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1
Examining Relationships in Wind Speeds and other Parameters
with Tornadic and Non-tornadic Supercells
Ron Harris
Candidate for B.S. Degree in Meteorology
State University of New York, College at Oswego
College Honors Program
May, 2010
2
Abstract
Photographs of a tornadic supercell in Lagrange, Wyoming, on 5 June 2009,
suggest that rain near the updraft was key to tornadogenesis on that day. In this case any
growth or further development of the funnel cloud was preceded by an increase in
precipitation near the wall cloud. There was no funnel at all until the first rain was visible
within the wall cloud region. Then, as a heavy rain shaft developed near the wall cloud,
the funnel grew wider and closer to the ground, touching down. The hypothesis for the
current work is that weaker mid-level wind speeds in close proximity to a storm are more
favorable for tornadogenesis than strong wind speeds at mid-levels. We believe weaker
winds at lower levels can help to keep rain falling near the wall cloud, as opposed to
stronger winds which would blow rain farther away. Nearby rain produces vorticity
baroclinically that can aid the tornadogenesis process, and can cool the air locally causing
lower lifted condensation level (LCL) heights. We tested this hypothesis by comparing
the tornadic supercell from 5 June to a non-tornadic supercell on 4 June 2009. The storm
on 4 June was also in southeastern Wyoming. Thermodynamic soundings launched
before and near the storms as part of The Verification of the Origins of Rotation in
Tornadoes Experiment 2 (VORTEX2) were used to examine wind speeds at various
heights, and to calculate convective available potential energy (CAPE), convective
inhibition (CIN), and to plot hodographs. Mid-level wind speeds were lighter near the
tornadic storm on 5 June than they were near the non-tornadic storm on 4 June. However,
examining CAPE, CIN and hodographs showed that conditions were much more
favorable for severe weather on 5 June than on 4 June. Radar imagery confirmed that the
supercell on 5 June was much larger and more intense than the supercell on 4 June. A
better comparison can be made in this fashion by using many more cases.
3
Table of Contents
Advice to Future Honors Students ……………………………………………………… 4
Acknowledgements …………………………………………………………………....... 6
Author’s Reflections ……………………………………………………………………. 7
Introduction …………………………………………………………………………… 10
Data and Methods ……………………………………………………………………… 14
Results ………………………………………………………………………………….. 16
Discussion and Conclusions …………………………………………………………… 19
Tables and Figures …………………………………………………………………….. 22
Appendix ………………………………………………………………………………. 33
References ……………………………………………………………………………… 34
4
Advice to Future Honors Students
Be bold. Break out of your high school shell and don’t worry about preserving an
image. Find out who you want to become and what type of person you are. Have a life
outside your school work. Take up some activities or just go places and do things with
friends. Try new foods, listen to new types of music, read a book, watch your favorite
television show, find an activity to attend on campus or go into town, etc.
Don’t expect to like every single class you take, but take them seriously. The
honors program isn’t hard. If you like something that you discussed in class, don’t be
afraid to talk about it outside of class. It’s likely you will need to improve your writing
skills to do well in the honors program. Writing comes easier to some people more than
others. Reading books and scholarly articles in your field does help you to understand
how your writing should sound.
Although it may seem logical to find a balance between leisure and work for your
various classes, you eventually might want to embrace your major and the work that it
entails. Balancing your various classes and activities will make you feel well-rounded,
but once you start feeling overwhelmed with work for your major, that is when you will
start learning the most. For me as a meteorology major, it was when I found myself stuck
in Piez Hall for most of my days that I realized I was starting to understand weather on a
higher level because I was studying it vigorously. However, sometimes you may find you
want something to take your mind off of school work entirely. I have found that naps are
very powerful.
When starting your thesis, do whatever interests you, whether or not it’s in your
major. You will have plenty of time to mull over any ideas that you have, and switching
topics is never a bad thing, at least not at earlier stages in the process. Start out by
thinking about a couple different ideas for your thesis. Have a backup plan so you have
5
something else to work with if your first idea falls through. Don’t be afraid to ask
questions to professors or other honors students. Professors and advisors are supposed to
help you. If you encounter a tough professor, don’t be timid. Be respectful, but stand your
ground. Overall, the thesis project is not difficult. Typically independent studies only take
place over the course of one semester, but the thesis spans two years. When you finish
you will have learned a lot and maybe even had some fun. You will also be proud.
If you find you genuinely don’t like your major or Oswego, don’t be afraid to
change majors or transfer somewhere else. People change their minds all the time, and
there are plenty of people on campus whose job it is to help you adjust. Just make sure
you are happy. Also, don’t take friends or family for granted.
6
Acknowledgements
Dr. Scott Steiger and Dr. Steve Skubis have helped me through my time in the
Meteorology program at SUNY Oswego, being my friends, mentors, and co-authors of
this thesis. Dr. Bob Ballentine and Dr. Al Stamm have also led our class through our time
at Oswego. The meteorology professors are always available to help students and they
seem willing to do almost anything to help us. We laugh with them too, so we all leave
with good memories. I want to also acknowledge those who led me through my time
outside of the Meteorology program. As my honors advisors, Dr. Norm Weiner and Dr.
