Methodology for the Determination of 12-Foot Sea Radii for Tropical Cyclones

The advent of satellite altimetry and ocean surface vector wind observations has significantly increased the amount of observational data available to marine meteorologists. The Tropical Analysis and Forecast Branch (TAFB) at the National Hurricane Center (NHC) actively incorporates these cutting edge data sets into their ocean analyses, providing the most accurate assessment of the current sea state in their tropical North Atlantic and Northeast Pacific areas of responsibility. This poster will detail the use of the recently-enhanced observational dataset available to TAFB for the determination of the 12-ft sea radii associated with a tropical cyclone (TC).

The Jason-1 polar-orbiting satellite observes global significant wave height data at 3-n mi horizontal resolution with a precision of 0.1 ft. Observations from the Jason altimeter provide wave height information in data-sparse regions of the tropical Atlantic and East Pacific oceans. To “fill in” the sea state analysis in other locations, gridded wave heights from a short-term forecast (most often a six-hour forecast) of a numerical wave model are used as a first-guess field and adjusted accordingly for any reliable observations. One such model used is the Geophysical Fluid Dynamics Laboratory (GFDL) version of the Wave Watch model, which incorporates a more detailed initial wind field for a TC than other numerical wave models. Over data-sparse regions such as the open waters of the tropical East Pacific or Atlantic oceans, ship data are the only in-situ observations available to assist in analyzing the 12-ft sea radii for a TC. Once a TC moves into the Gulf of Mexico, the southwest North Atlantic, or the Caribbean Sea, the observational networks are denser with NOAA Buoys in the offshore waters and Coastal Monitoring Automated Network (CMAN) sites in the near shore waters, primarily along the Gulf Coast. These dense observational networks provide enough data to ensure a much more accurate analysis of the 12-ft sea radii for the TC as compared to instances when a TC is located in the open ocean. Additionally, the dense observations in the western portion of the Atlantic basin allow for a better assessment of the performance of the various numerical models. In the future, TAFB should be able to perform a real-time evaluation of aircraft-based wave height information from the Joint Hurricane Test Bed Scanning Radar Altimeter Project, which could further improve the estimation of the 12-foot sea radii.

The active utilization of remotely-sensed ocean observations allows for a more exact analysis of the initial sea state in regions of little or no in-situ data and provides an “accuracy check” for ship and buoy observations that may contain errors. In turn, more reliable sea state analyses permit a better appraisal of the quality of numerical wave model output. While long-term verification is difficult due to a lack of historic data, we believe all of these advances have significantly improved the estimation of the 12-ft sea radii for TCs over previous methods.