Effects of surface exchange coefficients on the intensity and structure of axisymmetric hurricanes

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 (C_k) and drag (C_d). 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 C_k/C_d ratios of 0.5, 1.0 and 2.0 for constant values of C_d of 1.5x10^-3, 2x10^-3 and 3x10^-3. In addition, two simulations examined the effect of no-cap on C_d (i.e. increasing C_d with wind speed) and constant C_d (C_d = 2.4x10^-3) with C_k 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 C_k/C_d ratios of 1.0 and 2.0. A C_k/C_d of 0.5 produced the most realistic hurricanes and results consistent with Bryan (2012). In the case of capped versus no-cap C_d, stronger azimuthal and radial winds were present in the no-cap case. A comparison of structure for the C_k/C_d 0.5 cases shows for a larger value of C_d, the size of the radial flows increase, but between radius 20 – 40 km a weaker C_d 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.