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WEATHER AND FORECASTING
VOLUME 17
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 at
the surface within the rear-flank downdrafts of the storms. Furthermore, the downdraft air parcels, which have
been shown to enter the tornado in past observational and modeling studies, were associated with substantial
convective available potential energy and small convective inhibition. The detection of only small equivalent
potential temperature deficits (1–4 K) within the downdrafts may imply that the downdrafts were driven primarily
by nonhydrostatic pressure gradients and/or precipitation drag, rather than by the entrainment of potentially cold
environmental air at midlevels.
1. Introduction
Direct measurements of meteorological variables near
tornadoes have been relatively scarce, owing to the rarity of the phenomenon and the spatial and temporal
frequency of standard observations. A few fortuitous
datasets have been analyzed by Tepper and Eggert
(1956) and Fujita (1958). Storm intercept field programs, first organized in the 1970s, have obtained additional observations within supercell storms (e.g.,
Golden and Morgan 1972; Bluestein 1983; Davies-Jones
1986); however, our collection of in situ measurements
within supercells remains miniscule.
A fleet of instrumented automobiles was designed by
Straka et al. (1996) for use in the Verification of the
Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994), conducted on the U.S.
Great Plains during the springs of 1994 and 1995.
Coined the ‘‘mobile mesonet,’’ this platform collected
surface data within severe storms with unprecedented
spatial (100–1000 m) and temporal (10–60 s) resolution. The mobile mesonet also has been used in subsequent years following VORTEX, with the spring of
1999 being exceptionally fruitful for operations. This
paper summarizes a few noteworthy observations made
on 3 May 1999, during a significant outbreak of tornadoes in Oklahoma and Kansas.
During the evening of 3 May, the mobile mesonet
intercepted a pair of tornadic supercells in southwestern
and central Oklahoma (Fig. 1). The first supercell was
Corresponding author address: Dr. Paul Markowski, The Pennsylvania State University, 503 Walker Building, University Park, PA
16802.
E-mail: [email protected]
q 2002 American Meteorological Society
intercepted approximately 25 km north of Fort Sill,
Oklahoma, shortly after 2200 UTC (this storm will be
referred to as storm A to be consistent with National
Weather Service surveys of the event). A second supercell was intercepted farther west around 0000 UTC
(this storm will be referred to as storm B). Detailed
accounts of these intercepts appear in section 3, following a description of the analysis techniques in section
2. A few final comments are provided in section 4.
2. Mobile mesonet data
The mobile mesonet senses temperature, relative humidity, pressure, and wind velocity. Time and position
are recorded using a global positioning system receiver.
In 1999, data were recorded at 2-s intervals. A complete
list of instrument specifications (including response
times and errors) is available from Straka et al. (1996).
Vehicle velocities were removed from the wind velocity data when vehicle accelerations were small. In
the presence of significant vehicle accelerations (.1 m
s 22 ), accurate wind data could not be obtained. Furthermore, field operations relied on the use of radio
communication. On occasion, radio frequency interference caused large errors to arise in the meteorological
measurements. Data with these gross errors were removed prior to analysis. In addition, biases in the data
were removed by way of intercomparisons between vehicles. The intercomparisons were made in relatively
quiescent weather conditions while the vehicles were
moving as a caravan (e.g., en route to a storm).
The quality-controlled observations used in the analyses were averaged over 12-s intervals. Data were plotted relative to radar echoes using time-to-space con-
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MARKOWSKI
431
FIG. 1. The tracks of storms A and B, with the regions indicated where mobile mesonet data
were collected. The radar echoes outline the 30-dBZ base reflectivity values at the 0.58 elevation
angle of the Twin Lakes, OK, KTLX WSR-88D; times (UTC) also are included for each radar
echo. 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 did
not change significantly over the time interval during
which the measurements were made (the ‘‘Taylor hypothesis’’). Supercells are not steady. If they were, then
tornadogenesis could not occur. However, steadiness assumptions, at least for short time intervals, are virtually
unavoidable in observational studies (e.g., multiple
Doppler radar analysis). For the research herein, features
were assumed to be steady for 63 min, with respect to
an analysis time. This is approximately the length of
time that it takes for a Weather Surveillance Radar-1998
Doppler (WSR-88D) to complete a volume scan. There
is some confidence that such steadiness assumptions
were not too severe, for the analyzed fields tended to
be free of noise [inappropriate steadiness assumptions
lead to the artificial creation of warm and cold (or moist
and dry) pockets in the analysis following a time-tospace conversion].
