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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

Aircraft observations taken in the course of three flights of the Tornado Research Airplane on 11 June 1956 were analyzed. These flights were made over the eastern slope of the Rocky Mountains where a squall line was predicted. The analysis revealed the structure of a dry frontal surface which played an important role upon the development of the squall line.

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TETSUYA FUJITA

Abstract

An attempt is made to explain the mechanism of a hook-echo formation on the southwestern edge of an eastward-moving cumulonimbus cell. The conditions necessary for originating a hook-echo circulation through this proposed mechanism are: significant absolute vorticity within the subcloud layer, intense updraft to bring the low-level moist air into the cloud, and a vertical wind shear which steers the cumulonimbus cell with a velocity which is considerably different from that of the low-level winds. The Magnus effect involving the steering current and the spiraling up-draft is considered to be the force which directs the hook-echo circulation at low levels toward the southwestern edge of the major thunderstorm cell. A kinematic diagram with the absolute tangential speed and the radius of the cyclone as coordinates is also presented, as well as some speculation on the conservation of absolute circulation and absolute vorticity.

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TETSUYA FUJITA

Abstract

Doppler winds measured by an instrumented aircraft are of great value in determining the wind field accompanying large-scale atmospheric disturbances. When they are utilized in interpreting and computing the wind fields of so-called mesoscale disturbances with their horizontal dimensions of a few to a few hundred miles, slight errors in the vector quantities forming the navigation triangle result in fictitious winds which differ considerably from the real winds. In the first part of this paper, the wind velocity errors due to the backscattering water particles illuminated by Doppler beams, designated as wet, beams, are discussed. The influence of wet beams upon Doppler winds was calculated theoretically under various conditions to allow an estimate of maximum wind velocity error. Following the solution of wet-beam cases, theoretical consideration was given to the fluctuation of the measured winds caused by the constant errors in the true air speed, the aircraft heading, the Doppler ground speed, and the Doppler drift angle. For the purpose of investigating whether this type of error occurs or not, test flights were made over Florida and Oklahoma along a number of loops with varying diameters. Results of the evaluation revealed that the error in aircraft heading is of least importance and that the other errors can be determined and corrected with a high degree of accuracy provided only a few specific loops are flown during each mission. It has become feasible to calculate both divergence and vorticity associated with mesoscale disturbances from the calibrated Doppler winds measured along well-designed flight tracks. It is expected that the basic research presented in this paper will stimulate the use of Doppler wind systems in the determination of the detailed structure of winds accompanying mesoscale meteorological systems.

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TETSUYA FUJITA

Abstract

No Abstract Available.

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Tetsuya Fujita
and
William Bandeen

Abstract

For the purpose of evaluating the resolution of the High Resolution Infrared Radiometer (HRIR) flown on board the Nimbus I meteorological satellite, three cloud-free regions in the western United States—the Grand Canyon, Death Valley, and Sierra Nevada—were selected. Enlarged HRIR pictures and the analog frequency traces of the scan lines in the pictures were examined in an attempt to investigate the types of noise superimposed on the signals. Two types of noise which appear in periodic and oscillatory fashions were found. The latter can be eliminated by taking running-mean values at one-degree scan angle intervals. The equivalent blackbody temperatures thus obtained were analysed over these three regions, leading to the determination of the apparent temperature lapse rate inside the Grand Canyon atmosphere and of the temperature of Lake Tahoe and other lakes in the Sierra Nevadas. Death Valley was found to be about 10C warmer than the surrounding desert area 5000 ft high. An attempt was also made to produce an HRIR picture with isoneph contours similar to the iso-echo presentation of radar pictures.

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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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Tetsuya Fujita
and
Henry A . Brown

Five mesosystems, occurring on 4 to 5 June 1953 in an area 500 by 600 mi near Chicago, were analysed with the use of data from regular synoptic stations. Four of the systems were of the well known type having a pressure-surge line, a thunderstorm high, and a wake depression. The fifth system, a trough 500 mi in length, traveled at the rate of 60 kn.

By using radar pictures taken every minute, the movement of echoes was also studied. The echo velocities, which were approximately 20 kn while a pressure-surge line was 100 mi distant, increased up to 40 kn with the approach of the line. The individual echoes in the line moved in an east-northeast direction while the pressure-surge line, oriented southwest to northeast, proceeded towards the south-southeast. The centers of the mesosystems moved in the same direction as the individual echoes in the squall line.

Hourly precipitation amounts accompanying each system were integrated with respect to time and “time integrated” precipitation charts were then constructed.

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