North Atlantic OSSEs in support of improved hurricane forecasting: Nature Run evaluation
The system is based on the Hybrid Coordinate Ocean Model (HYCOM) and has been applied on a 1/25 degree Mercator mesh for the free-running Nature Run (NR) and on a 1/12 degree Mercator mesh for the data assimilative forecast model (FM); both are nested in the 1/12 degree global HYCOM. A “fraternal twin” system is employed, using two different realizations for NR and FM, each configured to produce substantially different physics and truncation errors. Building two different HYCOM configurations is a unique capability of the HYCOM code, as it contains multiple choices of numerical algorithms and subgrid-scale parameterizations that can be varied to produce the different physics and truncation errors between the two models, as required for a credible OSSE system. The chosen difference in horizontal resolution introduces additional truncation errors. The NR and FM mean and RMS amplitude maps are in good agreement with ocean climatology, such as the AVISO-derived Sea Surface Height Anomaly. Detailed NR evaluation is based on data products, specifically developed from RSMAS and AOML archives. These include upper ocean profiles from air-borne instruments, surface velocity maps derived from the historical drifter data set and tropical cyclone heat potential maps derived from altimetry observations.