In addition to the raw thermodynamic data recorded
by the mobile mesonet, several derived variables were
computed. Virtual potential temperature u y was computed, with the inclusion of liquid water effects, in addition to the contribution from water vapor. The liquid
water content was parameterized from the WSR-88D
reflectivity at the lowest elevation angle, using the method of Rutledge and Hobbs (1984). Equivalent potential
temperature u e was computed by lifting a surface parcel
adiabatically to 100 hPa, where the potential temperature of the parcel was assumed to be equal to the u e of
the parcel. Pressure p was reduced to the average height
of the vehicle observations (407 m) using U.S. Geological Survey Level-2 Digital Elevation Model data.
Convective available potential energy (CAPE) and convective inhibition (CIN) were computed for parcels by
inserting surface thermodynamic measurements obtained from the mobile mesonet into the 0000 UTC 4
May sounding at Norman, Oklahoma (located approximately 25 km south of Oklahoma City; see Fig. 2).1
The buoyancy integrations in the CAPE calculations
were terminated at 500 hPa, owing to the loss of the
sounding data at upper levels. Virtual temperature effects were not included in the CAPE and CIN calculations. Whereas u y (more precisely, its fluctuation) can
be considered a measure of parcel buoyancy, CAPE and
CIN values can be viewed as measures of the potential
buoyancy of a parcel.
The fluctuations of u y , u e , and p (denoted by a prime,
e.g., u9y ) from a base state (denoted by an overbar, e.g.,
u y , where u9y 5 u y 2 u y ) were analyzed instead of the
absolute quantities themselves. The base state was de-
1
It always is possible to debate the representativeness of a sounding. The 0000 UTC Norman sounding was obtained during the lifetimes of the tornadic supercells and from within 75–100 km of the
storms that were intercepted; however, it will be evident later that
the environment sampled by the Norman sounding, at least near the
surface, differed from the environment to the west in which the supercells were occurring (Fig. 3).
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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 s 21 speed
rings 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 smooth
subjectively analyzed contours obtained from Oklahoma
Mesonet stations (Brock et al. 1995; Fig. 3). All definitions of a ‘‘base state’’ are arbitrary (and potentially
problematic); the method used herein is similar to that
used by Fujita (1955) and Charba and Sasaki (1971).
The uncertainty of perturbation variables (denoted by
d) depends on the propagation of instrument errors (e.g.,
temperature, relative humidity, and pressure errors), uncertainties associated with the estimation of liquid water
in the case of u9y , and specification of the base state. A
detailed analysis of these effects appears in Markowski
(2000). On 3 May 1999, it has been estimated that dp9
ø 0.8 hPa, du9y ø 0.5 K, du9e ø 2.5 K, dCAPE ø 110
J kg 21 , and dCIN ø 9 J kg 21 .
The mobile mesonet comprised three vehicles on 3
May 1999, in contrast to the much larger assembly used
for VORTEX. The goal of the field operations of 1999
was to sample the rear-flank downdraft (RFD) region
of supercells (Lemon and Doswell 1979), rather than
attempt to sample a broad inflow region, as was the case
during VORTEX (Rasmussen et al. 1994). Thus, a
smaller array of vehicleborne instruments could accomplish the sampling goal. Furthermore, a smaller fleet
had some logistical advantages; for example, coordination among mobile mesonet crews and with the nowcaster was considerably easier.
The mobile mesonet RFD sampling strategy was favored for several reasons. In environments initially absent of vertical vorticity (i.e., vortex lines are quasihorizontal), a downdraft is necessary for intense vertical
vorticity to arise at the ground (Davies-Jones 1982; Da-
vies-Jones and Brooks 1993; Walko 1993). Even as early as 1975, Fujita (1975) hypothesized that the angular
momentum transport by a downdraft may be critical to
tornadogenesis. Burgess et al. (1977) and Barnes (1978)
also made similar speculations. Many studies have
found that the air parcels that supply the tornado pass
through the RFD. For example, observations by Brandes
(1978), Lemon and Doswell (1979), Rasmussen et al.
