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


Tropopause

500 hPa

Surface

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Motivation


Tropopause

500 hPa

Surface

Vort. Max

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Motivation


Tropopause

500 hPa

Surface

composite analyses of tropical convective systems prior to tropical cyclogenesis

Documents

case study

viable systems

marginal systems

compositing study

large number of cases

general sense

greatest level

homogeneous subsetselect