Mobile Mesonet Observations on 3 May 1999

430 VOLUME 17W E A T H E R A N D F O R E C A S T I N G

q 2002 American Meteorological Society

Mobile Mesonet Observations on 3 May 1999

PAUL M. MARKOWSKI

Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

(Manuscript received 19 January 2001, in final form 7 May 2001)

ABSTRACT

Two long-lived tornadic supercells were sampled by an automobile-borne observing system on 3 May 1999.The ‘‘mobile mesonet’’ observed relatively warm and moist air, weak baroclinity, and small pressure excess atthe surface within the rear-flank downdrafts of the storms. Furthermore, the downdraft air parcels, which havebeen shown to enter the tornado in past observational and modeling studies, were associated with substantialconvective available potential energy and small convective inhibition. The detection of only small equivalentpotential temperature deficits (1–4 K) within the downdrafts may imply that the downdrafts were driven primarilyby nonhydrostatic pressure gradients and/or precipitation drag, rather than by the entrainment of potentially coldenvironmental air at midlevels.

1. Introduction

Direct measurements of meteorological variables neartornadoes have been relatively scarce, owing to the rar-ity of the phenomenon and the spatial and temporalfrequency of standard observations. A few fortuitousdatasets have been analyzed by Tepper and Eggert(1956) and Fujita (1958). Storm intercept field pro-grams, first organized in the 1970s, have obtained ad-ditional observations within supercell storms (e.g.,Golden and Morgan 1972; Bluestein 1983; Davies-Jones1986); however, our collection of in situ measurementswithin supercells remains miniscule.

A fleet of instrumented automobiles was designed byStraka et al. (1996) for use in the Verification of theOrigins of Rotation in Tornadoes Experiment (VOR-TEX; Rasmussen et al. 1994), conducted on the U.S.Great Plains during the springs of 1994 and 1995.Coined the ‘‘mobile mesonet,’’ this platform collectedsurface data within severe storms with unprecedentedspatial (100–1000 m) and temporal (10–60 s) resolu-tion. The mobile mesonet also has been used in sub-sequent years following VORTEX, with the spring of1999 being exceptionally fruitful for operations. Thispaper summarizes a few noteworthy observations madeon 3 May 1999, during a significant outbreak of tor-nadoes in Oklahoma and Kansas.

During the evening of 3 May, the mobile mesonetintercepted a pair of tornadic supercells in southwesternand central Oklahoma (Fig. 1). The first supercell was

Corresponding author address: Dr. Paul Markowski, The Penn-sylvania State University, 503 Walker Building, University Park, PA16802.E-mail: [email protected]

intercepted approximately 25 km north of Fort Sill,Oklahoma, shortly after 2200 UTC (this storm will bereferred to as storm A to be consistent with NationalWeather Service surveys of the event). A second su-percell was intercepted farther west around 0000 UTC(this storm will be referred to as storm B). Detailedaccounts of these intercepts appear in section 3, follow-ing a description of the analysis techniques in section2. A few final comments are provided in section 4.

2. Mobile mesonet data

The mobile mesonet senses temperature, relative hu-midity, pressure, and wind velocity. Time and positionare recorded using a global positioning system receiver.In 1999, data were recorded at 2-s intervals. A completelist of instrument specifications (including responsetimes and errors) is available from Straka et al. (1996).

Vehicle velocities were removed from the wind ve-locity data when vehicle accelerations were small. Inthe presence of significant vehicle accelerations (.1 ms22), accurate wind data could not be obtained. Fur-thermore, field operations relied on the use of radiocommunication. On occasion, radio frequency interfer-ence caused large errors to arise in the meteorologicalmeasurements. Data with these gross errors were re-moved prior to analysis. In addition, biases in the datawere removed by way of intercomparisons between ve-hicles. The intercomparisons were made in relativelyquiescent weather conditions while the vehicles weremoving as a caravan (e.g., en route to a storm).

