The North Pacific Experiment (NORPEX-98): Targeted Observations for Improved North American Weather Forecasts

Abstract

The objectives and preliminary results of an interagency field program, the North Pacific Experiment (NORPEX), which took place between 14 January and 27 February 1998, are described. NORPEX represents an effort to directly address the issue of observational sparsity over the North Pacific basin, which is a major contributing factor in short-range (less than 4 days) forecast failures for land-falling Pacific winter-season storms that affect the United States, Canada, and Mexico. The special observations collected in NORPEX include approximately 700 targeted tropospheric soundings of temperature, wind, and moisture from Global Positioning System (GPS) dropsondes obtained in 38 storm reconnaissance missions using aircraft based primarily in Hawaii and Alaska. In addition, wind data were provided every 6 h over the entire North Pacific during NORPEX, using advanced and experimental techniques to extract information from multispectral geostationary satellite imagery. Preliminary results of NORPEX data impact studies using the U.S. Navy and National Weather Service forecast models include reductions of approximately 10% in mean 2-day forecast error over western North America (30°?60°N, 100°?130°W) from assimilation of targeted dropsonde and satellite wind data (when measured against control forecasts that contain no special NORPEX observations). There are local reductions of up to 50% in 2-day forecast error for individual cases, although some forecasts are degraded by the addition of the special dropsonde or satellite wind data. In most cases, the positive impact of the targeted dropsonde data on short-range forecast skill is reduced when the full set of advanced satellite wind data is already included in the model analyses. The NORPEX dataset is being used in research to improve objective methods for targeting observations, to study the "mix" of in situ and space-based observations, and to understand the structure and dynamics of fast-growing errors that limit our ability to provide more accurate forecasts of Pacific winter storms.