Howard B. Bluestein

Formation of Mesoscale Lines of Pirecipitation: Severe Squall Lines in Oklahoma during the Spring

Abstract Four distinct kinds of severe, mesoscale convective-line development are identified in Oklahoma during the spring based on the analysis of an 11-year period of reflectivity data from the National Severe Storms Laboratorys 10-cm radar in Norman, Oklahoma. The primary classes of fine formation are broken line, back building, broken areal and embedded areal. Each is described in detail, along with illustrative examples. Comparisons are made with other observations and with numerical model simulations. The former two classes of line formation have been previously documented, while the latter two have not. Only the broken-areal squall line has been realistically simulated numerically. The environment for each of the types of line development was determined from data from the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. It was found that broken-line formation tends to occur along cold fronts in a multicell environment, while back building occur...

Tornadoes and Tornadic Storms

Tornadoes, with measured wind speeds of 125 m s−1 to perhaps 140 m s −1, are the most violent of atmospheric storms (Fig. 5.1). A tornado is defined here as a violently rotating, narrow column of air, averaging about 100 m in diameter, that extends to the ground from the interior of a cumulonimbus (or occasionally a cumulus congestus) cloud and appears as a condensation funnel pendant from cloud base and/or as a swirling cloud of dust and debris rising from the ground. Significant damage can occur at the ground even when the condensation funnel does not reach the surface. A condensation funnel associated with a tornadic vortex that fails to contact the ground is called a funnel cloud. A waterspout is a tornado over a body of water.

Observations of Tornadoes and Other Convective Phenomena with a Mobile, 3-mm Wavelength, Doppler Radar: The Spring 1999 Field Experiment

Abstract In the spring of 1999 a field experiment was conducted in the Southern Plains of the United States, during which a mobile, millimeter–wavelength pulsed Doppler radar from the University of Massachusetts, Amherst, was used by a storm–intercept team from the University of Oklahoma to collect data in tornadoes and developing tornadoes. With a 0.18° beam antenna, resolution as high as 5–10 m in the azimuthal direction was attained in a tornado on 3 May. Data collected in three supercell tornadoes are described. Features such as eyes, spiral bands, and multiple vortices/wavelike asymmetries along the edge of the eyewall are discussed. Winds approaching 80 m s–1 were resolved without folding using the polarization diversity pulse pair technique. Two tornadoes formed at an inflection point in reflectivity where the hook echo and apparent rear–flank downdraft intersected. Finescale transverse bands of reflectivity were evident in one hook echo. Data in a dust devil are also described. Numerous other data...

The Second Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX2

The second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), which had its field phases in May and June of 2009 and 2010, was designed to explore i) the physical processes of tornadogenesis, maintenance, and demise; ii) the relationships among tornadoes, tornadic storms, and the larger-scale environment; iii) numerical weather prediction and forecasting of supercell thunderstorms and tornadoes; and iv) the wind field near the ground in tornadoes. VORTEX2 is by far the largest and most ambitious observational and modeling study of tornadoes and tornadic storms ever undertaken. It employed 13 mobile mesonet–instrumented vehicles, 11 ground-based mobile radars (several of which had dual-polarization capability and two of which were phased-array rapid scan), a mobile Doppler lidar, four mobile balloon sounding systems, 42 deployable in situ observational weather stations, an unmanned aerial system, video and photogrammetric teams, damage survey teams, deployable disdrometers, and othe...

Close-range observations of tornadoes in supercells made with a dual-polarization, X-band, mobile doppler radar

Abstract A mobile, dual-polarization, X-band, Doppler radar scanned tornadoes at close range in supercells on 12 and 29 May 2004 in Kansas and Oklahoma, respectively. In the former tornadoes, a visible circular debris ring detected as circular regions of low values of differential reflectivity and the cross-correlation coefficient was distinguished from surrounding spiral bands of precipitation of higher values of differential reflectivity and the cross-correlation coefficient. A curved band of debris was indicated on one side of the tornado in another. In a tornado and/or mesocyclone on 29 May 2004, which was hidden from the view of the storm-intercept team by precipitation, the vortex and its associated “weak-echo hole” were at times relatively wide; however, a debris ring was not evident in either the differential reflectivity field or in the cross-correlation coefficient field, most likely because the radar beam scanned too high above the ground. In this case, differential attenuation made identificat...