(1982), and Jensen et al. (1983) have shown or implied
a near total occlusion of the low-level mesocyclone by
the RFD prior to tornadogenesis. Furthermore, Brooks
et al. (1993), Wicker and Wilhelmson (1995), and Adlerman et al. (1999) have found that the trajectories
entering the near-ground circulations in their numerical
simulations passed through the hook echo and RFD.
Given the prior emphasis on the RFD in the tornadogenesis process and the apparent consensus that RFD
air parcels enter the tornado, it is believed that the thermodynamic characteristics of hook echoes and RFDs
naturally assume importance. For this reason, the focus
of the next section is on the observations obtained largely within the RFDs of the intercepted tornadic supercells.
3. Observations
a. Storm A
The mobile mesonet approached storm A around 2200
UTC. The supercell was associated with two brief, weak
tornadoes at 2151 and 2155 UTC, before an intercept
could be engaged. At 2220 UTC, the supercell produced
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MARKOWSKI
its first significant tornado.2 The tornado persisted for
15 min, although it did not do significant damage along
its 10-km track (F1 Fujita-scale rating). The RFD associated with the circulation was sampled by the mobile
mesonet west-southwest through east-southeast just prior to tornadogenesis, and during the lifetime of the tornado all three vehicles trailed the tornado to the southwest, within the RFD.
At 2219 UTC, 1 min before tornadogenesis, the mobile mesonet detected relatively small u y deficits within
the RFD and inflow of less than 2 K (Fig. 4). The RFD
parcel temperatures were 1.0–1.5 K warmer than the
inflow parcels. The lowest u e values, located within the
hook echo, were approximately 4 K less than the average
inflow values (Fig. 5); however, these u e values (;345
K) were larger than any u e values observed on the 0000
UTC Norman sounding (Fig. 2), because of the sounding being launched east of the axis of maximum u e
values (Fig. 3). Therefore, estimates cannot be made of
the altitude from which RFD parcels may have descended (nor could estimates be made even if u e could be
assumed to be conserved in the absence of entrainment).
The relatively small u y and u e deficits (and temperature
excess) of the downdraft air parcels also were associated
with small CIN and substantial CAPE (Fig. 6). CIN
values as small as 2 J kg 21 were detected in the hook
echo, and CAPE values in the RFD generally exceeded
900 J kg 21 in the lowest 500 hPa (it is estimated that
the total CAPE associated with the RFD air parcels was
;3000 J kg 21 ).
The mobile mesonet also sampled a small pressure
excess (,1 hPa) in the RFD at 2219 UTC, except within
a few hundred meters of the intensifying circulation
center, where a small pressure deficit (about 21 hPa)
was detected (Fig. 7). A small pressure deficit also was
sampled in the inflow east of the gust front. Streamlines
revealed difluence at the surface within the RFD, with
the strongest difluence (and divergence) apparently situated in the hook echo (Fig. 4). Anticyclonic vertical
vorticity also was present within the RFD to the south
and east of the incipient tornado.
The mobile mesonet was unable to sample the RFD
within 1 km of the tornado at later times; however, data
were collected within the RFD approximately 2–5 km
southwest of the tornado until its demise. At 2229 UTC
(6 min prior to tornado dissipation), only small u y deficits were detected again in the RFD; however, larger
u e deficits were sampled (and correspondingly smaller,
yet significant, CAPE values of ;200–400 J kg 21 below
500 hPa), with deficits as large as 7–8 K recorded a few
kilometers west-southwest of the tornado (Fig. 8). By
2234 UTC (1 min prior to tornado dissipation), the u y
and u e deficits within the RFD appeared to increase
slightly, with u y deficits exceeding 2 K and u e deficits
2
The tornado is referred to as significant because of its longevity.
433
as large as 10 K being detected within 3 km of the
tornado to its west and southwest (Fig. 9).
The relatively long-lived tornado dissipated at 2235
UTC, and another tornado was produced by storm A at
2246 UTC. The tornado produced F3 damage and lasted
until 2310 UTC, but the tornado could not be closely
intercepted for logistical reasons. The road network did
not allow sampling within several kilometers of the tornado, and many roads became obstructed by debris.
