14A.6 Effects of surface exchange coefficients on the intensity and structure of axisymmetric hurricanes

Thursday, 3 April 2014: 2:45 PM
Garden Ballroom (Town and Country Resort )
Tristan J. Shepherd, The University of Melbourne, Melbourne, VIC, Australia; and K. J. E. Walsh and J. D. Kepert
Manuscript (444.6 kB)

Understanding the effects of surface exchange coefficients in the hurricane environment is important because of their role in the intensification process. The maximum potential intensity (MPI) of hurricanes is proportional to the ratio of enthalpy (Ck) and drag (Cd). We present a new study testing MPI and the importance of this ratio on both intensity and structure. Unlike previous studies, we compare model results to MPI theory for simulated intensity and consistency of the theory with internal storm structure.

Hurricanes were simulated for a range of drag and enthalpy coefficients using the axisymmetric model CM1. The model was initialized with the moist tropical sounding of Dunion with an SST of 29°C. Nine simulations tested the Emanuel MPI theory, with Ck/Cd ratios of 0.5, 1.0 and 2.0 for constant values of Cd of 1.5x10-3, 2x10-3 and 3x10-3. In addition, two simulations examined the effect of no-cap on Cd (i.e. increasing Cd with wind speed) and constant Cd (Cd = 2.4x10-3) with Ck held constant at 1.2x10-3 in both runs. All simulations used a horizontal turbulence length scale of 1000 m and vertical length scale of 50 m.

Unrealistically strong hurricanes resulted for Ck/Cd ratios of 1.0 and 2.0. A Ck/Cd of 0.5 produced the most realistic hurricanes and results consistent with Bryan (2012). In the case of capped versus no-cap Cd, stronger azimuthal and radial winds were present in the no-cap case. A comparison of structure for the Ck/Cd 0.5 cases shows for a larger value of Cd, the size of the radial flows increase, but between radius 20 – 40 km a weaker Cd produced a stronger azimuthal wind, and a greater radius of maximum winds. The change in momentum balance has implications for the consistency of the sensitivity of the CM1 model with MPI theory.

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