180 likes | 297 Views
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).
E N D
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
Motivation Motivation – Gabrielle (2013)
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
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
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
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
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)
Methodology Vortex Tracker - Variables 100% = non-divergent cyclonic 0% = irrotational -100% = non-divergent anticyclonic
Methodology Idealized Example
Methodology Track Examples Cape Verde TCs Merging Circulations Non-developing System ???? 850 hPa Vortex Idealization
Methodology Pre-genesis Phase Space Organizational Environmental Organizational Mixed Environmental Mixed
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
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)
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
Results Phase Space2010 Atlantic Hurricane SeasonPre-genesis and Non-developing
Future Work Future Data Sources • Reanalyses • ERA-Interim, NCEP/NCAR, MERRA • Operational • GFS, ECMWF, CMC • Observational • CIMSS satellite winds, dropsondes, satellites
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