Towards modular analysis of tropical-cyclone structure: the boundary layer

Abstract

In the early 1970s, George Carrier and coworkers undertook a modular approach to modelling the internal thermofluid-dynamics of tropical cyclones of tropicaldepression-or-greater intensity. A novel, relatively simplistic, approximate analysis of the vortex, idealized as axisymmetric, was carried out in the asymptotic limit of large Reynolds number, so that inviscid and diffusive subdomains of the structure were distinguished. Little subsequent work has followed this line of investigation. The indifference has proven problematic because accurate prediction of tropical-cycling intensity remains a challenge for operational forecasting, despite decades of effort at direct integration of comprehensive boundary/initial-value formulations. A contributing factor is that, to achieve solution in real time, such computational treatment of the entire vortex invariably resorts to coarse gridding, and key features remain inadequately resolved. Accordingly, here the modular approach is revisited, with the assistance of: recent observational insights; greatly enhanced computer-processing power; and convenient computational software, which facilitates implementation of a semi-analytic, semi-numerical methodology. Focus is largely, but not exclusively, on the dynamics and energetics occurring in the nominally kilometre-thick, ocean-surfacecontiguous boundary layer, especially on influx to the boundary layer and efflux therefrom. The modular approach not only permits the boundary layer, which develops its own highly significant substructure under the high-speed portion of the inviscid vortex, to be well-resolved, but also allows the layer to be investigated in the context of the other tropical-cyclone-structure subdivisions.