A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis
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Transcript of 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
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?