Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis
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Transcript of Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis
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Composite Analyses of Tropical Convective Systems Prior to
Tropical Cyclogenesis
Chip HelmsJason DunionLance Bosart
University at Albany
Cyclone Workshop27 September 2013
Funding through NSF AGS-0849491 and NASA HSRP #NNX12AK63G
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Motivation
Motivation – Gabrielle (2013)
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Motivation
Motivating Questions and Working Hypotheses
• Why do some marginal systems develop despite the presence of inhibiting factors?– External features enhance vorticity generation– 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
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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
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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
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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
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Vortex Tracker• Limited best track data for pre-genesis and
non-develop systems
• Based on NCEP vortex tracker (Marchok 2002)– Multiple fields to generate center fix– Link fixes using steering flow and previous motion
• Currently using Climate Forecast System Reanalysis (CFSR)
Methodology
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Vortex Tracker - Variables
100% = non-divergent cyclonic0% = irrotational
-100% = non-divergent anticyclonic
Methodology
NCEP vortex tracker Pre-genesis vortex trackerVariable Levels Variable Levels
Vorticity 850, 700 hPa Tangential Velocity 850 hPa
Pressure Surface MSLP gradient Surface
Pressure gradient Surface Vortex Idealization 850, 700, 500 hPa
Geo. height 850, 700 hPa
Wind 850, 700 hPa
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Idealized ExampleMethodology
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Merging CirculationsNon-developing SystemCape Verde TCs ????
Track ExamplesMethodology
850 hPa Vortex Idealization
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Mixed
Methodology
Organizational
Environmental
Environmental
Mixed
Organizational
Pre-genesis Phase Space
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Phase Space: Organization Metrics
• 500-850 hPa center 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
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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
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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
Methodology
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Phase Space
2010 Atlantic Hurricane Season
Pre-genesis and Non-developing
Results
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Future Data Sources
• Reanalyses– ERA-Interim, NCEP/NCAR, MERRA
• Operational– GFS, ECMWF, CMC
• Observational– CIMSS satellite winds, dropsondes, satellites
Future Work
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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
Future Work
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