A Synoptic and Photographic Climatology of Low-Precipitation Severe Thunderstorms in the Southern Plains

Abstract The occurrence of an unusual type of severe thunderstorm, difficult to identify by radar, has been documented in the Southern Plains. All of the three storms previously studied occurred near the dryline. Although tornadoes and large hail were produced, little rain was observed at the ground and there was no evidence of a strong surface downdraft; two were described as “bell-shaped”. Ten more cases of this rare type of storm have been recently documented visually and on radar. A photographic and synoptic climatology of this class of storm, the low-precipitation severe storm, is suggested on the basis of an analysis of the comparison among 11 of these cases. It is found that these storms form in a supercell-like environment which is drier in the moist layer and more weakly sheared than that of well-documented supercells in Oklahoma. Implications of our findings and additional storm characteristics are discussed in light of the larger sample size.

Severe Thunderstorm Development in Relation to Along-Dryline Variability: A Case Study

Abstract Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations. Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm i...

Investigation of the Dryline and Convective Storms Initiated along the Dryline: Field Experiments during COPS–91

Abstract The dryline is recognized as a major factor in the initiation of severe thunderstorms in the central and southern plains of the United States during the spring. Although severe thunderstorm forecasters often use the strength and position of the dryline to help determine prime areas for convective development, relatively little is known of the exact mechanisms by which thunderstorms form in the dryline environment. In the spring of 1991 experiments were carried out to study the dryline and convective storms near the dryline as part of the Cooperative Oklahoma Profiler Studies program, which was supported by the National Oceanic and Atmospheric Administration, the National Science Foundation, and the National Aeronautics and Space Administration. Observing systems deployed in these experiments included a research aircraft equipped with both in situ instrumentation and a Doppler radar, two mobile laboratories capable of remote release of rawinsondes, a surface mesonetwork, the Profiler Demonstration...

Attenuation Correction and Hydrometeor Classification of High-Resolution, X-band, Dual-Polarized Mobile Radar Measurements in Severe Convective Storms

Abstract X-band and shorter radar wavelengths are preferable for mobile radar systems because a narrow beam can be realized with a moderately sized antenna. However, attenuation by precipitation becomes progressively more severe with decreasing radar wavelength. As a result, X band has become a popular choice for meteorological radar systems that balances these two considerations. Dual-polarization provides several methods by which this attenuation (and differential attenuation) can be detected and corrected, mitigating one of the primary disadvantages of X-band radars. The dynamics of severe convective storms depend, to some extent, on the distribution and type of hydrometeors within the storm. To estimate the three-dimensional distribution of hydrometeors using X-band radar data, it is necessary to correct for attenuation before applying commonly used hydrometeor classification algorithms. Since 2002, a mobile dual-polarized Doppler weather radar designed at the University of Massachusetts, Amherst has ...

Unusually Strong Vertical Motions in a Caribbean Hurricane

Abstract Unusually strong updrafts and downdrafts in the eyewall of Hurricane Emily (1987) during its rapidly deepening phase are documented by both in situ aircraft measurements and a vertically pointing Doppler radar. Updrafts and downdrafts as strong as 24 and 19 m s−1, respectively, were found. Mean updrafts and downdrafts were approximately twice as strong as those found in other hurricanes. Updrafts had approximately the same width as downdrafts. The most vigorous updrafts were located in the front quadrants of the storm, and most of the strongest downdrafts were found in the rear quadrants. The downdrafts could not be explained in terms of evaporative or melting cooling, or precipitation drag. Evidence is presented that moist symmetric instability initiated by precipitation loading may have been responsible for the strong downdrafts.