Tropical Cyclone (TC) genesis in an operational Numerical Weather Prediction (NWP) forecast model is investigated, using the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS). Results from three 48-hour simulations are presented in which a mature vortex of at least Tropical Storm intensity develops from a relatively benign initial state, without the use of a vortex bogus. In each case an intense PV core associated with surface-based deep convection (cumulonimbus hot tower) was embedded in a largely disorganized environment of cyclonic PV anomalies. Axisymmetrization of the environmental PV anomalies about this core, served to focus and align the environmental PV into a deep, large-scale coherent vortex that developed into the TC. We propose that the vortex interactions between the vortex core and environmental vorticity anomalies provide an upscale vorticity cascade to the TC-scale.

Results from two additional 48-hour simulations are presented in which genesis did not occur. In the first, strong low-level vertical shear tilted and tore apart the intense vortex core before the alignment and focussing of environmental PV through axisymmetrization could be completed. In the second case, the surface-based deep convection developed outside a PV-rich mid-level trough. The vortex core was of insufficient intensity to interact with the relatively remote mid-level PV source, and was short lived. These two non-developing examples illustrate the importance of a vertically coherent intense vortex core in the vicinity of sufficient environmental vorticity at both mid- and low-levels, for the upscale vorticity cascade to proceed to a point where the vortex becomes self-sustaining

The process is consistent with aspects of a number of observational genesis studies and does not contradict any such studies that we are aware of. It provides a possible and very plausible explanation for the much-speculated mechanism for the development of a low-level warm core vortex below a MCS (a commonly observed precursor to TC genesis).