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A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis. Authors: Ron McTaggart -Cowan, et. Al. Monthly Weather Review June 2013. Purpose.

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a global climatology of baroclinically influenced tropical cyclogenesis

A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis

Authors: Ron McTaggart-Cowan, et. Al.

Monthly Weather Review

June 2013

  • The authors had previously produced a dynamically based genesis pathway climatology for the North Atlantic in hopes of expanding upon the understanding, and thusly the prediction of TC genesis.

(McTaggart-Cowan, 2008)

  • This paper expands upon the climatology to include all of the world’s active tropical basins.
data and methodology
Data and Methodology
  • Period of 1948-2010
  • “combined reanalysis”
    • NCEP-NCAR reanalysis I
    • ERA-40
    • ERA-Interim
    • JRA-25 (Japan Meteorological Agency)
  • T₀ defined as first report in IBTrACS
data and methodology1
Data and Methodology
  • 2 metrics were used for characterization
  • Q – average convergence of the 400-200hPa Q vector.
    • Represents synoptic scale forcing for ascent.
  • TH – max difference between 1000–700-hPa thickness values in two hemispheres within 10⁰ of point of interest.
    • lower level thickness asymmetry.

Note: Background state rather than vortex itself.

data and methodology2
Data and Methodology

Do these seem like good metrics to use?

From figure 1 of Ron McTaggart-Cowan, et. Al. 2007

data and methodology3
Data and Methodology
  • Linear discriminant analysis is employed.
  • 5 development pathways are identified.
  • Note: different classification technique was used here from the Atlantic paper.
    • As a result 1 less category is used (transient trough interaction pathway is omitted)
the 5 development pathways
The 5 development pathways

The reclassification of the North Atlantic TC pathways provides a look at the classification metric space.

global climatology frequency
Global climatology frequency
  • Non baroclinic –within 15⁰ of equator
  • Low level baroclinic – areas with land-sea contrasts or SST gradients
  • Trough induced – equatorward and west of Rossby wave breaking centers
  • Weak TT – near TUTT axis
  • Strong TT – higher latitudes due to Rossby wave breaking.
classification a ccuracy assessment
Classification Accuracy Assessment
  • Do you think this is a good technique to accomplish the objective?
  • Do you foresee any issues that might influence the accuracy of the classification?
  • A thorough comparison with previous studies and a subjectively classificatied sample was done.
  • [Gray (1968), Hess et al.(1995), Elsner et al.(1996)]
  • Their classification compared very favorably!
relative frequency of tc development pathway by basin
Relative frequency of TC development pathway by basin

What can we learn from this figure?

  • Non-baroclinic pathway dominates in all basins.
  • But, secondary pathway varies.
  • Strong TT is generally least prolific
east pacific
East Pacific

Figure 9

west pacific
West Pacific

Figure 10

south indian
South Indian

Figure 13

development efficiency
Development Efficiency

What can we learn form this figure?

Figure 15

Equivalent to dividing figure 7 by figure 4

climatology in metric space
Climatology in Metric Space
  • Left: are all of the formation events plotted in metric space along with formation density.
    • Note maximum in non-baroclinic space
  • Right: is the percentage of time that the metrics lie close to a given position in metric space.

How do we interperate these figures?

Figure 16

Genesis events


tc efficiency in metric space
TC efficiency in Metric Space
  • Yield value of 1 is average
  • The presence of an upper level disturbance is more favorable than purely non-baroclinic (Gray1988)



Figure 17

Figure 16

Fig. B divided by fig. A

discussio n
  • Non-baroclinic TC developments dominant globally, but a “rich spectrum” of formation events happen under baroclinic conditions.
    • 70% non-baroclinic
    • 8% Low level baroclinic
    • 6% Trough induced
    • 11% weak Tropical transition
    • 5% Strong TT
  • Could be combined with traditional genesis potential evaluation techniques to develop a pathway conditional index.
    • It is hoped that this could be an improvement on current indices.
  • So, what do you think?
    • Were they successful?
    • Was this a worthwhile exercise?
    • Could it help improve genesis prediction?
    • Do you feel there are any other important takeaways from the climatology?