A monsoon depression over northwestern Australia part II: a numerical model study

A numerical shallow-water model is used to investigate dynamical aspects of the evolution of an initial weak and broad vortex circulation in the absence of a basic flow. The initial vortex lies partly over land and is centred over the coast which is straight and runs from southwest to northeast. The situation is relevant to the case study of the development of a monsoon depression near the northwestern coast of Australia presented in Part I. The model incorporates a modified form of the convective parametrisation scheme developed by Ooyama. The modifications were found to be necessary when the cyclonic circulation has its centre in the proximity of land. On an f-plane the initial vortex intensifies to a strength comparable to that of the observed monsoon depression and moves eastwards over the land. The eastward drift is caused by asymmetries in the convection which are a result of the land-sea distribution. On a beta-plane the motion is dominated by the flow associated with the beta-induced vorticity asymmetries and the vortex moves southwestwards and further inland. Accordingly, its maximum intensity is less than that on the f-plane. On a beta-plane in the absence of land, the vortex attains the strength of an intense tropical cyclone with maximum winds of 48 m s-1 and drifts towards the southwest.