Robert Moore have always talked to me about almost anything and answered questions
when I did not know who else to ask. Also, Dr. Eric Schmitz and Trevor Jorgensen have
taught me so much about music, which is a passion of mine. They are friends and role
models for the musician in me. All of the professors mentioned above have helped me
shape my life as a student.
Thanks to Dr. Steiger for supplying all the pictures of the tornado on 5 June
2009. Thanks to Dr. Skubis for all the computer codes to make calculations and plots.
Thanks to Lindsay Phillips for the time lapse video of the tornado.
Radar data are available through the National Climatic Data Center website
(http://www.ncdc.noaa.gov) and VORTEX2 sounding data were provided by
NCAR/EOL under sponsorship by the National Science Foundation
(http://catalog.eol.ucar.edu/vortex2_2009).
7
Author’s Reflections
So many thoughts and memories come to mind when I hear ―Oswego‖, like
sunsets, snow storms, classes and professors, friends, residence and dining halls, campus
activities, hockey games, concerts, plays, Hewitt Union (when it used to be the hot spot,
with Late Night and 4-7 dining), parties, bars, restaurants, trips to Syracuse, Rochester
and Fulton, life off-campus, Christmas shopping on First Street, and so much more.
Memories from high school still remain, but they fade with time. I feel like there are not
many things from my high school years that are still present in my daily life, especially
things that I learned in my classes. Fond memories are intact, but there seem to be only
several useful things remaining from what we learned in high school. Almost everything I
learned in college is sticking with me though. In just four short years I have made great
bonds with new friends and many of the professors have left lasting impressions. I have
learned a lot about myself and all sorts of subjects. Instead of being told to memorize
facts, we have been taught how to think for ourselves, and the learning is much more
rewarding.
I have realized we are growing up, becoming professionals and preparing to face
the real world. The world shouldn’t be scary; it should just be about being happy. For me
to be happy, my friends and family need to be happy too. Otherwise, I don’t feel right. I
have pets too, and they are wonderful because their love is always unconditional. The
time spent at home on breaks from college is almost just as important as the time when
classes are in session. I shaped myself by having a job while I was home, and I hope to
have an internship before going to graduate school. But the time at home is also for
relaxing, resting, and being with friends and family. While I am away they miss me like I
miss them. I have great memories of winter and summer breaks with my family and
8
friends from home, just like I have great memories with my friends from college during
class sessions.
Considering the knowledge and skills I have acquired at Oswego, one of the
skills I am most proud of is writing. I do not care so much about having my own voice
and expressing myself; rather, I just write for whoever is reading and grading my paper,
or for someone who is considering me for admission or hire. In doing that, I suppose I
have my own voice to a certain degree. I am glad that I have learned how to cohesively
communicate ideas on paper. Struggling to figure out what to write or how to explain an
idea to a reader is a good way to better understand an idea (just like teaching a subject is
one of the best ways to learn about the material). I am also glad that I have had many
opportunities to present my undergraduate research, and I have begun to enjoy public
speaking. College has been about learning and overall improvement. I have improved my
knowledge and skills in meteorology, writing, public speaking, and also in music and
other subject areas outside of meteorology.
When beginning my honors thesis project I had some ideas, but they fell through.
The summer after my junior year I went storm chasing with SUNY Oswego’s Storm
Forecasting and Observation program, coordinated by Dr. Scott Steiger. When I returned
to school that fall, I had a lot of interest in severe weather, particularly the tornado we
witnessed while in the Midwest. A simple independent study for that semester turned into
two semester’s worth of work on studying that tornadic thunderstorm, learning a great
deal about severe weather. Thus, I present my honors thesis.
Storm chasing was an extraordinary experience. The Great Plains is probably the
best place in the world to study thunderstorms. Conditions are usually favorable for
severe thunderstorms and the terrain is flat enough for everyone to observe storms in their
entirety. I enjoyed my time with my classmates and with the professors on that trip. We
bonded with each other and we learned a lot about weather. We also were privileged to
9
be a part of a large collaborative field experiment called VORTEX2 (Verification of the
Origins of Rotation in Tornadoes Experiment 2). We worked with over 100 top scientists
and students in our field, and met people from The Weather Channel and The Discovery
Channel. The constant everyday hustle and traveling was comparable to being in a rock
band, being in a different town every night. The attention that we all received was also
comparable to being a rock star. Many local residents recognized storm chasing vehicles
from what they had seen on The Weather Channel, and they welcomed us. They asked
questions and took pictures. The whole experience was exciting.
My first three years at Oswego were full of many experiences, giving me a well-
rounded education. Then, starting with the storm chasing excursion between my junior
and senior years, I began focusing on my major, learning about the weather by reading,
analyzing data, writing and presenting. With this thesis I sum up everything I learned
from storm chasing until now, and I reflect on the many things that shaped my entire
education experience. I feel that signing the final cover sheet to this document will be
only the beginning of my learning and my career.
