2005 LAYERED OCEAN MODEL WORKSHOP

Past observational, theoretical and numerical studies have substantiated the fact that tropical cyclone intensity is sensitive to the upper ocean heat potential in the directly forced region of the storm. Observational and numerical investigation of ocean response during hurricane Gilbert in the western Gulf of Mexico suggested that the upper ocean heat and mass budgets have a strong dependence on the entrainment mixing scheme. Thus, one of the major uncertainties in a coupled hurricane ocean forecasting model is the choice of mixing scheme as significant oceanic mixed layer cooling and deepening during a storm passage is due to mixing. Using the Hybrid Coordinate Ocean Model (HYCOM) configured with different entrainment mixing schemes, this issue is investigated in detail. Available high-resolution oceanic observations during the passage of three tropical cyclones (Gilbert 1988, Isidore 2002 and Lili 2002) in the Atlantic provide the data set to evaluate model results. Our objective is to identify the best mixing schemes for different forcing characteristics and background oceanographic conditions by comparing 702 temperature, 102 salinity and 210 current profiles acquired during these storms to model simulated results.

Realistic pre-storm ocean conditions for the Gilbert case are derived from objectively analyzed three dimensional observations of temperature and salinity blended with climatology in the Gulf of Mexico. There is excellent comparison of initial conditions to the pre-storm data as indicated by the sea surface height and current magnitudes. Initial conditions for hurricane Isidore are derived from the 0.08 degree North Atlantic HYCOM that assimilates altimetric height. The simulated subsurface structure is validated with pre-storm in situ data from expendable probes. Surface wind fields derived using buoy and aircraft measurements are blended with the large scale atmospheric model fields to provide boundary layer forcing every 3 hours for the three storms. While Gilbert (1988) is treated separately with boundary conditions provided from climatology, Isidore and Lili simulations are combined into a single continuous simulation due to their proximity in space and time with open boundary conditions derived from the North Atlantic HYCOM. Upper ocean response during these storms will be discussed and preliminary comparisons of the model results for five mixing parameterizations with profiler data will be presented in this paper.