North Atlantic OSSEs in Support of Coupled Hurricane Intensity Forecast Improvement: Impact of Ocean Observing System Enhancements

A prototype ocean OSSE system, first developed and data validated in the Gulf of Mexico, has been set up in the tropical and subtropical Atlantic Ocean with the initial goal of determining the impact of existing and planned ocean observing systems on improving hurricane intensity forecasts. The capability of the system to provide credible impact assessments is demonstrated by comparing Observing System Experiments to OSSEs. Two types of OSSEs are then performed for the purpose of improving ocean model initialization in coupled hurricane forecast models: seasonal and storm-relative. Seasonal OSSEs focus on long-term deployments of instruments during entire hurricane seasons. These OSSEs use a multi-year Nature Run (NR) that provides daily ocean fields that have been evaluated for realism, and a Data Assimilation (DA) system that uses a daily update cycle. The first seasonal OSSEs have been performed to evaluate deployment strategies for ocean gliders and profiling floats during the 2009 hurricane season. By contrast, storm-relative OSSEs are designed to test the impact of rapid-response ocean measurement programs that target individual storms and measure pre-storm, in-storm, and post-storm ocean conditions. The setup used for seasonal OSSEs is inadequate for storm-relative OSSEs because of the short time scales of atmospheric forcing changes, and of the ocean response beneath the storm. For each storm, a special NR must be performed that is nested within the long NR and that (1) is forced by higher-resolution and higher-frequency atmospheric forcing, and (2) saves ocean fields every two hours. The DA system is then run with a two-hourly update cycle. The first storm-relative OSSEs have been set up for hurricane Bill (2009) to determine the relative impacts of airborne ocean profile surveys along with rapid-response deployments of instruments such as profiling floats, gliders, and surface drifters with thermistor chains.