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D. L. Porter
and
D. R. Thompson

Abstract

This paper presents preliminary results on the use of synthetic aperture radar (SAR) imagery to extract oceanographic information about the continental shelf. From late spring through early fall the thermocline on the shelf is fully developed and the entire Mid-Atlantic Bight is replete with internal waves. The surface manifestation of these internal waves is observed using the SAR on board the European Research Satellite-1. The depth of the pycnocline and the density of the surface layer can be estimated from the dispersion relation obtained from a two-layer fluid model. The other parameters that are required to make this estimate are obtained by assuming climatological bottom density, estimating the speed of propagation from the tidal-generated wave packets, and calculating the depth of the pycnocline based on the isobath where the internal waves are dissipated. This robust method for obtaining quantitative information about the structure of the shelf’s water column shows good agreement with the in situ observations from that area.

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R. L. Thompson
,
J. M. Lewis
, and
R. A. Maddox

Abstract

The return of tropical air from the Gulf of Mexico is examined in the autumnal cool season. Results from the thermodynamic equilibrium model of Betts and Ridgway are used to calculate the equilibrium equivalent potential temperature (θ e ) over the gulf and the northwestern Caribbean Sea. With a climatological study as a backdrop, a case of severe weather outbreak in mid-November 1988 is analyzed with emphasis on the analysis of low-level θ e that flowed into the storm region from the Gulf of Mexico.

The primary results of the study are the following:

  1. The climatological distribution of equilibrium θ e over the gulf and the Caribbean in November serves as a useful tool for the analysis of the 1988 case study.

  2. Between 5 and 15 November 1988, equilibrium in the marine layer was established over the gulf due to the absence of any deep cold-air penetrations during this period.

  3. The high-valued θ e that streamed into the severe storm region on 15 November 1988 tracked from the Yucatán straits and the northwestern Caribbean over a three-day period.

  4. This air was able to maintain its high-θ e property because of an anomalously warm gulf.

  5. Significant increases in available energy for deep convection could have been anticipated by means of the upper bounds on coastal θ e predicted by the Betts and Ridgway formulation, which was supported by observations along the Texas coast.

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C. A. Doswell III
,
R. Edwards
,
R. L. Thompson
,
J. A. Hart
, and
K. C. Crosbie

Abstract

The notion of an “outbreak” of severe weather has been used for decades, but has never been formally defined. There are many different criteria by which outbreaks can be defined based on severe weather occurrence data, and there is not likely to be any compelling logic to choose any single criterion as ideal for all purposes. Therefore, a method has been developed that uses multiple variables and allows for considerable flexibility. The technique can be adapted easily to any project that needs to establish a ranking of weather events. The intended use involves isolating the most important tornado outbreak days, as well as important outbreak days of primarily nontornadic severe convective weather, during a period when the number of reports has been growing rapidly from nonmeteorological factors. The method is illustrated for both tornadic and primarily nontornadic severe weather event day cases. The impact of the secular trends in the data has been reduced by a simple detrending scheme. The effect of detrending is less important for the tornado outbreak cases and is illustrated by comparing rankings with and without detrending. It is shown that the resulting rankings are relatively resistant to secular trends in the data, as intended, and not strongly sensitive to the choices made in applying the method. The rankings are also consistent with subjective judgments of the relative importance of historical tornado outbreak cases.

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Evan S. Bentley
,
Richard L. Thompson
,
Barry R. Bowers
,
Justin G. Gibbs
, and
Steven E. Nelson