10
1. Introduction
The origins of rotation in supercell thunderstorms have been studied and
documented, but the mechanisms that cause tornadoes to form and develop are still not
fully understood. Using a cloud model, Rotunno and Klemp (1985) showed that storm
rotation at mid levels originates from the tilting of horizontal vorticity into the vertical by
the storm’s updraft. This horizontal vorticity is created as a result of the environmental
vertical wind shear. They also explained that low-level storm rotation involves tilting of
horizontal vorticity into the vertical, but the horizontal vorticity at low levels is formed
from a different mechanism than that at mid levels. They believed that low-level
horizontal vorticity is baroclinically generated by the merging of evaporatively-cooled,
negatively buoyant outflow air with warm, positively buoyant inflow air. Also noted by
Rotunno and Klemp is the formation of the lowered cloud base known as the wall cloud.
The potential temperature contours in their cloud model showed that the air composing
the wall cloud was much cooler than ambient. They attributed the wall cloud to rain-
cooled air reaching its lifted condensation level (LCL) at a lower height than the LCL of
the ambient air. The idea of evaporatively-cooled air being ingested into a storm’s updraft
raises questions about the role of the distribution of precipitation particles around the
storm. This is something that Rasmussen and Straka (1998) have examined. They did a
climatological analysis of soundings associated with low-precipitation (LP) supercells
(e.g. Burgess and Davies-Jones 1979; Bluestein and Parks 1983), classic (CL) supercells
(e.g. Browning 1964; Lemon and Doswell 1979), and high-precipitation (HP) supercells
(e.g. Doswell et al. 1990; Moller et al. 1994), concluding that the anvil-level storm-
relative (SR) flow is most important in terms of precipitation distribution near the
updraft. They noted that SR environmental flow at the anvil level is much stronger for LP
storms than HP, with the environments supporting CL storms having intermediate upper-
11
flow strengths. It appeared to them that for LP storms, the stronger flow aloft transported
hydrometeors further from the updraft, greatly reducing the number of hydrometeors that
could re-enter the updraft, which would greatly limit production of precipitation in the
updraft itself. The findings of Brooks et al. (1994b) are slightly contrary to the findings of
Rasmussen and Straka. Brooks et al. (1994b) found in their cloud model runs that the
magnitude of the midlevel shear had the most effect on whether or not precipitation was
blown away from the updraft. They explained that more low-level baroclinic vorticity
was generated by evaporative effects in the low- and middle-shear cases, with
substantially more precipitation falling near the updraft. They also noted that the low-
shear case was first to develop large values of vertical vorticity at low levels, but the low-
level vorticity endured for much longer with the middle-shear case.
There are minor discrepancies between the findings of Rasmussen and Straka (1998)
and Brooks et al. (1994b). One discrepancy, as Rasmussen and Straka noticed, is that
Brooks et al. did not vary the flow above 7 km AGL in their cloud model, so any
sensitivity to upper-level flow strength was not detected. Another difference to note is
that the precipitation embryos at the anvil level are important in terms of the distribution
of precipitation growth below the anvil, which in turn influences the amount of
precipitation particles around the updraft that are available to either be lofted again or fall
to the ground. The midlevel flow would then determine the distribution of bigger
precipitation particles, like large rain drops and hail, which had been through the updraft
more than once.
One of the objectives of the current work is to investigate wind speeds and shear in
the environments of a tornadic and a non-tornadic supercell because of the influence that
winds have on the distribution of precipitation. Previous work by the authors involved
examining and analyzing time-stamped photographs of a tornadic supercell that occurred
on 5 June 2009 in LaGrange, Wyoming. The photographs revealed that the funnel cloud
12
first appeared four minutes after a light rain shaft became visible in the wall cloud region
(Figure 1). The tornado touched down ten minutes after the funnel first appeared, then
stayed on the ground for roughly twenty-five minutes. As the funnel developed into the
tornado, there was a strong relationship between the distribution of rain around the wall
cloud and the development of the funnel. The funnel grew closer to the ground whenever
the narrow rain shaft was concentrated on one side of the wall cloud. Conversely, the
funnel appeared to retract when the rain was evenly distributed around the wall cloud.
The authors believe that the rain occurring within the wall cloud region was critical to the
development of the tornado. The ideas that Rotunno and Klemp (1985) involved in
explaining the wall cloud and low-level mesocyclone may explain the relationship
between the rain in the wall cloud region and the funnel development for this case. Those
observations along with current theory suggest that the narrow rain shaft helped to
baroclinically generate horizontal vorticity and lower the LCL height of the rising air
near the center of the updraft. These two mechanisms explain both the increase in
rotation and the lowering of the funnel cloud. Thus, the formation of the wall cloud and
funnel cloud are nearly identical, with the exception of their spatial scales. Evaporatively-
cooled air from the storm’s main downdraft area (forward flank) is ingested into the
updraft, lowering the LCL and forming the wall cloud. In a similar manner, a small rain
shaft within the wall cloud region allows cool air on a smaller spatial scale to be ingested
into the updraft and give way to the lowering funnel cloud. Rotation on the scales of both
the wall cloud and funnel cloud can be attributed to baroclinically generated vorticity.
