical Cyclone Intensity Brian
    Midlevel Ventilation’s Constraint on Tropical Cyclone Intensity by Brian Hong-An Tang B.S. Atmospheric Science, University of California Los Angeles, 2004 B.S. Applied Mathematics, University of California Los Angeles, 2004 Submitted to the Department of Earth, Atmospheric and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Atmospheric Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2010 c Massachusetts Institute of Technology 2010. All rights reserved. Author.............................................. ................ Department of Earth, Atmospheric and Planetary Sciences June 14, 2010 Certified by.......................................... ................ Kerry A. Emanuel Breene M. Kerr Professor of Atmospheric Science Director, Program in Atmospheres, Oceans and Climate Thesis Supervisor Accepted by.......................................... ............... Maria Zuber Earle Griswold Professor of Geophysics and Planetary Science Head, Department of Earth, Atmospheric and Planetary Sciences 2 Midlevel Ventilation’s Constraint on Tropical Cyclone Intensity by Brian Hong-An Tang Submitted to the Department of Earth, Atmospheric and Planetary Sciences on June 14, 2010, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Atmospheric Science Abstract Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a TC’s intensity. An idealized framework is developed to assess how ventilation affects TC intensity via two pathways: downdrafts outside the eyewall and eddy fluxes directly into the eyewall. Three key aspects are found: ventilation has a detrimental effect on TC intensity by decreasing the maximum steady state intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper ventilation bound beyond which no steady TC can exist.
    [Show full text]
  • Downloaded 09/29/21 04:52 AM UTC JANUARY 2015 N O W O T a R S K I E T a L
    272 MONTHLY WEATHER REVIEW VOLUME 143 Supercell Low-Level Mesocyclones in Simulations with a Sheared Convective Boundary Layer CHRISTOPHER J. NOWOTARSKI,* PAUL M. MARKOWSKI, AND YVETTE P. RICHARDSON Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania GEORGE H. BRYAN 1 National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 2 May 2014, in final form 5 September 2014) ABSTRACT Simulations of supercell thunderstorms in a sheared convective boundary layer (CBL), characterized by quasi-two-dimensional rolls, are compared with simulations having horizontally homogeneous environments. The effects of boundary layer convection on the general characteristics and the low-level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion, as well as with and without the effects of cloud shading. Bulk measures of storm strength are not greatly affected by the presence of rolls in the near-storm en- vironment. Though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed, simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low-level mesocyclones. Initially, CBL vertical vorticity perturba- tions are enhanced along the supercell outflow boundary, resulting in nonnegligible near-ground vertical vorticity regardless of roll orientation. At later times, supercells that move perpendicular to the axes of rolls in their environment have low-level mesocyclones with weaker, less persistent circulation compared to those in a similar horizontally homogeneous environment. For storms moving parallel to rolls, the opposite result is found: that is, low-level mesocyclone circulation is often enhanced relative to that in the corre- sponding horizontally homogeneous environment.
    [Show full text]
  • South Florida Seabreeze/Outflow Boundary Tornadoes
    South Florida Seabreeze/Outflow Boundary Tornadoes Russell L. Pfost, Pablo Santos, Jr., and Thomas E. Warner National Weather Service/Weather Forecast Office Miami, Florida Wednesday, August 3 2005 revised form Friday, September 9 2005 ABSTRACT In the late afternoon and evening of 7 August 2003 two tornadoes produced significant damage across parts of metropolitan Palm Beach County, Florida. These tornadoes were produced as a strengthening updraft encountered cyclonic shear along an enhanced east/west sea breeze convergence line meeting a southward moving outflow boundary from the north. The second tornado in particular produced substantial damage to a trailer park and industrial areas in both Palm Beach Gardens and Riviera Beach and crossed a major interstate highway (Interstate 95). Detection and warning of the tornadoes was a challenge for National Weather Service (NWS) forecasters at the Weather Forecast Office (WFO) in Miami due to the distance from both the Miami (KAMX) Weather Service Doppler radar (WSR-88D) and the Melbourne (KMLB) WSR-88D resulting in beam elevation and sampling issues. 1. Introduction Florida ranks as the top state in number of tornadoes per 10,000 square miles from 1953 to 2003 (DOC, 2003). South Florida's peak months for tornadoes are June and August (Gregoria, 2005), reflecting both the convective "rainy" season from late May through mid October as well as a tropical cyclone influence which peaks around mid August through the end of September each year. Because convection in South Florida during rainy season is almost always related in some way to sea and/or lake breeze convergence, the development of tornadoes is also greatly affected by the boundaries created as the sea and lake breezes begin their diurnal migration.
    [Show full text]
  • Conventional Wisdom
    COVERSTORY n October 2006, following a series of a mesoscale convective system near the The study of convection deals with fatal crashes, the U.S. National Trans- equator. More recently, the fatal crash of vertical motions in the atmosphere portation Safety Board (NTSB) issued a medical helicopter in March 2010 in caused by temperature or, more pre- a safety alert describing procedures Brownsville, Tennessee, U.S., was related cisely, density differences. The adage Ipilots should follow when dealing with to a “mesoscale convective system with a “warm air rises” is well known. In “thunderstorm encounters.” Despite bow shape.”1 meteorological parlance, a parcel of air these instructions, incidents continued to To improve the warning capabilities will rise if it is less dense than air in the occur. One concern is that terminology of the various weather services, convec- surrounding environment. Warmer air often used by meteorologists is unfamil- tion has been studied extensively in is less dense and will rise. Conversely, iar to some in the aviation community. recent years, leading to many new dis- colder air, being denser, will sink. As For example, the fatal crash of the Hawk- coveries. Although breakthroughs in the pilots, especially glider pilots, know, you er 800A at Owatonna, Minnesota, U.S., science have increased our understanding don’t need moisture — that is, clouds — in June 2008 (ASW, 4/11, p. 16) involved and improved convection forecasts, the to have rising and sinking currents of a “mesoscale convective complex.” The problem of conveying the information to air. However, when air rises it expands crash of Air France Flight 447 in June those who need it remains, complicated and cools.
    [Show full text]