Storm A went on to produce another significant tornado
near 2323 UTC, which produced F5 damage in the suburbs of Oklahoma City. This tornado also was not intercepted for logistical reasons (e.g., increasing amounts
of traffic near the metropolitan area, debris-filled roadways). Instead, the mobile mesonet abandoned storm A
in pursuit of another tornadic supercell to the west
(storm B).
b. Storm B
The mobile mesonet arrived at storm B at approximately 0000 UTC (4 May). Storm B produced several
brief, weak tornadoes from 2236 to 2324 UTC. Another
tornado was reportedly on the ground for 21 min from
2338 to 2359 UTC, but this tornado dissipated before
data could be collected. From 0000 to 0100 UTC, numerous additional tornadoes (as many as eight) were
reported, most of which were brief and weak (F0–F1
damage ratings), although two tornadoes persisted for
longer than 5 min. Data were collected within a few
kilometers of the surface circulation centers at several
times from 0000 to 0100 UTC.
At 0026 UTC, storm B was not producing a tornado,
but brief tornadoes were reported just before and after
the analysis time (at 0020 and 0034 UTC). The mobile
mesonet obtained data within 0.5–2.0 km of the mesocyclone center at the surface. Within 3 km of the
circulation center, the most negatively buoyant air sampled within the hook echo and RFD region was associated with a u y deficit of less than 1 K (Fig. 10). Furthermore, the baroclinity at the surface within the hook
echo was very weak, with maximum | = h u y | values of
less than 0.5 K km 21 (where = h is the horizontal gradient
operator). The relatively warm air within the downdraft
also was associated with small u e deficits (Fig. 11).
Within the hook echo, u e values were nearly the same
as in the inflow, and u e values were only 1–2 K less
than inflow values in a zone that appeared to wrap
around the north side of the circulation center. As was
the case for storm A, all of the surface u e values measured by the mobile mesonet in storm B were larger
than the maximum u e value observed on the 0000 UTC
Norman sounding. It also can be inferred that the CAPE
(CIN) within the RFD of storm B at 0026 UTC was
large (small).
The mobile mesonet made additional penetrations of
the hook echo and RFD near 0043 UTC. A tornado,
which formed at 0037 UTC, was observed at the anal-
FIG. 3. Meso-a-scale hourly subjective analyses of u y (K), u e (K), and pressure [hPa; reduced to the mean elevation of the mobile mesonet observations (407 m MSL)] from 2200 to 0100
UTC 3–4 May. Data were obtained from the Oklahoma Mesonet. Station models display (reading anticlockwise, beginning with the top-left value) temperature (8C), dewpoint (8C), u y (K),
u e (K), and reduced pressure (hPa). Wind barbs are relative to the ground, with each full (half ) barb being equal to 5 (2.5) m s 21 .
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FIG. 3. (Continued)
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FIG. 4. Subjective analysis of u9y (K) in storm A at 2219 UTC 3 May 1999 (u y 5 304.4 K).
Contours are dashed where significant uncertainty exists owing to sparseness of observations. The
analysis time is 1 min prior to tornadogenesis. Mobile mesonet station models include (reading
anticlockwise, beginning with the numeral at the top left) temperature (in degrees Celsius to the
nearest 0.18C with the decimal omitted), dewpoint temperature (in degrees Celsius to the nearest
0.18C with the decimal omitted), u y (in kelvins to the nearest 0.1 K with the decimal omitted),
and u e (in kelvins to the nearest 1 K). Wind barbs depict storm-relative winds [each full (half )
barb equals 5 (2.5) m s 21 ]. A few streamlines have been drawn in gray. Mobile mesonet observations have been averaged over 12-s intervals, and a steadiness was assumed for a period of 63
min (with respect to the analysis time) in the time-to-space conversion. Observations obtained
more than 1 min before or after the analysis reference time are ‘‘flagged’’ with a vertical bar
through the center of the station model. Storm-scale fronts are depicted using conventional frontal
symbology, and their placement has been aided by inspection of mobile radar data provided by
J. Wurman. The M indicates the position of the mesocyclone center on the lowest radar elevation
angle available. Base radar reflectivity data were obtained from the 0.58 elevation angle of the
KTLX WSR-88D, with the reflectivity scale (dBZ ) included at the bottom. The region analyzed
is indicated in the larger-scale inset.