This appeared to be the case in June 2009 in LaGrange, Wyoming.
Following the study of the tornado photographs, the authors developed a
hypothesis which gave way to the current work. The hypothesis is that weaker low- to
mid-level winds are more favorable than stronger winds at these levels for
tornadogenesis, at least in cases where rain could begin falling very near the updraft. The
13
reason for this hypothesis is that less environmental shear at low and middle levels would
keep rain falling more near a storm’s updraft instead of being blown far away. Thus, with
more rain able to fall near the updraft, a storm can have the potential for more
baroclinically generated vorticity and lower LCL heights. There is also a risk of having
too much rain near the updraft. Since this would lead to a storm’s demise, a fine balance
is necessary. The tornadic supercell from LaGrange, Wyoming on 5 June 2009 is
compared to non-tornadic supercelluar convection on 4 June 2009 near Cheyenne,
Wyoming. The storms on both of those days were observed by the Verifications of the
Origins of Rotation in Tornadoes Experiment Part 2 (VORTEX2), which is a large
collaborative field experiment through which a lot of data are available.1 When
examining sounding data from a large field experiment like this, some questions may
arise as to what constitutes a good proximity sounding, which is a sounding launched
spatially and temporally close to a storm or tornado.
Beebe (1958) studied tornadic storms and defined a proximity sounding to be one
taken near a storm and within fifty miles and roughly an hour from a tornado forming.
They also used the definition of a precedent or pre-storm sounding being one that was
characteristic of the air mass in which a tornado occurred but removed in time and/or
space from the vicinity of the tornado. Pre-storm soundings were found to have shallower
moisture in the vertical and a capping inversion, whereas proximity soundings had deep
moisture and no inversion. Other studies can have more complicated means of selecting
proximity soundings, like Thompson et al. (2003) where storms and their respective
soundings were selected based on radar characteristics, storm severity, and quantities
derived from models. Brooks et al. (1994a) stresses the difficulties associated with
forecasting tornadic environments. They discussed the lack of a complete understanding
1 See website: http://catalog.eol.ucar.edu/vortex2_2009/
14
of supercell tornadogenesis, mentioning that important environmental parameters such as
convective available potential energy (CAPE) and helicity can be altered greatly by
ongoing convection. They describe the forecaster’s challenge as needing to anticipate
what environmental conditions will be in the vicinity of a storm, before conditions are
affected by convection. In other words, a pre-storm sounding represents the environment
in which a storm will form, whereas a proximity sounding represents the environment
surrounding a storm that has formed. In a study of over 250 tornado proximity soundings,
Maddox (1976) found that there is a wide range of conditions associated with tornadoes.
They stated that proximity soundings (for tornadic storms) with similar characteristics are
often associated with tornado occurrences of greatly differing intensity and number. Even
with 250 proximity soundings, there was a great deal of variability for tornadic behavior.
However, they did find that many tornadic proximity wind profiles similarly
demonstrated strong environmental flow at low levels, weak flow in the middle levels,
and strong flow again at upper levels.
The current work compares the tornadic supercell on 5 June 2009 to a non-
tornadic supercell on 4 June 2009. The storms of interest from both days took place in
southeastern Wyoming. The objectives of this work are to compare environmental wind
speeds in proximity to the storms, along with hodographs and helicity, pre-storm CAPE
and convective inhibition (CIN), and radar imagery from the two days, in order to better
understand tornadogenesis. There may be many ways for supercellular tornadogenesis to
occur and the findings presented herein may or may not be generalized.
2. Data and Methods
This study made use of raw sounding data from VORTEX2 launches. All
VORTEX2 sounding data files are in the same format, making it relatively simple to sort
15
through them with computer programs. Wind speeds were averaged over 100 hPa layers
(i.e., from surface to 800 hPa, 800 to 700 hPa, etc.). The surface pressure is roughly
around 850 hPa in southeastern Wyoming due to higher elevation, so up to 700 hPa is
considered roughly the depth of the boundary layer, 700–500 hPa is considered mid-
levels, and above 500 hPa is treated in general as upper levels.
Two proximity soundings (near the storms) from each day were used to plot
mean hodographs. The u and v components of wind from points at half-kilometer
intervals were hand-selected from the raw sounding data and recorded into a spreadsheet.
