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DOPPLER RADAR OBSERVATIONS OF N OKLAffONADO~DRST*

tirilp WolfSon Uncoln Lsboratov Mssach”setts Institute of Tectiology Letington, NA 02173

1. INTRODUCTION phenomnon in th”ndersto- is a -tcer of sow debate ad one which till b investigated in .’Domhrsts”, first discovered by ti. T. the current wrk. T. F“jita, are s=ll intense do-drafts at very low ticitudes which impact the surface md 2. RADAR ECMOES cause a divergent outflow of tind. They can occur under a variety of =teorological con- Much effort tis hen spent relating speci- ditions as -s &monstrated during the JAWS fic radar echoes to ground da=g. and reports 1982 field experiwnt which took Place in the of eevere weather. 1“ the course of Ms vicinity of Stapleton airport in Denver, CO. investigation of the JFK accident Fujita (1976) Many do-bursts Wre detected ht mst of them associated da~ging domb”rst tinde on the were of the type now bing called “dry”’or g.oand ~th a “’radarechO ~th a pOinted aPpen- “cualue” or ‘“virga”domtirsts. A distinction dage extending toward the direction of the echo must b mde %tween these and the ‘“wet”or motion” whfch ti =lled a ,“spearhead’”echo. ,’thunderscon” dombarsts tiich are the s=bject ‘.The appendage wves mch faster than the of this study. me two phenomna are very dif- parent echo which is king dram Into the ferent. They are USy to distinmisb: the appendage. ~ring the mture stage, the appen- formr co= from beni~ looking c.alue dage turns into a =j or echo and the parent and fall through a vev deep ad d~ subclo.d echo loses its identity.‘“ layer and the latter are associated tith thun- dersto-. Thunderstorm domhrsts tive ben After further observational work a WI. detected throughout the Great Plains and the general type of echo tith tiich domhrsts mre Midwest, on the eaet coast, md in Florida, associated was identifled by Fujita (1978) as “bile the virga bve ken detected the ““bow”echo fiich then takes the stipe of a airily over the hfgh plains east of the ~ckies. spearhead echo during the strong domburst stag. end which so-t 1-s develops a “weak echo The word “domb”rst<’ was first introduced ctinnel”’in the area of strongest tinds. mere by Fujtta (1976) afCer the investigation of a is sow question = to tiether the dowtirst is plane crash at ~K airport, to describe the simply associated tith or actually -uses situation in which a thunderston do-draft these distinctive echo configurations. h beco=s ti.ardo”s to the operation of jet excenstve survey of the ground d~ge caused by aircraft on take-off or landing. At first, a bw echo thunderstorm in Illinois by Forbes F“jita (1979) thought that the domburst md and Wakimto (1983) revealed mny dom~rsts, the =11 bow thunderstorm domdraf t =re ticrobuzsts (domb”rsts tith dimnsione less essentially the sa= but that, in the saw way than 4 b), =d tornadoes. Their results in- a funnel aloft is wt ailed a tornado, a sistently show the strong cyclonic mrvature mid-level downdraft in a thunderstorm muld not and tornado paths to h on the north side of be al lad a domburst. The concept was later the diverging tind pattern of the dombursts. refined when it -s decided that the do-hrst must induce “an outhrst of &mgfng tinds on The Line Echo Wave Pattern (WWP ), defined or war the ground” (Fujita and Wakimto, 1981) by Nolen (1959) as a “confi~rat ion of radar where “damging tinds” refers to tinds that -U echoes in which a line of echoes ks Men sub- be esttmced on the F-scale (for *tch the jected to an acceleration along one portion tiniam threshold is 18 nts). These damging andtor a &celeration along that portion of the winds can b either straight or curved ht they line imediacely dja.ent, tith a resulting must k Wghly divergent (Fujita, 1981). Thus, sinusoidal =soscale wave pattern in the line,” even in its mst recent and wre =teorological is quite sitilar to the bow echo and h. also definition, the tem dowburst is =ant to been associated tith reports of Wgh tinds ad signify a potential humn hzard. Whether or tornadoes. &tilton (1970) “as able to &duce “ot it also signifies a dymtically distinct a =SO-1OW surface pressure area associated with the crest of the ~WP from the shape of the squall line as depicted on radar. *The work described here -s sponsored by the National Science Foundation Grant 81-15712 md Fujita (1978) &s docu=nted dombursts is taken from the author’s S. M. Thesis in the associated with kok echoes, a distinctive con- ad Physical Oceanography %partm.t figuration hom to b a good indicator of at of the Wssachusetts Institute of Technology. least a msocyclone and often a tornado. He The Uni,tedStates Governmnc asums no liabi- has docuwnted a series of doabrsts tiich all lity for its contents or use thereof. occurred on the south side of a =socyclone —. ,.

