Boundary Layer Structure and Surface Flux Estimates in Tropical Cyclones

Direct measurements of high-wind air-sea fluxes are notoriously difficult to make within tropical cyclones, which presents challenges in parameterizing or constraining these quantities in numerical models. Due to the scarcity of data in extreme conditions, hypotheses about how surface fluxes of energy and momentum behave in high winds, including the role of spray and/or waves, remains speculative. One method which attempts to estimate air-sea transfer in high winds relies on the so-called flux profile method, which has been used in several instances to retrieve surface momentum fluxes and the corresponding drag coefficient from mean velocity profiles obtained from either dropsondes or meteorological towers. In this work, we critically evaluate the reliability of this method for obtaining air-sea fluxes, and in particular, its validity as a function of storm radius. To do this, we utilize “virtual sondes” released throughout a turbulence-resolving large eddy simulation of an idealized tropical cyclone, in order to test whether prescribed (i.e. known) surface fluxes can be recovered from mean vertical profiles. While there are inherent uncertainties in this procedure, its use at radii near the radius of maximum winds likely violates key assumptions behind the flux theory, and must be adjusted. Potential corrections to the theory will be discussed.