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Tetsuya Fujita

Abstract

This paper presents a proposed mechanism of cold air production associated with precipitation. A dome of cold air is produced by the evaporation of raindrops falling beneath the cloud base. A quantitative relationship between the evaporated rain and the produced excess mass of cold air was obtained, which showed that the mass is directly proportional to the evaporation. The coefficient of proportionality is a dimensionless number which varies between 0 and 1 depending on the temperature lapse rate originally existing beneath the cloud base. Results of mesoanalyses of squall lines and thunderstorms were used to estimate the actual amount of evaporation. The mass ratio of evaporated rain to the surface rain was found to increase with the height of the cloud base, reaching 1.0 at a cloud base of 9000 ft. In-cloud evaporation obtained by Braham (1952) showed a very good agreement with the values obtained in this study.

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Tetsuya Fujita

Abstract

The original radar film of the Illinois tornadoes was analyzed with the additional use of surface observations from the available stations in the vicinity. This study shows that the tornadoes were associated with a tornado cyclone resembling a miniature hurricane in many respects. The tornado cyclone was only 30 mi in diameter, and it was characterized by an eye at its center, spiral echo bands, etc. The echo movement inside the tornado cyclone indicates that air converged at low levels then rose following the boundary of the eye. The location of the Champaign tornado with respect to the tornado cyclone center was carefully examined; it was placed beneath the ring of maximum wind, south of the cyclone center. Such a relative position was maintained at least during the developing stage of the tornado. It was also found that the direction of movement of the tornado cyclone formed a 25 deg angle with that of the echoes in outer fields.

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Tetsuya Fujita

Abstract

Mesosystems associated with stationary radar echoes were analyzed. Five of them occurred on 20 July 1956 over the area of the U. S. Weather Bureau's Severe Local Storms Network, and they grew to 300–400 mi in diameter. Divergence and vorticity at each 1000-ft level inside a composite mesosystem were computed up to 5000 ft. It is found that the wind field is rotational up to 3000 ft where it becomes irrotational. Appreciable divergence reaching over 100 × 10−5 per sec on the ground decreases linearly to 30 × 10−5 per sec at the 5000-ft level. Computed vertical velocity inside the mesosystems was about 1 ft per sec at 1000 ft, reaching 3 ft per sec at the 5000-ft level. A small system of 13 August 1947 over the Thunderstorm-Project area was also studied; it was only 20 mi in diameter. Comparison of the characteristics of these systems indicated that the systems, large and small, may be produced by similar processes.

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Tetsuya Fujita
and
Hector Grandoso

Abstract

Since the concept of a rotational thunderstorm was presented by Byers in 1942, little attention has been paid to this important characteristic. Through direct and indirect observations, as well as a series of numerical experiments, the authors, some 24 years later, now postulate that many large thunderstorms are rotating. The numerical experiments revealed that a thunderstorm in a strong environmental wind field deviates to the left of the mean wind unless it rotates slowly and cyclonically. It was also found that the maximum deviation, either to the right or left, occurs when such a thunderstorm rotates with a critical tangential speed of only a few meters per second. This striking result contradicts the conventional expectation that the faster the rotation, the larger the storm's deviation. Further investigation of numerically produced clouds revealed that most of the peculiar motion of thunderstorms can be simulated by computing the momentum of clouds through step-by-step integration. A thunderstorm couplet formed by an echo split was successfully simulated numerically. Then the tracks of both cyclonic and anticyclonic storms, almost identical to those observed by radar, were obtained by a computer. The experimental results in comparison with actual storms lead us to conclude that a cloud cannot be treated as a well-mixed entity and that it does not deviate accidentally. Its motion is a consequence of various parameters, including slow rotation, mostly cyclonic but occasionally anticyclonic.

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