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Tornadoes and Downbursts in the Context of Generalized Planetary Scales

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Tornadoes and Downbursts in the Context of Generalized Planetary Scales T . THEODORE FUJITA Reprinted from JouRNAL OF THE ATMOSPHERIC SCIENCES, Vol. 38, No. 8, August 1981 American t-{eteorological Society Printed in l". S. /\. Reprinted from JOURNAL OF THE ATMOSPHERIC ScIE:-iCES, Vol. 38, No. 8, August 1981 American Meteorological Society . · ! Printed in U. S. A. ' Tornadoes and Downbursts in the Context of Generalized Planetary Scales T. THEODORE FUJITA Department of Geophysical Sciences, The University of Chicago, Chicago, IL 60637 (Manuscript received 24 December 1980, in final form 2 February 1981) ABSTRACT In order to cover a wide range of horizontal dimensiqns 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, I, 0, U. The dimen~ions decrease by two orders of magnitude per scale, beginning with the planet's 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 dis­ turbances 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. 1. Historical overview of mesoscale Severe weather events associated with small-scale disturbances, regarded as noise in daily weather The invention of the barometer led people to re­ analyses, became the focal point of storm research­ late the daily weather with barometric readings; high ers [a micro study by Williams (1948), pressure pressure symbolizes fine weather; low pressure, bad pulsations by Brunk (1949), a micro-analytical study weather; and extremely low-pressure storms. Dis­ by Fujita (1950), etc.]. covery of air masses and fronts in the early 1920' s by · The term meso, designating the subsynoptic the Norwegian school altered the simple-minded noise,. was first used by Ligda (1951) in emphasizing pressure-weather relationship into so-called synop­ the scale of radar echoes. Since then U.S. Army tic analysis, relating weather patterns with fronts Signal Corps used frequently but internally the term and cyclones. ''mesometeorological'' to identify small-scale weather Local storms in continental Europe and the phenomena which could affect the Army's operations. United States, thunderstorms in particular, are due Meanwhile, U.S. Weather Bureau defined the to. intense . heating in warm seasons and are not "mesoscale" to be centered between 10 and 100 always related to frontal activities. Since the 1920's, mi, leading to the publications of mesometeorologi­ both frontal and non-frontal thunderstorms were in­ cal papers [mesometeorological study of squall lines vestigated by various researchers: heat thunder­ by Fujita (1955) and mesoanalysis by Fujita, New­ storms by Brooks (1922), cold-air production by stein and Tepper (1956), both with color pages]. Suckstorff (1938), thunderstorm highs by Schaffer The original mesometeorological scale was shifted (1947) and many others. to 10-100 km after the adoption of the metric As air traffic increased in the 1940's, squall-line system. The Glossary of Meteorology (1959). de­ and thunderstorm-related accidents and difficulties fines synoptic as "in general, pertaining to or af­ . occurred in various parts of the world. Fact-finding fording an overall view.'' The key words for synop­ networks covering local areas were operated in both tic in meteorology are "overall" and " simul­ Germany and Japan. Some of their results had been taneous" data collected for weather analyses. classified until the end of the second world war. Petterssen (1956), applying this fundamental defini­ These networks were Lindenberger Boennetz tion, suggested the use of "macrosynoptic", (Squall Network) in 1939-41, 20 km east-southeast "mesosynoptic" and "microsynoptic", thus ex­ of Berlin reported by Koschmieder (1955) and tending the concept of the conventional term Maebashi Raiu Kansokumo (Maebashi Thunder­ "synoptic" into mesoscale and microscale. storm Network) in 1940, 100 km northwest of Tokyo Byers (1959) and Tepper (1959) attempted to; reported by .Fujiwara (1943). The Thunderstorm generalize the scale of atmospheric motion through Project in 1946 (Florida) and 1947 (Ohio) operated five orders of magnitude, ranging between a few the· most comprehensive data-collection networks, tenths of a kilometer to the dimensions of long as reported by Byers and Braham (1949). · waves. 0022-4928/81/081511-24$10. 00 1511 © 1981 American Meteorological Society 1512 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 38 HORIZONTAL DIMENSIONS IOO"""km IOl'Mkm I OOOkm IOO km IOkm l km IOOm IOm Im IOcm fem Imm YEAR 1951 L1 odo MESOME TEOROLOGICAL 1955 Fu1 ito MESOMETEOROLOG!CAL SCALE 19 56 Fu'ito,News1e1n Teoper M£SOANALYS IS 1956 Pe 11er\ sen MACROSYNOPTIC MESOSYNPTIC MICROSYNOPTjC 1959 BvtrS MACROMET.: SY NOP.MET. i MESOMETEOROLOGY MICROMET. 