A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis

Authors: Ron McTaggart-Cowan, et. Al.Monthly Weather Review

June 2013

Purpose

• 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

• 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 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 Methodology

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

Do these seem like good metrics to use?

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

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 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

• Non-baroclinic pathway dominates in all basins.

• But, secondary pathway varies.

• Strong TT is generally least prolific

What can we learn from this figure?

Location of developments via each pathway.

North Atlantic

Figure 8

East Pacific

Figure 9

West Pacific

Figure 10

Bay of Bengal – North Indian

Figure 11

Arabian Sea – North Indian

Figure 12

South Indian

Figure 13

South Pacific

Figure 14

Development Efficiency

Figure 15

What can we learn form this figure?

Equivalent to dividing figure 7 by figure 4

Climatology in Metric Space

Figure 16EnvironmentalGenesis events

How do we interperate these figures?• 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.

TC efficiency in Metric Space

Figure 16

Figure 17

• Yield value of 1 is average• The presence of an upper level disturbance is more

favorable than purely non-baroclinic (Gray1988)

Fig. B divided by fig. A

Trough

Ridge

Discussion• 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.

Discussion

• 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?