ysis time. The tornado dissipated at approximately 0048
UTC. Again, only small u y and u e deficits were detected
within the RFD and hook echo (Fig. 12), with substantial
CAPE and small CIN also being present within the
downdraft at the surface (not shown). Baroclinity within
the hook echo also was weak (maximum | = h u y | of ;1
K km 21 ).3
At 0052 UTC, the RFD and hook echo region of storm
B were again sampled relatively well, and another tornado was in progress at the time (the tornado formed
at 0047 UTC and dissipated at 0100 UTC). As at earlier
times, only small u y and u e deficits (,2 K) were observed, and baroclinity was weak (Fig. 13; maximum
3
The adjective ‘‘weak’’ is used to describe the baroclinity because
a value of | = h u y | ; 1 K is considered to be small on the storm scale.
However, by some standards (e.g., on the synoptic scale), a gradient
of this magnitude would be considered to be enormous.
| = h u y | of ;1 K km 21 detected north of the tornado).
Streamlines diverged within the hook echo, and a couplet of vertical vorticity appeared to straddle the hook
echo. Anticyclonic vertical vorticity, estimated to be
approximately 25 3 10 23 s 21 , was observed at the
surface on the side of the hook echo opposite the stronger, cyclonic vertical vorticity associated with the tornado. CIN values were very insignificant (,10 J kg 21
within 5 km of the tornado in the RFD), and RFD parcels
also were associated with large CAPE (.800 J kg 21
below 500 hPa; Fig. 14).
Surface pressure gradients were weak at 0052 UTC
at distances of more than 500 m from the tornado, similar to observations in storm A (Fig. 15). A small pressure excess (,1 hPa) was detected within the RFD, and
a small pressure deficit (also ,1 hPa) was measured
within the region of cyclonic vertical vorticity associated with the tornado parent circulation. The relatively
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FIG. 5. As in Fig. 4 but u9e (K) is analyzed (u e 5 350.0 K).
FIG. 6. As in Fig. 4 but CIN (J kg 21 ) is analyzed. CAPE (below 500 hPa) and CIN values
appear to the left and right, respectively, in each station model.
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WEATHER AND FORECASTING
FIG. 7. As in Fig. 4 but p9 (hPa) is analyzed (p 5 953.1 hPa). Reduced pressure values, with
the leading 9 omitted, appear in each station model. Values are to the nearest 0.1 hPa, with the
decimal omitted.
FIG. 8. As in Fig. 4 but at 2229 UTC 3 May 1999. The analysis time is 9 min after
tornadogenesis and 6 min prior to tornado dissipation. The T indicates the tornado location.
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FIG. 9. As in Fig. 4 but at 2234 UTC 3 May 1999. The analysis time is 14 min after
tornadogenesis and 1 min prior to tornado dissipation.
FIG. 10. As in Fig. 4 but storm B is analyzed at 0026 UTC 4 May 1999 (u y 5 304.0 K). No
tornado was in progress at the analysis time, but tornadoes were observed shortly before and after
0026 UTC (at 0020 and 0034 UTC).
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WEATHER AND FORECASTING
FIG. 11. As in Fig. 10 but u9e (K) is analyzed (u e 5 349.0 K).
FIG. 12. As in Fig. 10 but at 0043 UTC 4 May 1999. The analysis time is 6 min after
tornadogenesis and 5 min prior to tornado dissipation.
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FIG. 13. As in Fig. 10 but at 0052 UTC 4 May 1999. The analysis time is 5 min after tornadogenesis and 8 min prior to tornado dissipation (this is a different tornado than that which appears
in Fig. 12). A few streamlines also have been drawn.
FIG. 14. As in Fig. 13 but CIN (J kg 21 ) is analyzed. Station models are as in Fig. 6.
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FIG. 15. As in Fig. 13 but p9 (hPa) is analyzed (p 5 951.5 hPa). Station models are as in Fig. 7.
weak pressure gradients are somewhat consistent with
observations 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 communication).
Storm B produced at least six additional tornadoes,
some strong, after dark, which arrived between 0100
and 0130 UTC. Data collection operations by the mobile
mesonet were terminated near 0130 UTC. The supercell
dissipated in northern Oklahoma around 0400 UTC.
