Introduction
Accurate modeling of tropical cyclone motion and intensity requires both realistic numerical models and accurate representation of meteorological fields through the depth of the troposphere on a variety of scales. Reynolds et al. (1994) and Zhu et al. (1996) indicated that, in the midlatitudes, most synoptic-scale errors in global numerical weather prediction models are not due primarily to model deficiencies, and that the largest forecast improvements are likely to be achieved by decreasing the analysis error. They also found evidence of significant model improvements in the tropics in the six years leading to 1992, but suggest that both model and analysis aimprovements are necessary for accurate forecasting there. To improve the analysis of initial conditions for tropical cyclone forecasting, between 1982 and 1996, the Hurricane Research Division (HRD) of the National Oceanographic and Atmospheric Administration (NOAA) conducted twenty "Synoptic-Flow" experiments to gather observations in the data-sparse tropical cyclone core and environment in the North Atlantic basin and assess the impact of the observations on numerical guidance. The experiments were typically conducted two to three days before the projected landfall of a mature hurricane on the coastline of the United States. Using the NOAA WP-3D (P3) research aircraft and Omega-based dropwindsondes (ODWs), vertical profiles of wind, temperature, and humidity were gathered below about 400 hPa within 1000 km of the tropical cyclone center. The dropwindsonde observations produced significant improvements in the primary numerical guidance for the Tropical Prediction Center (TPC) official track forecasts ( Burpee et al. 1996). The size of the improvements (16% - 30% for 12 - 60 h forecasts) were as large as those obtained over the previous 20 - 25 years, and suggested that operational dropwindsonde missions would be highly effective in reducing forecast errors on a regular basis.

In 1996, NOAA procured a Gulfstream-IV jet aircraft (G-IV) to conduct operational "Synoptic Surveillance" missions in the environments of hurricanes threatening the coastline of the continental United States, Puerto Rico, and the Virgin Islands. The G-IV is a low-wing, twin turbofan, pressurized aircraft which can fly at an altitude of 13 716 m (45 000 ft) with a cruise speed of 226.5 ms-1 (440 kn), with an operational range of 7412 km (4000 nm) (White et al. 1998). A new dropwindsonde, based on the Global Positioning System (GPS), has been developed by the National Center for Atmospheric Research (NCAR) to replace the ODW ( Franklin et al. 1997, Hock and Franklin 1998). Approximately ten to fifteen missions are expected in a typical year.

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