2. Ocean initialization in the GFDL/URI coupled hurricane prediction system
b. New Loop Current and rings assimilation procedure for the Gulf of Mexico
In the new ocean initialization procedure, the LC position and structure are more realistically represented, and the ability to initialize LCEs (also known as warm-core rings (WCRs)) and cold-core rings (CCRs) in the GoM has been developed. In Falkovich et al. (2005), only the northernmost position of the LC was prescribed. Now, multiple points along the LC path can be specified, allowing the LC shape to be adjusted to match, for example, the observed shape given by near real-time satellite altimetry. Perhaps more importantly, the LC shape can be modified from the form suggested by altimetry (which may not be completely accurate) to a form that is more consistent with near real-time in situ temperature profiles, when available. Also, the LCPROFILE can be defined based on one or more of these in situ profiles.
LCEs (WCRs) and CCRs are prominent features that typically accompany the LC in the GoM. LCEs rotate anticyclonically in their geostrophically-adjusted state because they are warm-core and are located from near the ocean surface to ~1000-m depth, below which the horizontal temperature gradient is relatively small and the current velocity is relatively weak. A typical LCE has a diameter of ~200-300 km, a swirl speed of ~1.8-2 m s-1, and a westward translational speed of ~2-5 km day-1. LCEs are formed by eddy shedding of the LC. The physical processes that dictate the irregular time scale of the LC’s intrusion into the GoM and eddy shedding (~3-17 months) are not fully understood, but various modeling studies have made major strides towards understanding these processes (Oey et al. 2005 and references therein). Hurlburt and Thompson (1980), for example, conducted sensitivity studies using 1-layer, 1.5-layer reduced-gravity, and 2-layer models and concluded that eddy shedding is caused by horizontal shear instability of the internal mode and requires the planetary β-effect for westward spreading of the LC and subsequent eddy separation. Also, the presence of CCRs, which rotate cyclonically in their geostrophically-adjusted state for the same reason LCEs rotate anticyclonically, typically move clockwise around the periphery of the LC and are hypothesized to play a role in eddy shedding (e.g. Oey et al. 2005; Schmitz 2005). A typical CCR is ~50-150 km in diameter and extends ~1000 m deep (Oey et al. 2005).
In our feature-based approach, WCRs and CCRs are assumed to be elliptically-shaped, with major and minor axes that can be defined from available observations. Although neither WCRs nor CCRs are perfectly elliptical, this approximation is reasonable based on observations, and in the future, the feature-based approach could be improved to allow more variability in the ring shape. The temperature profile at the center of a given WCR (hereafter WCRnPROFILE, where “n” is an integer assigned to the WCR) can be defined either as a fraction of the LCPROFILE (e.g. 0.8, where the remaining 0.2 is the local climatological profile) or based on near real-time in situ temperature profile(s). For the temperature profile at the center of a given CCR (hereafter CCRnPROFILE), a negative temperature departure from the surrounding climatology (e.g. 2.0°C) can be assigned at a specified depth, currently set to 400 m (i.e. the 13th GDEM level). At the other 32 GDEM levels, the negative temperature departure from climatology is a fraction of the departure at the 13th level. This fraction for each of the 33 GDEM levels is 0, 0, 0, 0, 0, 0, 0.07, 0.15, 0.3, 0.45, 0.6, 0.75, 1, 0.9, 0.8, 0.65, 0.5, 0.35, 0.25, 0.2, 0.1, 0.05, 0.02, 0, 0, 0, 0, 0, 0, 0, 0, 0, and 0, respectively. Therefore, in CCRnPROFILE, the upper 75 m and depths at or below 1500 m are unmodified from climatology during the feature-based portion of data-assimilation, but recall that the OML is subsequently modified during the SST-assimilation phase. Alternatively, the CCRnPROFILE can be based on near real-time in situ temperature profile(s). For both WCRs and CCRs, once the ring size and the center profile are defined, cross-frontal sharpening is performed along the elliptical front between the center profile and the surrounding GoM temperature profiles.