Typically, a mean tornado proximity hodograph shows winds veering and increasing with
height, especially in the lowest few kilometers (Davies-Jones 1984). Veering winds have
been shown to be more favorable for right-moving supercells. Davies-Jones established a
positive correlation between storm updraft and vertical vorticity when storm-relative
winds veer with height, or equivalently, when the vorticity has a streamwise component
in the storm’s reference frame. Helicity, which is a measure of how much streamwise
vorticity is available in a storm’s environment, has been shown to be a fairly good
predictor of supercells and tornadoes. Given the occurrence of a tornado, the tornadic
strength increases with increasing values of helicity, and thus stronger tornadoes are
associated with high helicity (Kerr and Darkow 1996; Davies-Jones et al. 1990; Davies-
Jones 1993). But, as Davies-Jones (1993) cautions, a strong cap can prevent storms from
forming altogether, even when helicity (and CAPE) are favorable for supercells. The
average storm proximity hodographs plotted for this work are good for qualitatively
analyzing helicity and diagnosing streamwise vorticity, whereas a computer program (see
Appendix) can calculate helicity for each proximity sounding. The existence of
convection is implicit in the fact that they are storm proximity soundings, therefore the
hodographs can be assumed to show helicity that is available to the storm.
16
For the pre-storm soundings, CAPE and CIN for parcels at every point from the
surface up to 700 hPa were calculated. Vertical profiles of each were then plotted on the
same axes for a visual and quantitative comparison. These profiles were only examined
for the pre-storm soundings because once convection is initiated the cap is broken and
CAPE can be realized. Comparing vertical profiles of CAPE and CIN from the pre-storm
environment is useful for forecasting because CAPE is a good discriminator between
severe and non-severe thunderstorms (though not necessarily a good discriminator
between tornadic and non-tornadic storms; Brooks et al. 1994a).
Archived radar data for Cheyenne (KCYS) were downloaded from the National
Climatic Data Center website2 and the files were viewed in GR2 Analyst, which is a very
useful tool for viewing storms from different angles, zoomed in and out, and vertical
cross-sections. GR2 Analyst also allows the user to measure distances between points and
pinpoint latitude/longitude locations. Radar imagery that was saved includes views of the
supercells on each day, vertical cross-sections showing the bounded weak echo regions
(BWER) on each day, and zoomed-out views of the general convective situation, with
pinpoints at the locations and times of the pre-storm and proximity sounding launches.
3. Results
a. Synoptic overview of 4 June and 5 June 2009
Heights and isotachs at 300 hPa for 1200 UTC from both days are shown in
Figure 2. There is a closed low over California which deepens from 4 June to 5 June.
Winds are much stronger on 5 June, and downstream from the low there appears to be
more diffluence, which could potentially cause the surface pressure to fall in Wyoming
2 See website: http://www.ncdc.noaa.gov
17
and nearby states. Heights and relative humidity at 500 hPa for 1200 UTC from both days
are shown in Figure 3. On both days the low at 500 hPa is generally in the same location
as the 300 hPa low. As was the case at 300 hPa, the winds at 500 hPa are also stronger on
5 June, with more diffluence downstream from the low. There are also pockets of higher
relative humidity on 5 June. Enhanced water vapor satellite imagery for 1325 UTC from
both days is shown in Figure 4. There is more water vapor in the vicinity of Wyoming on
5 June than on 4 June. The synoptic overview of the two days suggests that 5 June was
more favorable for active weather than 4 June, just based on water vapor and upper-level
diffluence.
b. Radar and sounding results
Figure 5 shows radar images with the locations of the pre-storm sounding
launches for both days. On 4 June the pre-storm sounding location may not be a very
good representation of the air in which the storms in southeastern Wyoming formed
because of how far away it is (150 to 200 km) from where the targeted storms developed.
The locations of the pre-storm soundings on 5 June are closer to where storms formed
(Fig 5). Figure 6 shows the vertical profiles of CAPE and CIN from the pre-storm
soundings at the locations and times from Figure 5. There was a lot more convective
potential energy available to be realized on the tornadic day than on the non-tornadic day.
There was also more CIN on the non-tornadic day. Values of CAPE were up near 1600
J/kg at the surface, and higher values extended through most of the surface-to-700 hPa
layer (assumed boundary layer). The non-tornadic day had values of CIN near 400 J/kg
and the weak CAPE values barely reaching 200 J/kg. As will be discussed later, the pre-
storm sounding on the tornadic day may not be entirely representative of the environment
in which the storms formed. The pre-storm soundings for the tornadic day were a very
good representation of the conditions in which the storms formed, seeing as the pre-storm
18
launch location was approximately 30 km from each of the two later storm environment
launches, generally in between them.
Radar imagery revealed structural and temporal information about the storms.
Figure 7 shows radar imagery from times on both days with supercells. The zoom scale is
the same for both images, so it is easy to note that the supercell on 5 June is larger than
the supercell on 4 June. The supercell on 4 June was rather isolated in space and time,
and it later merged with the surrounding convective cells. On 5 June there appears to be
more down-shear precipitation which would correspond to the stronger upper-level
winds, as will later be discussed in the sounding analysis. Figure 8 is a zoomed-in view of
both supercells at the same time as in Figure 7. There is radar-indicated rotation in the
storm on 4 June and a radar-indicated tornado on 5 June. Figure 9 shows vertical cross-
sections of the two supercells at the same times as in Figure 8. The tornadic storm had a
much larger BWER with a lot of precipitation overhang and a hail core. The non-tornadic
storm has a BWER as well, but it is not as deep and likely does not include any hail,
based on reflectivity. The vertical extent of the tornadic storm is about 10,000 feet higher
than that of the non-tornadic storm. The storms were roughly the same distance from the
KCYS radar, so it is logical to assume the vertical cross-sections can be compared fairly.
