27 sept. 2013 future workresultsmethodologymotivation chip helmscomposite analyses of tropical...

27 Sept. 2013Future WorkResultsMethodologyMotivation

Chip Helms Composite Analyses of Tropical Convective Systems 1

Composite Analyses of Tropical Convective Systems Prior to

Tropical Cyclogenesis

Chip Helms

Jason Dunion

Lance BosartUniversity at Albany

Cyclone Workshop

27 September 2013Funding 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 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



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

• 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 (Marchok 2002)– 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

NCEP vortex tracker Pre-genesis vortex tracker

Variable 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



Idealized Example

Methodology



Merging CirculationsNon-developing SystemCape Verde TCs ????

Track ExamplesMethodology

850 hPa Vortex Idealization



Mixed

Methodology

Organizational

Environmental

Environmental

Mixed

Organizational

Pre-genesis Phase Space



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



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

Methodology



Phase Space

2010 Atlantic Hurricane Season

Pre-genesis and Non-developing

Results



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

Future Work



END

27 sept. 2013 future workresultsmethodologymotivation chip helmscomposite analyses of tropical...

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