How Does the Eye Warm? A Potential Temperature Budget and Trajectory Analysis of Idealized Tropical Cyclones

The mechanisms that accomplish the warming in the eye of tropical cyclones are investigated through a potential temperature budget and trajectory analysis of idealized simulations, both with and without vertical wind shear. During intensification of a tropical cyclone in a quiescent environment, time-averaged eye descent is maximized at 12-13 km height. Warming is not generally maximized at these levels, however, because the static stability is at a minimum. Consequently, the perturbation temperature is maximized at mid-levels. Each of the above results remain valid for sheared tropical cyclones, and therefore shear does not systematically alter the height of the warm core.

The spatial structure of warming varies substantially with time. At the start of rapid intensification (RI), total advection of potential temperature is the only significant term contributing to warming the eye. However, for a substantial portion of RI, the region of most rapid warming actually undergoes mean ascent. The net advective warming is shown to be a result of eddy radial advection of potential temperature, dominated by a wavenumber-1 feature that is likely due to a dynamic instability. At a sufficient intensity, mean vertical advective warming becomes concentrated in a narrow zone just inward of the eyewall. In agreement with prior studies, this advective tendency is largely canceled by diabatic cooling. Subgrid-scale horizontal diffusion of potential temperature plays a surprisingly large role in the maintenance of the warm-core structure, and when the storm is intense, yields a negative tendency that can be of the same magnitude as advective warming.

An analysis of over 90,000 parcel trajectories yields further insight into the mechanisms of eye warming, and addresses several outstanding questions regarding the character of eye descent. The rate at which parcels are stirred from the eye into the eyewall is a strong function of intensity. While stirring is large at the beginning of RI, once a sufficient intensity is achieved, most parcels originating near the storm center can remain inside of the eye for at least several days. Many parcels in the upper-troposphere are able to descend within the eye by 5-10 km. The above results are relatively insensitive to the presence of up to 10 m/s of shear. In contrast, stirring in the eye/eyewall interface region is substantially enhanced by shear.