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Composite Analysis of Thundersnow Events in the Central United States

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Composite Analysis of Thundersnow Events in the Central United States COMPOSITE ANALYSIS OF THUNDERSNOW EVENTS IN THE CENTRAL UNITED STATES ————————————————– A Thesis Presented to the Faculty of the Graduate School University of Missouri-Columbia ————————————————– In Partial Fulfillment for the Degree Master of Science ————————————————– by Angela M. Oravetz Dr. Patrick S. Market, Thesis Supervisor MAY 2003 Public Abstract Angela Oravetz, ID #713285 M.S. Atmospheric Science Composite Analysis of Thundersnow Events in the Central United States Advisor: Dr. Patrick Market ————————————— Graduation Term: Winter 2003 Snowfall can be an inconvenience no matter how little or how much falls. How- ever, conditions can become more inconvenient when the snowfall is heavy. Occa- sionally, these events may be due to the presence of convection, which results in thunder and lightning; such events are called thundersnow. Oftentimes, thunder- snow events form in bands of snowfall and tend to be localized, perhaps only a few tens of kilometers wide. Thundersnow events from 1961 through 1990 were examined. Chosen events occurred on synoptic hours and featured thundersnow only, a stratification leads to 97 cases of thundersnow. The thundersnow events were classified into cate- gories based upon the setting in which each event occurred; examples include: with a cyclone, with a front, in a lake effect situation, etc. Events that occurred with a cyclone were the focus of this study, and further classified based upon where the thundersnow occurred in relation to the cyclone (northwest of it, northeast, north, etc.) Mean fields of atmospheric variables at several levels (900 mb, 850 mb 700 mb, 500 mb, and 300 mb) were then generated with compositing software at the time of initiation as well as 12, 24, 36, and 48 hour prior to thundersnow initiation. Also, sounding profiles of the atmosphere were generated to get a more accurate picture of the thermodynamics present during thundersnow. Thundersnow associated with cyclones will on average occur in a dynamic en- vironment. Thundersnow occurring northwest of a cyclone center appears to be the result of upright convection. Whereas thundersnow occurring to the northeast of a cyclone appears to be more the result of a slantwise parcel displacement. One of the goals of this research was to document the environment in which thundersnow occurs. This will help in forecasting the thundersnow events, which can help prepare the public as well as emergency personnel for the future weather. COMMITTEE IN CHARGE OF CANDIDACY: Assistant Professor Patrick Market Chairperson and Advisor Assistant Professor Neil Fox Professor William Kurtz Assistant Professor Anthony Lupo Acknowledgements For aiding in the development of this research project, many thanks go to Dr. Patrick Market. His ability to plan out such a complex research project deserves a commendation. His positive perspective that things would work out and in- credible belief in my ability to complete this project kept me going so many days throughout this degree. Thanks also to Rebecca Ebert for doing some of the ”busy-work” and other repetitive tasks associated with making great scientific discoveries. I would not have been able to make this thesis as complete as it is without the many long hours she poured into the final stretches of this project. This research was funded by a grant from the University of Missouri-Columbia Graduate Research Board. Additional funding was provided from the Cooperative Program for Operational Meteorology, Education and Training (COMET). Thanks go to my parents for encouraging me to choose a career in meteorology, a field of study that I was always interested in and also for always believing that I could tackle any hurdle that came my way. My new family also provided much support and encouragement through the toughest parts of the degree. To Brian, my husband, whose unconditional love and support helped me make it through so many days. Thank you for sharing in all the joys and frustrations associated with this degree. I would not be the person I am today without your constant love, support, guidance, and humor. The past two years have seemed so long at times, and you have helped me make it through them. i Contents 1 Introduction 1 1.1 Purpose&Objectives............................ 2 1.1.1 Purpose . 2 1.1.2 Objectives . 2 1.2 Statementofthesis ............................. 3 2 Literature Review 4 2.1 Compositing of Thundersnow . 4 2.2 Dynamic Forcing Associated with Thundersnow . 13 2.2.1 Frontogenesis . 13 2.2.2 Conditional Symmetric Instability . 14 2.3 Literature Summary . 18 3 Methodology 21 3.1 Data . 21 3.2 Case Categorization . 22 3.2.1 Categorization by Location . 22 3.2.2 Categorization by Intensity . 22 3.3 CompositingMethod............................ 25 3.4 Special Diagnostics . 27 3.4.1 Frontogenesis . 27 3.4.2 Equivalent Potential Vorticity . 29 ii 3.5 Methodology Summary . 