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

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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.

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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

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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

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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.

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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).

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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

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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

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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.

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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-

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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

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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

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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/

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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

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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.

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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

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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

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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-

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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

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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

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