25th Conference on Hurricanes and Tropical Meteorology


Tropical cyclone enthalpy and angular momentum fluxes

William D. Ramstrom, MIT, Cambridge, MA

Tropical cyclones are known to be very sensitive to air-sea fluxes of enthalpy and angular momentum. One way of representing these fluxes is to use the bulk aerodynamic formulae to quantify angular momentum and enthalpy fluxes at the sea surface. Using these formulae allows estimates of these fluxes as a function of wind speed, without having to consider the full complexity of the physics of the air-sea interface. Of particular interest, modeling results show that the intensification of a hurricane is very sensitive to the value of the ratio of these coefficients.

The coefficients to these equations, Cd and Ck, have been empirically determined in previous studies, either by direct measurements on platforms and ships, or by budget analyses from airborne data, as was done by Hawkins and Rubsam (1968) for Hurricane Hilda of 1964. Similar budgets were analyzed in this study. Three cases of strong hurricanes that were monitored using GPS dropsondes have been selected for this study, allowing analysis of these coefficients for conditions with surface winds up to 65 m/s. Recent advances in dropsonde technology provide improved range and accuracy from earlier methods, with reliable, high-resolution measurements of wind and thermodynamic variables down to within 10m of the sea surface. Budgets of angular momentum and heat were generated using this sounding data; these budgets yielded estimates of Cd and Ck. Turbulent fluxes in the vertical at mid-levels are shown to significantly affect the enthalpy budget.

The budget calculations in this study show that the value of the 10m drag coefficient, Cd, is in the range of 0.0026 to 0.0030 for wind speeds in the 40-60 m/s range. The 10m enthalpy transfer coefficient, Ck, ranges from 0.0029 to 0.0036 under these conditions. Thus, the ratio of Ck/Cd is approximately 1.0.

extended abstract  Extended Abstract (424K)

Supplementary URL: http://wind.mit.edu/~ramstrom/flux/

Session 16C, Tropical Cyclone Air-Sea Interaction III (Parallel with Sessions 16A and 16B)
Friday, 3 May 2002, 8:30 AM-10:30 AM

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