30 4 Composite results 31 4.1 T-48 . 31 4.1.1 All cyclones . 31 4.1.2 Northwest of cyclone . 35 4.1.3 Northeast of cyclone . 37 4.2 T-36 . 41 4.2.1 All cyclones . 41 4.2.2 Northwest of cyclone . 43 4.2.3 Northeast of cyclone . 47 4.3 T-24 . 51 4.3.1 All cyclones . 51 4.3.2 Northwest of cyclone . 53 4.3.3 Northeast of cyclone . 56 4.4 T-12 . 59 4.4.1 All cyclones . 59 4.4.2 Northwest of cyclone . 62 4.4.3 Northeast of cyclone . 64 4.5 T-00 . 67 4.5.1 All cyclones . 67 4.5.2 Northwest of cyclone . 71 4.5.3 Northeast of cyclone . 73 4.6 Comparisons . 78 4.6.1 All cyclones . 78 4.6.2 Northwest of cyclone . 78 4.6.3 Northeast of cyclone . 79 4.7 Snowfallintensity.............................. 81 iii 4.7.1 Light . 81 4.7.2 Moderate . 84 4.7.3 Heavy . 87 5 Sounding Profiles 91 5.1 Proximity Soundings . 91 5.2 DerivedProfiles............................... 94 5.2.1 Equivalent Potential Temperature . 96 5.2.2 Equivalent Potential Vorticity . 97 5.2.3 Frontogenesis . 100 5.3 Typical Profiles . 101 5.3.1 Equivalent Potential Temperature . 102 5.3.2 Equivalent Potential Vorticity . 103 5.3.3 Frontogenesis . 104 5.3.4 Examples . 104 5.4 Summary . 107 6 Summary 108 6.1 Discussion . 108 6.2 Conclusions .................................110 Appendix A REFERENCES ...................................118 iv List of Figures 2.1 Reanalyzed skew-T log p diagram using data from the Curran and Pearson (1971) mean proximity sounding. The skew-T log p chart depicts temperature ( C; solid) and dew point ( C; dashed). The column, labeled “K FT,” is a plot of ¡ ¢ versus height. .......... 6 2.2 The locations with the most frequent occurrence for elevated thun- derstorms in the United States. Reproduced from Colman (1990a). 8 2.3 The locations with the most frequent occurrence for elevated thun- derstorms during the months October through March in the United States. Reproduced from Colman (1990a). ............... 9 2.4 A cross section of a PSI environment oriented North-South with an ordinate of pressure, decreasing with height. The dashed green lines are lines of constant absolute geostrophic momentum, £ ¤ . The solid red lines are lines of constant equivalent potential temperature, ¡ ¢ . The solid blue shaded region is a region where the relative hu- midity is greater than 80 percent. ..................... 17 v 2.5 A cross section of a CSI environment oriented North-South with an ordinate of pressure, decreasing with height. The dashed green lines are lines of constant absolute geostrophic momentum, £ ¤ . The solid red lines are lines of saturated equivalent potential tem- perature, ¡ ¢ . The solid blue shaded region is a region where the relative humidity is greater than 80 percent and in the blue shaded region only, CSI=PSI. ............................ 19 ¢ ¡ 3.1 The ¡ grid used in the compositing of the thundersnow cases. The map background is for a frame of reference only, and does not exclusively represent the area composited. 27 4.1 The 900-mb heights in geopotential meters (gpm) and temperature § ¥ ¨ advection ( £ ¤ ¥ ¦ K ) for ALL cyclones at T-48. With this and all similar figures, the map is provided for reference only. Also, standard contour intervals are not employed in all cases such that more detail may be rendered in each analysis. .................... 32 £ ¤ § ¥ ¨ 4.2 The 850-mb heights (gpm) and divergence ( ¥ © ) fields for ALL cyclones at T-48. .............................. 33 ¡ ¢ £ ¤ § ¥ ¨ 4.3 The 700-mb (K) and temperature advection ( ¥ ¦ K ) for ALL cyclones at T-48. .............................. 34 4.4 The 300-mb heights (gpm) and isotachs (ms ¥ ¨ ) for ALL cyclones at T-48. ...................................... 34 £ ¤ § ¥ ¨ 4.5 The 900-mb heights (gpm) and temperature advection ( ¥ ¦ K ) for NWC cases at T-48. ........................... 35 vi £ ¤ § ¥ ¨ 4.6 The 850-mb heights (gpm) and divergence ( ¥ © ) fields for NWC cases at T-48. ................................ 36 ¡ ¢ § ¥ ¨ 4.7 The 700-mb (K) and temperature advection ( £ ¤ ¥ ¦ K ) for NWC cases at T-48. ................................ 37 4.8 The 300-mb heights (gpm) and isotachs (ms ¥ ¨ ) for NWC cases at T-48. ...................................... 38 £ ¤ § ¥ ¨ 4.9 The 900-mb heights (gpm) and temperature advection ( ¥ ¦ K ) for NEC cases at T-48. ........................... 39 4.10 The 850-mb heights (gpm) and divergence ( £ ¤ ¥ © § ¥ ¨ ) fields for NEC cases at T-48. ................................ 40 ¡ ¢ £ ¤ § ¥ ¨ 4.11 The 700-mb (K) and temperature advection ( ¥ ¦ K ) for NEC cases at T-48. ................................ 40 4.12 The 300-mb heights (gpm) and isotachs (ms ¥ ¨ ) for NEC cases at T-48. 41 § ¥ ¨ 4.13 The 900-mb heights (gpm) and temperature advection ( £ ¤ ¥ ¦ K ) for ALL cases at T-36. ............................ 42 £ ¤ § ¥ ¨ 4.14 The 850-mb heights (gpm) and divergence ( ¥ © ) fields for ALL cases at T-36. ................................ 43 ¡ ¢ £ ¤ § ¥ ¨ 4.15 The 700-mb (K) and temperature advection ( ¥ ¦ K ) for ALL cases at T-36. ................................ 44 4.16 The 300-mb heights (gpm) and isotachs (ms ¥ ¨ ) for ALL cases at T-36.
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