The importance of low-deformation vorticity in tropical cyclone formation Centre for Australian Weather and Climate Research, Melbourne, Australia
22 Feb 2013
Received: 12 April 2012 – Published in Atmos. Chem. Phys. Discuss.: 18 July 2012 Abstract. Studies of tropical cyclone (TC) formation from tropical waves have shown
that TC formation requires a wave-relative quasi-closed circulation: the
"marsupial pouch" concept. This results in a layerwise nearly contained
region of atmosphere in which the modification of moisture, temperature and
vorticity profiles by convective and boundary layer processes occurs
undisturbed. The pouch concept is further developed in this paper. TCs
develop near the centre of the pouch where the flow is in near solid body
rotation. A reference-frame independent parameter is introduced that
effectively measures the level of solid-body rotation in the lower
troposphere. The parameter is the product of a normalized Okubo-Weiss
parameter and absolute vorticity (OWZ).
Revised: 16 January 2013 – Accepted: 20 January 2013 – Published: 22 February 2013
Using 20 yr of ERA-interim reanalysis data and the IBTrACS global TC
database, it is shown 95% of TCs including, but not limited to, those
forming in tropical waves are associated with enhanced levels of OWZ on both
the 850 and 500 hPa pressure levels at the time of TC declaration, while
90% show enhanced OWZ for at least 24 h prior to declaration. This
result prompts the question of whether the pouch concept extends beyond
wave-type formation to all TC formations world-wide.
Combining the OWZ with a low vertical shear requirement and lower
troposphere relative humidity thresholds, an imminent genesis parameter is
defined. The parameter includes only relatively large-scale fluid properties
that are resolved by coarse grid model data (>150 km), which
means it can be used as a TC detector for climate model applications. It is
also useful as a cyclogenesis diagnostic in higher resolution models such as
real-time global forecast models.
Citation: Tory, K. J., Dare, R. A., Davidson, N. E., McBride, J. L., and Chand, S. S.: The importance of low-deformation vorticity in tropical cyclone formation, Atmos. Chem. Phys., 13, 2115-2132, doi:10.5194/acp-13-2115-2013, 2013.