The quality-controlled observations used in the anal-yses were averaged over 12-s intervals. Data were plot-ted relative to radar echoes using time-to-space con-

JUNE 2002 431M A R K O W S K I

FIG. 1. The tracks of storms A and B, with the regions indicated where mobile mesonet datawere collected. The radar echoes outline the 30-dBZ base reflectivity values at the 0.58 elevationangle of the Twin Lakes, OK, KTLX WSR-88D; times (UTC) also are included for each radarecho. Tornado paths are gray (storm A) and black (storm B). The surface stations SWO, CSM,OKC, and FSI are Stillwater, Clinton, Oklahoma City, and Fort Sill, respectively.

version, assuming that the features being analyzed didnot change significantly over the time interval duringwhich the measurements were made (the ‘‘Taylor hy-pothesis’’). Supercells are not steady. If they were, thentornadogenesis could not occur. However, steadiness as-sumptions, at least for short time intervals, are virtuallyunavoidable in observational studies (e.g., multipleDoppler radar analysis). For the research herein, featureswere assumed to be steady for 63 min, with respect toan analysis time. This is approximately the length oftime that it takes for a Weather Surveillance Radar-1998Doppler (WSR-88D) to complete a volume scan. Thereis some confidence that such steadiness assumptionswere not too severe, for the analyzed fields tended tobe free of noise [inappropriate steadiness assumptionslead to the artificial creation of warm and cold (or moistand dry) pockets in the analysis following a time-to-space conversion].

In addition to the raw thermodynamic data recordedby the mobile mesonet, several derived variables werecomputed. Virtual potential temperature uy was com-puted, with the inclusion of liquid water effects, in ad-dition to the contribution from water vapor. The liquidwater content was parameterized from the WSR-88Dreflectivity at the lowest elevation angle, using the meth-od of Rutledge and Hobbs (1984). Equivalent potentialtemperature ue was computed by lifting a surface parceladiabatically to 100 hPa, where the potential tempera-ture of the parcel was assumed to be equal to the ue of

the parcel. Pressure p was reduced to the average heightof the vehicle observations (407 m) using U.S. Geo-logical Survey Level-2 Digital Elevation Model data.Convective available potential energy (CAPE) and con-vective inhibition (CIN) were computed for parcels byinserting surface thermodynamic measurements ob-tained from the mobile mesonet into the 0000 UTC 4May sounding at Norman, Oklahoma (located approx-imately 25 km south of Oklahoma City; see Fig. 2).1

The buoyancy integrations in the CAPE calculationswere terminated at 500 hPa, owing to the loss of thesounding data at upper levels. Virtual temperature ef-fects were not included in the CAPE and CIN calcu-lations. Whereas uy (more precisely, its fluctuation) canbe considered a measure of parcel buoyancy, CAPE andCIN values can be viewed as measures of the potentialbuoyancy of a parcel.

The fluctuations of uy , ue, and p (denoted by a prime,e.g., ) from a base state (denoted by an overbar, e.g.,u9y

y , where 5 uy 2 y ) were analyzed instead of theu u9 uy

absolute quantities themselves. The base state was de-

1 It always is possible to debate the representativeness of a sound-ing. The 0000 UTC Norman sounding was obtained during the life-times of the tornadic supercells and from within 75–100 km of thestorms that were intercepted; however, it will be evident later thatthe environment sampled by the Norman sounding, at least near thesurface, differed from the environment to the west in which the su-percells were occurring (Fig. 3).


FIG. 2. (left) Skew T–logp diagram from Norman, OK, at 0000 UTC 4 May 1999. The hodograph is shown in the inset, with 10 m s21 speedrings and numerals along the hodograph indicating heights AGL (km). (right) Vertical profile of ue derived from the Norman sounding.

fined at the storm location by interpolating from smoothsubjectively analyzed contours obtained from OklahomaMesonet stations (Brock et al. 1995; Fig. 3). All defi-nitions of a ‘‘base state’’ are arbitrary (and potentiallyproblematic); the method used herein is similar to thatused by Fujita (1955) and Charba and Sasaki (1971).

The uncertainty of perturbation variables (denoted byd) depends on the propagation of instrument errors (e.g.,temperature, relative humidity, and pressure errors), un-certainties associated with the estimation of liquid waterin the case of , and specification of the base state. Au9ydetailed analysis of these effects appears in Markowski(2000). On 3 May 1999, it has been estimated that dp9ø 0.8 hPa, d ø 0.5 K, d ø 2.5 K, dCAPE ø 110u9 u9y e

J kg21, and dCIN ø 9 J kg21.The mobile mesonet comprised three vehicles on 3

May 1999, in contrast to the much larger assembly usedfor VORTEX. The goal of the field operations of 1999was to sample the rear-flank downdraft (RFD) regionof supercells (Lemon and Doswell 1979), rather thanattempt to sample a broad inflow region, as was the caseduring VORTEX (Rasmussen et al. 1994). Thus, asmaller array of vehicleborne instruments could accom-plish the sampling goal. Furthermore, a smaller fleethad some logistical advantages; for example, coordi-nation among mobile mesonet crews and with the now-caster was considerably easier.

