2. Ocean initialization in the GFDL/URI coupled hurricane prediction system
The starting point for the ocean initialization in any operational GFDL model forecast is the Generalized Digital Environmental Model (GDEM) monthly ocean temperature and salinity climatology (Teague et al. 1990), which has 1/2° horizontal grid spacing and 33 vertical z-levels at 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 4500, 5000, and 5500-m depth. Other climatologies have been developed since this GDEM version, such as a newer GDEM climatology and a Levitus climatology that both have 1/4° grid spacing (NAVOCEANO 2006; Boyer and Levitus 1997), but tests with these climatologies in the GFDL model do not show increased skill over the original GDEM version used operationally (Yablonsky et al. 2006). The GDEM climatology is then modified by employing a feature-based modeling procedure that incorporates observations. This procedure is summarized in subsections 2a and 2b and is a major focus of this paper.
After feature-based modifications, the three-dimensional temperature and salinity fields are interpolated onto the 23 vertical sigma levels that are used in the subsequent integrations of the Princeton Ocean Model (POM) (Blumberg and Mellor 1987; Mellor 2006), which is the ocean component of the GFDL model and currently has 1/6° grid spacing (Bender et al. 2007).1 Next, the OML temperature is modified by assimilating the real-time daily SST data (with 1° grid spacing) that is used in the operational NCEP GFS global analysis (Reynolds and Smith 1994), and the POM is integrated for two days for dynamic adjustment, keeping the SST constant. This ocean model integration is referred to as “phase 1” (Falkovich et al. 2005). Then, in “phase 2”, the cold wake at the ocean surface and the currents produced by the hurricane prior to the beginning of the coupled model forecast are generated by a three-day integration of POM with the observed hurricane surface wind distribution provided by the NOAA’s National Hurricane Center (NHC) along the storm track (Falkovich et al. 2005).