Exploring Eyewall Contraction Using the FLIGHT+ Dataset

It has long been known that the eyewall and radius of maximum wind (RMW) in tropical cyclones (TCs) tend to contract inwards during intensification. Recently, Stern et al. (2015) found from idealized simulations (as well as observed Vortex Data Messages) that contraction generally ceases during the middle of the intensification period, and that intensification continues at a fixed size. Here, we further explore eyewall contraction from the full radial profiles of flight-level data, using The Extended Flight Level Dataset for Tropical Cyclones (FLIGHT+). Identifying hurricanes that are well-sampled throughout their intensification periods, we assess the rate of contraction and how this varies relative to intensity change.

We also examine a key finding of Stern et al. (2015): that contraction slows down in association with a rapid increase in the “peakedness” of the wind profile. Kinematically, the rate of change of the RMW is determined by the ratio of the radial gradient of the wind tendency to the curvature of the radial profile, both evaluated at the RMW itself. Stern et al. (2015) found from a simulation that it is the increasing curvature that eventually renders further contraction nearly impossible. By calculating the curvature of the observed radial profiles of tangential wind, we can determine whether this type of evolution is typical. We also can examine the inward displacement of the eyewall updraft from the RMW, and how this varies with intensity and size.

The eyewall and RMW systematically slope outwards throughout the troposphere. However, the reasons for this slope in the free atmosphere and within the boundary layer are dynamically distinct. Due to surface friction and the unbalanced flow it induces within the boundary layer, the RMW at the surface is displaced inwards from that above. Powell et al. (2009) examined the ratio of the surface to flight-level RMW from SFMR/aircraft data. However, they didn’t examine the dependence on RMW size itself. Kepert (2017) recently showed that theoretically, the eyewall slope should increase with increasing size. Here, we examine this relationship in observations. Finally, for all of our observational analyses using the FLIGHT+ dataset, we perform comparisons to a set of idealized simulations.