The mobile mesonet RFD sampling strategy was fa-vored for several reasons. In environments initially ab-sent of vertical vorticity (i.e., vortex lines are quasi-horizontal), a downdraft is necessary for intense verticalvorticity to arise at the ground (Davies-Jones 1982; Da-

vies-Jones and Brooks 1993; Walko 1993). Even as ear-ly as 1975, Fujita (1975) hypothesized that the angularmomentum transport by a downdraft may be critical totornadogenesis. Burgess et al. (1977) and Barnes (1978)also made similar speculations. Many studies havefound that the air parcels that supply the tornado passthrough the RFD. For example, observations by Brandes(1978), Lemon and Doswell (1979), Rasmussen et al.(1982), and Jensen et al. (1983) have shown or implieda near total occlusion of the low-level mesocyclone bythe RFD prior to tornadogenesis. Furthermore, Brookset al. (1993), Wicker and Wilhelmson (1995), and Ad-lerman et al. (1999) have found that the trajectoriesentering the near-ground circulations in their numericalsimulations passed through the hook echo and RFD.Given the prior emphasis on the RFD in the tornado-genesis process and the apparent consensus that RFDair parcels enter the tornado, it is believed that the ther-modynamic characteristics of hook echoes and RFDsnaturally assume importance. For this reason, the focusof the next section is on the observations obtained large-ly within the RFDs of the intercepted tornadic super-cells.

3. Observations

a. Storm A

The mobile mesonet approached storm A around 2200UTC. The supercell was associated with two brief, weaktornadoes at 2151 and 2155 UTC, before an interceptcould be engaged. At 2220 UTC, the supercell produced


its first significant tornado.2 The tornado persisted for15 min, although it did not do significant damage alongits 10-km track (F1 Fujita-scale rating). The RFD as-sociated with the circulation was sampled by the mobilemesonet west-southwest through east-southeast just pri-or to tornadogenesis, and during the lifetime of the tor-nado all three vehicles trailed the tornado to the south-west, within the RFD.

At 2219 UTC, 1 min before tornadogenesis, the mo-bile mesonet detected relatively small uy deficits withinthe RFD and inflow of less than 2 K (Fig. 4). The RFDparcel temperatures were 1.0–1.5 K warmer than theinflow parcels. The lowest ue values, located within thehook echo, were approximately 4 K less than the averageinflow values (Fig. 5); however, these ue values (;345K) were larger than any ue values observed on the 0000UTC Norman sounding (Fig. 2), because of the sound-ing being launched east of the axis of maximum ue

values (Fig. 3). Therefore, estimates cannot be made ofthe altitude from which RFD parcels may have descend-ed (nor could estimates be made even if ue could beassumed to be conserved in the absence of entrainment).The relatively small uy and ue deficits (and temperatureexcess) of the downdraft air parcels also were associatedwith small CIN and substantial CAPE (Fig. 6). CINvalues as small as 2 J kg21 were detected in the hookecho, and CAPE values in the RFD generally exceeded900 J kg21 in the lowest 500 hPa (it is estimated thatthe total CAPE associated with the RFD air parcels was;3000 J kg21).

The mobile mesonet also sampled a small pressureexcess (,1 hPa) in the RFD at 2219 UTC, except withina few hundred meters of the intensifying circulationcenter, where a small pressure deficit (about 21 hPa)was detected (Fig. 7). A small pressure deficit also wassampled in the inflow east of the gust front. Streamlinesrevealed difluence at the surface within the RFD, withthe strongest difluence (and divergence) apparently sit-uated in the hook echo (Fig. 4). Anticyclonic verticalvorticity also was present within the RFD to the southand east of the incipient tornado.

