One of the fundamental unsolved questions as for a hurricane is the cause-and-effect sequence of events that contribute to the increase of the maximum tangential velocity. In order to clarify the important physical variables in a time-dependent manner, we perform a sensitivity analysis of the maximum tangential velocity (Vmax) at the top of the boundary layer in a hurricane by using a cloud-permitting nonhydrostatic axisymmetric adjoint model. This method is basically the backward time integration of signal for Vmax tendency to isolate the important physical variables only associated with the hurricane intensification.
As a result of a backward integration to a few minutes prior to the specified time, the inflow around the region of Vmax is shown to be very sensitive. Furthermore, 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 means that the intensification occurs after the introduction of positive (negative) perturbations in potential temperature or in the mixing ratio of water vapor in the interior (exterior). Using term balance analysis, one can interpret this to mean that the stronger (weaker) convection in the hurricane interior (exterior) induces the changes in inflow around the region of Vmax and that it turns into changes in the Vmax in a quarter of local inertial period, a few minutes. Note that the signal is not related to the changes in the central pressure field. It suggests that the changes in the central pressure field is the result rather than the cause of the intensification as expected by the argument of local Rossby's deformation radius.
With further backward time integration, the sensitivity signals reach down to the surface and are located in the exterior region of the hurricane indicating that the sea surface state is critically important in terms of intensification, at least, in the framework of current simplified experiment. Another important finding is that both "stronger friction" and "reduction in moist air supply" in the exterior region of the hurricane can serve to strengthen the Vmax through shrinking the size of the vortex. To the authors' knowledge, this effect has not been explained physically in previous studies. See Ito et al. (2011, JAS) for details.