Examining Relationships in Wind Speeds and other Parameters with Tornadic and Non-tornadic Supercells

Transcription

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,

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 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. 2

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 3

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 4

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. 5

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 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 ( and VORTEX2 sounding data were provided by NCAR/EOL under sponsorship by the National Science Foundation ( 6

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 7

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 8

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 wellrounded 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. 9

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 raincooled 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 stormrelative (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- 10

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 lowshear case was first to develop large values of vertical vorticity at low levels, but the lowlevel 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 11

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. Evaporativelycooled 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 12

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: 13

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 nontornadic supercell on 4 June 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 14

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, hpa is considered midlevels, 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. 15

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 website 2 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: 16

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 prestorm 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 17

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 crosssections 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- 18

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 m 2 /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 m 2 /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 m 2 /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 midlevels 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 19

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 nontornadic 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. 20

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. 21

22 Table 1 4 June, 2310 Z [kts] 5 June, 2143 Z [kts] 824 (surface) to 700 mb (surface) to 700 mb to to to to to to to to Figure 1 Table 1. Table of average storm environment wind speeds for 100 hpa layers for the tornadic case on 5 June and the nontornadic case on 4 June, 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. 22

23 Figure 2 Fig 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 ( 23

24 Figure 3 Fig 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 ( 24

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 ( 25

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. 26

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. 27

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. 28

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. 29

30 Figure 9 Fig. 9. Vertical cross-sections of the two supercells. Note a larger BWER and deeper vertical extent on 5 June. 30

31 Figure UTC Proximity Launch 2308 UTC Proximity Launch 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. 31

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. 32

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("NCAR _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 ), 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); } } 33

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