The mobile mesonet was unable to sample the RFDwithin 1 km of the tornado at later times; however, datawere collected within the RFD approximately 2–5 kmsouthwest of the tornado until its demise. At 2229 UTC(6 min prior to tornado dissipation), only small uy def-icits were detected again in the RFD; however, largerue deficits were sampled (and correspondingly smaller,yet significant, CAPE values of ;200–400 J kg21 below500 hPa), with deficits as large as 7–8 K recorded a fewkilometers west-southwest of the tornado (Fig. 8). By2234 UTC (1 min prior to tornado dissipation), the uy

and ue deficits within the RFD appeared to increaseslightly, with uy deficits exceeding 2 K and ue deficits

2 The tornado is referred to as significant because of its longevity.

as large as 10 K being detected within 3 km of thetornado to its west and southwest (Fig. 9).

The relatively long-lived tornado dissipated at 2235UTC, and another tornado was produced by storm A at2246 UTC. The tornado produced F3 damage and lasteduntil 2310 UTC, but the tornado could not be closelyintercepted for logistical reasons. The road network didnot allow sampling within several kilometers of the tor-nado, and many roads became obstructed by debris.Storm A went on to produce another significant tornadonear 2323 UTC, which produced F5 damage in the sub-urbs of Oklahoma City. This tornado also was not in-tercepted for logistical reasons (e.g., increasing amountsof traffic near the metropolitan area, debris-filled road-ways). Instead, the mobile mesonet abandoned storm Ain pursuit of another tornadic supercell to the west(storm B).

b. Storm B

The mobile mesonet arrived at storm B at approxi-mately 0000 UTC (4 May). Storm B produced severalbrief, weak tornadoes from 2236 to 2324 UTC. Anothertornado was reportedly on the ground for 21 min from2338 to 2359 UTC, but this tornado dissipated beforedata could be collected. From 0000 to 0100 UTC, nu-merous additional tornadoes (as many as eight) werereported, most of which were brief and weak (F0–F1damage ratings), although two tornadoes persisted forlonger than 5 min. Data were collected within a fewkilometers of the surface circulation centers at severaltimes from 0000 to 0100 UTC.

At 0026 UTC, storm B was not producing a tornado,but brief tornadoes were reported just before and afterthe analysis time (at 0020 and 0034 UTC). The mobilemesonet obtained data within 0.5–2.0 km of the me-socyclone center at the surface. Within 3 km of thecirculation center, the most negatively buoyant air sam-pled within the hook echo and RFD region was asso-ciated with a uy deficit of less than 1 K (Fig. 10). Fur-thermore, the baroclinity at the surface within the hookecho was very weak, with maximum | =huy | values ofless than 0.5 K km21 (where =h is the horizontal gradientoperator). The relatively warm air within the downdraftalso was associated with small ue deficits (Fig. 11).Within the hook echo, ue values were nearly the sameas in the inflow, and ue values were only 1–2 K lessthan inflow values in a zone that appeared to wraparound the north side of the circulation center. As wasthe case for storm A, all of the surface ue values mea-sured by the mobile mesonet in storm B were largerthan the maximum ue value observed on the 0000 UTCNorman sounding. It also can be inferred that the CAPE(CIN) within the RFD of storm B at 0026 UTC waslarge (small).

The mobile mesonet made additional penetrations ofthe hook echo and RFD near 0043 UTC. A tornado,which formed at 0037 UTC, was observed at the anal-


FIG

.3.

Mes

o-a

-sca

leho

urly

subj

ecti

vean

alys

esof

u y(K

),u e

(K),

and

pres

sure

[hP

a;re

duce

dto

the

mea

nel

evat

ion

ofth

em

obil

em

eson

etob

serv

atio

ns(4

07m

MS

L)]

from

2200

to01

00U

TC

3–4

May

.D

ata

wer

eob

tain

edfr

omth

eO

klah

oma

Mes

onet

.S

tati

onm

odel

sdi

spla

y(r

eadi

ngan

ticl

ockw

ise,

begi

nnin

gw

ith

the

top-

left

valu

e)te

mpe

ratu

re(8

C),

dew

poin

t(8

C),

u y(K

),u e

(K),

and

redu

ced

pres

sure

(hP

a).

Win

dba

rbs

are

rela

tive

toth

egr

ound

,w

ith

each

full

(hal

f)ba

rbbe

ing

equa

lto

5(2

.5)

ms2

1.


FIG

.3.

