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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