Variability of the Outer Wind Profiles of Western North Pacific Typhoons: Classifications and Techniques for Analysis and Forecasting

Stephen B. Cocks Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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William M. Gray Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

The hazard posed by tropical cyclones (TCs) is often quantified in terms of the minimum central pressure (MCP) or maximum wind speed. While significant, these qualities must be balanced against considerations for the strength and spatial extent of the outer-core circulation, which also significantly determines the total peril posed by these storms. To this end, the results of recent studies on the time evolution of the outer wind structure of western North Pacific typhoons are presented. The results include a technique for diagnosing the azimuthally averaged wind profile from a combination of regional synoptic data and estimates of MCP. Aircraft reconnaissance data, augmented with available synoptic and scatterometer data, are used to determine the radial extent of 15 (R15), 25 (R25), and 33 (R33) m s−1 winds for 50 typhoons. Thirty-five of these typhoons were designated as the developmental dataset and 15 were reserved for use as independent data.

Using the developmental data, concurrent time series were constructed for outer wind radii and MCP values for TCs whose life cycles were unperturbed by proximity to land. Analysis of these time series revealed a distinct type of TC with very large R15 and weak intensity values. These “gyre” systems were examined as a special TC class. The remaining TCs were divided according to the size of their outer circulations into groups termed small, medium, and large. Observations stratified for these size categories revealed that for about 40% of the TCs, significant increases of R15 [greater than 50 km day−1] occurred during the 1.5-day period before maximum intensity. Composite analyses revealed that mature large (small) TCs develop as larger (smaller) TCs early in their life cycle. Composite analysis of the gyre systems showed several unusual characteristics including very large initial R15 values (much larger than even large TCs) and weak intensities. Spatial analyses indicate that the gyre and larger TCs tend to attain maximum R15 values in locations west of 135°E whereas the smaller TCs tend to maximize farther east. Analyses of surface pressure showed that the gyres are associated with broad areas of comparatively low pressure (versus other TCs), in agreement with previous studies.

Statistical associations between R15 values and an array of environmental parameters revealed that although area coverage of intense convection was poorly correlated with R15, the radial extent of the 1004-hPa isobar (ER04) enveloping the systems was well correlated with R15. Whereas the overall relationship between MCP and R15 is weak, it is noted that when data for the gyre TCs are removed (14% of the observations), the total variance explained between MCP and R15 is more than doubled. Consequently, a regression equation using both MCP and ER04 was developed and used in conjunction with a Rankine vortex model to successfully estimate values for R15 and R25. The independent data were used to evaluate this analysis technique versus a procedure used by the Joint Typhoon Warning Center (the Huntley method) for estimating the radius of 17 m s−1 winds (R17). The new ER04/MCP-based technique created in this study suffered no loss of skill when applied to the independent data and both methods appear to be quite useful for estimating the properties of symmetric outer wind profiles of northwest Pacific typhoons.

Corresponding author address: Dr. William Gray, Dept. of Atmospheric Science, Colorado State University, Fort Collins, CO 80523. Email: barb@tutt.atmos.colostate.edu

Abstract

The hazard posed by tropical cyclones (TCs) is often quantified in terms of the minimum central pressure (MCP) or maximum wind speed. While significant, these qualities must be balanced against considerations for the strength and spatial extent of the outer-core circulation, which also significantly determines the total peril posed by these storms. To this end, the results of recent studies on the time evolution of the outer wind structure of western North Pacific typhoons are presented. The results include a technique for diagnosing the azimuthally averaged wind profile from a combination of regional synoptic data and estimates of MCP. Aircraft reconnaissance data, augmented with available synoptic and scatterometer data, are used to determine the radial extent of 15 (R15), 25 (R25), and 33 (R33) m s−1 winds for 50 typhoons. Thirty-five of these typhoons were designated as the developmental dataset and 15 were reserved for use as independent data.

Using the developmental data, concurrent time series were constructed for outer wind radii and MCP values for TCs whose life cycles were unperturbed by proximity to land. Analysis of these time series revealed a distinct type of TC with very large R15 and weak intensity values. These “gyre” systems were examined as a special TC class. The remaining TCs were divided according to the size of their outer circulations into groups termed small, medium, and large. Observations stratified for these size categories revealed that for about 40% of the TCs, significant increases of R15 [greater than 50 km day−1] occurred during the 1.5-day period before maximum intensity. Composite analyses revealed that mature large (small) TCs develop as larger (smaller) TCs early in their life cycle. Composite analysis of the gyre systems showed several unusual characteristics including very large initial R15 values (much larger than even large TCs) and weak intensities. Spatial analyses indicate that the gyre and larger TCs tend to attain maximum R15 values in locations west of 135°E whereas the smaller TCs tend to maximize farther east. Analyses of surface pressure showed that the gyres are associated with broad areas of comparatively low pressure (versus other TCs), in agreement with previous studies.

Statistical associations between R15 values and an array of environmental parameters revealed that although area coverage of intense convection was poorly correlated with R15, the radial extent of the 1004-hPa isobar (ER04) enveloping the systems was well correlated with R15. Whereas the overall relationship between MCP and R15 is weak, it is noted that when data for the gyre TCs are removed (14% of the observations), the total variance explained between MCP and R15 is more than doubled. Consequently, a regression equation using both MCP and ER04 was developed and used in conjunction with a Rankine vortex model to successfully estimate values for R15 and R25. The independent data were used to evaluate this analysis technique versus a procedure used by the Joint Typhoon Warning Center (the Huntley method) for estimating the radius of 17 m s−1 winds (R17). The new ER04/MCP-based technique created in this study suffered no loss of skill when applied to the independent data and both methods appear to be quite useful for estimating the properties of symmetric outer wind profiles of northwest Pacific typhoons.

Corresponding author address: Dr. William Gray, Dept. of Atmospheric Science, Colorado State University, Fort Collins, CO 80523. Email: barb@tutt.atmos.colostate.edu

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