(Con

tinu

ed)


FIG. 4. Subjective analysis of (K) in storm A at 2219 UTC 3 May 1999 ( y 5 304.4 K).u9 uy

Contours are dashed where significant uncertainty exists owing to sparseness of observations. Theanalysis time is 1 min prior to tornadogenesis. Mobile mesonet station models include (readinganticlockwise, beginning with the numeral at the top left) temperature (in degrees Celsius to thenearest 0.18C with the decimal omitted), dewpoint temperature (in degrees Celsius to the nearest0.18C with the decimal omitted), uy (in kelvins to the nearest 0.1 K with the decimal omitted),and ue (in kelvins to the nearest 1 K). Wind barbs depict storm-relative winds [each full (half )barb equals 5 (2.5) m s21]. A few streamlines have been drawn in gray. Mobile mesonet obser-vations have been averaged over 12-s intervals, and a steadiness was assumed for a period of 63min (with respect to the analysis time) in the time-to-space conversion. Observations obtainedmore than 1 min before or after the analysis reference time are ‘‘flagged’’ with a vertical barthrough the center of the station model. Storm-scale fronts are depicted using conventional frontalsymbology, and their placement has been aided by inspection of mobile radar data provided byJ. Wurman. The M indicates the position of the mesocyclone center on the lowest radar elevationangle available. Base radar reflectivity data were obtained from the 0.58 elevation angle of theKTLX WSR-88D, with the reflectivity scale (dBZ ) included at the bottom. The region analyzedis indicated in the larger-scale inset.

ysis time. The tornado dissipated at approximately 0048UTC. Again, only small uy and ue deficits were detectedwithin the RFD and hook echo (Fig. 12), with substantialCAPE and small CIN also being present within thedowndraft at the surface (not shown). Baroclinity withinthe hook echo also was weak (maximum | =huy | of ;1K km21).3

At 0052 UTC, the RFD and hook echo region of stormB were again sampled relatively well, and another tor-nado was in progress at the time (the tornado formedat 0047 UTC and dissipated at 0100 UTC). As at earliertimes, only small uy and ue deficits (,2 K) were ob-served, and baroclinity was weak (Fig. 13; maximum

3 The adjective ‘‘weak’’ is used to describe the baroclinity becausea value of | =huy | ; 1 K is considered to be small on the storm scale.However, by some standards (e.g., on the synoptic scale), a gradientof this magnitude would be considered to be enormous.

| =huy | of ;1 K km21 detected north of the tornado).Streamlines diverged within the hook echo, and a cou-plet of vertical vorticity appeared to straddle the hookecho. Anticyclonic vertical vorticity, estimated to beapproximately 25 3 1023 s21, was observed at thesurface on the side of the hook echo opposite the stron-ger, cyclonic vertical vorticity associated with the tor-nado. CIN values were very insignificant (,10 J kg21

within 5 km of the tornado in the RFD), and RFD parcelsalso were associated with large CAPE (.800 J kg21

below 500 hPa; Fig. 14).Surface pressure gradients were weak at 0052 UTC

at distances of more than 500 m from the tornado, sim-ilar to observations in storm A (Fig. 15). A small pres-sure excess (,1 hPa) was detected within the RFD, anda small pressure deficit (also ,1 hPa) was measuredwithin the region of cyclonic vertical vorticity associ-ated with the tornado parent circulation. The relatively


FIG. 5. As in Fig. 4 but (K) is analyzed ( e 5 350.0 K).u9 ue

FIG. 6. As in Fig. 4 but CIN (J kg21) is analyzed. CAPE (below 500 hPa) and CIN valuesappear to the left and right, respectively, in each station model.


FIG. 7. As in Fig. 4 but p9 (hPa) is analyzed ( 5 953.1 hPa). Reduced pressure values, withpthe leading 9 omitted, appear in each station model. Values are to the nearest 0.1 hPa, with thedecimal omitted.

FIG. 8. As in Fig. 4 but at 2229 UTC 3 May 1999. The analysis time is 9 min aftertornadogenesis and 6 min prior to tornado dissipation. The T indicates the tornado location.


FIG. 9. As in Fig. 4 but at 2234 UTC 3 May 1999. The analysis time is 14 min aftertornadogenesis and 1 min prior to tornado dissipation.

FIG. 10. As in Fig. 4 but storm B is analyzed at 0026 UTC 4 May 1999 ( y 5 304.0 K). Noutornado was in progress at the analysis time, but tornadoes were observed shortly before and after0026 UTC (at 0020 and 0034 UTC).


