The Structure and Dynamics of Coherent Vortices in the Eyewall Boundary Layer of Tropical Cyclones

Abstract:

The boundary layer within the eyewall of intense tropical cyclones has been shown to be both highly turbulent and to contain coherent small-scale (of order 1 km) vortices. Dropsonde observations have indicated that extreme updrafts of 10-25 m/s can occur in the lowest 2 km, sometimes as low as a few hundred meters above the surface. These updrafts are often collocated with or found very nearby to local extrema in horizontal wind speed, which sometimes exceed 100 m/s.

Here, the CM1 model is used to simulate intense tropical cyclones in an idealized framework, with horizontal grid spacing as fine as ~30 meters. At this grid spacing, the scales of the vortices are well resolved. By examining individual features and compositing over many updrafts, we find that there is a consistent structure and relationship between vorticity, vertical velocity, and near-surface windspeeds. We quantitatively show that buoyancy is not responsible for the acceleration of strong boundary layer updrafts. Instead, the updrafts are forced by dynamical pressure gradients associated with strong gradients in the velocity fields.

It is currently unknown whether dropsonde observations represent quasi-vertical profiles through the features, or if instead the sondes are horizontally advected through the features. Using simulated dropsonde trajectories, we show that sondes are likely to be horizontally advected through features, and therefore apparent vertical variability in observed kinematic and thermodynamic profiles may actually be primarily in the horizontal. In observations, extreme updrafts are almost exclusively found in Category 4 and 5 hurricanes. We conduct simulations at varying intensity to investigate whether or not similar features exist in weaker storms. Finally, we have developed an objective algorithm that allows us to track individual updrafts/vortices in time, and we use this to investigate the evolution and lifecycle of these features and to gain further insight into their dynamics.