How do anomalous heat fluxes affect the maximum tangential velocity in a tropical cyclone?

The tropical cyclone often encounters small scale oceanic variabilities due to oceanic mesoscale warm eddies and warm western boundary currents. These features act to enhance latent and sensible heat fluxes at the sea surface [Lin et al. 2005; Wu et al. 2007]. According to the energy balance obtained from the current state-of-the-art theoretical framework, these oceanic factors can potentially have an impact on tangential maximum velocity via changes in sensible and latent heat fluxes [Emanuel et al. 2004; Lin et al. 2005]. One of the fundamental questions unsolved is how perturbations bring about changes in the maximum tangential velocity. One may identify changes in the central pressure field as a mechanism capable of intensifying the tropical cyclone vortex after the enhancement of condensation in the eyewall. However, Wu et al. (2006) showed perturbation-like inputs to the central pressure field are not likely to affect the subsequent maximum tangential velocity substantially when the radius of the eyewall is smaller than Rossby's deformation radius, which is typically the case. In this study, a sensitivity analysis is performed to trace the sensitivity of maximum tangential velocity by using an adjoint model. As a result of integration backwards to four minutes prior to the specified time, a dipole pattern appears in the sensitivity fields with respect to potential temperature and the mixing ratio of water vapor. A positive (negative) sensitivity is found inside (outside) the target region, which exhibits an increase of tangential velocity four minutes after the introduction of positive (negative) perturbations in potential temperature or in the mixing ratio of water vapor inside (outside) the target region. The sensitivity signal in the central pressure field is quite weak. With further backward integration, the sensitivity signals reach down to the surface within 20 minutes. It indicate that changes in potential temperature and water vapor mixing ratio can affect the tangential velocity very locally around the target region without the changes in the central pressure field. The processes associated with the sensitivities are investigated by means of the term balance analysis of the adjoint equations. The result indicates that the stronger radial inflow around the eyewall region is induced locally following the changes in the vertical velocity through enhanced condensation and changes in buoyancy forces. Changes in radial velocity alter the maximum tangential velocity through an inertial behavior. Thus, this short-timescale process is not associated with the changes in the central pressure field. See details in Ito et al. (2011, JAS).