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Curtis R. Alexander
and
Joshua Wurman

Abstract

On the evening of 30 May 1998 atmospheric conditions across southeastern South Dakota led to the development of organized moist convection including several supercells. One such supercell was tracked by both a Weather Surveillance Radar-1988 Doppler (WSR-88D) from Sioux Falls, South Dakota (KFSD), and by a Doppler On Wheels (DOW) mobile radar. This supercell remained isolated for an hour and a half before being overtaken by a developing squall line. During this time period the supercell produced at least one strong and one violent tornado, the latter of which passed through Spencer, South Dakota, despite the absence of strong low-level environmental wind shear. The two tornadoes were observed both visually and with the DOW radar at ranges between 1.7 and 12.9 km. The close proximity to the tornadoes permitted the DOW radar to observe tornado-scale structures on the order of 35 to 100 m, while the nearest WSR-88D only resolved the parent mesocyclone in the supercell. The DOW observations revealed a persistent Doppler velocity couplet and associated ring reflectivity signature at the tip of the hook echo.

The DOW radar data contained tornado strength winds over 35 m s−1 within 100 m AGL approximately 180 s prior to both the first spotter report and visual confirmation of the first tornado associated with this supercell. Following the formation of the second tornado, the DOW radar observations revealed a tornado-strength Doppler velocity couplet within 150 m AGL between two separate tornado tracks determined by a National Weather Service (NWS) damage survey. Based upon the DOW Doppler velocity data it appears that the second and third damage tracks from this supercell are produced from a single tornado.

The time–height evolution of the Doppler velocity couplet spanning both tornadoes revealed a gradual increase in vertical vorticity across each tornado's core region within a few hundred meters AGL from near 0.2 to over 2.0 s−1 over a 45-min period. A corresponding reduction in vertical vorticity was observed aloft especially near 1000 m AGL where vorticity values decreased from near 1.0 to about 0.5 s−1 during this same time interval. The shear across the Doppler velocity couplet appears to undergo strengthening both at the surface and aloft during both tornadoes. An oscillatory fluctuation in the near-surface shear across the tornado core developed during the second tornado, with peak shear values as high as 206 m s−1, Doppler velocities over 106 m s−1, and peak ground-relative wind speeds reaching 118 m s−1. The period of this intensity oscillation appears to be around 120 s and was most prominent just prior to and during the passage of the tornado through Spencer. Coincident with the tornado passage through Spencer was a rapid descending of the reflectivity eye in the core of the tornado. A detailed comparison of surveyed tornado damage and radar-calculated tornado winds in Spencer is discussed in Part II.

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Joshua Wurman
and
Curtis R. Alexander

Abstract

A violent supercell tornado passed through the town of Spencer, South Dakota, on the evening of 30 May 1998 producing large gradients in damage severity. The tornado was rated at F4 intensity by damage survey teams. A Doppler On Wheels (DOW) mobile radar followed this tornado and observed the tornado at ranges between 1.7 and 8.0 km during various stages of the tornado's life. The DOW was deployed less than 4.0 km from the town of Spencer between 0134 and 0145 UTC, and during this time period, the tornado passed through Spencer, and peak Doppler velocity measurements exceeded 100 m s−1. Data gathered from the DOW during this time period contained high spatial resolution sample volumes of approximately 34 m × 34 m × 37 m along with frequent volume updates every 45–50 s.

The high-resolution Doppler velocity data gathered from low-level elevation scans, when sample volumes are between 20 and 40 m AGL, are compared to extensive ground and aerial damage surveys performed by the National Weather Service (NWS) and the National Institute of Standards and Technology (NIST). Idealized radial profiles of tangential velocity are computed by fitting a model of an axisymmetric translating vortex to the Doppler radar observations, which compensates for velocity components perpendicular to the radar beam as well as the translational motion of the tornado vortex.

