The maximum intensity of tropical cyclones in axisymmetric numerical model simulations

AMS Citation:
Bryan, G. H., and R. Rotunno, 2009: The maximum intensity of tropical cyclones in axisymmetric numerical model simulations. Monthly Weather Review, 137, 1770-1789, doi:10.1175/2008MWR2709.1.
Resource Type:article
Title:The maximum intensity of tropical cyclones in axisymmetric numerical model simulations
Abstract: An axisymmetric numerical model is used to evaluate the maximum possible intensity of tropical cyclones. As compared with traditionally formulated nonhydrostatic models, this new model has improved mass and energy conservation in saturated conditions. In comparison with the axisymmetric model developed by Rotunno and Emanuel, the new model produces weaker cyclones (by ∼10%, in terms of maximum azimuthal velocity); the difference is attributable to several approximations in the Rotunno-Emanuel model. Then, using a single specification for initial conditions (with a sea surface temperature of 26°C), the authors conduct model sensitivity tests to determine the sensitivity of maximum azimuthal velocity (υmax) to uncertain aspects of the modeling system. For fixed mixing lengths in the turbulence parameterization, a converged value of υmax is achieved for radial grid spacing of order 1 km and vertical grid spacing of order 250 m. The fall velocity of condensate (Vt) changes υmax by up to 60%, and the largest υmax occurs for pseudoadiabatic thermodynamics (i.e., for Vt > 10 m s⁻¹). The sensitivity of υmax to the ratio of surface exchange coefficients for entropy and momentum (CE/CD) matches the theoretical result, υmax ∼(CE/CD)1/2, for nearly inviscid flow, but simulations with increasing turbulence intensity show less dependence on CE/CD; this result suggests that the effect of CE/CD is less important than has been argued previously. The authors find that υmax is most sensitive to the intensity of turbulence in the radial direction. However, some settings, such as inviscid flow, yield clearly unnatural structures; for example, υmax exceeds 110 m s⁻¹, despite a maximum observed intensity of ∼70 m s⁻¹ for this environment. The authors show that turbulence in the radial direction limits maximum axisymmetric intensity by weakening the radial gradients of angular momentum (which prevents environmental air from being drawn to small radius) and of entropy (which is consistent with weaker intensity by consideration of thermal wind balance). It is also argued that future studies should consider parameterized turbulence as an important factor in simulated tropical cyclone intensity.
Subject(s):Tropical cyclones
Peer Review:Refereed
Copyright Information:Copyright 2009 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be "fair use" under Section 107 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Law (17 USC, as revised by P.L. 94-553) does not require the Society's permission. Republication, systematic reproduction, posting in electronic form on servers, or other uses of this material, except as exempted by the above statements, requires written permission or license from the AMS. Additional details are provided in the AMS Copyright Policies, available from the AMS at 617-227-2425 or Permission to place a copy of this work on this server has been provided by the AMS. The AMS does not guarantee that the copy provided here is an accurate copy of the published work.
OpenSky citable URL: ark:/85065/d7cv4jsm
Publisher's Version: 10.1175/2008MWR2709.1
  • George Bryan - NCAR/UCAR
  • Richard Rotunno - NCAR/UCAR
  • Random Profile


    Recent & Upcoming Visitors