INVESTIGATING THE ROLE OF THE UPPER-LEVELS IN TROPICAL CYCLONE GENESIS

Despite decades of theoretical research and observational studies, a good understanding of tropical cyclone genesis(TCG) remains elusive. One school of theories proposes that TCG within an African Easterly Wave results from “bottom-up” development of cyclonic vorticity that is contingent upon favorable conditions in the lower-troposphere and boundary layer. Our observational study suggests that while lower-tropospheric forcing is a necessary condition for this type of TCG, it may not be sufficient in some cases, and that environmental conditions in the upper levels can have an influence. Specifically, we find evidence to suggest that pre-TCG upper-tropospheric flow patterns characterized by core-connecting outflow vents to the environment can in certain situations provide a modulating effect on Atlantic tropical disturbances trying to develop. Patterns of nearenvironment upper-level inertial stability, divergence, outflow setup, and mass evacuation are identified and related to surface development. The study employs high-resolution satellite-derived wind data, aircraft GPS dropwindsondes, composite fields, multivariate objective analyses, and case studies to help identify conditions in the upper-level environment that can play a role in Atlantic TCG events.

Towards an objective historical tropical cyclone dataset for the Australian region

The appropriate design of infrastructure in tropical cyclone(TC)prone regions requires an understanding of the hazard risk profile underpinned by an accurate,homogenous long-term TC dataset.The existing Australian region TC archive,or’best track’(BT),suffers from inhomogeneities and an incomplete long-term record of key TC parameters.This study assesses mostly satellite-based objective techniques for 1981-2016,the period of a geostationary satellite imagery dataset corrected for navigation and calibration issues.The satellite-based estimates of Australian-region TCs suffer from a general degradation in the 1981-1988 period owing to lower quality and availability of satellite imagery.The quality of the objective techniques for both intensity and structure is compared to the reference BT 2003-2016 estimates.For intensity the Advanced Dvorak Technique algorithm corresponds well with the BT 2003-2016,when the algorithm can use passive microwave data(PMW)as an input.For the period prior to 2003 when PMW data is unavailable,the intensity algorithm has a low bias.Systematic corrections were made to the non-PMW objective estimates to produce an extended(1989-2016)homogeneous dataset of maximum wind that has sufficient accuracy to be considered for use where a larger homogeneous sample size is valued over a shorter more accurate period of record.An associated record of central pressure using the Courtney-Knaff-Zehr wind pressure relationship was created.For size estimates,three techniques were investigated:the Deviation Angle Variance and the’Knaff’techniques(IR-based),while the’Lok’technique used model information(ECMWF reanalysis dataset and TC vortex specification from ACCESS-TC).However,results lacked sufficient skill to enable extension of the reliable period of record.The availability of scatterometer data makes the BT 2003-2016 dataset the most reliable and accurate.Recommendations regarding the best data source for each parameter for different periods of the record are summarised.