mvi”g from northwest to southeast across the of O.1 w md 200 b, mst docu-nted dom- ~nsas-Misso.ri krder, & bs docu=nted mny ,b”rsts are on the order of 5 km acroes md are twisting donbursts tiich show rotational as much s=ller and stronger than the =in well as divergent ti”d patterns, and & tis even dondraf ts. ~is discrepancy led E-nuel inferred the efistence of a domburst from the (1981) to speculate that donbrsts my k due damge pattern left by a hurricane over land. to a dy~tically distinct mchsnism. He It is difficult to Ignore these coincidental suggests that dombursts are manifestations of occ”rre.ces of do=bursts @th etrong .yclonic the ‘“penetrativedo-draft’” which could account rotation. Yet wst explanations for the do=- for their strength ad s-11 scale. ~. pOteU- burst do exactly ttit. tial for penetrative domdrafts inside a thun- derstorm -ists when cool dry tir overlies 3. PROPOSED ~EOSSTICAL EXPLANATIONS cloudy air of Wgb water content. The updraft+ owdraft confipration in a supercell F“jita (1976) =d F“jita and Byers (1977) thunderstorm my provide this setting. Ewnuel developed a mdel of the domb”rst which is the first theoretician to suggest so= con- accounted for the spearhead echo. They proposed nection htween the storm rotation ad the that the domb”rst is -used by tbe collapse of dowburst tithough, in h2s scenario, the rota- a“ on a large tall cell. The tion serves only to trap air of h2gh liquid potential energy of the cloud top is converted water content and smll vertical velocity into tinetic energy of the descendi~ air which, directly below a region of inflowlng poten- by tict”e of its krge horizontal mw.tum, tially cold air, thus setting up a conducive moves faster th.” wighbortng parts of the sa= environment for Fnetratlve do~draft.. echo. The dom”ard wtion is accelerated by the evaporation of water droplets into the dry None of the tioremntioned =chanism tive air tht mst b entrained along tbe way. been kmnstrated to & the actual cause of dombursts although they are all plausible. The -in domdraft in a =t.re thun- They do provide so- s“ggestion of what co look derstorm is a result of the cooling of dry dd- f.= in the observations. level tic tithin the storm mdlor the cooling of sub-cloudbase air by evaporation. The In sumry, the recurring parts of the downdraft producee an outflow of air kneath puzzle appear to k: a particularly strong =11 the stocm kt the vertical velocities are weak within a line of th”ndersto-; a tiw echo or when the cooled air reaches the surface. There LEWP i“ the =ture stage of the cell; a pst ie often a wst front at the edge of the fronc; eow smll scale rotation; decay of the outflow titb associated tind shear and a dram- parent cell - the echo shape bgins to tic temperat”ra drop. ~e sidlarity htween resemble a spearhead; strong surface tinde, the Fujitats proposed mchanism for domhrsts -d dombrsc, ad =ybe a tornado; =d a possible the Mchanism knom to produce the thunderstorm weak echo trench in the vicinity of the downdraft led SO= scientists to the conclusion strongest tinds. In tb=e latter stages, the that F“jita =s observing ground dmge =used storm is decaying rapidly. W rest of this by the wst front itself. As observations “ork till h concerned with trying to recopize accumulated, it hcam clear ttit the @st these phenomna in t~ radar observations of an front was me of the by ingredients tit that Oklahom thunderston =d tith understanding the dowb.rst -8 . smiler scale, separate just how thy combine to produce the dowburst. phenownon. Caracena (1978) suggests that a large domdraft my mt”rally contain an 4. OVERVIEW OF TSE STNOPZIC S1TUATION ensemble of SU1l iqulsive components of various intensities, and that domb”rsts and On 13 April 1981 during the NSSL Spring microbursts my SIWIY be the stronger ones of Program a wam hudd eomtherly airflow wae pre- these. = also notes that they WY occur wre sent over Oklaho-, tith a cold front oriented comonly than one tight expect from the relati- southwest to northeast mvtng into the state vely few Wblished case studies. from the north”est. Surface and upper air ma- lyses -n b found in the mre detailed descrip- A study was do”. by Caracena and ~ier tions by DiStefano (1983) and Wolfson (19S3). (1979) of a ticroburst associated tith a thun- derstorm which passed over the Florida ties A sounding taken at Tuttle, OK (Tuttle is Cumlus Experimnt surface =sonetwork, Thy arked tith a triangle in figure 2) shows warn, concluded tht the spearhead echo was moist surface &r, a slisht capping inversion .,eymptomtic of strong b“ndary layer forcing at 850 d and an approximately dry adiabatic and wi. t”re f1“% convergence”. This, however, lapse rate up to 500 mb (figure 1). This did not explain why or how ticrobursts so”ndi”g was taken at 2005 CST, = hour kf ore occurred. The authors noted chat a technique the thunderstorm arrived, and is representative by Foster (1958), based on mist adiabatic of the pre-stem environwnt. The surface descent of domdraft air consisting of a tix- winds are light from the southwest kt at t“re of tidlevel air and “pdraft air, failed to slightly higher levels a southerly low level account for the strength of the observed duds. jet is present. me tinds turn gradually to They suggest that the wcessary additional becom mre wsterly md stronger tith height. sources of negative boyancy dght h “the “ntixed entrain=nt of environ=ntal air into 5. ~PPLER KADAS OATA ANALYSIS the stift a“dlor tbe mlting of a large qnantity of ”’. Much i“fomtion c.. b gained by exa- mining the reflectivity ad radial =locity Although domb”rsts coa in -ny different fields observed by the Normn, OK Wppler radar sizes (Garacena, 1978; Fujita and Wakim.to, (wBO) while the was occurring. Te” 1981) ranging from 1 km to 40 km tith extrems tilt sequences were recorded during the fifty . .

