Adjoint Sensitivity Diagnosis of Hurricane Irma (2017) Intensity Change

Tropical cyclone intensity change remains a significant forecast challenge due to model deficiencies and poor and uncertain initial conditions. Hurricane Irma (2017) posed a significant forecast challenge as it rapidly intensified in the central Atlantic prior to landfall in the northeastern Caribbean. This presentation will explore sensitivities of tropical cyclone intensity to the wind and thermodynamical variables including mixing ratio and potential temperature using the WRF adjoint model. In addition, sensitivities to kinematic properties of simulated two-dimensional flows (vorticity and divergence) are diagnosed and described.

Sensitivities to the horizontal wind field in the lower-troposphere reveal that cyclonic perturbations to the horizontal wind field prior to the response function time, results in a stronger cyclone. These sensitivity are maximized well outside the radius of maximum wind (RMW) for 24h adjoint integrations, but as lead times shorten, the sensitivities are maximized closer to the RMW. Sensitivities to vorticity suggest an “Orr-like” mechanism at work – in which vorticity perturbations spiral anticyclonically inward and are oriented against the flow of the swirling flow of the vortex. For 6h to 15h adjoint integrations, the sensitivities in the upper-troposphere/lower-stratosphere to vorticity are negative - indicating a reduction in the inertial stability of the outflow layer. Azimuthally averaged radial-height sections of sensitivities to the radial flow show weak sensitivities for 24h adjoint integrations. Sensitivities to potential temperature and water vapor mixing ratio suggest increases in the equivalent potential temperature in the hurricane eye, or in the environment outside the radius of maximum winds would increase the cyclone intensity at later times.