ACCESS-TC: Vortex Specification, 4DVAR Initialization, Verification, and Structure Diagnostics

Noel E. Davidson Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Yi Xiao Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Yimin Ma Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Harry C. Weber Fachhochschule des Bundes für öffentliche Verwaltung, Fachbereich Wetterdienst, Fürstenfeldbruck, Germany

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Xudong Sun Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Lawrie J. Rikus Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Jeff D. Kepert Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Peter X. Steinle Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Gary S. Dietachmayer Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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Charlie C. F. Lok Centre for Australian Weather and Climate Research,* Melbourne, Victoria, Australia

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James Fraser National Meteorological and Oceanographic Centre, Australian Bureau of Meteorology, Melbourne, Victoria, Australia

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Joan Fernon National Meteorological and Oceanographic Centre, Australian Bureau of Meteorology, Melbourne, Victoria, Australia

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Hakeem Shaik Northern Territory Regional Office, Australian Bureau of Meteorology, Darwin, Northern Territory, Australia

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Abstract

The Australian Community Climate and Earth System Simulator (ACCESS) has been adapted for operational and research applications on tropical cyclones. The base system runs at a resolution of 0.11° and 50 levels. The domain is relocatable and nested in coarser-resolution ACCESS forecasts. Initialization consists of five cycles of four-dimensional variational data assimilation (4DVAR) over 24 h. Forecasts to 72 h are made. Without vortex specification, initial conditions usually contain a weak and misplaced circulation pattern. Significant effort has been devoted to building physically based, synthetic inner-core structures, validated using historical dropsonde data and surface analyses from the Atlantic. Based on estimates of central pressure and storm size, vortex specification is used to filter the analyzed circulation from the original analysis, construct an inner core of the storm, locate it to the observed position, and merge it with the large-scale analysis at outer radii.

Using all available conventional observations and only synthetic surface pressure observations from the idealized vortex to correct the initial location and structure of the storm, the 4DVAR builds a balanced, intense 3D vortex with maximum wind at the radius of maximum wind and with a well-developed secondary circulation. Mean track and intensity errors for Australian region and northwest Pacific storms have been encouraging, as are recent real-time results from the Australian National Meteorological and Oceanographic Centre. The system became fully operational in November 2011. From preliminary diagnostics, some interesting structure change features are illustrated. Current limitations, future enhancements, and research applications are also discussed.

Centre for Australian Weather and Climate Research is a partnership between the Bureau of Meteorology and Commonwealth Scientific and Industrial Research Organisation.

Corresponding author address: Noel Davidson, CAWCR, P.O. Box 1289, Melbourne, VIC 3000, Australia. E-mail: n.davidson@bom.gov.au

Abstract

The Australian Community Climate and Earth System Simulator (ACCESS) has been adapted for operational and research applications on tropical cyclones. The base system runs at a resolution of 0.11° and 50 levels. The domain is relocatable and nested in coarser-resolution ACCESS forecasts. Initialization consists of five cycles of four-dimensional variational data assimilation (4DVAR) over 24 h. Forecasts to 72 h are made. Without vortex specification, initial conditions usually contain a weak and misplaced circulation pattern. Significant effort has been devoted to building physically based, synthetic inner-core structures, validated using historical dropsonde data and surface analyses from the Atlantic. Based on estimates of central pressure and storm size, vortex specification is used to filter the analyzed circulation from the original analysis, construct an inner core of the storm, locate it to the observed position, and merge it with the large-scale analysis at outer radii.

Using all available conventional observations and only synthetic surface pressure observations from the idealized vortex to correct the initial location and structure of the storm, the 4DVAR builds a balanced, intense 3D vortex with maximum wind at the radius of maximum wind and with a well-developed secondary circulation. Mean track and intensity errors for Australian region and northwest Pacific storms have been encouraging, as are recent real-time results from the Australian National Meteorological and Oceanographic Centre. The system became fully operational in November 2011. From preliminary diagnostics, some interesting structure change features are illustrated. Current limitations, future enhancements, and research applications are also discussed.

Centre for Australian Weather and Climate Research is a partnership between the Bureau of Meteorology and Commonwealth Scientific and Industrial Research Organisation.

Corresponding author address: Noel Davidson, CAWCR, P.O. Box 1289, Melbourne, VIC 3000, Australia. E-mail: n.davidson@bom.gov.au
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