Both the larger BWER and the deeper vertical extent of the tornadic storm indicate that it
had a vigorous updraft, one much stronger than the one on the non-tornadic day.
Figure 10 shows the radar images from the times when proximity soundings were
launched, with points indicating the locations of the launches. The proximity soundings
launched on 4 June were roughly an hour and a half after the occurrence of the supercell
in Figures 7–9. The proximity soundings launched on 5 June were only ten minutes
before the time in Figures 7–9. Mean hodographs from the soundings in Figure 10 are
shown in Figure 11. Qualitatively, the mean hodograph for the tornadic day is much more
favorable for streamwise vorticity than the near-linear mean hodograph for the non-
19
tornadic day. Quantitatively, the calculated helicity values for the proximity soundings on
the non-tornadic day were low. The 0–6km and 0–3km helicity values at 2308 UTC on 4
June were 45 m2/s
2 and 137 m
2/s
2, respectively. The 0–6km and 0–3km helicity values at
2310 UTC on 4 June were both negative and small. The 0–6km and 0–3km helicity
values at 2143 UTC on 5 June were 340 m2/s
2 and 158 m
2/s
2, respectively. The 0–6km
and 0–3km helicity values at 2150 UTC on 5 June were 165 m2/s
2 and 234 m
2/s
2,
respectively.
A wind speed comparison is shown in Table 1 (the locations of the proximity
soundings compared in Table 1 are shown in the radar image in Figure 10). The
comparison of wind speeds supports the hypothesis of weaker wind speeds at mid-levels
for the tornadic storm (Table 1). For the tornadic day, environmental winds (taken from
proximity soundings) were slightly stronger at low-levels, much stronger at upper-levels,
and weaker at mid-levels (around 17 to 26 knots, as opposed to 30 to 40 knots at mid-
levels for the non-tornadic day).
4. Discussion and Conclusions
The hypothesis that weaker low- to mid-level wind speeds are more favorable for
tornadogenesis is supported by some of the findings in this study. The comparison of
layer-average wind speeds for the tornadic storm from 5 June and the non-tornadic storm
on 4 June, 2009, shows that compared to the non-tornadic storm environment, the storm
environment winds on the tornadic day were stronger at low levels, weaker at mid-levels,
and stronger again at upper levels. The observed wind profile on the tornadic day (Table
1) also matches well with the findings of Maddox (1976), where the flow is strong at low
levels, weaker at mid-levels, and stronger again at upper levels. The findings shown in
Table 1 support the hypothesis that weaker mid-level winds are more favorable to
20
tornadogenesis, at least in the event that rain falling near the updraft plays an important
role. This is because weaker mid-level winds will keep precipitation from blowing too far
away from the updraft and more baroclinic vorticity can be generated, along with lower
LCL heights occurring once rain-cooled air near the updraft was ingested again. A caveat
to note with this comparison is that based on previous work by the current authors,
weaker low- to mid-level wind speeds can only be assumed to favor tornadogenesis in the
event that rain can and will start falling very near the updraft, in the wall cloud region.
Photographic evidence and personal observation strongly suggest that the 5 June tornadic
supercell in LaGrange, WY, developed because of rain occurring within the wall cloud
region. In order to accurately compare any tornadic supercell to any non-tornadic
supercell, photographs would need to be examined from both storms to see if rain did
occur very near the updraft. Tornadoes forming due to rain near the updraft form from
different means than tornadoes occurring along gust fronts or non-supercell storms,
which tend to be weaker (Wakimoto and Wilson 1989). Further comparisons of storm
environment wind speeds in this fashion should be limited to tornadoes occurring at the
occlusion point of the gust fronts, at the center of the storm (Lemon and Doswell 1979),
as this is typically where the wall cloud forms and baroclinic vorticity is greatest.
The two storms compared in this study may have been too drastically different
for a fair comparison. The amount and depth of CAPE was far more on the tornadic day
(Fig. 6), along with less CIN. As would be assumed with higher CAPE values, and as was
seen in Figure 9, the tornadic storm had a much more intense updraft than the non-
tornadic storm. Also more favorable for strongly rotating tornadic supercells is the mean
environment hodograph for the tornadic day (Fig. 3). There was a great deal of
streamwise vorticity available in the storm’s environment on 5 June, whereas the storm
environment on 4 June favored more crosswise vorticity (linear hodograph) and thus was
less favorable for rotating storms.
