Composite Analyses of Tropical Convective Systems Prior to Tropical Cyclogenesis
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9 Sept. 2013Future WorkResultsMethodologyMotivation
Chip Helms Composite Analyses of Tropical Convective Systems
Composite Analyses of Tropical Convective Systems Prior to
Tropical Cyclogenesis
Chip Helms
Cyclone Research Group9 September 2013
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Motivation
Questions
• Why do viable systems fail to develop?
• Why do some marginal systems develop despite the presence of inhibiting factors such as dry air, high shear, or low SSTs?
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Motivation
Hypotheses
• Why do viable systems fail to develop?– Insufficient vorticity generation and excess vorticity
destruction at low and mid-levels due to conditions hostile to sustained deep convection and vorticity preservation
• Why do some marginal systems develop despite the presence of inhibiting factors?– These systems develop as a result of external features
acting to enhance low and mid-level vorticity generation
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Motivation
Method Motivation
• Two general approaches to studying genesis– Case Studies
• Detailed analyses, may not be representative – Composite Studies
• Represenative features, loss of detail
• Solution: Composite on homogeneous subset– Select cases with similar structures
• Make subset selections using phase space
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Methodology
Phase Space
Avg(|𝑉|−𝑉 λ
|⃑𝑉| )=Avg (𝑊𝑆𝑃𝐷−𝑇𝑉𝐸𝐿𝑊𝑆𝑃𝐷 )
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Idealized ExampleMethodology
Idealization Deficit Mean Vλ
+
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Planned Variable Changes• ‘Idealization Deficit’ to ‘Vortex Idealization’
– Still measures of how close the wind field is to purely tangential cyclonic flow
– Old: 0% = Cyc, 100% = Irrot, 200% = Anti– New: -100% = Anti, 0% = Irrot, 100% = Cyc
• Still need to find a good moisture metric– 500-300 hPa RH > 70% coverage?
Methodology
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Moving Beyond NHC INVESTs• Using INVEST files introduces a selection
bias and reduces potential data ranges– Only NHC basins from 2005 onwards– Biased by system impact potential
• Vortex detection and tracking algorithm– Based on NCEP Vortex Tracker (Marchok 2002)– 850 hPa, 700 hPa, and 500 hPa idealization– Surface pressure gradient, 850 hPa tangential
velocity
Methodology
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Example Vortex IdentificationMethodology
850 Ideal 700 Ideal (3°) 500 Ideal (5°) MSLP grad. (5°) 850 Vλ (3°)
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Example Vortex IdentificationMethodology
850 Ideal 700 Ideal (3°) 500 Ideal (5°) MSLP grad. (5°) 850 Vλ (3°)
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Tracking Algorithm• Approximate steering flow
– Average of 850 hPa and 500 hPa mean wind– Implied motion must be within 60° of steering flow– Search distance
• Steering wind speed?• Previous motion?
– Allow system to jump any direction by up to 2°• System must last for 24 hours
Methodology
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Future Work• Finish implementing tracking algorithm• QC tracking algorithm• Finalize phase space variables
– Decide on moisture metric• Examine composites
– e.g. Dev vs Non-dev• Apply composite values to equations
– Vorticity tendency, PV tendency• Use continuity and hydrostatic to better understand mid-level
vorticity generation as a function of upper level temperature anomaly (and by extension moisture)
Future Work
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Future Work
• What is the best way to measure dry air?– 500-300 hPa RH >70% coverage over area– Radius at which azimuthal mean RH drops
below 70%– RH in vicinity of 500 hPa vorticity max?– Something else?
Future Work
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END
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EXTRA SLIDES
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Motivation
Genesis Process Hypothesis
Tropopause
500 hPa
SurfaceWave Axis
Convergenceand ascentalong wave
Cooling (Melting, Evaporation, Radiation?)
Concentration of background vorticity produces low-level vortex
Deep convectionforms along
convergence line
Deep convection fuels formation of stratiform
sheild downshear
+PVMid-Level Vortex
Low-Level Vortex
Latent Heat Release
ShearHydrostatic response to
heating profile results in PV convergence
and positive(?) feedback due to
thermal wind balance
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Results
N=516, Red=15
Year: 2010
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Results
N=107, Red=6
Year: 2010
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Results
N=25, Red=6
Year: 2010
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Results
N=45, Red=6
Year: 2010
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Results
N=16, Red=4
Year: 2010
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REMOVED SLIDES
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TheorySimpson et al. (1997) and Ritchie and Holland (1997)
Prior Work
Evaporative Cooling
StratiformLatent
Heating
p
gfP )(
+ PV Anomaly
Mergers of PV anomalies add PV while averaging
thermal propertiesNew PV Anomaly
Out of balance with thermal structure
Forced Ascent andEvaporative Cooling
Act to cool sub-cloud layer
Warm anomaly growth not detailed by theory, but would be accomplished by forced subsidence or increased LHR
Forced Convergence+
pf
tVkV)(
p-f)(V
Concentration term
Stretching termMCS
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Motivation
Issues with Traditional Composites
• Mid-level features will appear weaker– High variability in system tilt
• Vertically-aligned systems tend to be stronger– Composites will favor upright systems
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Methodology/Data• Locate center at 850 and 500 hPa
1) Maximum Vλ (0.5° search grid)2) Minimum Difference of Vλ and V (0.25°)3) Minimum Difference of Vλ and V (0.10°)
• Datasets: CFSRv2, HURDAT2+INVESTs– Convenient for testing methodology– CFSR: Uniform in time– Complete with all the selection bias caveats of
the INVEST files
Methodology
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Motivation
Genesis Process Hypothesis
Tropopause
500 hPa
Surface
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Motivation
Genesis Process Hypothesis
Tropopause
500 hPa
Surface
Vort. Max
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Chip Helms Composite Analyses of Tropical Convective Systems
Motivation
Genesis Process Hypothesis
Tropopause
500 hPa
Surface