Both the original single-Doppler velocity data and the interpolated velocity fields are compared with damage survey Fujita scale (F-scale) estimates throughout the town of Spencer. This comparison on a structure-by-structure basis revealed that radar-based estimates of the F-scale intensity usually exceeded the damage-survey-based F-scale both inside and outside the town of Spencer. In the town of Spencer, the radar-based wind field revealed two distinct velocity time series inside and outside the passage of the core-flow region. The center of the core-flow region tracked about 50 m farther north than the damage survey indicated because of the asymmetry induced by the 15 m s−1 translational motion of the tornado. The radar consistently measured the strongest winds in the lowest 200 m AGL with the most extreme Doppler velocities residing within 50 m AGL. Alternate measures of tornado wind field intensity that incorporated the effects of the duration of the extreme winds and debris were explored. It is suggested that damage may not be a simple function of peak wind gust and structural integrity, but that the duration of intense winds, directional changes, accelerations, and upwind debris loading may be critical factors.

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Branden Katona
,
Paul Markowski
,
Curtis Alexander
, and
Stanley Benjamin

Abstract

Relatively little is known about how topography affects convective storms. The first step toward understanding these effects is to investigate how topography affects storm environments. Unfortunately, the effects of topography on convective environments are not easily observed directly. Instead, it is necessary to resort to using output from the High-Resolution Rapid Refresh (HRRR). The HRRR’s 3-km grid spacing can resolve some of the larger-scale topographic effects. Popular convective storm forecasting parameters obtained from the HRRR are averaged on convective days from February to September 2013–15. It is surmised that most of the day-to-day variability attributable to synoptic- and mesoscale meteorological influences is removed by averaging; the remaining horizontal heterogeneity in parameters related to instability and vertical wind shear is due to the hemispheric-scale meridional temperature and pressure gradient, and likely also topographic influences, especially where recurring longitudinal variations in instability, wind shear, etc. are found. Anomalies are sensitive to the ambient low-level wind direction (i.e., whether winds are locally blowing upslope or downslope), especially for parameters that depend on the low-level vertical shear. The statistical significance of local maxima and minima is demonstrated by comparing the amplitudes of the anomalies to bootstrapped estimates of the standard errors.

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Joshua Wurman
,
Curtis Alexander
,
Paul Robinson
, and
Yvette Richardson

Using an axisymmetric model of tornado structure tightly constrained by high-resolution wind field measurements collected by Doppler on Wheels (DOW) mobile radars, the potential impacts of intense tornadoes crossing densely populated urban areas are evaluated. DOW radar measurements combined with in situ low-level wind measurements permit the quantification of low-level tornadic winds that would impact structures. Axisymmetric modeled wind fields from actual and hypothetical tornadoes are simulated to impact high-density residential and commercial districts of several major cities. U.S. census block data, satellite imagery, and other sources are used to characterize and count the number of structures impacted by intense winds, up to 132 m s−1, and estimate the level and cost of resulting damage. Census data are used to estimate residential occupancy and human casualties.

Results indicate that a large and intense tornado crossing through residential portions of Chicago, Illinois, could result in tragic consequences with winds in excess of 76 m s−1 impacting 99 km2 , substantially destroying up to 239,000 single-and dual-family housing units, occupied by up to 699,000 people, resulting in 4,500–45,000 deaths, and causing substantial damage to over 400,000 homes occupied by over 1,100,000 people. Widespread damage caused by winds exceeding 102 m s−1 could occur over a broad area of the high-rise office and apartment districts causing permanent structural damage to many such buildings. Smaller and less intense tornadoes would cause lesser, but still substantial, levels of damage and mortality. Tornadoes crossing Houston and Dallas-Fort Worth, Texas; New York, New York; Saint Louis, Missouri; Washington, D.C., and Atlanta, Georgia, could cause varying levels of damage and mortality.