Abstract

Previous work has considered tornado occurrence with respect to radar data, both WSR-88D and mobile research radars, and a few studies have examined techniques to potentially improve tornado warning performance. To date, though, there has been little work focusing on systematic, large-sample evaluation of National Weather Service (NWS) tornado warnings with respect to radar-observable quantities and the near-storm environment. In this work, three full years (2016–18) of NWS tornado warnings across the contiguous United States were examined, in conjunction with supporting data in the few minutes preceding warning issuance, or tornado formation in the case of missed events. The investigation herein examines WSR-88D and Storm Prediction Center (SPC) mesoanalysis data associated with these tornado warnings with comparisons made to the current Warning Decision Training Division (WDTD) guidance. Combining low-level rotational velocity and the significant tornado parameter (STP), as used in prior work, shows promise as a means to estimate tornado warning performance, as well as relative changes in performance as criteria thresholds vary. For example, low-level rotational velocity peaking in excess of 30 kt (15 m s−1), in a near-storm environment, which is not prohibitive for tornadoes (STP > 0), results in an increased probability of detection and reduced false alarms compared to observed NWS tornado warning metrics. Tornado warning false alarms can also be reduced through limiting warnings with weak (<30 kt), broad (>1 n mi; 1 n mi = 1.852 km) circulations in a poor (STP = 0) environment, careful elimination of velocity data artifacts like sidelobe contamination, and through greater scrutiny of human-based tornado reports in otherwise questionable scenarios.

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Richard L. Thompson
,
Bryan T. Smith
,
Jeremy S. Grams
,
Andrew R. Dean
, and
Chris Broyles

Abstract

A sample of 22 901 tornado and significant severe thunderstorm events, filtered on an hourly 40-km grid, was collected for the period 2003–11 across the contiguous United States (CONUS). Convective mode was assigned to each case via manual examination of full volumetric radar data (Part I of this study), and environmental information accompanied each grid-hour event from the hourly objective analyses calculated and archived at the Storm Prediction Center (SPC). Sounding-derived parameters related to supercells and tornadoes formed the basis of this investigation owing to the dominance of right-moving supercells in tornado production and the availability of supercell-related convective parameters in the SPC environmental archive. The tornado and significant severe thunderstorm events were stratified by convective mode and season. Measures of buoyancy discriminated most strongly between supercell and quasi-linear convective system (QLCS) tornado events during the winter, while bulk wind differences and storm-relative helicity were similar for both supercell and QLCS tornado environments within in each season. The larger values of the effective-layer supercell composite parameter (SCP) and the effective-layer significant tornado parameter (STP) favored right-moving supercells that produced significant tornadoes, as opposed to weak tornadoes or supercells that produced only significant hail or damaging winds. Additionally, mesocyclone strength tended to increase with increasing SCP for supercells, and STP tended to increase as tornado damage class ratings increased. The findings underscore the importance of convective mode (discrete or cluster supercells), mesocyclone strength, and near-storm environment (as represented by large values of STP) in consistent, real-time identification of intense tornadoes.

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Alexandra K. Anderson-Frey
,
Yvette P. Richardson
,
Andrew R. Dean
,
Richard L. Thompson
, and
Bryan T. Smith
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Alexandra K. Anderson-Frey
,
Yvette P. Richardson
,
Andrew R. Dean
,
Richard L. Thompson
, and
Bryan T. Smith

Abstract

In this study, a 13-yr climatology of tornado event and warning environments, including metrics of tornado intensity and storm morphology, is investigated with particular focus on the environments of tornadoes associated with quasi-linear convective systems and right-moving supercells. The regions of the environmental parameter space having poor warning performance in various geographical locations, as well as during different times of the day and year, are highlighted. Kernel density estimations of the tornado report and warning environments are produced for two parameter spaces: mixed-layer convective available potential energy (MLCAPE) versus 0–6-km vector shear magnitude (SHR6), and mixed-layer lifting condensation level (MLLCL) versus 0–1-km storm-relative helicity (SRH1). The warning performance is best in environments characteristic of severe convection (i.e., environments featuring large values of MLCAPE and SHR6). For tornadoes occurring during the early evening transition period, MLCAPE is maximized, MLLCL heights decrease, SHR6 and SRH1 increase, tornadoes rated as 2 or greater on the enhanced Fujita scale (EF2+) are most common, the probability of detection is relatively high, and false alarm ratios are relatively low. Overall, the parameter-space distributions of warnings and events are similar; at least in a broad sense, there is no systematic problem with forecasting that explains the high overall false alarm ratio, which instead seems to stem from the inability to know which storms in a given environment will be tornadic.

