Climatological Adjustments to Objective Tropical Wave Diagnostics

Aidan Mahoney

The National Hurricane Center’s (NHC) Tropical Analysis and Forecast Branch (TAFB) analyzes and forecasts the positions of tropical wave axes in the Atlantic and Eastern Pacific Oceans. Tropical wave diagnostics provide an important aid to forecasters in a data limited area of responsibility. They were developed at the University at Albany, and are used operationally at NHC/TAFB. The tropical wave diagnostics use relative vorticity to objectively analyze 700 hPa trough axes. Relative vorticity is separated into its shear and curvature components, and curvature vorticity is then used to determine the tropical wave trough axes. In the Caribbean Sea, the Colombian/Panamanian Low impacts the 700 hPa curvature vorticity field, causing the diagnostics to persistently analyze a standing trough axis near this feature. In many cases, this climatological feature dominates the model fields and obscures the signal from tropical waves that propagate through this region. It is hypothesized that a climatological adjustment to the 700 hPa curvature vorticity field will lessen the impact of the Colombian/Panamanian Low and improve the objective tracking of tropical wave axes in the Caribbean Sea. A climatology of curvature vorticity is created for the period of 1981-2010, using the Climate Forecast System Reanalysis (CFSR) dataset. The climatology is a 45-day boxcar average, and is computed at six-hourly intervals to account for diurnal and semidiurnal oscillations in the tropics. The curvature vorticity climatology is subtracted from the curvature vorticity in the model fields, producing curvature vorticity anomalies. The objective analysis is then conducted on the curvature vorticity anomalies. In some cases analyzed beginning in July 2021, the climate adjusted diagnostics produce subtle differences in analyzed wave axes across the domain. This includes the Caribbean, where the primary goal of this study is to improve the objective analysis. It remains inconclusive if analyzing the curvature vorticity anomalies produces an improvement, but this procedure does not appear to degrade the quality of the analysis. This study represents just one method to attempt to remove the signal in the curvature vorticity field from the climatological low over the Caribbean. Future research is planned using different approaches to assess if a clear improvement can be made to the tropical wave diagnostics.

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