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Kyoko Ikeda
,
Matthias Steiner
,
James Pinto
, and
Curtis Alexander

Abstract

The hourly updating High-Resolution Rapid Refresh (HRRR) model is evaluated with regard to its ability to predict the areal extent of cold-season precipitation and accurately depict the timing and location of regions of snow, rain, and mixed-phase precipitation on the ground. Validation of the HRRR forecasts is performed using observations collected by the Automated Surface Observing System (ASOS) stations across the eastern two-thirds of the United States during the 2010–11 cold season. The results show that the HRRR is able to reliably forecast precipitation extent during the cold season. In particular, the location and areal extent of both snow and rain are very well predicted. Depiction of rain-to-snow transitions and freezing rain is reasonably good; however, the associated evaluation scores are significantly lower than for either snow or rain. The analyses suggest the skill in accurately depicting precipitation extent and phase (i.e., rain, snow, and mixed phase) depends on the size and organization of a weather system. Typically, larger synoptically forced weather systems are better predicted than smaller weather systems, including the associated rain-to-snow transition or freezing-rain areas. Offsets in space or time (i.e., causing misses and false alarms) have a larger effect on the model performance for smaller weather systems.

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Joshua Wurman
,
Yvette Richardson
,
Curtis Alexander
,
Stephen Weygandt
, and
Peng Fei Zhang

Abstract

Dual-Doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field in and near a tornado occurring near Kiefer, Oklahoma, on 26 May 1997. Analyses of the dual-Doppler vector wind fields document in detail, for the first time, several structures associated with the tornado: a proximate updraft region, a rear-flank downdraft wrapping around the tornado, a double gust front structure occluding near the tornado, and a region of enhanced vorticity separated from the tornado that may have been associated with cyclic tornadogenesis. The analyses are compared to conceptual and computer models of tornadic storms.

A subsequent tornadogenesis was observed with radar every 18 s, providing a finescale temporal view of the genesis process. The genesis process was complex and the evolution of tornado intensity parameters was not monotonic in time. Low-level rotation contracted and intensified, then broadened, then contracted and intensified a second time to form the tornado.

The initial tornadogenesis was coincident with the merger of the main supercell and a much smaller convective storm. This tornado, which was always surrounded by substantial precipitation originating from both storms, began to dissipate just a few minutes after genesis, and the rotation both aloft and near the surface weakened substantially. A second storm merger, with a much larger and supercellular storm, was coincident with a reintensification of the mesocyclone aloft, a new hook echo development, and the genesis of a short-lived tornado. After the dissipation of this second tornado, the merger disrupted the structure of the supercell storm, the hook echo was absorbed, and the mesocyclone dissipated. The current analysis suggests a process in which storm mergers may, in sequence, aid tornadogenesis by enhancing surface convergence, or through another mechanism, but subsequently disrupt the tornado’s parent supercell perhaps by cooling the inflow air, with the result being short-lived tornadoes.

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Joshua Wurman
,
Yvette Richardson
,
Curtis Alexander
,
Stephen Weygandt
, and
Peng Fei Zhang

Abstract

Three-dimensional dual-Doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field and vorticity generation terms in and near a weak, short-lived tornado. The beam widths of the two Doppler on Wheels (DOW) mobile radars, at the range of the tornado, are 250 m with gate lengths of 75 m, resulting in a resolution of less than 107 m3. One of the DOWs collected data during the 240 s prior to the formation of the tornado, enabling examination of the genesis process. A single set of volumetric scans suitable for dual-Doppler analyses were completed by both DOWs, permitting the calculation of vertical and horizontal vorticity, divergence, and stretching and tilting terms in the vorticity budget of the large but weak tornado and its surroundings, but no local tendency terms. Analyses of the dual-Doppler vector wind fields document, for the first time in a supercellular tornado, revealed several structures expected to be associated with tornadoes, including the tilting of horizontal vorticity into the vertical near the tornado, and stretching of vertical vorticity in the region of the tornado. The analyses are compared to conceptual and computer models of tornadic storms, confirming the existence of various phenomena and processes, although some model predictions, particularly those concerning the distribution of horizontal vorticity, could not be verified through these analyses. The observed magnitudes of stretching of vertical vorticity and tilting of horizontal vorticity are consistent with those necessary for generating the observed vorticity near the tornado.

