Modeling and Understanding Rapidly Intensifying Extratropical Cyclones

1. Modeling and Understanding Rapidly Intensifying Extratropical Cyclones Christopher Castellano Cornell University Atmospheric Science ‘10 NWS/NCEP/OPC Joseph Sienkiewicz, Branch Chief, OAB

2. Outline • Project Goals (Page 3) • Overview: Unique Storms (Page 4) • Methodology (Page 5) • Positive Cases vs. Null Cases (Pages 6-18) • Pressure and Winds (Pages 6-9) • Upper Level Dynamics (Pages 10-11) • Cross Sections (Pages 12-14) • Thermal and Diabatic Properties (Pages 15-18) • Summary: Significant Findings (Page 19)

3. Project Goals • Become familiar with WRF EMS software, develop atmospheric modeling skills, and build upon concepts from college courses • Increase awareness and knowledge about rapidly intensifying cyclones in the Atlantic and Pacific Oceans • Compare large-scale dynamics and diabatic/thermal properties in order to determine distinguishing characteristics between rapid intensifiers and null events 3

4. “Rapidly Intensifying”Extratropical Cyclone? • Rapid intensification defined as pressure decrease of 24+ mb in 24 hrs • Typically occur in the Northern Pacific and Northern Atlantic between October and May (much more common than originally thought) • Generate violent winds and waves that are hazardous to commercial activity, enhance coastal erosion and flooding, and endanger human lives • Better understanding of their formation is necessary for the Ocean Prediction Center to fulfill its responsibilities and succeed in its mission 4

5. Methodology: Modeling Case Studies • Software: WRF Environmental Modeling System (Version 3.0) • Specified a geographic domain for each case study (consistent 12.5 km grid resolution) • Used GFS forecast files (0.5 deg, 3-hrly) to create initial and boundary conditions for each storm (specifying date, cycle, and simulation length) • Ran six simulations (three rapid intensifiers, three null cases), creating hourly output files • Used NMAP2 to visualize the graphics and interpret the results 5

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9. Pressure & Winds • Pressure tendencies much more pronounced in rapidly intensifying storms 12 – 36 hrs before min pressure • Substantially greater pressure gradients and wind speeds in rapidly intensifying cyclones • Pressure gradient field strengthens and weakens fairly quickly in positive cases, but exhibits less drastic temporal changes in null cases • Max pressure gradient and winds occur at least 12 hours before min pressure in positive cases, with time lag between pressure gradient and wind response • No observable pattern for pressure gradient or winds among null cases 9

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11. Upper Level Dynamics • Polar jet structure is a manifestation of the large-scale dynamics • 300 mb winds 24 - 36 hrs preceding final cyclone maturity may signal whether or not cyclone undergoes explosive cyclogenesis • 300 mb winds (24 hrs before min pressure) considerably higher in positive cases • Rapidly intensifying cyclones typically develop slightly downstream of very impressive 300 mb jet streaks (near left-exit region) • 300 mb divergence is stronger along bent-back occlusion in positive cases • Upper level divergence has important implications to surface pressure because it serves as a measure of mass evacuation from a column of air 11

12. Cross Sections: Atl Jan ’08 vs. Atl Feb ‘07 12

13. Cross Sections: Pac Feb ‘08 vs. Pac Dec ‘07 13

14. Cross Sections • 2 IPV potential vorticity surface (cyan) extends much further downward (665 mb vs. 460 mb for the Atlantic storms, and 565 mb vs. 485 mb for the Pacific storms) in rapidly intensifying cyclones • Contouring of 2 IPV potential vorticity surface in rapid intensifiers illustrates tropopause folding and downward intrusion of stratospheric air • Polar jet core exhibits greater vertical extent in rapidly intensifying cyclones • Polar jet appears fairly symmetrical (wind speed decreases uniformly above and below jet core) in positive cases, but asymmetrical (wind speed decreases faster above jet core) in null cases • Upward vertical motion much stronger near bent-back occlusion in rapidly intensifying cyclones (consistent with stronger upper level divergence) 14

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18. Diabatic & Thermal Properties • Horizontal thermal gradients generally much more well-defined in positive cases • Thermal gradients reiterate the significance of both dynamical and diabatic features • Despite relatively weak thermal gradients, Jan ‘08 storm occurs near a very strong 300 mb jet (175 kts at min pressure) • Despite a steadily weakening 300 mb jet (140 kts at min pressure), Jan ‘09 storm develops very large thermal gradients • Despite impressive thermal gradients, Dec ’07 storm occurs near a relatively week 300 mb jet (125 kts at min pressure) • Warm-core seclusion (from thickness analysis) evident in most mature cyclones, but tends to develop much earlier in rapidly intensifiers • Stability typically more negative (environmental lapse rate exceeds standard lapse rate by 2 – 4 C/km) at min pressure in rapidly intensifying cyclones, and near neutral in null cases 18

19. Summary: Distinguishing Characteristics • Explosive cyclogenesis produces significantly lower pressures, stronger winds, and tighter pressure gradients • Greatest pressure tendencies, wind speeds, and pressure gradients occur 12 – 36 hrs before min pressure in rapid intensifiers (no clear pattern in null events) • Vigorous polar jet and substantial descent of tropopause (2 IPV surface) coincident with rapidly intensifying cyclones • Polar jet typically symmetrical or inverse asymmetrical, with large vertical depth • Enhanced upper level divergence and low-to-mid level upward motion near bent-back occlusion • Stability clearly more negative at min pressure • Strong thermal gradient a potentially important factor, but primarily in cases of marginal jet strength • Warm-core evolution usually more accelerated and long-lasting 19