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Methods Lor Identifying Severe Thunderstorms by Radar

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Methods Lor Identifying Severe Thunderstorms by Radar Ralph J. Donaldson, Jr. methods lor Identifying Air Force Cambridge Research Laboratories severe thunderstorms Bedford, Mass. by radar: a guide and bibliography' 1. Introduction Thunderstorms have always been prominent among the meteorological targets observed by radar. They command attention because of their greater precipitation intensity and height. Moreover, some of the more severe and hazardous thunderstorms pose an im- portant short-range forecasting problem to which radar may be profitably applied. This paper is intended as an aid to the forecaster of severe thunderstorms. An attempt will be made to sketch the historical development and evaluate the current success of con- ventional radar techniques which are sensitive to the identification of severe thunder- storms. The bibliography includes most of the pertinent references by Canadian and United States authors and a few citations representative of significant work in this field in other countries. Radar meteorologists quite early learned how to distinguish between echoes from con- vective and stratiform precipitation. With convective precipitation, in which vertical and horizontal air speeds are comparable, the echoes are cellular, rather similar in verti- cal and horizontal scale, and display a tendency to form into linear groups. Convective echo intensities were qualitatively observed to be greater, in general, than the intensities of stratiform echoes, as indicated by echo brightness on the oscilloscope display. Further- more, convective echoes never, except in their decaying stages, displayed the melting-layer "bright band." Some of these early discoveries were reported by Bent (1946) and Wexler (1947a) and were confirmed and organized by Byers and Braham (1949) in their compre- hensive report on the Thunderstorm Project. Convective storms may be classified conveniently into four categories of increasing dis- turbance to human activity: showers, ordinary thunderstorms (showers accompanied by lightning but no hail or damaging wind at the ground), hailstorms, and severe thunder- storms (usually these contain large hail as well as damaging wind). Many of the radar investigations of convective storms have been concerned with techniques for distinguish- ing among the various categories. However, the four types of storms are not clearly sepa- rated from one another: they form a continuum with class boundaries defined only by conveniently observable events. For example, the presence of small hail at the ground depends on surface temperature and the height of the wet-bulb OC isotherm as well as on favorable hail growth conditions within the convective cloud. The definition of a severe thunderstorm is quite arbitrary. In this discussion, a thun- derstorm will be classified as severe if it produces a tornado or straight-line wind capable of inflicting major structural damage to dwellings and snapping off tree trunks and large limbs. An additional criterion of severity is the appearance of hailstones with major di- mension greater than 3/4 inch, whether or not tornadoes or damaging winds are also released. Large hail, tornadoes, and other damaging winds may be regarded not only as a severe weather hazard, but also an indicator of the intensity and persistence of convection in a thunderstorm. Large hail requires sufficient time to grow in an environment rich in wa- ter droplets at temperatures below OC. This implies an adequate supply of low-level moisture and an unusually intense updraft to retard the fallout of the growing hailstone. Long hail swaths deposited by some severe storms require a persistent as well as intense i Under the title, "Radar as a Severe Thunderstorm Sensor," an earlier version of this paper was presented at a Radio Meteorology Symposium during the XIII General Assembly of the International Union of Geodesy and Geophysics, Berkeley, Calif., 24 August 1963. 174 Vol. 46, No. 4, April 1965 Unauthenticated | Downloaded 10/05/21 10:48 AM UTC Bulletin American Meteorological Society updraft; therefore, the storm must be organized in such a manner that its updraft draws upon a continuing supply of moist, buoyant low-level air. The initiation and mainte- nance of a tornado is in itself a highly organized event, and it seems to occur in a storm in which some degree of organized air motion persists during the life of the tornado. The identification of a severe thunderstorm by radar, then, depends on the observation of storm features which reflect either the intensity or the persistent state of organization of the convective process. Not surprisingly, the most successful of the conventional radar methods involve measurement of echo top heights; echo intensity or reflectivity; and un- usual, suggestive, and generally persistent echo configurations. A high echo top, and especially an unusually reflective echo at high altitudes, indicates an updraft of sufficient intensity to carry radar-detectable particles to the observed height. The intensity of a storm echo is a direct indication of the size and