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Bryan T. Smith
,
Richard L. Thompson
,
Andrew R. Dean
, and
Patrick T. Marsh

Abstract

Radar-identified convective modes, peak low-level rotational velocities, and near-storm environmental data were assigned to a sample of tornadoes reported in the contiguous United States during 2009–13. The tornado segment data were filtered by the maximum enhanced Fujita (EF)-scale tornado event per hour using a 40-km horizontal grid. Convective mode was assigned to each tornado event by examining full volumetric Weather Surveillance Radar-1988 Doppler data at the beginning time of each event, and 0.5° peak rotational velocity (V rot) data were identified manually during the life span of each tornado event. Environmental information accompanied each grid-hour event, consisting primarily of supercell-related convective parameters from the hourly objective mesoscale analyses calculated and archived at the Storm Prediction Center. Results from examining environmental and radar attributes, featuring the significant tornado parameter (STP) and 0.5° peak V rot data, suggest an increasing conditional probability for greater EF-scale damage as both STP and 0.5° peak V rot increase, especially with supercells. Possible applications of these findings include using the conditional probability of tornado intensity as a real-time situational awareness tool.

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Joe L. Sutherland
,
John R. Thompson
,
Don A. Griffith
, and
Bruce Kunkel

Abstract

In Michigan in early 1977, an experiment was conducted to test the ability of silver iodide (AgI) ice nucleus curtains generated by vertical-fall pyrotechnics to produce clearings in supercooled stratus. A second objective of the experiment was to determine how well a clearing could be targeted over a preselected ground location. Previous stratus clearing tests had primarily involved curtains of dry ice particles or horizontal lines of AgI nuclei. Silver iodide pyrotechnics were chosen because of their logistical advantages over dry ice.

Results of the Michigan testing were favorable. Clearings were produced in cloud decks up to 1400 m thick and as warm as −8°C. In thicker cloud decks, glaciation occurred only to a depth equal to the fall distance of the pyrotechnics. There were indications of “overseeding” from the relatively poor visibility through the cleared area that likely was caused by high ice-crystal concentrations. Targeting was successful when accurate wind data were available.

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Roger Edwards
,
Andrew R. Dean
,
Richard L. Thompson
, and
Bryan T. Smith

Abstract

A gridded, hourly, three-dimensional environmental mesoanalysis database at the Storm Prediction Center (SPC), based on objectively analyzed surface observations blended with the Rapid Update Cycle (RUC) model-analysis fields and described in Parts I and II of this series, is applied to a 2003–11 subset of the SPC tropical cyclone (TC) tornado records. Distributions of environmental convective parameters, derived from SPC hourly mesoanalysis fields that have been related to supercells and tornadoes in the midlatitudes, are evaluated for their pertinence to TC tornado occurrence. The main factor differentiating TC from non-TC tornado environments is much greater deep-tropospheric moisture, associated with reduced lapse rates, lower CAPE, and smaller and more compressed distributions of parameters derived from CAPE and vertical shear. For weak and strong TC tornado categories (EF0–EF1 and EF2–EF3 on the enhanced Fujita scale, respectively), little distinction is evident across most parameters. Radar reflectivity and velocity data also are examined for the same subset of TC tornadoes, in order to determine parent convective modes (e.g., discrete, linear, clustered, supercellular vs nonsupercellular), and the association of those modes with several mesoanalysis parameters. Supercellular TC tornadoes are accompanied by somewhat greater vertical shear than those occurring from other modes. Tornadoes accompanying nonsupercellular radar echoes tend to occur closer to the TC center, where CAPE and shear tend to weaken relative to the outer TC envelope, though there is considerable overlap of their respective radial distributions and environmental parameter spaces.

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