tinutes htme” 2045 and 2135 CST and the AC 2106 the ,,hole,sin the 45 dsz contour rapidly chansing mt”re of this storm required is still evident ht th 40 dBZ contour bs now that all of them be analyzed. Both the protruded southeastward ad a 8=11 hllet Cartesian and radial coordinate systems cen- $~ped resio. of b2gh radial velocities hs tered at NSO, as well as the location of the developed in the sa= place. The east~est storm at various analysis ti=s are shorn in gust front is characterized by a reflectivity fiwre 2. A series of =Ps shining the evolu- cell of greater than 25 dBZ. The outflow air tion of the reflectivity and ~ppler velocity behind the wst fronts appears to b -rging, fields at L.0 km above the ground is presented suggesting = occlusion process. The weak i“ fig”.. 3. Negative values of tippler velo- reflectivity region at the southwestern side of city signify radial mtion toward tbe radar. tk picture b. groin and curved in an ati- cyclonic sense, although this does not appear The storm is very strong at 2047 CST md clearly in the ~ppler velocity field. Notice 2050 CST when reflectivity values ~eatar than also the anticyclonic hook in the 40 dBZ con- 55 dBZ -n k found. Based upon analyses of tour. much larger extent (not shorn kre ) it is clear that there is a very well defined Wst front The dowburst, characterized by low oriented i“ approxi-tely the east-est dicec- equivalent potential temperature (EPT) and tion, as evidenced at 2047 by the east-est ~xi~m tind ~sts, is bom to bve tit the line or arc of enhanced reflectivity titch surface WSonet station just south of CIM (-30, intercepts the right horder at y-35. The ast 25) at 2110 CST (DiStefano, 19a3). I sugge8t front cnrves to kcome mre parallel tith the that the velocity mxim”m and 40 dSZ protru- cold front slightly farther to the east. It is sion at 2106 are due to the domburst. The not clear “hich th””derstorm cell tis produced reflectivity tiniwm or “’hole’”at 2102 appears the outflow responsible for this east-est to b related to the dowburst and mY be an oriented W.t front ht it is probably a cell indication of a newly fo-d updraft.“ Note to the northeast of the one depicted at 2047, that these features are quite distinct brizon- or perhaps it is a number of different cells tally from the dry region In the southwest. along the front whose outflows bve =rged. There is another ~st front present which.is At 21lZ CST there is continued dry intr”- definitely d“e to the outflow from the &picted sion from the west =d a suggestion of a “weak cell. It is oriented northeast to southwest echo trench”’or “’spearheadtrench”’tith tbe and is evidenced by the tight reflectivity gra- spearhead hing the deflection of the reflec- dient from 15 to 40 dBZ (Wakimto, 1982) on the tivity contours probbly due to the do=bursc. southeast side of the hfgh reflectivity core. It is not at ~1 clear tbt the dry air to the In the following discussion I refer to these west ad the spearhead are -usally connected. as two sepsrate mst fronts although, as the The stem b. =.kened ~eatly -d eve. the 50 cell evolves, this distinction bcows so%wbat dBZ region is breaki% up md shritiing in artificial. size. me lobe of Mgh reflecttvity extending southeastward tith dry air bhtnd it is the At 2047 CST there is a closed 15 dBZ con- old east~est @stfront. There WY actually ~ tour on the east-est @st front. BY 20S0 this another do=krs t occurring at this ti= at has grown to a 25 dBZ closed mntour and at x--33, Y-35 where the reflectivity tinfmm 2057 CST there is m longer any distinction exists in approxi=telv the sa= Dlace relative between this region and tbe min echo. The to the mre of the sto~m and the astfronts as =in echo b., however, chansed shpe con- did ttit at 2102 CST. siderably. The rapid growth of this =11 ws probably due to tbe increased convergence of In the series of pictures from 2117 to i“flowins air -a. the junction of the ast 2132 not shorn kre (see Wolfson, 19S3) the frente. In a less detailed view, this behavior min echo continues to elongate and curve, might suggest tbe fomtion of a spearhead echo developing into a bw shape while increased with the parent echo king dram into the cyclonic rotation is evident in the Doppler appendage. At the saw tim there is a south- velocity field. This cyclonic rotation is con- ward protnsion and a suggestion of cy.ionic firud in the tind field from a Z-Doppler ana- turning of the outflow air behind the north- lysis at 2130 CST. The weak reflectivity south ~st front. This mtion is particularly regfon.in tbe wst COntinUes tO infiltrate in evident is the -25 d. isodop (line of constant an anticyclonic =nner what was the -in core Doppler velocity) and in the increasing wlo- of the cell and the Wstfront structure city gradient btween 2047 and 2057. gradually disintegrates.

