The results of recent numerical simulations suggest that the classical axisymmetric paradigm for tropical cyclone spin up applies primarily to explain the spin up of the outer core of the vortex. This paradigm invokes the collective effects of deep convection within the inner region of a nascent vortex to produce inflow in the lower troposphere. Above the frictional boundary layer, spin up occurs as this inflow draws absolute angular momentum (M-)surfaces inwards because M is approximately materially conserved in the absence of friction. Here M is defined as rv + □(1/2)fr2, where r is the radius, v is the tangential velocity component and f is the Coriolis parameter (assumed constant). Within the boundary layer, M is reduced by friction, but the tangential velocity of a converging air parcel (v = M/r - □(1/2)fr) may become large, indeed larger than the local tangential wind above the boundary layer, if the rate at which the radius of the air parcel decreases (i.e. its inward velocity) is larger than the rate of loss of M. Both observations and a range of numerical modelling studies show that the spin up of the maximum tangential wind occurs in the layer of strong frictionally induced inflow, i.e. the boundary layer.
Azimuthally-averaged fields from three-dimensional numerical model simulations together with those from a minimal axisymmetric model suggest that the eyewall, itself, is spun up by the vertical advection of absolute angular momentum from the boundary layer and not by the classical mechanism. Comparisons of the boundary layer flow in the model with results from a steady slab boundary layer model highlight an important boundary-layer control mechanism on vortex evolution. The methodology enables one to break into the tightly-coupled feedback processes involved in vortex evolution. These ideas offer an attractively simple explanation for the tendency of mature storms to expand in size as they age. References: Kilroy, G., R. K. Smith, and M. T. Montgomery, 2015: Why do model tropical cyclones grow progressively in size and decay in intensity after reaching maturity? J. Atmos. Sci., 72, in press Persing, J., M. T. Montgomery, J. McWilliams, and R. K. Smith, 2013: Asymmetric and axisymmetric dynamics of tropical cyclones. Atmos. Chem. Phys., 13, 12299-12341. Schmidt, C. and R. K. Smith, 2015: Tropical cyclone evolution in a minimal axisymmetric model revisited. Quart. Journ. Roy. Meteor. Soc., 140, submitted.