Our current method for the generation of synthetic observations is to define observations of pressure, temperature, and wind at specific points around the TC, as represented by a Rankine wind vortex. The details of the Rankine vortex are defined by the parameters in the warning message: the location of the TC; the radii of the 34, 50, and 65 kt winds in each of four quadrants; and the radius and intensity of the maximum wind. Synthetic observations are placed at 41 points around the TC, with 1 point at the storm center, and then 8 points on each of 5 different radii out from the storm, at ½-, 1-, 2-, 3-, and 4-degrees from the center of the storm. For weak systems, an additional 8 points are included at 6-degrees from the TC center. At each of these locations, synthetic wind observations are defined at 1000, 965, 925, 850, 775, 700, 600, 500, and 400 hPa; with the height and temperature specified only at 1000 hPa. Prior to inserting the observations within NAVDAS, the TC circulation in the COAMPS-TC first-guess field is relocated to the observed TC location. During the NAVDAS analysis, the geostrophic constraint is relaxed and the correlation lengths are shortened in the vicinity of the tropical cyclone. By initializing the tropical cyclone circulation in this manner, we have found that while the initial conditions created often exhibit the general structure of the observed TC, our ability to more precisely fit the observed structure was occasionally hampered by specifying the synthetic observations at fixed locations relative to the TC, and that the initial circulation within the TC vortex was often unbalanced.
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