21
Overall, the comparison of the two storms was a successful study. A lot can be
learned from closely examining two different storms. They were similar in that they
occurred at the same elevation, and even in the same state. They were different in that
one was much stronger than the other, and that one produced a tornado. The weaker
supercell on 4 June did not form in favorable conditions for convection, let alone
supercells. There could have been some sort of external forcing or boundaries that caused
cells to initiate that day. The pre-storm sounding on 4 June was within roughly 150 km to
the south of the location where convection occurred, whereas the pre-storm soundings on
5 June were within roughly 60 miles east of the storm’s eventual location. Despite the
less than ideal location of the pre-storm sounding on 4 June, the radar imagery showed
storms that likely would have formed in the conditions that the sounding reflected. A
better comparison can be made if more storms were included in the study, as well as
photographs of all of them. Regardless of the differences between the storms here, the
wind speeds do support the hypothesis that weaker mid-level wind speeds allow rain to
fall near the updraft, which would in turn support more baroclinic vorticity and lowering
of LCL heights, aiding tornadogenesis. Although it is not known whether the stronger
winds in the non-tornadic supercell from 4 June blew precipitation far away from the
updrafts, the rain near the updraft on 5 June was likely of key importance to the
development of the tornado. In the case of a much stronger supercell than what occurred
on 4 June, but with a similar wind profile, stronger winds at mid levels may or may not
have reduced the possibility for a tornado. The key to future research comparing tornadic
and non-tornadic supercells is to look at more cases.
22
Table 1
4 June, 2310 Z [kts] 5 June, 2143 Z [kts]
824 (surface) to 700 mb 10.63 856 (surface) to 700 mb 11.32
700 to 600 31.29 700 to 600 17.71
600 to 500 40.27 600 to 500 26.72
500 to 400 40.85 500 to 400 44.88
400 to 300 42.67 400 to 300 58.30
Figure 1
Table 1. Table of average storm environment wind speeds for 100 hPa layers for the tornadic case on 5 June and the non-
tornadic case on 4 June, 2009. Mid-level (700 to 500 hPa) winds are weaker in the tornadic case, favoring the hypothesis of the current work. Note that the locations of these soundings is shown in Figure 10.
Fig 1. Photograph taken of tornadic storm on 5 June, 2009, in LaGrange, WY. Funnel first appeared four minutes after rain first occurred within wall cloud region. Photograph courtesy of Scott Steiger.
23
Figure 2
Fig. 2. 300 hPa heights and isotachs at 1200 UTC for 4 June (top) and 5 June (bottom). On 5 June note stronger low over
California with more diffluence, and also stronger wind speeds. From VORTEX2 briefing packages
(http://catalog.eol.ucar.edu/vortex2_2009).
24
Figure 3
Fig. 3. 500 hPa heights and RH(%) at 1200 UTC for 4 June (top) and 5 June (bottom). On 5 June note stronger low over
California with more diffluence, and also higher RH in the Midwest. From VORTEX2 briefing packages (http://catalog.eol.ucar.edu/vortex2_2009).
25
Figure 4
Fig. 4. Water vapor imagery for 1325 UTC on both 4 June (top) and 5 June (bottom). Note much more water vapor in the Midwest on 5 June. From VORTEX2 briefing packages (http://catalog.eol.ucar.edu/vortex2_2009).
26
Figure 5
Fig. 5. Top image shows location of pre-storm sounding on 4 June. Bottom image shows locations of pre-storm soundings
on 5 June. The pre-storm sounding on 4 June may not be a good representation of the air in which storms formed in SE Wyoming (note Denver, CO and Cheyenne, WY). Pre-storm soundings on 5 June are much closer to where storms
eventually formed.
27
Figure 6
Fig. 6. Top graph shows CAPE and CIN for the tornadic day, bottom graph shows same for non-tornadic day. There was significantly more CAPE and less CIN on the tornadic day.
28
Figure 7
Fig. 7. Top shows radar imagery of a supercell on 4 June, propagating southeastward. Bottom shows radar imagery from a
supercell on 5 June, propagating eastward and notably larger the storm on 4 June.
29
Figure 8
Fig. 8. Zoomed-in on images from Fig. 7. Top is 4 June, with rotation indicated by radar. Bottom is 5 June with a tornado
indicated by radar.
30
Figure 9
Fig. 9. Vertical cross-sections of the two supercells. Note a larger BWER and deeper vertical extent on 5 June.
31
Figure 10
Fig. 10. Top is radar image from 4 June showing locations of proximity soundings, occurring later than the supercell in
previous figures. Bottom is radar image from 5 June also showing locations of proximity soundings.
2310 UTC
Proximity Launch
2308 UTC
Proximity Launch
32
Figure 11
Fig 11. Average storm environment hodographs for the tornadic and non-tornadic days. The hodograph for the tornadic day
favors streamwise vorticity, whereas the non-tornadic day is more linear. Points every half kilometer.
