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Composite Analyses of Midwestern Convective Snow Events

Composite Analyses of Midwestern Convective Snow Events. Angela Oravetz 1 , Patrick Market 1 , David Gaede 2 , Evan Bookbinder 2 , and Anthony Lupo 1. 1 Department of Atmospheric Science University of Missouri - Columbia. 2 National Weather Service Office Springfield, Missouri.

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Composite Analyses of Midwestern Convective Snow Events

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  1. Composite Analyses of Midwestern Convective Snow Events Angela Oravetz1, Patrick Market1, David Gaede2, Evan Bookbinder2, and Anthony Lupo1 1 Department of Atmospheric Science University of Missouri - Columbia 2 National Weather Service Office Springfield, Missouri Presented at the NWA Annual Meeting – 24 October 2002

  2. Support University of Missouri – Columbia Graduate Research Board

  3. Introduction • Analyses of 99 thundersnow (TS) events • All associated w/ a transient mid-latitude cyclone • 1961-1990 • Market et al. (2003) • Northeast and northwest quadrants of surface cyclones exhibit the bulk of thundersnow occurrence

  4. TS Distribution w/ Transient Midlatitude Cyclone L

  5. Motivation • On the synoptic and meso- scales, why are these regions preferred for thundersnow? • Assumption 1: thundersnow results from convection • Assumption 2: convection results from the release of instability

  6. Hypothesis • Northeast of SFC cyclone • North of warm front • Anticyclonic shear more likely • Release of conditional symmetric instability (CSI) in the presence of F • (Young cyclone?) • Northwest of SFC cyclone • Strong cyclonic shear • Release of potential instability (PI) within a trowal • (Occluded cyclone?)

  7. Method • Assemble composite fields of TS events Northeast and Northwest of SFC cyclone • 9x9 grid with a grid spacing of 190.5 km. • Composite cases processed • 19 cases of TS NE of cyclone • 28 cases of TS NW of cyclone

  8. Results - Surface • Northeast (TS NE of Low center) • 19 cases • Mean central pressure of 999.0 mb • S.D. • Only 2 cyclones (11%) clearly occluded • Northwest (TS NW of Low center) • 28 cases • Mean central pressure of 998.0 mb • S.D. • 7 cyclones (25%) clearly occluded

  9. Thundersnow Northeast of SFC Cyclone

  10. NE - 300 mb Heights & Isotachs

  11. NE - 300 mb Temp. Advection 10-1 K hr-1

  12. NE - 500 mb Heights & Temp.

  13. NE - 500 mb Heights & Abs. Vort. X

  14. NE - 700 mb Qe

  15. NE - 850 mb Heights & Isotachs L

  16. NE - 850 mb Heights & Temp. L

  17. NE - 850 mb RH & Moist. Conv.

  18. NE – Surface Example

  19. NE – Instability • Examined cases with TS at 00Z or 12Z • N=5 • 4 of 5 systems open wave (1 occluded) • NO potential instability • Weak conditional symmetric stability (CSS) • Median value of equivalent potential vorticity (EPV) = 0.1*10-6(m2 K s-1 kg-1) • Median level elevation = 650 mb

  20. NE - Summary • TS occurs ahead of 300 mb trough • Jet core SE of event • Temp. advect. max S of event • TS beneath 500 mb ridge axis • Vort. max (16) SW of TS event • TS in zonal 700 mb Qe pattern • TS in 850 mb southerly flow • Nose of LLJ • Warm air advection • Moisture convergence

  21. Thundersnow Northwest of SFC Cyclone

  22. NW - 300 mb Heights & Isotachs

  23. NW - 300 mb Temp. Advection 10-1 K hr-1

  24. NW - 500 mb Heights & Temp.

  25. NW - 500 mb Heights & Abs. Vort. X

  26. NW - 700 mb Qe

  27. NW - 850 mb Heights & Isotachs L

  28. NW - 850 mb Heights & Temp. L

  29. NW - 850 mb RH & Moist. Conv.

  30. L NW – Surface Example

  31. NW - Instability • Examined cases of TS at 00Z and 12 Z • N = 12 • 11 of 12 systems open wave (1 occluded) • Potential Instability in 6 of 12 (50%) • Median level of base of unstable layer = 700 mb • Weak CSS in remaining 6 • No potential instability in these profiles • Median value of EPV = 0.2 *10-6 (m2 K s-1 kg-1)

  32. NW - Summary • TS occurs ahead of 300 mb trough • Jet streak SE of event (more meridional) • Temp. advect. max over event • TS ahead of negatively tilted 500 mb trough • Vort. max (19) SSW of TS • TS along 700 mb Qe ridge • TS NW of 850 mb Low • Warm air advection diminished • Moisture convergence persists • TS resides in RH max (85%)

  33. Conclusions • Surface analysis alone is inadequate for locating TS • Not all TS events NW of a cyclone center are associated with occluded cyclones • Not all TS events NE of a cyclone center are associated with nascent cyclones • Upper air analyses suggest TS NW of cyclone does occur with more mature cyclones

  34. Conclusions • Preliminary stability analysis • Northeast of a cyclone • The occurrence of CSI in the presence of frontogenesis more prevalent • Northwest of a cyclone • The occurrence of CSI in the presence of frontogenesis occurs ~50% of the time • Dynamics here? • The occurrence of PI occurs ~50% of the time • Presence of a trowal airstream?

  35. Questions, Comments, Criticism? marketp@missouri.edu amo598@mizzou.edu http://weather.missouri.edu/

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