FIG. 11. As in Fig. 10 but (K) is analyzed ( e 5 349.0 K).u9 ue

FIG. 12. As in Fig. 10 but at 0043 UTC 4 May 1999. The analysis time is 6 min aftertornadogenesis and 5 min prior to tornado dissipation.


FIG. 13. As in Fig. 10 but at 0052 UTC 4 May 1999. The analysis time is 5 min after torna-dogenesis and 8 min prior to tornado dissipation (this is a different tornado than that which appearsin Fig. 12). A few streamlines also have been drawn.

FIG. 14. As in Fig. 13 but CIN (J kg21) is analyzed. Station models are as in Fig. 6.


FIG. 15. As in Fig. 13 but p9 (hPa) is analyzed ( 5 951.5 hPa). Station models are as in Fig. 7.p

weak pressure gradients are somewhat consistent withobservations made by the mobile mesonet crews of‘‘light winds’’ within a few kilometers of the tornadoes,both in storms A and B (J. Straka 1999, personal com-munication).

Storm B produced at least six additional tornadoes,some strong, after dark, which arrived between 0100and 0130 UTC. Data collection operations by the mobilemesonet were terminated near 0130 UTC. The supercelldissipated in northern Oklahoma around 0400 UTC.

4. Discussion and closing remarks

The observational data presented herein have severallimitations. First, road networks do not allow continuoussampling of moving updrafts for periods longer thanabout 5 min before repositioning of the vehicles requiresthat they temporarily forfeit data collection in criticalregions of the storm; therefore, the time evolution offeatures is difficult to document. Furthermore, steadi-ness reluctantly was assumed for durations as long as6 min (63 min from the analysis reference times) inconstructing the analyses, to maximize the coverage ofdata (gathered by a finite number of vehicles). Second,thermodynamic fields and their gradients cannot be as-certained above the surface by direct means. At best,only the sign of the gradients can be inferred above thesurface, based on assumptions of the lapse rates beneathand at a distance from the storm. Third, time historiesof air parcels are important, possibly more than 30 minprior to tornadogenesis. It was not possible to compute

trajectories at the surface because of inadequate obser-vation density.

Despite the above impediments, substantial evidencewas presented of the following characteristics of the 3May 1999 tornadic supercells:

1) RFDs were associated with small uy and ue deficits,2) RFDs contained substantial amounts of CAPE and

small amounts of CIN,3) hook echoes lacked strong baroclinity at the surface,

and4) pressure fluctuations and gradients were small at dis-

tances greater than approximately 250 m from thecirculation centers.

It is perhaps most intriguing that the RFDs were as-sociated with what one might consider to be ‘‘small’’signatures, both kinematically and thermodynamically.

The relatively small uy and ue deficits within the RFDsmay imply a descent largely forced by nonhydrostaticpressure gradients, rather than the entrainment of low-ue environmental air at midlevels, and subsequent gen-eration of negative buoyancy by evaporative chilling,as in long-standing conceptual models (e.g., Browningand Ludlam 1962; Browning and Donaldson 1963). Vi-sual observations by mobile mesonet volunteers (e.g.,J. Straka 1999, personal communication) suggested thatthe hook echoes were composed of only a thin veil ofmainly large raindrops; thus, precipitation loading mayonly have been of secondary importance in the down-draft forcing. Furthermore, it is worth reiterating thatthe observations of RFD thermodynamic properties sim-


ilar to the properties representative of the inflow do notnecessarily imply small vertical downward excursionsfor RFD air parcels. The depth of descent cannot beascertained from the surface ue measurements alone.

Some evidence also was presented of vertical vortic-ity couplets straddling the divergence maxima withinthe RFDs (roughly collocated with the hook echo), aswas presented by numerous other studies at low levels(e.g., Ray et al. 1975, 1981; Brandes 1977, 1978, 1981;Fujita and Wakimoto 1982; Wurman et al. 1996). Thisprobably is evidence that RFDs are involved in a down-ward displacement of vortex lines, perhaps necessarilysupplying angular momentum to the tornado, as manyothers have previously conjectured (section 1).

