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Howard B. Bluestein and Douglas A. Speheger

Abstract

Hourly wind data from the National Oceanic and Atmospheric Administration's demonstration network of 404-MHz profilers in the central United States and hourly wind data from the standard National Weather Service surface network are used to determine the validity of the geostrophic momentum (GM) approximation in the vicinity of a sharp trough in the baroclinic westerlies by using observations alone. The ageostrophic and geostrophic components of the wind are retrieved from time-filtered and objectively analyzed wind data and from kinematically computed vertical velocities using the inviscid form of the horizontal equations of motion in pressure coordinates. The success of the retrieval technique is discussed critically in terms of the difference between the horizontal divergence of the wind and the horizontal divergence of the retrieved ageostrophic wind and in terms of the retrieval of the geostrophic wind field from simulated wind data using a primitive equation numerical model of a growing baroclinic wave.

It is shown that for a system that moved through the network in Kansas and Oklahoma on 7–8 October 1992, the GM approximation breaks down, owing to strong curvature around the trough. In particular, the geostrophic advection of ageostrophic momentum, which is neglected in the GM formulation of the equations of motion, is comparable in magnitude to the ageostrophic advection of geostrophic momentum, which is retained in the GM formulation. However, along- and downstream from the trough axis, the effects of geostrophic advection of ageostrophic momentum are counteracted by the local time rate of change of ageostrophic momentum and the vertical advection of ageostrophic momentum, effects not included in the GM formulation.

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Douglas A. Speheger, Charles A. Doswell III, and Gregory J. Stumpf

Abstract

The tornado events of 3 May 1999 within the county warning area of the Norman, Oklahoma, office of the National Weather Service are reviewed, emphasizing the challenges associated with obtaining accurate information about the existence, timing, location, and intensity of individual tornadoes. Accurate documentation of tornado and other hazardous weather events is critical to research, is needed for operational assessments, and is important for developing hazard mitigation strategies. The situation following this major event was unusual because of the high concentration of meteorologists in the area, relative to most parts of the United States. As a result of this relative abundance of resources, it is likely that these tornadoes were reasonably well documented. Despite this unique situation in central Oklahoma, it is argued that this event also provides evidence of a national need for a rapid-response scientific and engineering survey team to provide documentation of major hazardous weather events before cleanup destroys important evidence.

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Howard B. Bluestein, James G. Ladue, Herbert Stein, Douglas Speheger, and Wesley F. Unruh

Abstract

A storm-intercept team from the University of Oklahoma, using the Los Alamos National Laboratory portable, continuous wave/frequency modulated–continuous wave, 3-cm Doppler radar, collected close-range data at and below cloud base in six supercell tornadoes in the southern plains during the springs of 1990 and 1991. Data collection and analysis techniques are described. Wind spectra from five weak-to-strong tornadoes and from one violent tornado are presented and discussed in conjunction with simultaneous boresighted video documentation, photogrammetric analysis, and damage surveys.

Maximum Doppler wind speeds of 55–105 m s−1 were found in five of the tornadoes; wind speeds as high as 120–125 m s were found in a large tornado during an outbreak on 26 April 1991. These may be the highest wind speeds ever measured by Doppler radar and the first radar measurements of F-5 intensity wind speeds. The variation in the spectrum across the 26 April 1991 tornado is presented. Standard and mobile soundings, and surface data, used to determine the “thermodynamic speed limit” indicate that it was usually exceeded by 50%–100%. A comparison of actual Doppler spectra with simulated spectra suggests that the maximum in radar reflectivity in supercell tornadoes lies well outside the core.

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Charles M. Kuster, Pamela L. Heinselman, Jeffrey C. Snyder, Katie A. Wilson, Douglas A. Speheger, and James E. Hocker

Abstract

Many public safety officials (e.g., emergency managers and first responders) use weather-radar data to inform many life-saving decisions, such as sounding outdoor warning sirens and directing storm spotters. Therefore, to include this important user group in ongoing radar applications research, a knowledge coproduction framework is used to interact with, learn from, and provide information to public safety officials. From these interactions, it became clear that radar-based products that estimate a tornado’s location, intensity, or both could be valuable to public safety officials. Therefore, a survey was conducted and a focus group formed to 1) collect feedback on several of these products currently under development, 2) identify potential decisions that could be made with these products, and 3) examine the impact of radar update time on product usefulness. An analysis of the survey and focus group responses revealed that public safety officials preferred simple interactive products provided to them using multiple communication methods. Once received, any product that could clearly communicate where a tornado may have occurred would likely help public safety officials focus search and rescue efforts in the immediate aftermath of a tornado. Additionally, public safety officials preferred products created using rapid-update data (1–2-min volumetric updates) over conventional-update data (4–5-min volumetric updates) because it provided them with more complete information.

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Bryan T. Smith, Richard L. Thompson, Douglas A. Speheger, Andrew R. Dean, Christopher D. Karstens, and Alexandra K. Anderson-Frey

Abstract

The Storm Prediction Center (SPC) has developed a database of damage-surveyed tornadoes in the contiguous United States (2009–17) that relates environmental and radar-derived storm attributes to damage ratings that change during a tornado life cycle. Damage indicators (DIs), and the associated wind speed estimates from tornado damage surveys compiled in the Damage Assessment Toolkit (DAT) dataset, were linked to the nearest manual calculations of 0.5° tilt angle maximum rotational velocity V rot from single-site WSR-88D data. For each radar scan, the maximum wind speed from the highest-rated DI, V rot, and the significant tornado parameter (STP) from the SPC hourly objective mesoscale analysis archive were recorded and analyzed. Results from examining V rot and STP data indicate an increasing conditional probability for higher-rated DIs (i.e., EF-scale wind speed estimate) as both STP and V rot increase. This work suggests that tornadic wind speed exceedance probabilities can be estimated in real time, on a scan-by-scan basis, via V rot and STP for ongoing tornadoes.

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Bryan T. Smith, Richard L. Thompson, Douglas A. Speheger, Andrew R. Dean, Christopher D. Karstens, and Alexandra K. Anderson-Frey

Abstract

A sample of damage-surveyed tornadoes in the contiguous United States (2009–17), containing specific wind speed estimates from damage indicators (DIs) within the Damage Assessment Toolkit dataset, were linked to radar-observed circulations using the nearest WSR-88D data in Part I of this work. The maximum wind speed associated with the highest-rated DI for each radar scan, corresponding 0.5° tilt angle rotational velocity V rot, significant tornado parameter (STP), and National Weather Service (NWS) convective impact-based warning (IBW) type, are analyzed herein for the sample of cases in Part I and an independent case sample from parts of 2019–20. As V rot and STP both increase, peak DI-estimated wind speeds and IBW warning type also tend to increase. Different combinations of V rot, STP, and population density—related to ranges of peak DI wind speed—exhibited a strong ability to discriminate across the tornado damage intensity spectrum. Furthermore, longer duration of high V rot (i.e., ≥70 kt) in significant tornado environments (i.e., STP ≥ 6) corresponds to increasing chances that DIs will reveal the occurrence of an intense tornado (i.e., EF3+). These findings were corroborated via the independent sample from parts of 2019–20, and can be applied in a real-time operational setting to assist in determining a potential range of wind speeds. This work provides evidence-based support for creating an objective and consistent, real-time framework for assessing and differentiating tornadoes across the tornado intensity spectrum.

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