Tetsuya Theodore. Fujita

Tornadoes and Downbursts in the Context of Generalized Planetary Scales

Abstract In order to cover a wide range of horizontal dimensions of airflow, the author proposes a series of five scales, maso, meso, miso (to be read as my-so), moso and muso arranged in the order of the vowels, A, E, 1, O, U. The dimensions decrease by two orders of magnitude per scale, beginning with the planets equator length chosen to be the maximum dimension of masoscale for each planet. Mesoscale highs and lows were described on the basis of mesoanalyses, while sub-mesoscale disturbances were depicted by cataloging over 20 000 photographs of wind effects taken from low-flying aircraft during the past 15 years. Various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent. mesoscale disturbances.

Five Scales of Airflow Associated with a Series of Downbursts on 16 July 1980

Abstract A series of destructive windstorms on 16 July 1980 in a 50 km (30 mi) wide zone from Chicago to Detroit was surveyed both from the air and the ground. In spite of the initial suspicion of 10–20 tornadoes in the area, the nature of the windstorms was confirmed to be downbursts and microbursts characterized by multiple scales of airflows with their horizontal dimensions extending tens of meters to hundreds of kilometers. An attempt was made to estimate the wind speed based on three types of airborne objects: a 180 kg (390 lb) chimney, a 1000 kg (one ton) corn storage bin, and lumber from damaged roofs found inside downburst areas, obtaining the maximum wind speed of 63 ± 10 m -1(140 ± 25 mph). A total of

AN ANALYSIS OF THREE WEATHER-RELATED AIRCRAFT ACCIDENTS

500 million damage reported was caused by thunderstrom-induced non-tornadic storms which affected very large areas. SMS/GOES pictures showed that the parent cloud was oval-shaped with its lifetime in excess of 12 h. The overshooting areas enclosed by the −66°C isotherms shrunk rapidly at the onset...

Spearhead Echo and Downburst in the Crash of an Airliner

Abstract Two aircraft accidents in 1975, one at John F. Kennedy International Airport in New York City on 24 June and the other at Stapleton International Airport in Denver on 7 August, were examined In detail. A third accident on 23 June 1976 at Philadelphia International Airport is being investigated. Amazingly, there was a spearhead echo just to the north of each accident site. The echoes formed from 5 to 50 min in advance of the accident and moved faster than other echoes in the vicinity. These echoes were photographed by National Weather Service radars, 130–205 km away. At closer ranges, however, one or more circular echoes were depicted by airborne and ground radars. These cells were only 3–5 km in diameter, but they were accompanied by downdrafts of extreme intensity, called downbursts. All accidents occurred as aircraft, either descending or climbing, lost altitude while experiencing strong wind shear inside downburst cells.

PRECIPITATION AND COLD AIR PRODUCTION IN MESOSCALE THUNDERSTORM SYSTEMS

Abstract Meteorological conditions leading to the crash of an airliner short of the runway of a New York airport were studied. Thunderstorm downdrafts much stronger than those measured on the 1946–47 Thunderstorm Project were found. These exceptional downdrafts have been designated as “downbursts.” The violent cloud systems that produce downburst cells can be identified in the form of forward extensions of radar echoes designated as “spearhead echoes” which move with unusual speed. The development of downburst cells appears to he tied in with overshooting tops of clouds at the anvil level.

Downbursts: meteorological features and wind field characteristics

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.

Split of a Thunderstorm into Anticyclonic and Cyclonic Storms and Their Motion as Determined from Numerical Model Experiments

Since the term “downburst” was coined by the author back in 1976, it has been subclassified into “microburst” and “macroburst” according to the horizontal dimension of the wind system. Introduced in this paper is the basic observational evidence of the microburst investigated by three fact-finding projects in 1978, 1982, and 1986, at three different climatological locations in the United States. Described briefly are the three major aircraft accidents in microbursts and some examples of the wind damage by strong microbursts. Current estimates of the maximum microburst wind at the 10−5 per year probability is as high as 150 mph, which could result in the tornado-like damage.

PALM SUNDAY TORNADOES OF APRIL 11, 1965

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 storms 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 thunde...

The Joint Airport Weather Studies Project

Abstract An extensive aerial survey was made over a large portion of the area affected by the outbreak of tornadoes on Palm Sunday on Apr. 11, 1965. The destruction from the tornadoes extended over parts of six Midwestern States. Aerial and ground damage surveys were combined with eyewitness reports to determine the exact location and time of each tornado occurrence and its path. Radar pictures of the squall line clouds were used to verify the direction and speed of the tornado-producing clouds. Almost simultaneously with the first tornado touchdown in eastern Iowa, TIROS IX took pictures of the Midwest United States that showed a large tongue of cloud-free dry air behind the cold front. The vertical structure of the cold dome is discussed in connection with its role in the development of the tornadoes. Two predictive parameters, namely, the best lifted index (BLI) and material differential advection (MDA) were developed and evaluated with data gathered on this outbreak of tornadoes. The wind speed of a t...

Analytical Mesometeorology: A Review

Abstract The Joint Airport Weather Studies (JAWS) Project will investigate the microburst event, having 2–10 km spatial and 2–10 min temporal scales, at Denvers Stapleton International Airport during the summer of 1982. JAWS applications and technology transfer objectives include: broadening the data base; providing data for real-time detection of thunderstorm hazards for dissemination to the public and avaiation communities; examining aircraft performance in wind shear; providing a real-time test for display software; identifying which scales of atmospheric motion are pertinent to applied objectives; providing a test of optimal Doppler radar placement suitable for metropolitan and airport terminal coverage; and describing in more detail the microburst hazard.