The importance of initial size on the size and structure of tropical cyclones

Sujan Pal1, Elizabeth Ritchie2

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2School of Physical, Environmental, and Mathematical Science, University of New South Wales, Canberra, New South Wales

Empirical observations show that the structure and size of tropical cyclones (TCs) have dramatic impacts at landfall, including wind damage and storm surge. TCs have been observed to change size under a variety of environmental conditions. Several numerical studies conducted within the last decade have looked at the environmental conditions and associated physical mechanisms that likely cause TC size changes. For example, Stovern and Ritchie (2016) found that cooling (warming) the environmental temperature resulted in larger (smaller) TCs mostly because of the changes in the resulting surface energy fluxes. Initial size of the cyclone is also an important factor behind rapid change of structure and size of the storm (Stovern and Ritchie 2016). This modeling study was designed to further investigate how the environment affects the size and structure of real TCs.

This study examines historical TC cases where the wind field expanded or contracted during its lifetime. Size changes are evaluated using the North Atlantic Hurricane Database second generation (HURDAT2) data set, which contains the maximum radial extent of the 64-, 50- and 34-kt wind in four quadrants. The average 34-kt wind radius (R34) is used as an indicator of the size of the TC. For the purposes of this study the environment of a TC is investigated if the wind field either expanded or contracted in size at least 15 n mi radially in a 12-hour period. The model used is the Weather Research and Forecasting- Advanced Research WRF (WRF-ARW) developed at NCAR. WRF is configured with multiple nests with the finest resolution of the inner nest being 2km, the WRF double-moment cloud microphysics scheme, Mellor-Yamada planetary boundary layer. Both inner meshes are vortex following and initial and boundary conditions are derived from the 6-hourly NCEP FNL data set.  In this presentation, we will use simulations of Hurricanes Igor (2010) and Katrina (2005) as they undergo size changes to explore how environmental forcing affect TC size and structure.

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