The effects of convection and baroclinicity on the motion of tropical‐cyclone‐like vortices
Authors: Dengler Klaus , Reeder Michael J. 
Published: February 2007

A three‐layer shallow‐water numerical model is used to study the motion of tropical‐cyclone‐like vortices in a westerly baroclinic current. the structure of the vortex and its environment are chosen to be representative of real tropical cyclones. Numerical experiments with and without convection are discussed. Several different environmental flows are considered.

In calculations on a northern‐hemisphere f‐plane without convection, anomalies of upper‐layer potential vorticity (PV) have a major influence on the vortex motion in the middle layer. This is true even in numerical experiments that include a PV‐gradient in the middle layer associated with an upper‐level westerly jet. For example, when a broad anticyclone is included in the upper layer, the middle‐layer vortex moves initially to the east‐northeast. If the middle‐layer PV‐gradient were the dominant factor, the middle‐layer vortex would, instead, move to the south‐east. When there is no middle‐layer PV‐gradient, the middle‐layer vortex is deflected to the right of the environmental shear vector if the upper‐layer vortex is cyclonic and to the left of the environmental shear vector if the upper‐layer vortex is anticyclonic. the size of the vortex affects the drift velocity, but has a negligible effect on the drift direction.

Calculations incorporating a convective parametrization based on that developed by Ooyama (1969) show that an initially weak vortex can intensify to hurricane strength in a conditionally neutral environment. This calculation underscores the importance of surface fluxes in Ooyama's scheme and subsequent schemes based on it, as it shows that the scheme does not rely necessarily on a large amount of convective instability in the initial state.

In calculations on a northern‐hemisphere f‐plane with an upper‐level westerly jet and convection, the middle‐layer vortex moves to the south‐east because of the middle‐layer PV‐gradient. When the middle‐layer PV‐gradient is removed but the convection retained, the middle‐layer vortex moves first to the south‐east and then to the northeast. In this case the motion of the middle‐layer vortex is controlled by the upper‐layer PV‐anomaly. At early times, the vertical shear tilts the vortex towards the east so that the western side of the upper‐layer cyclonic core lies above the middle‐layer vortex centre. Consequently, the upper‐layer cyclonic core induces a southward component to the flow across the middle‐layer vortex centre. As time proceeds, the upper‐layer westerlies advect the broad anticyclonic outer part of the upper vortex over the middle‐layer vortex centre, inducing a northward component across its centre.

Relatively minor changes to the convective parametrization have a relatively large effect on the motion of the vortex. Changing the way the parametrized clouds transport momentum, changes the structure of the upper‐layer vortex. Changes to the pattern of PV in the upper layer in turn affect the middle‐layer flow attributable to it. the effect of the upper‐layer PV on the motion of the middle‐layer vortex is weakened when the outflow from a tropical cyclone is confined to a comparatively shallow upper layer as is commonly observed.