33
Appendix
Computing Helicity
/**Java program to compute SREH for all V2 soundings
* Must put desired sounding file under "File sounding"
* Put in Cx and Cy manually (obtained from radar)
* In data file, cut out data above 6km (surfAlt+6000m)
*/
package helicity2;
import static java.lang.Math.*;
import java.util.Scanner;
import java.io.*;
public class Main {
public static void main(String[] args) throws Exception {
File sounding = new File("NCAR20090604_2310.txt");
Scanner sc1 = new Scanner(sounding);
Scanner sc2 = new Scanner(sounding);
double x = 10.92;
double y = -9.16; // Storm motion vector components, Cx and Cy (m/s)
double u1,u2,v1,v2,SREHtempo,SREH;
SREHtempo = SREH = 0;
//Advance scanner-1 past the file header and to first line
//Advance scanner-2 to the second line
for(int h=0; h<15; h++){
sc1.nextLine();
}
for(int k=0; k<16; k++){
sc2.nextLine();
}
double u2tempo, v2tempo, u1tempo, v1tempo;
while(sc2.hasNext()){
for(int q=0; q<8; q++){
//advances scanners up to Ucmp
sc1.next();
sc2.next();
}
u2tempo = sc2.nextDouble();
v2tempo = sc2.nextDouble();
u1tempo = sc1.nextDouble();
v1tempo = sc1.nextDouble();
// Make sure not to read in erroneous numbers (like 999.99), use <50 as cut-off
if(abs(u2tempo)<50 && abs(v2tempo)<50 && abs(u1tempo)<50 && abs(v1tempo)<50){
u2 = u2tempo;
v2 = v2tempo;
u1 = u1tempo;
v1 = v1tempo;
SREHtempo = ((u1-x)*(v1-v2) + (v1-y)*(u2-u1));
// Discretation of SREH = Integral (0 to 6km) [(V-C) dot k cross dV/dz] Dz (derived by hand)
SREH = SREH + SREHtempo; //Sum up all levels
}
sc1.nextLine();
sc2.nextLine();
//Advance scanners each by one, sums up discrete levels
}
System.out.println("SREH [m2/s2] is: " + SREH);
}
}
34
References
Beebe, R. G., 1958: Tornado proximity soundings. Bull. Amer. Meteor. Soc., 39, 195–
201.
Bluestein, H. B., and C. R. Parks, 1983: A synoptic and photographic climatology of low-
precipitation thunderstorms in the Southern Plains. Mon. Wea. Rev., 111, 2034–
2046.
Brooks, H. B., C. A. Doswell III, and J. Cooper, 1994a: On the environments of tornadic
and nontornadic mesocyclones. Wea. Forecasting, 9, 606–618.
––, ––, and R. B. Wilhelmson, 1994b: The role of midtropospheric winds in the evolution
and maintenance of low-level mesocyclones. Mon. Wea. Rev., 122, 126–136.
Browning, K. A., 1964: Airflow and precipitation trajectories within severe local storms
which travel to the right of the winds. J. Atmos. Sci., 21, 634–639.
Burgess, D.B., and R. P. Davies-Jones, 1979: Unusual tornadic storms in eastern
Oklahoma on 5 December 1975. Mon. Wea. Rev., 107, 451–457.
Davies-Jones, R., 1984: Streamwise vorticity: The origin of updraft rotation in supercell
storms. J. Atmos. Sci., 41, 2991–3006.
––, 1993: Helicity trends in tornado outbreaks. Preprints, 17th Conf. on Severe Local
Storms, St. Louis, MO, Amer. Meteor. Soc., 56-60.
—, D. Burgess and M. Foster, 1990: Test of helicity as a tornado forecast parameter.
Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, AB, Canada,
Amer. Meteor. Soc., 588–592.
Doswell, C. A., A. R. Moller, and R. Przybylinski, 1990: A unified set of conceptual
models for variations on the supercell theme. Preprints, 16th Conf. Severe Local
Storms, Kananaskis Park, AB, Canada, Amer. Meteor. Soc., 40–50.
Kerr, B., and G. L. Darkow, 1996: Storm-relative winds and helicity in the tornadic
thunderstorm environment. Weather and Forecasting, 11, 489–505.
Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm evolution and
mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107, 1184–
1197.
Maddox, R. A., 1976: An evaluation of tornado proximity wind and stability data. Mon.
Wea. Rev., 104, 133–142.
Moller, A. R., C. A. Doswell III, M. P. Foster, and G. R. Woodall, 1994: The operational
recognition of supercell thunderstorm environments and storm structures. Wea.
Forecasting, 9, 327–347.
35
Rasmussen, E. N., and J. M. Straka, 1998: Variations in supercell morphology. Part I:
Observations of the role of upper-level storm-relative flow. Mon. Wea. Rev., 126,
2406–2421.
Rotunno, R., and J. Klemp, 1985: On the rotation and propagation of simulated supercell
thunderstorms. J. Atmos. Sci., 42, No. 3, 271–292.
Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. Markowski, 2003: Close
proximity soundings within supercell environments obtained from the Rapid
Update Cycle. Wea. Forecasting, 18, 1243–1261.
Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev.,
117, 1113–1140.