It is speculated that some of the above intriguingfeatures of the 3 May 1999 tornadic supercells, partic-ularly the relative warmth and moistness of the down-drafts, may be relevant to the problem of tornadoge-nesis. Does tornadogenesis probability, longevity, andintensity increase as RFD parcels become more buoy-ant? If buoyant RFDs are propitious for tornadogenesis,are there any large-scale environmental conditions fromwhich we may anticipate warm, moist RFDs? It is gen-erally accepted that only a relatively small percentageof supercells are tornadic; yet, on 3 May 1999, nearlyall were tornadic. This fact may suggest that large-scalefactors may exist, at least in some cases, from whichfavorable RFD thermodynamic characteristics can beinferred. Additional cases and implications and, it ishoped, some answers to the above-posed questions willbe the subject of a series of companion papers.

Acknowledgments. I am most grateful to all of thevolunteers whose countless personal sacrifices madedata collection possible. I am indebted to Drs. JerryStraka and Erik Rasmussen for their support during fieldoperations and for stimulating discussions related to su-percells and tornadogenesis during the last several years.Data-quality checks were performed by Mr. Al Pietry-cha, and indispensable assistance in obtaining digitalelevation model data was provided by Mr. Rob Carver.Dr. Joshua Wurman provided data from the Doppler onWheels mobile radars, which were used to help to iden-tify the position of the gust front in storm A. Mr. MarkShafer provided Oklahoma Mesonet data. I also thankDr. Chuck Doswell and two anonymous reviewers forhelping to improve the manuscript. NSF Grant ATM-9617318 partially supported this research.

REFERENCES

Adlerman, E. J., K. K. Droegemeier, and R. P. Davies-Jones, 1999:A numerical simulation of cyclic mesocyclogenesis. J. Atmos.Sci., 56, 2045–2069.

Barnes, S. L., 1978: Oklahoma thunderstorms on 29–30 April 1970.Part I: Morphology of a tornadic storm. Mon. Wea. Rev., 106,673–684.

Bluestein, H. B., 1983: Surface meteorological observations in severe

thunderstorms. Part II: Field experiments with TOTO. J. ClimateAppl. Meteor., 22, 919–930.

Brandes, E. A., 1977: Gust front evolution and tornado genesis asviewed by Doppler radar. J. Appl. Meteor., 16, 333–338.

——, 1978: Mesocyclone evolution and tornadogenesis: Some ob-servations. Mon. Wea. Rev., 106, 995–1011.

——, 1981: Finestructure of the Del City-Edmond tornadic meso-circulation. Mon. Wea. Rev., 109, 635–647.

Brock, F. V., K. Crawford, R. Elliot, G. Cupernus, S. Stadler, H.Johnson, and M. Eilts, 1995: The Oklahoma Mesonet: A tech-nical overview. J. Atmos. Oceanic Technol., 12, 5–19.

Brooks, H. E., C. A. Doswell III, and R. P. Davies-Jones, 1993:Environmental helicity and the maintenance and evolution oflow-level mesocyclones. The Tornado: Its Structure, Dynamics,Prediction, and Hazards, Geophys. Monogr., No. 79, Amer. Geo-phys. Union, 97–104.

Browning, K. A., and F. H. Ludlam, 1962: Airflow in convectivestorms. Quart. J. Roy. Meteor. Soc., 88, 117–135.

——, and R. J. Donaldson, 1963: Airflow and structure of a tornadicstorm. J. Atmos. Sci., 20, 533–545.

Burgess, D. W., R. A. Brown, L. R. Lemon, and C. R. Safford, 1977:Evolution of a tornadic thunderstorm. Preprints, 10th Conf. onSevere Local Storms, Omaha, NE, Amer. Meteor. Soc., 84–89.

Charba, J., and Y. Sasaki, 1971: Structure and movement of the severethunderstorms of 3 April 1964 as revealed from radar and surfacemesonetwork data analysis. J. Meteor. Soc. Japan, 49, 191–214.

Davies-Jones, R. P., 1982: A new look at the vorticity equation withapplication to tornadogenesis. Preprints, 12th Conf. on SevereLocal Storms, San Antonio, TX, Amer. Meteor. Soc., 249–252.

——, 1986: Tornado dynamics. Thunderstorm Morphology and Dy-namics, E. Kessler, Ed., 2d ed., University of Oklahoma Press,197–236.

——, and H. E. Brooks, 1993: Mesocyclogenesis from a theoreticalperspective. The Tornado: Its Structure, Dynamics, Prediction,and Hazards, Geophys. Monogr., No. 79, Amer. Geophys.Union, 105–114.

