Data-assimilative Ocean Analyses that Accurately Represent the Initial Ocean State are Essential to Achieve Realistic HWRF Intensity Forecasts of Hurricane Michael

By varying only the initial ocean state provided to the HWRF hurricane prediction model,
the importance of realistic ocean model initialization to achieve accurate intensity
forecasts is demonstrated for Hurricane Michael (2018). Using the HWRF version
coupled to the HYbrid Coordinate Ocean Model (HYCOM), the same ocean model is
used to generate multiple initial ocean states. The most accurate initialization is provided
by a data-assimilative ocean analysis product, specifically the NOAA global Real-Time
Ocean Forecast System (RTOFS) produced by the Environmental Modeling Center. That
product reproduces key properties of the observed initial ocean state that contributed to
the rapid intensification of this storm. These properties include anomalously warm SST
throughout the Gulf, a strong warm-core anticyclonic eddy present in the east-central
Gulf that previously detached from the now-retracted Loop Current, and a weaker
warm-core anticyclonic eddy present in the extreme northeastern Gulf. The resulting
pre-storm SST and ocean heat content are generally much above normal near the storm
track, particularly north of ~25ºN. Forecast cycles using this initial state reveal that rapid
intensification (RI) begins as the model storm approaches 25ºN and interacts with the
strong anticyclonic eddy, and then continues to reach category 4 at landfall. To isolate the
contribution of the anonymously warm ocean conditions, the same forecast cycles are
repeated using a balanced climatological initial ocean state. In this case, RI does not
occur and the storms only reach category 1 or minimal category 2 intensity at landfall. To
demonstrate the importance of ocean data assimilation in generating a realistic initial
state, HWRF-HYCOM is initialized by the output from an unconstrained HYCOM
simulation. This simulation generally underestimates SST and ocean heat content, while
the strong warm-core anticyclonic eddy is not present and the Loop Current remains
retracted. RI again does not occur, and category 1 storms are predicted at landfall.
Accurate initialization of the ocean for coupled hurricane prediction requires the
simultaneous assimilation of satellite altimetry to constrain the horizontal structure of
ocean currents and eddies, ocean profilers at sufficient horizontal resolution to constrain
the temperature, salinity, and stratification profiles within these features, and satellite and
in-situ SST to correct mixed layer temperatures. This work is supported in part by
Hurricane Supplemental funding.