The intensity issue of hurricanes is addressed in this paper using a scale interaction approach and one that invokes the angular momentum budget of a storm in relative cylindrical coordinates.

The scale interaction approach is the familiar energetics in the wave number domain. Here, however, these are cast in a storm relative local cylindrical coordinate about the storm’s center as a point of reference. The wave numbers include the azimuthally averaged wave number 0, the principal hurricane scale asymmetries (wave numbers 1 and 2, determined from data sets) and the other scales. The main questions asked here relate to the role of the individual cloud scales in supplying energy to the hurricane scales, thus determining its intensity. A principal finding is that cloud scales carry most of their variance, via organized convection, directly on the scales of the hurricane. The generation of available potential energy and the transformation of eddy kinetic energy from the cloud scale are in fact directly passed on to the hurricane scale by these processes. Less of the kinetic energy is generated on the individual cloud scales, which are of the order of a few km. The other major component of the energetics is the kinetic to kinetic energy exchange and potential-to-potential energy exchange among different scales. These occur via triad interaction and were noted to be essentially downscale transfer, i.e., a cascading process. A second avenue for addressing the intensity explores the angular momentum budget in storm relative coordinates. Here large outer angular momentum of the hurricane is depleted continually along the inflowing trajectories. This depletion occurs via surface and planetary boundary layer friction, model diffusion, and ‘cloud torques’. The principal contributor to the dimunition of outer angular momentum is the cloud torques. The eventual angular momentum of the parcel near the storm center determines its final intensity.

The data sets for this study came from ECMWF, research aircrafts and satellite. Furthermore, based on realistic simulation of a hurricane (Bonnie of 19998) data sets were generated using a no hydrostatic microphysics meso-scale model (the MM5) in order to carry out these enquiries.