Tropical cyclone genesis in TC-LAPS: The importance of sufficient net deep convection and system scale cyclonic absolute vorticty

Tropical Cyclone (TC) genesis is explored in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational Numerical Weather Prediction (NWP) forecast model. Numerous developing and non-developing cases have been investigated. The simulations showed that in each developing case PV cores were enhanced through low- to mid-tropospheric vortex enhancement in model resolved updraft cores, and interacted to form larger cores through diabatic vortex merger. On the system-scale, vortex intensification resulted from the large-scale mass redistribution forced by net diabatic heating in excess of adiabatic cooling in the updraft cores, which resulted in vortex intensification through low- to mid-level convergence by the system scale secondary circulation. The non-developing cases illustrated that vertical wind-shear and an insufficient large-scale cyclonic environment, can hamper genesis by tilting and tearing apart the PV cores as they develop, and fail to trap and convert convectional energy to rotational energy on the system-scale, respectively.

The exact detail of the vortex interactions was found to be unimportant for qualitative genesis forecast success. Instead the critical ingredients were found to be, sufficient net deep convection in a sufficiently cyclonic environment in which vertical shear was less than some destructive limit. The often-observed TC genesis pattern of convection convergence, where the active convective regions converge into a 100 km diameter center, prior to an intense convective burst and development to Tropical Storm intensity, is evident in the developing TC-LAPS simulations.

A climatalogical study is currently underway in which all Tropical Depressions for the 2003/2004 and 2004/2005 seasons in the Australian region, are being investigated. To date, despite a tendency for more rapid genesis in TC-LAPS, the qualitative success is very good, which suggests TC genesis may be largely predictable, provided the large-scale environment (vorticity, vertical shear, and convective forcing) is sufficiently resolved and initialized.