By 2102 CST the cell looks vev different. RH11s constructed from sector scans of the The east~est @st front is still present ad a sterm taken while the domburst was occurring “ew closed 15 dBZ contour has appeared. A ‘“hole” clearly show the low level (below 2.5 km) hish has developed in the 45 dBZ contour &hind the speed outflow current, a strons Gadient of north-south Pst front close to the increased Doppler veloctiy across the Wstfront, and a area of mximm mppler velocity. The eaatern leading patch of tigh speed air at the head of portion of the -25 ds isodop &s becow wre the oucflow which corresponds to the domburst. rounded and extended southeastward while the Dry air entering the storm from tbe rear above northern edge tis been deflected 8trongly 6 b is ~so evident. southwestward suggesting a substantial increase in the cyclonic rotation. Note that the reflec- 6. A NZW STPOTSBSIS tivity field is less than 20 dsz at the -stern edge of the depicted do~in around Y-25 and that In this section I present a new hypothesis, a cell of greater than 45 dBZ is evident on the developed using the preceding Doppler radar southern edge of the do=in around x-6a. observations as a Wide line, for thunderstorm

1 r“ ,,

i dombursts. It is different from previone hypo- The .onfi@ration of the @stfront, mrked theees in that dynatic rather than therwdy~tic by che cold front hundary in fi~re 4, is very processes are proposed to b responsible for much like ttit shorn at 2102 CST. There is an the dombursts. Tkis =W hypothesis accounts updraft denoted by a solid contour =ar the well for the rotational characteristic of dom- occlusion point of the @stfront in 4 which is bursts reported in other observational studies in a region of strong convergence. It was in a which hve wt &en e~lained by the earlier coqarable region of the 13 April stem ttit proposals and it -kes the connection htween cells ahead of tbe -in stem cell were grming the uw, the hw ad spearhead echoes, the and subsequently mrging tith che ain =11. cyclonic rotation to the north ad the mti- It was also in this region that an updraft was cyclonic rotation to tbe south, the @st front, inferred from the observations. Tb domburst, the hook echo, and the tornado. The proposed denoted by th s-11 circular &shed contour in mechanism differs little from the =chanism fimre 4, is close to tit south and weet of the determined, in a ft.. wsh (.25 b) numrical updraft and is khind the mrth-south oriented simlation by Uemp ad Kotunno (1983), to pro- portion of the wstf rent, just as in the analy- duce a s-11 scale “occlusion domdraft<’ near sis at 2106. Notice also tbe cyclontc cur- the tornadic region in a supercell thunderstorm. vature of the flow aromnd the updraft-domburst This -w hypothesis is siqly stated klow and area ad the anticyclonic curvature south and not proved, for although diagnostics WY k per- we$t of there. for=d, they are kyond the scope of this work. The potentially cool and dry air (low EPT) The basic pretise is that tien a cell along thought to cbracterize the dowburst my well a line or front kco=s very strong and organized be a property of the entire thunderstorm it bgins to take on sow supercell chracterie- outflow. It is likely tbt the low EPT air is tics, particularly