Darren Van Cleave M.S. Thesis Defense

1. Relating Snowfall Patterns Over the Central and Eastern U.S. to Infrared Imagery of Extratropical Cyclone Comma Heads Darren Van Cleave M.S. Thesis Defense 07/07/09

2. Acknowledgements • Dr. Tom Vonder Haar – Advisor • Dr. Sue Van den Heever & Dr. Jeff Nieman – Committee • Jack Dostalek – All-purpose Answer Man • CIRA – Technical Support • 301 Officemates & Atmos. Friends – Distractions/Ideas • Family – Common Sense • NOAA Grant NA17RJ1228 - Funding

3. Outline • Background • Introduction & Origins • Datasets & Methodology • Results - Dynamic Cause of Comma-Head Separation i. Jets ii. Trowal - Forecasting Implications • Future Work

4. Background • Geostationary Satellites and IR imagery T=-30 C 6 km T=-20 C 4 km 2 km T=5 C

5. Background cont. • Extratropical Cyclones & Geostrophic Balance Cyclone Terminology Geostrophic Wind Comma Head Frontal Zone Actual Wind Ageostrophic Wind Geostrophic Wind

6. Background cont. • Review of jet concepts For this research, jets are defined as areas of wind maxima around 250 millibars. In an idealized jet, rising motion is preferred in the right entrance and left exit regions with sinking in the other quadrants. Curvature can enhance the rising and sinking in select regions.

7. Introduction • Research Ideas Differences were identified in snowfall over the central US, possibly associated with differences in the comma-head patterns as seen in infrared geostationary satellite imagery.

8. Introduction cont. • Case Collection Cases of extratropical cyclones over the central and eastern U.S. with at least 5 inches of snow and obvious comma heads were sought. The western U.S. was excluded because of frequent cyclolysis along the West Coast and the generally disorganized cyclones in this area. The archived satellite imagery was available from 1996 to the present, and so the period of interest was the winter of 1996/’97 to 2008/’09. Nearly 50 cases were found which met these criteria.

9. Introduction cont. • Datasets Satellite 10.7 μm IR – GOES (Geostationary Operational Environmental Satellite) Weather forecast model reanalysis – NARR (North American Regional Reanalysis) Snowfall data – COOP (Cooperative Observer Program)

10. Origins • Original Ideas At the beginning of the project, it was thought that snowfall swaths and the resulting snow swath could be connected to cloud-top temperatures to develop an operational product. Instead, a textbook by Bader et al. indicated a possible dichotomy in storm structure, and it was thought that this could possibly apply to the cases that had already been collected. Bader et al. 1995

11. Origins cont. • A New Discovery After looking through the cases, a cloud feature was found which was absent from the literature. This feature was similar to the second type of storm in Bader’s text wherein the comma head featured warmer cloud tops than the frontal cloud shield. However, in contrast to the Bader diagrams, the comma head was visibly separate from the frontal zone for a period of time as seen in the IR imagery. vs.

12. Research Motivation • Snowfall Differences Initial plots of snowfall contours over satellite imagery seemed to indicate that the “separated” cases featured snowfall which was concentrated in a narrow area on the southern side of the comma head, in contrast with the “classic” cases wherein the snowfall was distributed throughout the comma head with localized maxima. It was decided to investigate why the comma-head separation occurred, and how it affected snowfall. vs.

13. Category Definitions • Classic 500km Continuous cloud shield as seen in IR imagery, classic comma-head shape, comma-head temperatures within 15 K of frontal zone 1000km • Separated 250km 450km • Separated comma-head temperatures are within 15K of frontal zone, yet there is an area of temperatures around 30 K warmer than the frontal zone 200km

14. Case Categories • New Category Further analysis of separated comma-head cases revealed another possible category: warm separated comma heads. These cases had comma heads with temperatures around 15K warmer than the corresponding frontal zones, and the distances of separation between the comma heads and the frontal zones were less than most of the separated cases. Additionally, the warm-separated cases seemed to be a part of a different storm evolution than the other cases, thus meriting a separate category. vs.

15. Methodology • Storm-Relative Composites Three model times for each case were utilized in making the composites. The first and last hour were selected based on spatial definitions, and the middle hour was then defined as the hour between those two. The composites were storm-relative, with the surface low serving as the center of the composite grid.

16. Methodology cont. • Compositing Hour Examples Classic Separated Warm Separated

17. Methodology cont. • Cross Sections Cross sections were made at various angles through the cyclone at the middle hour of storm evolution, with the center of the surface low serving as the center of the cross section. To investigate the separated comma heads, two regions of interest were transected: the colder cloud-tops of the comma head, and the area of warmer cloud-tops between the comma head and the frontal zone. Classic – 17Feb08 Separated – 13Mar99 Warm Separated – 04Mar08

18. Methodology cont. • Omega Calculations Q-G omega (rising motion) was simplified to four terms following Pauley and Nieman (1992) Omega terms Temp. advection Vorticity advection

19. Methodology – Figure Explanations Cross Section Plan View Cyclone category or individual case Cyclone category or individual case False US states background Height in millibars units L Pa s-1 ms-1 Always centered on surface low Kilometers south of the low Kilometers north of the low Surface low units Consistent intervals for most plots of the same variable