Fujita, T. T., 1955: Results of detailed synoptic studies of squall lines.Tellus, 4, 405–436.

——, 1958: Tornado cyclone: Bearing system of tornadoes. Proc.Seventh Conf. on Radar Meteorology, Miami Beach, FL, Amer.Meteor. Soc., K31–K38.

——, 1975: New evidence from the April 3–4, 1974 tornadoes. Pre-prints, Ninth Conf. on Severe Local Storms, Norman, OK, Amer.Meteor. Soc., 248–255.

——, and R. M. Wakimoto, 1982: Anticyclonic tornadoes in 1980and 1981. Preprints, 12th Conf. on Severe Local Storms, SanAntonio, TX, Amer. Meteor. Soc., 213–216.

Golden, J. H., and B. J. Morgan, 1972: The NSSL/Notre Dame tor-nado intercept program, spring 1972. Bull. Amer. Meteor. Soc.,53, 1178–1179.

Jensen, B., T. P. Marshall, M. A. Mabey, and E. N. Rasmussen, 1983:Storm scale structure of the Pampa storm. Preprints, 13th Conf.on Severe Local Storms, Tulsa, OK, Amer. Meteor. Soc., 85–88.

Lemon, L. R., and C. A. Doswell, 1979: Severe thunderstorm evo-lution and mesocyclone structure as related to tornadogenesis.Mon. Wea. Rev., 107, 1184–1197.

Markowski, P. M., 2000: Surface thermodynamic characteristics ofrear-flank downdrafts, with implications for tornado genesis andmaintenance. Ph.D. dissertation, University of Oklahoma, 217pp. [Available from Bell and Howell Information and Learning,Ann Arbor, MI 48106-1346.]

Rasmussen, E. N., R. E. Peterson, J. E. Minor, and B. D. Campbell,1982: Evolutionary characteristics and photogrammetric deter-mination of windspeeds within the Tulia outbreak tornadoes 28May 1980. Preprints, 12th Conf. on Severe Local Storms, SanAntonio, TX, Amer. Meteor. Soc., 301–304.

——, J. M. Straka, R. P. Davies-Jones, C. A. Doswell, F. H. Carr,M. D. Eilts, and D. R. MacGorman, 1994: Verification of theOrigins of Rotation in Tornadoes Experiment: VORTEX. Bull.Amer. Meteor. Soc., 75, 995–1006.


Ray, P. S., R. J. Doviak, G. B. Walker, D. Sirmans, J. Carter, and B.Bumgarner, 1975: Dual-Doppler observation of a tornadic storm.J. Appl. Meteor., 14, 1521–1530.

——, B. C. Johnson, K. W. Johnson, J. S. Bradberry, J. J. Stephens,K. K. Wagner, R. B. Wilhelmson, and J. B. Klemp, 1981: Themorphology of several tornadic storms on 20 May 1977. J. At-mos. Sci., 38, 1643–1663.

Rutledge, S. A., and P. V. Hobbs, 1984: The mesoscale and microscalestructure and organization of clouds and precipitation in mid-latitude cyclones. Part XII: A diagnostic modeling study of pre-cipitation development in narrow cold-frontal rainbands. J. At-mos. Sci., 41, 2949–2972.

Straka, J. M., E. N. Rasmussen, and S. E. Fredrickson, 1996: A mobile

mesonet for finescale meteorological observations. J. Atmos.Oceanic Technol., 13, 921–936.

Tepper, M., and W. E. Eggert, 1956: Tornado proximity traces. Bull.Amer. Meteor. Soc., 37, 152–159.

Walko, R. L., 1993: Tornado spin-up beneath a convective cell: Re-quired basic structure of the near-field boundary layer winds.The Tornado: Its Structure, Dynamics, Prediction, and Hazards,Geophys. Monogr., No. 79, Amer. Geophys. Union, 89–95.

Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysisof tornado development and decay within a three-dimensionalsupercell thunderstorm. J. Atmos. Sci., 52, 2675–2703.

Wurman, J., J. M. Straka, and E. N. Rasmussen, 1996: Fine-scaleDoppler radar observations of tornadic storms. Science, 272,1774–1777.

Documents

Mobile Mesonet Observations on 3 May 1999