Near-surface coherent structures in an intense tropical cyclone: conditional eddies and vertical momentum fluxes

Abstract

The intermittency of coherent turbulent structures in the tropical cyclone boundary layer makes them challenging to fully characterize, especially regarding their impact on momentum dynamics in the eyewall. Furthermore, the fine spatial and temporal model resolution needed to resolve these structures has long impeded their understanding. Using the output of a large eddy simulation (with 31-m horizontal and 15-m vertical grid spacing), we investigate the three-dimensional structure of conditionally averaged eddies associated with extreme Reynolds stress events in the near-surface region of a category 5 tropical cyclone. For ejection events, we find (using two- and three-dimensional analysis) that the structure of the conditional eddy is a vortex core roughly inclined toward the direction of the mean flow in the inflowing boundary layer, outer and inner eyewall region. For sweep events, this vortex core is less coherent. The nature of the educed eddies found in this study suggests a similarity between the coherent structures in simpler wall-bounded turbulent flows and those in complex environmental flows.