OSSE Assessment of Ocean Observing Strategies to Improve Coupled Tropical Cyclone Prediction

A new, validated, ocean OSSE system implemented in the tropical/subtropical Atlantic Ocean is used to evaluate ocean observing strategies during the 2014 hurricane season with the goal of improving coupled tropical cyclone forecasts. Enhancements to the existing operational ocean observing system are evaluated prior to two storms, Edouard and Gonzalo, where pre-storm ocean measurements were obtained during field experiments supported by the 2013 Disaster Relief Appropriation Act. For Gonzalo, an OSSE is performed to evaluate the impact of two ocean gliders deployed north and south of Puerto Rico during the 2014 hurricane season. For Edouard, an OSSE is performed to evaluate the pre-storm ocean profile survey conducted by NOAA WP-3D aircraft. For both storms, additional OSSEs are then conducted to evaluate more extensive pre-storm ocean observing strategies. These include (1) deploying a larger number of synthetic ocean gliders, (2) deploying either synthetic thermistor chains or synthetic profiling floats along one or more “picket fence” lines that cross projected storm tracks, and (3) designing airborne profiling surveys to have larger impact than the actual pre-storm survey conducted for Edouard. Impacts are evaluated based on error reduction in parameters important to SST cooling and hurricane intensity such as tropical cyclone heat potential and the structure of the ocean eddy field. In all cases, ocean profiles that sample both temperature and salinity down to 1000m provide greater overall error reduction than shallower temperature profiles obtained from XBTs and thermistor chains. Error reduction due to assimilating pre-storm observations from any individual instrument is primarily confined to within 2 degrees of the measurement location. Therefore large spatial coverage with multiple instruments is necessary to sufficiently reduce ocean initialization errors over a region broad enough to significantly impact predicted surface enthalpy flux into the storm. The HYCOM-HWRF coupled prediction model can now be initialized with analyses produced by the OSSE system, so observing system impacts on actual intensity forecasts will also be presented.