20. Results – Causes of the Comma-Head Separation • Review of jet concepts Jet placement is associated with position of clouds Bader et al. (1995) ≈ Ageostrophic circulations from the jets can reinforce each other if the jet placement is correct Uccellini and Kocin (1987)

21. Results – Causes of the Comma-Head Separation • Jet Position Differences All the cyclone categories have a dual-jet configuration, although there are significant differences. The two jets of the classic cases are within close proximity of each other, whereas the jets of the separated cases are distant. ms-1

22. Results – Causes of the Comma-Head Separation • Jet Position Differences – Representative Cases ms-1

23. Results – Causes of the Comma-Head Separation • Cross Sections of Jet Circulations Two distinct ageostrophic circulations are present in a north-south cross section through the surface low for the classic composite. ageostrophic wind omega

24. Results – Causes of the Comma-Head Separation • Cross Sections of Jet Circulations Low-level frontal circulations are evident for the separated cases, but the reinforcing upper-level jet circulations are missing.

25. Results – Causes of the Comma-Head Separation • Cross Sections of Jet Circulations Low-level frontal circulations are evident for the separated cases, but the reinforcing upper-level jet circulations are missing.

26. Results – Causes of the Comma-Head Separation • TROWAL – Trough of Warm Air Aloft The trowal is an area of warm air elevated above the surface by frontal interactions near the intersection of the cold and warm fronts. Martin 1998

27. Results – Causes of the Comma-Head Separation • Trowal presence/absence Looking at individual cases reveals that the classic cases feature a trowal, whereas the separated cases do not, and the warm-separated cases show a weak or developing trowal. K θeat 750 hPa

28. Results – Causes of the Comma-Head Separation • Trowal presence/absence Looking at individual cases reveals that the classic cases feature a trowal, whereas the separated cases do not, and the warm-separated cases show a weak or developing trowal. K θeat 750 hPa

29. Results – Causes of the Comma-Head Separation • Trowal presence/absence cont. Martin (1998) connected the appearance of a “treble clef” potential vorticity anomaly with the presence of a trowal. Martin 1998 PV units (1.0 × 10−6 m2 s−1K kg−1)

30. Results – Causes of the Comma-Head Separation • Trowal presence/absence cont. Martin (1998) connected the appearance of a “treble clef” potential vorticity anomaly with the presence of a trowal. PV units (1.0 × 10−6 m2 s−1K kg−1)

31. Fig. 4 – Cross-sectional composites of omega (Pa s-1) overlaid with equivalent potential temperature (K) along a north-south line through the surface low. The y-axis is the height in hPa, and the x-axis is the distance from the surface low in km. Results – Causes of the Comma-Head Separation • Trowal airstream As noted by Martin (1998), the trowal airstream is associated with decreased stability and widespread clouds and precipitation. It is therefore reasonable that its absence could be linked to a discontinuity in the cloud field as observed in the separated cases. θe(K), omega (Pa s-1)

32. Results – Causes of the Comma-Head Separation • Links between the jets and trowal positions The positions of these two features can be related by the thermal wind concept. θ(K), wind (m s-1)

33. Results – Causes of the Comma-Head Separation • Comma heads of the separated cases are a result of the southern jets wind speed(m s-1), omega (Pa s-1)

34. Results – Snowfall Differences Classic cases – broad snow swath extending throughout southern half of the comma head with localized maxima Separated cases – narrow snow swath aligned to the south of the coldest cloud tops (similar to Johnston 1995) Warm-separated cases – elements of both classic and separated cases, i.e. extension southward of coldest clouds and localized maxima.

35. Results – Conceptual Models • Classic Category L

36. Results – Conceptual Models • Separated Category L

37. Results – Conceptual Models • Warm-separated Category L

38. Forecasting Implications • Snowfall Forecasting For classic cases, snowfall tends to be spread throughout the southern half of the comma head. For both the separated and warm-separated cases, the snowfall is focused in a narrow area southward of the coldest cloud tops. vs.

39. Forecasting Implications • Future storm development The warm-separated cases were found to be associated with a type of storm evolution which led to moderate cyclogenesis and development once the separated comma head reconnected (as it did in 7 of the 8 cases), with an average sea-level pressure drop of 8 millibars in 15 hours. 8 hours

40. Future Work • Devise metric for total snow (e.g. snow depth x area) to determine snow producing capabilities of the cyclone classifications. • Track air parcel trajectories through the comma head to more definitively determine the role of the trowal airstream in clouds and precipitation within that area. • Gather more cases to generate more robust results and eliminate “marginal” cases. • Analyze radar to determine which parts of the cloud were responsible for which regions of the resulting snow swath. • Explain the surface low strengthening associated with reconnecting warm-separated comma heads. • Create composites of cloud-top positions as seen in satellite imagery

41. Questions/Comments?

42. Methodology cont. • NARR Grid Remapping The native NARR grid features 32km x 32km grid-spacing. The storm-relative composites required a more consistent grid geometry since grids from different latitudes would have to be composited. To account for this, the data was remapped onto a 0.25o x 0.25o equirectangular projection.