The Differing Roles of the Large-scale Environment on
the Intensity Changes Of Three 1996 Atlantic Hurricanes
Principal Investigator:
John Kaplan
Collaborating scientist(s):
Christopher Landsea
Mark DeMaria (NHC/TPC)
Objective:
To investigate the relative importance of relative eddy-angular
momentum fluxes, vertical
shear, and sea-surface temperature on the intensity changes of three
hurricanes that
occurred during the 1996 Atlantic hurricane season.
Rationale:
Past research has shown that the
large-scale
environment plays an important role in the tropical cyclone (TC)
intensification process.
For instance, many modeling and observational studies have shown that
upper-level (~200 mb) troughs can have a positive impact on subsequent
TC intensification by physical processes such as the import of
relative eddy angular momentum (REAM). Some researchers have hypothesized
that the import of REAM leads to enhanced
middle-level inflow and upper-level outflow which is favorable for
future tropical cyclone development. However, other research suggests that
the positive
influence of upper-level troughs on TC intensification only
occurs provided that the vertical
shear of the horizontal wind (SHR) and sea-surface temperature (SST) are
also favorable.
Despite the potential beneficial
aspects of the REAM fluxes produced by upper-level troughs, there exists
some
disagreement as to their relative importance
since some research
suggests that SHR
plays the most crucial role in the
tropical cyclone intensification process. A key question which remains
unanswered is which of these large-scale physical processes is
more important and are either of
them alone sufficient to explain the intensification process.
Method:
The relative importance of fluxes of REAM
,SHR, and SST were examined
for three 1996 Atlantic hurricanes (Bertha, Edouard, and Isidore)
using data from operational runs of the SHIPS model. This
was accomplished by examining the temporal changes in the
200 mb fluxes of REAM, 850-200 mb
SHR, and SST for
each of these cases. Satellite imagery was also employed to help
identify salient features that may have affected the observed intensity
changes of these systems. These three particular hurricanes were chosen
since the relative importance of the fluxes of REAM, SHR, and SST
appear to have been quite different in the intensity changes observed
during the 72 h time periods studied for each of these TCs.
Accomplishment:
Time series plots of 200 mb REAM, 850-200 mb SHR, and SST were constructed
for hurricanes Bertha, Edouard, and Isidore (Fig.1). These time series
diagrams suggest that a favorable upper-level trough interaction may have
resulted in the rapid intensification of Bertha late
on 12 July.
Although these data
suggests that an upper-level trough may have played a key role in
the intensification of Bertha, Hurricane
Edouard appears to have intensified rapidly in a low SHR environment
without the presence of any significant fluxes of REAM. Moreover,
Isidore apparently
weakened due to the presence of high SHR despite the
presence of fairly large 200 mb fluxes of REAM. These three cases
suggest
that
TC intensity change can not be explained by one physical
mechanism. Work will continue on examining other select TCs that
developed during the 1995 and 1996 hurricane seasons to obtain a better
understanding of the relative importance of REAM, SHR, and SST on TC
intensity change.
Key reference:
Kaplan, J., C.W. Landsea, and M. DeMaria, 1997: The differing roles
of the large-scale
environment in the intensity changes of three 1996 Atlantic Hurricanes.
Preprints, 22nd Conf. on Hurricanes and Tropical
Meteorology, Miami, FL, Amer. Meteor. Soc., 358-359.
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