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Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis

Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis. Chip Helms Jason Dunion Lance Bosart University at Albany Cyclone Workshop 27 September 2013. Funding through NSF AGS-0849491 and NASA HSRP #NNX12AK63G. Motivatio n. Motivation – Gabrielle (2013).

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Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis

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  1. Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis Chip Helms Jason Dunion Lance Bosart University at Albany Cyclone Workshop 27 September 2013 Funding through NSF AGS-0849491 and NASA HSRP #NNX12AK63G

  2. Motivation Motivation – Gabrielle (2013)

  3. Motivation Motivating Questions and Working Hypotheses • Why do some marginal systems develop despite the presence of inhibiting factors? • External features enhance vorticitygeneration • Robust vorticity column dampens turbulent mixing • Why do viable systems fail to develop? • Insufficient vorticity generation • Excess vorticity destruction • Conditions hostile to sustained deep convection

  4. Motivation Previous Studies • McBride and Zehr (1981) on developing systems • More pronounced upper-level warm anomaly • Stronger large-scale low-level vorticity • Lower vertical wind shear • Stronger upper-level divergence • Lee (1989) on developing systems • Slightly lower environmental MSLP • Larger mid-level cyclonic circulations • Stronger low-level convergence • Moistening at mid-levels important • Environmental favorability dominant until just prior to genesis • Internal dynamics become dominant during final spin up

  5. Methodology Methodology • General approaches to studying genesis • Case Studies: • Detailed analyses, may not be representative • Composite Studies • Representative features, loss of detail • Solution: Hybrid approach • Composite subsets with similar structure and environments

  6. Methodology Creating Subset Composites • Metrics represent system evolution • System structure • Near-system environment • Metrics define a phase space • Phase spaces have proven useful in past studies • Wheeler and Hendon 2004; Hart 2006; McTaggart-Cowan et al. 2008

  7. Methodology Vortex Tracker • Limited best track data for pre-genesis and non-develop systems • Based on NCEP vortex tracker (Marchok2002) • Multiple fields to generate center fix • Link fixes using steering flow and previous motion • Currently using Climate Forecast System Reanalysis (CFSR)

  8. Methodology Vortex Tracker - Variables 100% = non-divergent cyclonic 0% = irrotational -100% = non-divergent anticyclonic

  9. Methodology Idealized Example

  10. Methodology Track Examples Cape Verde TCs Merging Circulations Non-developing System ???? 850 hPa Vortex Idealization

  11. Methodology Pre-genesis Phase Space Organizational Environmental Organizational Mixed Environmental Mixed

  12. Methodology Phase Space: Organization Metrics • 500-850 hPacenter offset • Conflicting tilts lower composite detail • Genesis occurs shortly after vertical alignment • Nolan (2007), Davis and Ahijevych (2012), Helms and Hart (2012) • Tangential velocity (850, 500 hPa) • Tracks intensity of system • Vortex idealization (850, 500 hPa) • Proxy for evolution of a closed circulation

  13. Methodology Phase Space: Near-system Environment Metrics • Deep layer environmental shear • High shear has a detrimental effect on genesis • Look for dev/nondev bifurcation in profiles • Saturation deficit (300-500, 500-850 hPa) • Important to convection (Rappin et al. 2010)

  14. Methodology Phase Space: Mixed Metrics • Δθe between 850 hPa and tropopause • Potential stability → near-system environment • Bulk diabatic heating → convective activity • Thermal vorticity(200-850 hPa) • Warm core cyclone • Upper-level anticyclone • synoptic scale feature or system-scale feature

  15. Results Phase Space2010 Atlantic Hurricane SeasonPre-genesis and Non-developing

  16. Future Work Future Data Sources • Reanalyses • ERA-Interim, NCEP/NCAR, MERRA • Operational • GFS, ECMWF, CMC • Observational • CIMSS satellite winds, dropsondes, satellites

  17. Future Work Analysis Goals • Examine differences between dev/non-dev in variety of composites • Kinematic, dynamic, and thermodynamic fields • Examine how parameters vary with phase space location • SST, OHC, MPI (Emanuel 1988), ventilation index (Tang and Emanuel 2012), genesis pathway (McTaggart-Cowan et al. 2008) • Will allow us to explore why viable systems sometimes fail to develop and marginal systems sometimes succeed

  18. END

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