Title: Short-Time-Scale Processes in a Mature Hurricane as a Response to Sea Surface Fluctuations

Abstract: To clarify the effect of fluctuations in surface stress and heat fluxes on the maximum wind speed of a mature-state hurricane, a sensitivity analysis is performed by using an adjoint model based on the cloud-permitting nonhydrostatic axisymmetric model of Rotunno and Emanuel (1987, JAS). The use of this adjoint model provides all the sensitivities associated with the maximum wind speed of the mature-state hurricane obtained by one-time backward integration. Compared with another type of experiment in which all the model variables are perturbed, this method is efficient and offers an integrated point of view.

As a result of an integration backward to 4 min prior to the specified time, a dipole pattern appears in the sensitivity fields with respect to the vertical velocity, the potential temperature, and the mixing ratio of water vapor. A positive (negative) sensitivity is found in the hurricane interior (exterior) relative to the verification region. It exhibits an increase of tangential velocity 4 min after the introduction of positive (negative) perturbations in potential temperature or in the mixing ratio of water vapor in the interior (exterior). These sensitivities are not related to the changes in the central pressure field. With further backward integration, the sensitivity signals reach down to the surface and are located in the exterior region of the hurricane. While the sensitivity with respect to surface friction (heat flux) is strongly negative (positive) within a certain radius, the sensitivity can be positive (negative) beyond that radius. This means that both stronger friction and a reduction in moist air supply in the exterior region of the hurricane can serve to strengthen the maximum tangential velocity. To the authors’ knowledge, this effect has not been explained in previous studies. See more details in Ito et al. (2011, JAS)

Student Host: Tim Cronin
Faculty Host: Kerry Emanuel