concentration of precipitation particles, and hence an unusually intense echo indicates an updraft magnitude and organization favorable for growing large particles and for converting the supply of water vapor to pre- cipitation at a high rate per unit volume. This change of phase of water will also release a great deal of latent heat over a limited region, which will contribute significantly to the buoyancy of the storm and raise its top. Finally, a persisting echo configuration suggests an organized pattern of convection. Later discussion will show how one type of organ- ized convection can produce long swaths of large hailstones one inch or more in diameter. Evidently tornadoes also occur frequently with this same pattern. The study of the radar characteristics of severe thunderstorms has revealed a few sur- prising discoveries, however, and has contributed to new understanding of these storms. For example, carefully measured storm echo tops show penetrations into the stratosphere by as much as 20,000 ft. A highly reflective echo observed in some severe storms has led to studies of the scattering properties of hail of various sizes and states and inferences re- garding the hail growth condition in these storms. Recent observations of hooked echoes and echo-free vaults have provided clues to the form of the air flow in storms with these unusual echo configurations. The intensive coverage of radar over a wide area provides an opportunity to locate storms and, hopefully, to recognize the hazardous ones. Unfortunately, a unique, reli- able, and objective radar echo signature for severe thunderstorms has not yet been found. However, research during the past decade has furnished a guide to certain radar echo characteristics which frequently attend very intense convection. The experience with ob- jective measures, such as echo height and reflectivity, is now sufficiently extensive at cer- tain locations that tentative working thresholds may be specified which divide frequent from rare occurrence of severe thunderstorms. The most successful methods are listed in the summary, following a discussion of their discovery and development. Doppler radar techniques have scarcely been applied as a tool for observing thunder- storms. However, recent developments in Doppler radar offer great promise, furnishing an attractive capability of direct measurement of wind velocity, and hence an unambigu- ous method for identification of tornadoes and other severe windstorms. A comprehen- sive discussion of the potential use of Doppler radar in severe storm identification was given by Lhermitte (1964). Also, the reviews of severe storm analysis and the entire field of radar meteorology by Atlas (1963, 1964) incorporate extensive, detailed surveys of Doppler radar techniques. 2. Arrangement and Severe weather occasionally appears with isolated thunderstorm echoes, but most of it motion of a group occurs in echoes forming a part of a line. In a case study, Bigler (1955a) prepared com- of convective echoes posite echo patterns from nine radars in the central plains of the United States, and he noted that tornadoes and hail occurred during the organization of echoes into a line pat- tern which eventually extended over a length of 1000 miles. Further study of many cases by Inman and Bigler (1958) showed that 82 per cent of tornado-producing situations could be classified as an echo line, even though some of the most destructive major tornadoes occurred with isolated echoes. A wave-like pattern in an echo line was first described by Wexler (1947a, b) and related to a synoptic pressure feature. Tepper (1950) noted the occurrence of a tornado at a bend in an echo line which he felt to be compatible with the intersection of two pressure- jump lines. Nolen (1959) described a characteristic line echo wave pattern, or LEWP, which appeared in near proximity to about three-fourths of the tornadoes he studied. 175 Unauthenticated | Downloaded 10/05/21 10:48 AM UTC Vol. 46, No. 4, April 1965 An example of the development of this kind of pattern is given in Fig. 1. Stout, Black- mer and Wilk (1960) made a detailed study of two cases of distortions in squall lines. They found that die LEWP, that part of the line which was distorted in the direction of line movement, corresponded closely with the areas of most hail and damaging winds. The upper wind field suggested a mid-tropospheric jet at 4-5 km in the vicinity of both LEWPs. An evaluation of LEWPs by Cook (1961) revealed that 40 of a total of 49 of them had severe weather, most of it in the more advanced section of the wave pattern. Although this is impressive evidence, which suggests a preference for tornadoes to occur in a mesoscale disturbance of sufficient intensity to cause distortions in an echo line, an observation of a LEWP is not entirely reliable because of its dependence on radar char- acteristics and range and because it may be mistaken for small irregularities in which severe weather is not especially likely. A study of echo motions in severe storms in the midwest United States by Stout and Hiser (1955) showed a high degree of association of echo convergence and vortex motion with severe winds and hail.
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