Rodger A. Brown

Tornado Detection by Pulsed Doppler Radar

Abstract Doppler radar measurements in the Union City, Okla., tornadic storm of 24 May 1973 led to discovery of a unique tornadic vortex signature (TVS) in the field of mean Doppler velocity data. The distinct character of this signature and its association with the tornado are verified using a model that simulates Doppler velocity measurements through a tornado. Temporal and spatial variations of the TVS reveal previously unknown tornado characteristics. The TVS originates at storm mid-levels within a parent mesocyclone, descends to the ground with the tornado (extending vertically at least 10 km), and finally dissipates at all heights when the tornado dissipates. NSSL Doppler radar data from 1973 through 1976 reveal 10 signatures; eight were associated with tornadoes or funnel clouds, while no reports are available for the other two. Since the TVS first appears aloft tens of minutes before tornado touchdown, the signature has decided potential for real-time warning.

Effects of Radar Sampling on Single-Doppler Velocity Signatures of Mesocyclones and Tornadoes

Abstract Simulated WSR-88D (Weather Surveillance Radar-1988 Doppler) radar data were used to investigate the effects of discrete azimuthal sampling on Doppler velocity signatures of modeled mesocyclones and tornadoes at various ranges from the radar and for various random positions of the radar beam with respect to the vortices. Results show that the random position of the beam can change the magnitudes and locations of peak Doppler velocity values. The important implication presented in this study is that short-term variations in tornado and far-range mesocyclone intensity observed by a WSR-88D radar may be due to evolution or due to the chance positions of the radar beam relative to the vortex’s maximum rotational velocities or due to some combination of both.

Improved Tornado Detection Using Simulated and Actual WSR-88D Data with Enhanced Resolution

Abstract The magnitude of the Doppler velocity signature of a tornado depends on the effective width of the radar beam relative to the size of the tornado. The effective beamwidth is controlled by the antenna pattern beamwidth and the azimuthal sampling interval. Simulations of Weather Surveillance Radar-1988 Doppler (WSR-88D) velocity signatures of tornadoes, presented in this paper, show that signature resolution is greatly improved when the effective beamwidth of the radar is reduced. Improved signature resolution means that stronger signatures can be resolved at greater ranges from the radar. Using a special recording device on the National Weather Services Radar Operations Centers KCRI test bed radar, Archive Level I time series data were collected during the Oklahoma–Kansas tornado outbreak of 3 May 1999. Two Archive Level II meteorological datasets, each having a different effective beamwidth, were created from the Archive Level I dataset. Since the rotation rate and time interval between pulses ...

Identifying predictive multi-dimensional time series motifs: an application to severe weather prediction

We introduce an efficient approach to mining multi-dimensional temporal streams of real-world data for ordered temporal motifs that can be used for prediction. Since many of the dimensions of the data are known or suspected to be irrelevant, our approach first identifies the salient dimensions of the data, then the key temporal motifs within each dimension, and finally the temporal ordering of the motifs necessary for prediction. For the prediction element, the data are assumed to be labeled. We tested the approach on two real-world data sets. To verify the generality of the approach, we validated the application on several subjects from the CMU Motion Capture database. Our main application uses several hundred numerically simulated supercell thunderstorms where the goal is to identify the most important features and feature interrelationships which herald the development of strong rotation in the lowest altitudes of a storm. We identified sets of precursors, in the form of meteorological quantities reaching extreme values in a particular temporal sequence, unique to storms producing strong low-altitude rotation. The eventual goal is to use this knowledge for future severe weather detection and prediction algorithms.

Effects of radar proximity on single-Doppler velocity signatures of axisymmetric rotation and divergence

Abstract Geometrical and mathematical relationships are developed to explain the variation with radar range of idealized single-Doppler velocity patterns of axisymmetric rotation and divergence regions. The velocity patterns become distorted as they approach a Doppler radar site. As a consequence, the apparent core diameters and locations of the centers of the features depart from the true values. Equations are derived to estimate the true values from the distorted Doppler velocity fields.

On the interpretation of single-doppler velocity patterns within severe thunderstorms

Abstract Although the flow field within a severe thunderstorm is complex, it is possible to simulate the basic features using simple analytical flow models (such as uniform flow, axisymmetric rotation, axisymmetric divergence). Combinations of such flow models are used to produce simulated Doppler velocity patterns that can be used as “signatures” for identifying quasi-horizontal flow features within severe thunderstorms. Some of these flow features are: convergence in the lower portions of a storm and divergence in the upper portions associated with a strong updraft, surface divergence associated with a wet or dry downdraft, mesocyclone (rotating updraft), flow around an updraft obstacle, and tornado. Recognition of the associated Doppler velocity patterns can aid in the interpretation of single-Doppler radar measurements that include only the radial component of flow in the radar viewing direction.

Error Sources and Accuracy of Vertical Velocities Computed from Multiple-Doppler Radar Measurements in Deep Convective Storms

Abstract Actual data are used in one case to investigate the nature and source of vertical velocity errors resulting from analyses of multiple-Doppler radar measurements. Consistent with earlier analytical works, larger errors are found than would be expected from previous theoretical studies. It is shown that the reconstructed maximum updraft speed in strong updrafts (>20 m s−1) is accurate, on the average, to within about 10% (standard deviation of 10%). Storm advection, incomplete sampling of low-altitude divergence caused by the radar horizon, top boundary errors, and uneven terrain are studied and all are dismissed as dominant sources of error in the case considered here. The inability to determine a dominant error source has important consequences for the formulation of vertical velocity adjustment schemes.

New WSR-88D Volume Coverage Pattern 12: Results of Field Tests

Abstract For the first time since the installation of the national network of Weather Surveillance Radar-1988 Doppler (WSR-88D), a new scanning strategy—Volume Coverage Pattern 12 (VCP 12)—has been added to the suite of scanning strategies. VCP 12 is a faster version of VCP 11 and has denser vertical sampling at lower elevation angles. This note discusses results of field tests in Oklahoma and Mississippi during 2001–03 that led to the decision to implement VCP 12. Output from meteorological algorithms for a test-bed radar using an experimental VCP were compared with output for a nearby operational WSR-88D using VCP 11 or 21. These comparisons were made for severe storms that were at comparable distances from both radars. Findings indicate that denser vertical sampling at lower elevation angles leads to earlier and longer algorithm identifications of storm cells and mesocyclones, especially those more distant from a radar.

The North Dakota Thunderstorm Project: A Cooperative Study of High Plains Thunderstorms

Abstract The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification. Of primary interest were tracer experiment that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide. Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunder...

Tornadic Storm Airflow and Morphology Derived from Single-Doppler Radar Measurements

Abstract Single-Doppler Velocity data reveal that a dominant feature in the Union City, Okla., tornadic thunderstorm is a core mesocyclonic circulation, 2–6 km in diameter, extending to at least 9 km above ground. There is an apparent flow through the precipitation echo at low levels and divergence at high levels. Considerable similarity appears between mid-level flow structure around the mesocyclone core and that observed around a solid rotating cyclinder embedded in classical potential flow. As tornado time approaches, core circulation tangential velocities increase while diameter decreases. Simultaneously, the collapse of storm top and extensive echo overhang suggest updraft weakening.