Editorial Type:
Article
Article Type:
Research Article

Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region

Linda A. Paterson Bureau of Meteorology, Western Australia Regional Office, Perth, Australia

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Barry N. Hanstrum Bureau of Meteorology, Western Australia Regional Office, Perth, Australia

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Noel E. Davidson Bureau of Meteorology, Western Australia Regional Office, Perth, Australia

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Harry C. Weber Bureau of Meteorology, Western Australia Regional Office, Perth, Australia

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Print Publication:
01 Dec 2005
DOI:
https://doi.org/10.1175/MWR3041.1
Page(s):
3644–3660
Restricted access

Abstract

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

* Current affiliation: Bureau of Meteorology, New South Wales Regional Office, Sydney, Australia

+ Current affiliation: Bureau of Meteorology Research Centre, Melbourne, Australia

# Current affiliation: Meteorological Institute, University of Munich, Munich, Germany

Corresponding author address: Linda A. Paterson, WARO, P.O. Box 1370, West Perth, 6872 Western Australia, Australia. Email: l.paterson@bom.gov.au

Abstract

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

* Current affiliation: Bureau of Meteorology, New South Wales Regional Office, Sydney, Australia

+ Current affiliation: Bureau of Meteorology Research Centre, Melbourne, Australia

# Current affiliation: Meteorological Institute, University of Munich, Munich, Germany

Corresponding author address: Linda A. Paterson, WARO, P.O. Box 1370, West Perth, 6872 Western Australia, Australia. Email: l.paterson@bom.gov.au

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