Evaluation of “Climatology of Storm Reports Relative to Upper-Level Jet Streaks” by Clark et al.

1. Evaluation of “Climatology of Storm Reports Relative to Upper-Level Jet Streaks” by Clark et al. By: Kristin Bremer & Sean Archer

2. Outline • Introduction • Background • Previous work • Extended work • Data & Methodology • Data retrieval • How jet streak was defined • How data was analyzed • Results • Jet streak position relative to storm reports • Jet streak composite fields • Jet streak curvature • Jet streak direction • Discussion & Conclusions

3. Introduction • Ageostrophic winds result in upper-level divergence in the right-entrance and left exit regions of a jet streak • Jet streaks are said to be important in the development of severe wx because they help to: • Decrease static stability • Cool air to saturation in a layer of air • Release CAPE • Enhance moisture convergence at low-levels • Enhance vertical wind shear

4. Background • Based on paper by Rose et al. • 10 year climatological study of F1 and above tornado occurrences relative to 250 hPa jet streaks • Used 4 quadrant model • Did not considered other major jet dynamics • Found that the majority of the tornado reports were in the exit quadrants of jet, specifically the left-exit • Concluded that there were errors when using the 4 quadrant model • Clark et al. extended these results by not only looking at all tornado reports in U.S. but also hail reports and wind reports • Clark et al. will also be looking at other cloud dynamics such as upper-level divergence, curvature, and direction of jet streaks

5. Data & Methodology • Upper-level jet streaks: North American Regional Reanalysis (NARR) • Storm reports: National Climatic Data Center (NCDC) on the Storm Prediction Center Website • March-September 1994-2004 • Only jet streaks that occurred at 0000 UTC with storm reports being within 3 hour period • Any storm report within this time period but was outside of the jet streak was deemed a “non-jet-related” report

6. Data & Methodology • How jet was defined: • Enclosed area of 25 m/s+ winds • Mark jet core and jet endpoints • Jet core defined as point within the area of max wind speed where the acceleration became zero • Max major jet axis at 1000 km • After each jet was defined, it was placed on 77 x 37 grid with the jet core at the center using Grid Analysis and Display System (GrADS)

7. (Clark et al.)

8. Results • A) Jet streak position relative to storm reports • 105,987 storm reports out of 126, 864 analyzed fell within jet streak-84% • Peak month for both jet-related and non-jet-related was June and May was the peak month for jet-related storm reports • Most likely due to north migration of jet stream • Non jet-related reports were most likely due to MCS which usually occur outside a jet streak and during July and August when these reports were at its peak

9. Results • A) Jet streak position relative to storm reports cont’d • March and April-Storm reports majority in left-exit • April-Storm reports majority in right-entrance • After April the right-exit region was favored which is not consistent with 4QM • Tornado reports-Exit region favored, specifically the left exit • Hail reports-More evenly distributed but a majority were found in the left exit • Wind reports-majority in right entrance

10. Results • B) Jet streak Composite fields • Upper-level divergence does not exactly coincide with the 4QM-slight displacement toward cyclonically sheared side of jet streak • Number of storm reports matched up with areas of strong upper-level divergence in entrance regions, specifically right-entrance • Exit regions had most reports just south of area of high upper-level divergence (right-exit) • Exit-regions seemed to coincide better with low-level convergence

11. Results • C) Jet Streak Curvature • Radius of curvature • R = 𝐿/∆𝑑 • R* = (1/R) x 〖10〗^4 • Linear (-1.5≤R*≤1.5) • Cyclonic (R*≥5.0) • Anticyclonic (R*≤-5.0) • Jet streaks with values outside above intervals are not used

12. Jet Streak Curvature: Storm Report Distribution

13. Results • C) Jet Streak Curvature (Distribution Results) • Matched conceptual models (Beebe and Bates, 1955) • Matched numerical simulations (Moore and VanKnowe, 1992) • Anticyclones – stronger max divergence/convergence and stronger vertical velocity contradicts Moore and Vanknowe • Areas of storm report maxima experienced max divergence aloft and max convergence at the surface with a low pressure centered in the region

14. Jet Streak Curvature: Atmospheric Conditions

15. Results • D) Jet streak Direction • Jet streaks categorized into 6 ranges of directions • Easterly jet streaks were ignored • SW and WSW were the most active and had the most storm reports

16. Jet Streak Direction: Storm Report Distribution

17. Jet Streak Direction: Atmospheric Conditions

18. Discussion & Conclusions The left exit (right entrance) region contained the most storm reports in April and May (after April) Storm reports were concentrated on the axis in the exit region and the center of the right entrance region Anticyclonic curvature had the stronger vertical velocity while linear had the weakest More storm reports in WSW and SW oriented jet streaks Based on the results by Clark et al., jet streaks play a major role in the development of severe wx

19. Questions?