The Dynamics of Boundary Layer Jets within the Tropical Cyclone Core. Part I: Linear Theory

Observations of wind profiles within the tropical cyclone boundary layer have until recently been quite rare. The recent massive increase in observations due to the operational implementation of the global positioning system dropwindsonde has emphasised that a low-level wind speed maximum is a common feature of the tropical cyclone boundary layer. Here is proposed a mechanism for producing such a maximum, whereby strong inward advection of angular momentum generates the supergradient flow. The processes that maintain the necessary inflow against the outward acceleration resulting from gradient wind imbalance are identified as being (i) vertical diffusion, (ii) vertical advection, and (iii) horizontal advection. Two complementary tools are used to diagnose the properties and dynamics of the jet. The first, presented here, is a linear analytical model of the boundary layer flow in a translating tropical cyclone. It is an extension of the classical Ekman boundary layer model, as well as of earlier work on stationary vortex boundary layers. This simplifies the vertical diffusion, omits the vertical advection, and linearizes the horizontal advection. The solution is shown to have three components, a symmetric one due to the cyclone, and two asymmetric ones that result from the interaction of the moving cyclone with the earth's surface. The asymmetric components are shown to be equivalent to a frictionally stalled inertia wave. It is argued that an Ekman-type model may be appropriate in tropical cyclones since diurnal effects are weak or absent, turbulence is dominantly shear-generated, and baroclinicity is weak.