Extended Abstract (148K)1C.1
Tropical Cyclone formation mechanisms in the BMRC Tropical Cyclone Limited Area Prediction System (TC-LAPS)
Kevin J. Tory, CAWCR, Melbourne, Vic, Australia; and M. T. Montgomery and N. E. Davidson
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).
Session 1C, Tropical Cyclogenesis I
Monday, 3 May 2004, 8:30 AM-9:45 AM, Napoleon II Room
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