in its postmture stage. already in the bu.dary layer when the dom- Horizontal vorticity, which is present in the burst occurs. low level environ~ntal shear and tiich is generated by the horizontal hoyan.y ~adients ln s“-ry, I view the thunderstorm dow- in the stem, is tilted into the vertical near burst as a inherently low level pheno-non, the -i” updraft. Strong low level convergence, “Ith essentially all of the iwortant ingre- caused by the domdraft .mtfIow from the col- dients contained in the lowest 3 to 4 b of the lapsing cell, forces stretching of the now ver- atmosphere. It till appear in the post=ture tically oriented vortex tubes. Siwificant to collapsing ptise of the cell’s evolution. CYC1OUIC rotation &gins at low levels and, as DPatic vertical accelerations could wll h a“ a result, the wst front &gIns to occlude. order of -gnitude larger than them. dymtic Anticyclonic vorticity is aleo generated tit (buoyancy) accelerations at low levels. It is the cyclonic vorticity appears to b favored. apparently tisleadlng to look for the source The horizontal curvature of the flow resulting height of CM domb”rsc. It is induced by a from tbe cyclonic rotation promtes do-ndraft horizontal =soacale circulation at low levels intensification along the backside of the @st- which marly, or =t”ally does, hcom tor- frent, especially near the circulation -nter, nadic. tithough comonly thought to b -inly by dynamically inducing a vertical pressure divergent, tb domburst is really a rotational gradient. ~is acts to rapidly accelerate dr phenownon. It is also apparently misleading domward causing the dowb.rst. The domburst to we coneerved properties such as EPT or dght even form first at low levels and then horizontal mmnt”m to trace the domb”rst dr; extend upward as the flow adjusts to the dyna- those properties only Wlp indicate the source tic pressure gradient. A sche=tic tiagram height of the outflow air from the whole storm. depicting the proposed =soscale circulation responsible for distorting the radar echo md This is a novel view of the thunderstorm inducing the dombursts is presented in fimre 4. downburst =d it bs not ken proved. It does account for the observations and link phenomna which were ~om to occur simultaneously ht were thought to k disconnected. Calculations must h done before its real relevance can b detehned.

?. CONCLUSIONS ~ FUTW WOW

The mjor thrust of this mrk tis ken the analysis and presentation of detailed Doppler radar data collected during an Oklaho= thun- derston i. which domb.rsts developed. me distinctive Ww or -velike radar echo asso- ciated tith this type of thunderstorm was determined co k caused by the low leYel cyclo- nic rotation. me spearhead echo, identified as a pointed appendage in the direction of motion, ma determined to k a cell growing Fig. 4. Propoeed circulation inducing dow- along an occluded wst front ahead of the min bursts. Dashed contours indicate domdraf ts, storm cell. me ~owth of this cell is an solid contours, updrafts and the stippled area indication of the Increased convergence in the represents reflectivity greater than 30 dBZ. region of Wst front occlusion. The s=ll scale do-draft near the center of circulation is the doab”rst. After ~e~ and Rotunno. 1983. , ,. . , . . f

