On the Vertical Decay Rate of the Maximum Tangential Winds in Tropical Cyclones

Daniel P. Stern Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

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David S. Nolan Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

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Abstract

In this study, it is shown that the maximum tangential winds within tropical cyclones decrease with height at a percentage rate that is nearly independent of both the maximum wind speed and the radius of maximum winds (RMW). This can be seen by normalizing the profiles of maximum tangential winds Vmax by their respective values at 2-km height. From Doppler radar analyses, profiles of maximum normalized tangential wind Vmaxnorm are found to share a common shape, despite spanning a great range of intensities. There is a systematic dependence of Vmaxnorm on intensity and size, but it is shown to be small, and the mean profile of Vmaxnorm can be used to accurately “predict” the individual profiles of Vmax. Using Emanuel’s steady-state analytical vortex model, it is shown that Vmaxnorm is essentially independent of the size of the RMW. It is shown mathematically that the near independence of Vmaxnorm from size is due to the facts that the RMW is nearly a surface of constant absolute angular momentum M and that its outward slope increases linearly with radius. As the slope of the RMW is not a function of intensity, Vmaxnorm is also nearly independent of intensity in theory, and this is confirmed using Emanuel’s simple time-dependent model. In contrast to intensity, it is shown that Vmaxnorm increases with potential intensity. A suite of idealized simulations using the Weather Research and Forecasting model (WRF) are used to further examine the manner in which the maximum winds change with height. Above 2-km height, vertical profiles of Vmaxnorm are nearly independent of both intensity and size. Occasional deviations from this near-universal profile in these simulations are due to unbalanced winds, and it is proposed that this is the cause of occasional observations of maximum winds that are nearly constant with height through the midtroposphere, as in Hurricane Gloria (1985) and Hurricane Dennis (2005).

Current affiliation: Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania.

Corresponding author address: Dr. Daniel P. Stern, Department of Meteorology, The Pennsylvania State University, University Park, PA 16802. E-mail: dps19@psu.edu

Abstract

In this study, it is shown that the maximum tangential winds within tropical cyclones decrease with height at a percentage rate that is nearly independent of both the maximum wind speed and the radius of maximum winds (RMW). This can be seen by normalizing the profiles of maximum tangential winds Vmax by their respective values at 2-km height. From Doppler radar analyses, profiles of maximum normalized tangential wind Vmaxnorm are found to share a common shape, despite spanning a great range of intensities. There is a systematic dependence of Vmaxnorm on intensity and size, but it is shown to be small, and the mean profile of Vmaxnorm can be used to accurately “predict” the individual profiles of Vmax. Using Emanuel’s steady-state analytical vortex model, it is shown that Vmaxnorm is essentially independent of the size of the RMW. It is shown mathematically that the near independence of Vmaxnorm from size is due to the facts that the RMW is nearly a surface of constant absolute angular momentum M and that its outward slope increases linearly with radius. As the slope of the RMW is not a function of intensity, Vmaxnorm is also nearly independent of intensity in theory, and this is confirmed using Emanuel’s simple time-dependent model. In contrast to intensity, it is shown that Vmaxnorm increases with potential intensity. A suite of idealized simulations using the Weather Research and Forecasting model (WRF) are used to further examine the manner in which the maximum winds change with height. Above 2-km height, vertical profiles of Vmaxnorm are nearly independent of both intensity and size. Occasional deviations from this near-universal profile in these simulations are due to unbalanced winds, and it is proposed that this is the cause of occasional observations of maximum winds that are nearly constant with height through the midtroposphere, as in Hurricane Gloria (1985) and Hurricane Dennis (2005).

Current affiliation: Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania.

Corresponding author address: Dr. Daniel P. Stern, Department of Meteorology, The Pennsylvania State University, University Park, PA 16802. E-mail: dps19@psu.edu
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