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ICAS External Seminar: The dynamics of tropical cyclones

ICAS External Seminar: The dynamics of tropical cyclones

Date: Tuesday 11th October

Time: 14:00 to 15:00

Presenter: Roger K. Smith (Host Juliane Schwendike), Ludwig-Maximilians University of Munich, Germany

Location: SEE L8 Semianr Rooms

In this talk I will review some recent research on the formation and intensification of tropical cyclones in the prototype problem for intensification, which considers the evolution of a prescribed, initially cloud-free axisymmetric vortex in a quiescent environment on an f-plane. Spin up involves the collective action of rotating deep convection in converging absolute vorticity in the lower troposphere to increase the local circulation about fixed circuits within the flow. This process will be illustrated for a case of tropical cyclogenesis, when the initial vortex is weak.

Part of the talk will focus on an axisymmetric view of the intensification process and present some new ideas on spin up. The classical axisymmetric paradigm for tropical cyclone invokes the collective effects of deep convection within the inner region of a nascent vortex to produce lower-tropospheric inflow. Above the frictional boundary layer, this inflow draws absolute angular momentum (M-) surfaces inwards and because M is approximately materially conserved in the absence of friction, the tangential wind above the boundary layer increases. However, both observations and a range of numerical modelling studies show that the spin up of the maximum tangential wind occurs in the layer of strong frictionally induced inflow, i.e. in the boundary layer, a result that might seem counterintuitive. An explanation for this boundary-layer spin up process will be given. The results of recent numerical simulations suggest that the classical axisymmetric paradigm for tropical cyclone spin up applies primarily to explain the spin up of the outer core of the vortex, while the eyewall region, itself, is spun up by the vertical advection of absolute angular momentum from the boundary layer and not by the classical mechanism. Comparisons of the azimuthally-averaged boundary layer flow in the model with results from a steady, axisymmetric, slab boundary layer model highlight an important boundary-layer control mechanism on vortex evolution. The methodology enables one to break into the tightly-coupled feedback processes involved in vortex evolution. These ideas offer an attractively simple explanation for the tendency of the eyewall in mature storms to expand in size as the storms age.

Roger Smith is an Emeritus Professor at the Ludwig Maximilians University of Munich. A graduate of the University of Manchester
in the 1960's, he spent nearly 20 years in the Mathematics Department at Monash University before moving to a Chair of Meteorology at the LMU in Munich. He was a founding member of the Australian Branch of the Royal Meteorological Society and served as Secretary of the Branch and later as Chairman. When the Australian Branch became an independent society, The Australian Meteorological and Oceanographical Society, in 1988, he became its first President before moving to Munich. His main research interests have been in the dynamics of the Australian summertime cool change, cold fronts over the Australian subtropics and tropics, the Morning Glory phenomenon of northeastern Australia, which is a type of atmospheric undular bore, heat lows, rotating tropical convection, and last, but not least, tropical cyclones. He officially retired in 2008, but continues an active interest in tropical cyclone research together with close colleagues Professor Michael Montgomery at the Naval Postgraduate School in Monterey, California, and his coworker at the LMU, Dr. Gerard Kilroy. This research, which is currently supported by the US Office of Naval Research Global, has led to more than 50 publications on tropical cyclones, including 34 in the Quarterly Journal in the last 8 years.