Angular momentum diffusion and the initiation of cyclones

Abstract

IT is possible to explain^1,2 the intensification of a cyclone vortex once the stream lines in a weak depression close around a core of higher vorticity, but it is less clear how the process starts. Scorer³ proposed that convection might diffuse angular momentum within a field of initially uniform rotation to localised regions, thereby concentrating cyclonic vorticity. Experiments failed to demonstrate the effect^4–7, and although strong vortices could be produced through the breaking of inertial waves⁸, the effects were open to misinterpretation. We present here new experimental results showing that first, angular momentum or relative vorticity can be diffused at a rate comparable with the diffusion of relative velocity (turbulent ‘viscosity’). This distinctive property seems to be intrinsic to weak turbulence in rotation, and can be parameterised very simply. Second, such diffusion occurs only when the turbulence is sufficiently weak to be constrained by rotation to an essentially two-dimensional structure. This might explain the failure of earlier experiments. Our results could have many applications to geophysics, including tropical cyclogenesis.