3. Evaluation of the new initialization procedure
a. Ocean initialization using 15 September 2005 satellite and in-situ data
In advance of hurricane Rita, on 15 September 2005, 18 AXBTs were deployed successfully in the GoM, providing a unique opportunity to test the capabilities of the initialization procedure described herein. During the winter months, the LC and ring positions are often identifiable from the SST field (e.g. Schmitz 2005). On 15 September 2005, however, the SST is nearly homogeneous in the GoM (Fig. 1a), as is typical during the month of September (Fig. 1b). Therefore, another data source is required. Since the LC contains water originating from the Caribbean Sea, which has a considerably deeper OML base and thermocline than Gulf Common Water in the GoM, the LC (and associated rings) can be identified from the surrounding Gulf Common Water by the difference in the dynamic topography (i.e. sea surface height). One suitable data source, therefore, is satellite altimetry (e.g. Leben 2005), from which the daily surface height anomaly (SHA) is currently processed in near real-time (Fig. 1c). The composite SHA product used herein is created at Stennis Space Center in Mississippi by blending multiple ground tracks from the GFO and Jason-1 satellites and applying appropriate corrections, as described by Mainelli et al. (2007).
The altimetric SHA alone is insufficient for monitoring the LC because of the large contribution of the mean circulation to the dynamic topography in the eastern GoM (Leben 2005). To remedy this problem, it is necessary to either (1) have an independent estimate of the long-term altimetric mean sea surface height or (2) use the SHA to adjust the three-dimensional ocean temperature climatology. Scientists at NHC employ method (2) by utilizing the Stennis SHA product, a 1.5-layer reduced-gravity ocean model, and a blend of the GDEM and Levitus ocean climatologies to calculate the depth of the 20°C isotherm, the depth of the 26°C isotherm (hereafter d26) (Fig. 1d), and the tropical cyclone heat potential (hereafter TCHP) (AOML 2006; Goni et al. 1996), all of which are currently integrated into the Statistical Hurricane Intensity Prediction Scheme (SHIPS) (Demaria et al. 2005; Mainelli et al. 2007). The TCHP (also commonly referred to as oceanic heat content or OHC), which is a measure of the integrated heat content from the ocean’s surface to d26, provides a quantitative measure of the heat energy available to an approaching tropical cyclone (Leipper and Volgenau 1972; Shay et al. 2000).
Using the 15 September 2005 d26 map as a starting point (Fig. 1d), more accurate feature identification can be achieved by subjectively adjusting the LC path and ring size and locations based on the available AXBT profiles, whose geographical locations are indicated in Fig. 1d. Here, AXBTs #1-10 are used to adjust the parameters of the LC path (Table 1). This LC path is defined such that the LC axis bends at a specified location, effectively breaking the LC axis into two straight axes. Five points are then provided to create the LC path: (1) the northernmost LC intrusion, (2) the LC position as it enters the GoM, (3) the LC position as it merges with the Florida Current, (4) the western edge of the LC axis bend, and (5) the eastern edge of the LC axis. AXBTs #11-13 and #16 are used to adjust the parameters of an elliptical WCR’s perimeter (hereafter WCR1) (Table 2), and AXBTs #14-15 are used to adjust the parameters of an elliptical CCR’s perimeter (hereafter CCR1) (Table 2). In addition, the AXBT #13 and AXBT #14 profiles are used to define the upper 300 m of WCR1PROFILE and CCR1PROFILE, respectively.
For simplicity, we refer to the resulting modifications to the original GDEM climatological temperature after d26/AXBT-assimilation, cross-frontal sharpening, SST-assimilation, and a 2-day POM spinup (i.e. end of phase 1) as “data-assimilated”. In the following section, results from this data-assimilated simulation are evaluated and compared to another ocean data assimilation technique currently available. These results are also compared to a modified simulation (hereafter “data2-assimilated”) in which the AXBT #6 profile (instead of the Caribbean profile) is used to define the upper 400 m of LCPROFILE (Table 3). AXBT #6 is chosen for this purpose because the NHC d26 map (Fig. 1d) suggests that this AXBT is closest to the LC axis. In the future, the AXBT deployment strategy could be improved by dropping a higher concentration of AXBTs close to the perceived LC axis in an effort to obtain the best possible LCPROFILE.