4. Discussion and closing remarks
The observational data presented herein have several
limitations. First, road networks do not allow continuous
sampling of moving updrafts for periods longer than
about 5 min before repositioning of the vehicles requires
that they temporarily forfeit data collection in critical
regions of the storm; therefore, the time evolution of
features is difficult to document. Furthermore, steadiness reluctantly was assumed for durations as long as
6 min (63 min from the analysis reference times) in
constructing the analyses, to maximize the coverage of
data (gathered by a finite number of vehicles). Second,
thermodynamic fields and their gradients cannot be ascertained above the surface by direct means. At best,
only the sign of the gradients can be inferred above the
surface, based on assumptions of the lapse rates beneath
and at a distance from the storm. Third, time histories
of air parcels are important, possibly more than 30 min
prior to tornadogenesis. It was not possible to compute
trajectories at the surface because of inadequate observation density.
Despite the above impediments, substantial evidence
was presented of the following characteristics of the 3
May 1999 tornadic supercells:
1) RFDs were associated with small u y and u e deficits,
2) RFDs contained substantial amounts of CAPE and
small amounts of CIN,
3) hook echoes lacked strong baroclinity at the surface,
and
4) pressure fluctuations and gradients were small at distances greater than approximately 250 m from the
circulation centers.
It is perhaps most intriguing that the RFDs were associated with what one might consider to be ‘‘small’’
signatures, both kinematically and thermodynamically.
The relatively small u y and u e deficits within the RFDs
may imply a descent largely forced by nonhydrostatic
pressure gradients, rather than the entrainment of lowu e environmental air at midlevels, and subsequent generation of negative buoyancy by evaporative chilling,
as in long-standing conceptual models (e.g., Browning
and Ludlam 1962; Browning and Donaldson 1963). Visual observations by mobile mesonet volunteers (e.g.,
J. Straka 1999, personal communication) suggested that
the hook echoes were composed of only a thin veil of
mainly large raindrops; thus, precipitation loading may
only have been of secondary importance in the downdraft forcing. Furthermore, it is worth reiterating that
the observations of RFD thermodynamic properties sim-
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MARKOWSKI
ilar to the properties representative of the inflow do not
necessarily imply small vertical downward excursions
for RFD air parcels. The depth of descent cannot be
ascertained from the surface u e measurements alone.
Some evidence also was presented of vertical vorticity couplets straddling the divergence maxima within
the RFDs (roughly collocated with the hook echo), as
was 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). This
probably is evidence that RFDs are involved in a downward displacement of vortex lines, perhaps necessarily
supplying angular momentum to the tornado, as many
others have previously conjectured (section 1).
It is speculated that some of the above intriguing
features of the 3 May 1999 tornadic supercells, particularly the relative warmth and moistness of the downdrafts, may be relevant to the problem of tornadogenesis. Does tornadogenesis probability, longevity, and
intensity increase as RFD parcels become more buoyant? If buoyant RFDs are propitious for tornadogenesis,
are there any large-scale environmental conditions from
which we may anticipate warm, moist RFDs? It is generally accepted that only a relatively small percentage
of supercells are tornadic; yet, on 3 May 1999, nearly
all were tornadic. This fact may suggest that large-scale
factors may exist, at least in some cases, from which
favorable RFD thermodynamic characteristics can be
inferred. Additional cases and implications and, it is
hoped, some answers to the above-posed questions will
be the subject of a series of companion papers.
Acknowledgments. I am most grateful to all of the
volunteers whose countless personal sacrifices made
data collection possible. I am indebted to Drs. Jerry
Straka and Erik Rasmussen for their support during field
operations and for stimulating discussions related to supercells and tornadogenesis during the last several years.
Data-quality checks were performed by Mr. Al Pietrycha, and indispensable assistance in obtaining digital
elevation model data was provided by Mr. Rob Carver.
Dr. Joshua Wurman provided data from the Doppler on
Wheels mobile radars, which were used to help to identify the position of the gust front in storm A. Mr. Mark
Shafer provided Oklahoma Mesonet data. I also thank
Dr. Chuck Doswell and two anonymous reviewers for
helping to improve the manuscript. NSF Grant ATM9617318 partially supported this research.
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