Mesoscale Air-Sea Coupling in Tropical Cyclones Isidore and Lili

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The long-term goal of this research is to improve understanding of how mesoscale ocean processes modify the near surface wind structure of tropical cyclones through air sea interactions. Of particular importance are the interactions occurring as tropical cyclones pass over warm oceanic features off the Atlantic and Gulf coasts of the United States. Understanding lower boundary effects is not only important from a research perspective, but also has far reaching consequences with respect to forecasting the surface wind field and intensity changes for landfalling storms. The main research theme involves the physical effects of oceanic vertical mixing and advection on heat, moisture, and momentum fluxes in the atmospheric boundary layer (ABL) as tropical cyclones encounter mesoscale ocean features. Specific research objectives are:1. Improve pre and post storm ocean heat content estimates (OHC) using in situ and satellite derived fields.2. Examine the forcing by warm ocean features on enhanced surfaces stresses, fluxes and surface waves in the ABL embedded within the direct forcing region, and the role of the ocean mixed layer (OML) feedback on the ABL.3. Quantify the roles of mixing and horizontal advection of mass, heat, and salt by analyzing their budgets in the OML, focusing especially on the effects of precipitation.4. Document the variability in the atmospheric environment such as equivalent potential temperature and wind shear relative to the storm and upper ocean variability.During Hurricane Isidore and Lili (in 2002), the Principal Investigator and collaborators were able to collect one of the most comprehensive ocean atmosphere data sets to date under strong hurricane forcing conditions. These observations included measurements of upper ocean current, temperature and salinity profiles along with simultaneous atmospheric temperature, moisture and wind profiles. Additionally, the large scale atmospheric variability was observed using several hundred atmospheric sondes released from a research aircraft from the National Oceanic and Atmospheric Administration. To achieve the research objectives, the Principal Investigator will utilize a combined observational and numerical approach using the aforementioned data and the HYbrid Coordinate Ocean Model (HYCOM). An important aspect of this research will be to develop a set of consistent metrics in order to assess the performance of the ocean model in simulating the upper ocean response to TC forcing. Successful completion of this research eventually could lead to improvements in forecast models that could lead to better intensity forecasts for tropical cyclones.