1959 Teooer MACROSCALE i MESOSCALE LOCAL SCALE 1959 Glossary AMS MACROME T. ; CYCLONE SCALE : MESOMET EOROLOGY : MICROMET. 1963 0Quro I ' II Ill IV v p ' 1963 Fu 11 0 NelwOfk Scale : MESO a ' r 1970s Modelln11 Scale . /3 y 1972 Glossary, Brit ish SY NOPTIC MET. ME SOMETEOROLOGY MICROMET. 1976 Aoee Snow Clore I I I !1 I Ill I IV I v VI 1979 Fu i.to I M AS 0 I ME S V I M I SO I M O SO I MUS O I 1979 Hobbs Macro a I /3 I Mesa a I /3 I y I Micro a I /3 I y I 8 I < 1 2 7 WA VE NUMBER 10° 2 " • 10 10 10' 10· 10' 1p• 10 10• 10• JPIO MASOsco le I MESOscole I MISOscole I MOSOsco le I MGSOscole I 400km 4km 4 0m 4mm E M2J9!>!9b f)1_e_s_o_hjgb f)1J~QIJ.'9b *M9JQl]igb 40<m * f)1_U§Qf)igQ ->c \0 PRESSURE DOME IHydrostoflc !'-. I 0 'tr £x1stence not conflfmed ' H IGHS 0 I ANTICVCLONE I ' I PRESSURE NOSE INonhydrostollc '<t 0 I : ' ' II ' : ---'M1croburs1L.- ' I ; * LAB HIGH Cl: I ' I DOWN BURS T ' ' '.·1Burst Swath ! ! ' I 2 ' 'I ' <t ,' . ' ' ' I' ' :::i I' BURST FRONT j ../iswoth Front j ' ' ' i ' 0 I FRONT I ' ' I LAB FRO NT i l.L.i ' FRON TS u.. GUST FRONT 0 i I l: f-- ! CYCLONE I··--· ····f MESOCYCLONE I· ------! T O RNA DO Cb I ~ IHURRICANE! -........... * ::::::::J .. ·1 SUCTION VOR TEX I LAB VORTE X l.L.i I I LOWS ...J ~-Q..S..OfYf:tC?.flt! 400km ~-e_S,9fYf!tlflt! 4km ~j ~QCYflOJ!..~ 40m ~-o_s_o_s:yf!~IJ~ 40cm ~-U~Q~ Yf lO_n_ft 4mm A SCALE I E SCALE I I SCALE I 0 SCALE I u SCAL E I 5 4 3 2 1 1 3 10 10 10 10 . 10 10° 10· 10-2 10" 10-4 10·5 km F1G. I. A diagram showing the historical definitions of the mesoscale along with the five scales, maso-, meso- , miso-, moso- and musoscale proposed by the author. In their articles on severe local storms, Ogura suspected rotation was confirmed by Fujita et al. (1963) and Fujita (1963) independently suggested (1972) on the basis of the swirl pattern of sweet­ subclassifications of scales for every " one order of potato vines in the wake of a tornado near Tokyo, magnitude" in horizontal dimensions. Ogura's Japan. (1963) Group Numbers I, II, III, IV and V are In an attempt to accommodate the dimensions of centered at 10 000, 1000, 100, 10 and I km, respec­ tornadoes and suction vortices, Agee et al. (1976) tively, with corresponding phenomena ranging be­ proposed a subclassification of mesoscale into I-VI tween planetary waves to Rayleigh-type convec­ which extend through four orders of magnitude be­ tions. Meanwhile, Fujita (1963) emphasized a need tween 10 m and 100 km. for three-density networks with station densities classified into meso-a, meso-/3, and meso-y for depicting all types of mesoscale disturbances. Cur­ rently, the grid size of numerical modeling of mesoscale phenomena is categorized into a, 13, and y which correspond to Ogura's groups III, IV and V, respectively. The diameter of tornadoes rarely exceeds 1000 m or the meso-y scale. Fujita 1 found that subtornado­ scale vortices, called "suction vortices" orbit around the core of large tornadoes. These vortices are one order of magnitude smaller than their parent tornado. Initiall y, Fujita ( 1970) used the term "suction spot" with a remark that there are evidences that suction spots are in a state of rotation. The 1 Fujita, T . T ., 1971: Proposed mechanism of suction spots FIG. 2. Cloud-motion vectors around Jupiter's Great Red Spot, accompanied by tornadoes. Preprints 7th Conf. Severe lorn/ computed by the author based on four pictures taken by Voyager I Storms , Kansas City, MO, Amer. Meteor. Soc., 208-213. on 2 and 3 February 1979. AUGUST 1981 T . THEODORE FUJITA 1513 Fig. 1 summarizes the geometric scales proposed by various researchers, along with both confirmed an.d unconfirmed disturbances associated with highs, fronts and lows. It is seen that there are two schemes of scaling atmospheric motion. The first scheme is based on the familiar sequence, macro, meso and micro with their dimensional increments, approx­ imately two orders of magnitude each. The second scheme designates scales in either numerical or alphabetic (Greek) order. Ogura's I, II, III, ... and meso-a, meso-,B , meso-y are separated by one order of magnitude while Agee et al. 's I, II, III, ... , by nonuniform scale separations ranging between approximately one-third to one order of magnitude. The macro, meso, micro sequence of the first scheme is useful in identifying the subfields of meteorology, because each scale extends through approximately two orders of magnitudes, including a variety of important scales of motion.
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