Careful e=tination of the &ta at dif- Foster, D. S. 1958: Thunderston @ecs compared ferent time MS led ‘co the identification of with computed do-draft speeds. ~n. Wea. low level convergence and vorticity as =j or ~., 86, 91-94. factors in downb”rst develop=nt. A new hypothesis was presented which identifies Fujita,T. T., 1976: Spearhead echo ad don- tilting of horizontal vorticity into the ver- burst war the approach end of a J. F. tical and a subsequent increase in vclonic -nnedy airport mnway, mC. SW rotation as the crucial elemnts in creating Research Paper 137, Univeristy of Chicago. the dynatic pressure gradient proposed to k Fujita, T, T., 1978: bnul of dombnrst iden- responsible for the dowbnrst. tification for Project NI~OD. SMSP Research Paper 156, University of Chicago. Since mst observations to &te hve been viewed tith thermodynamic =chanism in F“jita, T. T., 1979: ObjectIves, operation, mind as the -“se of the dombursts, 1 believe and results of project W~OD. Preprints, it would bs fruitful to review them in light Eleventh Conference on Severe Local of this new hypothesis. Esti-tea ued to k Stor-, tinsas City, ~. mde of the rate of vorticity production, the curvature of the flow, the mrtical gradients Fujita, T. T., 1981: Tornadoes and downbursts of vorticity md dynatic preesure, and the in the context of generalized planetary total vertical acceleration in the vicinity of scales. J. Atms. Sci., 38, 1511-1534. the dow”btirst. A &tailed, general scenario needs to h developed for the evolution of the Fujita, T. T., md H. Byers; 1977: Spearhead thunderstom downbursts for, &though the thun- echo and domb”rst in the crash of an derstorm tbewelves are fairly predictable and airliner. ~ 105, 129-146. easy to track, tbe accurate prediction of the downbursts &s eluded wteorologists. Fujite, T. T., and R. Wakimto, 1981: Five scales of tirflow associated tith a series The theoretical work on thutierstom dom- of domtirets m 16 July 1980. Mon. Wea. burscs ks bsrely bgun. Although the =alogy —*v., 109, 1438-1456. is wt complete, it my h possible to exploit som of the work which hss ben done on super- Hadlton, R. E., 1970: Use of &tailed inten- cell storm, -socyclones, and tornadoes, tith sity radar &ta in =soscale surface ana- a few modifications, to klp mderstand the lysis of the July 4 1969 storm in Ohio. line cell circulation. ~ce the dowhrst wt- Preprint8, Fourteenth Wdar &teorology tern is =11 recognized, ftit”retits gathering Conference, 339-346. e~eri=nts an h designed specifically to obeerve them. I aution the =teorological Kle~, J. B. md R. tit”nno, 1983: A study of cownity against drawing =eeping conclusions the tornadic region tithin a supercell about donbursts in all parts of the country th”nderstom. J. Atws. Sci., 40, 359-377. from the results of the JAWS experiwnt. The virga ticrob”rsts obeerved there are bzardous Nolen, R. H., 1959: A radar pattern =sociated and very difficult to predict tit they are “ith tornadoes. ~ 40, perhaps characteristic of that local environ- 277-279. ment. Theoreticians met mt channel all of their efforts into understanding virga ti.ro- Wakimto, R., 1982: T~ life cycle of thunder- bursts *il. the mre w=. md equally tizar- storm Wst fronts as viewed tith ~ppler do”s thunderston dombursts go unexplained. radar and rawinsode data. Mn. Wea. @v. , 110, 1060-182. REFERENCES Wolfson, M., 1983: Doppler radar observations Caracena, F., 1978t A comparison of 2 dom- of an Oklahom domb.rst. ~ster’s bursts of different size ecales. Thesis, tissach”setts Institute of Preprints, Conference on Weather Technology. Forecasting and -lysis ad Aviation Msceorology. Silver Springs, ~, 293-300.

Caracena, F. and M. tiier, 1979: halysia of a microburst in the FACE =teorological =sonet”ork. Pre.rints.. . Eleventh Conference on Severe bcal Storm, ~nsas City, MD, 279-286.

DiStefano, J., 1983: balysis of a thunderstorm downbnrst. Master’s Thesis, tissachusett8 Inetit”te of Technology.

Emnuel, K., 1981: A similarity theory for “neat”rated domdcafts tithtn clouds. J. Atmos. SCi., 38, 1541-1580.

Forbes, G., and R. Wakiwto, 1983: A concen- trated outbreak of tormdoes, dombursts, and dcrobrsts on 6 hwst 1977. k. Waa. Rev. 111, 220-235.

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