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Janice L. Bytheway
,
Christian D. Kummerow
, and
Curtis Alexander

Abstract

The High Resolution Rapid Refresh (HRRR) model has been the National Weather Service’s (NWS) operational rapid update model since 2014. The HRRR has undergone continual development, including updates to the Weather Research and Forecasting (WRF) Model core, the data assimilation system, and the various physics packages in order to better represent atmospheric processes, with updated operational versions of the model being implemented approximately every spring. Given the model’s intent for use in convective precipitation forecasting, it is of interest to examine how forecasts of warm season precipitation have changed as a result of the continued model upgrades. A features-based assessment is performed on the first 6 h of HRRR quantitative precipitation forecasts (QPFs) from the 2013, 2014, and 2015 versions of the model over the U.S. central plains in an effort to understand how specific aspects of QPF performance have evolved as a result of continued model development. Significant bias changes were found with respect to precipitation intensity. Model upgrades that increased boundary layer stability and reduced the strength of the latent heating perturbations in the data assimilation were found to reduce southward biases in convective initiation, reduce the tendency for the model to overestimate heavy rainfall, and improve the representation of convective initiation.

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David C. Dowell
,
Curtis R. Alexander
,
Joshua M. Wurman
, and
Louis J. Wicker

Abstract

High-resolution Doppler radar observations of tornadoes reveal a distinctive tornado-scale signature with the following properties: a reflectivity minimum aloft inside the tornado core (described previously as an “eye”), a high-reflectivity tube aloft that is slightly wider than the tornado core, and a tapering of this high-reflectivity tube near the ground. The results of simple one-dimensional and two-dimensional models demonstrate how these characteristics develop. Important processes in the models include centrifugal ejection of hydrometeors and/or debris by the rotating flow and recycling of some objects by the near-surface inflow and updraft.

Doppler radars sample the motion of objects within the tornado rather than the actual airflow. Since objects move at different speeds and along different trajectories than the air, error is introduced into kinematic analyses of tornadoes based on radar observations. In a steady, axisymmetric tornado, objects move outward relative to the air and move more slowly than the air in the tangential direction; in addition, the vertical air-relative speed of an object is less than it is in still air. The differences between air motion and object motion are greater for objects with greater characteristic fall speeds (i.e., larger, denser objects) and can have magnitudes of tens of meters per second. Estimates of these differences for specified object and tornado characteristics can be obtained from an approximation of the one-dimensional model.

Doppler On Wheels observations of the 30 May 1998 Spencer, South Dakota, tornado demonstrate how the apparent tornado structure can change when the radar-scatterer type changes. When the Spencer tornado entered the town and started lofting debris, changes occurred in the Doppler velocity and reflectivity fields that are consistent with an increase in mean scatterer size.

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Ming Hu
,
Stanley G. Benjamin
,
Therese T. Ladwig
,
David C. Dowell
,
Stephen S. Weygandt
,
Curtis R. Alexander
, and
Jeffrey S. Whitaker

Abstract

The Rapid Refresh (RAP) is an hourly updated regional meteorological data assimilation/short-range model forecast system running operationally at NOAA/National Centers for Environmental Prediction (NCEP) using the community Gridpoint Statistical Interpolation analysis system (GSI). This paper documents the application of the GSI three-dimensional hybrid ensemble–variational assimilation option to the RAP high-resolution, hourly cycling system and shows the skill improvements of 1–12-h forecasts of upper-air wind, moisture, and temperature over the purely three-dimensional variational analysis system. Use of perturbation data from an independent global ensemble, the Global Data Assimilation System (GDAS), is demonstrated to be very effective for the regional RAP hybrid assimilation. In this paper, application of the GSI-hybrid assimilation for the RAP is explained. Results from sensitivity experiments are shown to define configurations for the operational RAP version 2, the ratio of static and ensemble background error covariance, and vertical and horizontal localization scales for the operational RAP version 3. Finally, a 1-week RAP experiment from a summer period was performed using a global ensemble from a winter period, suggesting that a significant component of its multivariate covariance structure from the ensemble is independent of time matching between analysis time and ensemble valid time.

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