Operational ACCESS-TC: Vortex specification, 4DVAR initialization, verification and structure diagnostics

Abstract

The Australian Community Climate and Earth System Simulator, ACCESS (Puri et al., 2010, 2012) has been configured for operational and research applications on Tropical Cyclones. ACCESS is an implementation at the Australian Bureau of Meteorology of the full UK Meteorological Office’s Numerical Modelling System (Gregory and Rowntree, 1990; Wilson and Ballard, 1999; Webster et al., 2003; Davies et al., 2005; Rawlins et al., 2007 and references therein). The ACCESS-TC system runs at a resolution of 0.11° and 50 levels. The domain is re-locatable and nested in coarser-resolution forecasts. Initialization consists of five cycles of 4DVAR over 24 hours and forecasts to 72 hours are made. Without vortex specification, initial conditions usually contain a weak and misplaced circulation (Zou and Xiao, 2000). This makes it necessary to incorporate a synthetic TC vortex at the observed location in initial conditions and preferably during the data assimilation (e.g., Kurihara et al., 1995; Davidson and Weber, 2000). In ACCESS-TC, based on estimates of central pressure and storm size, vortex specification is used to filter the analysed circulation from the original analysis, construct the inner-core of the storm, merge it with the large scale analysis at outer radii, and relocate it to the observed position. Since TC reconnaissance is not conducted in the Australian Region, significant effort has been devoted to building physically based, synthetic inner-core structures, validated using historical dropsonde archives from the Atlantic and northeast Pacific. The power of the 4DVAR method allows the use of only synthetic mean sea level pressure observations to build the horizontal and vertical structure of storms.