Establishing more truth in true winds
Shawn R. Smith, Mark A. Bourassa, and Ryan J. Sharp
Techniques are presented for the computation and quality control of true winds from vessels at sea. These techniques are developed to improve the accuracy of true winds (defined as a wind relative to true north and the fixed Earth) calculated by automated weather systems (AWS). Quality true winds are desired to achieve improvements in flux fields over the ocean, coupled ocean-atmospheric modeling, operational forecasting, and over-water climatologies. Correct computation of true winds and quality control methods are demonstrated for complete data. Additional methods are presented for estimating true winds from incomplete data. Recommendations are made for both existing data and future applications.
Quality control of AWS data at the World Ocean Circulation Experiment Surface Meteorological Data Center revealed that only 20% of studied vessels report all parameters necessary to compute a true wind. Required parameters include the ship's heading, course over the ground (COG), speed over the ground, wind vane zero reference, and wind speed and direction relative to the vessel. When any parameter is omitted or incorrect averaging is applied, AWS true wind data display systematic errors. Quantitative examples of several problems are shown in comparisons between collocated winds from research vessels and the NASA scatterometer (NSCAT). Errors also arise because meteorologists, oceanographers, and the Merchant Marine use different definitions for various wind parameters. Procedures are developed to identify observational shortcomings and to quantify the impact of these shortcomings in the determination of true wind observations.
Methods for estimating true winds are presented for situations where heading or COG is missing. Empirical analysis of two vessels with high quality AWS data showed these estimates to be more accurate when the vessel heading is available. Large differences between the heading and COG angles at low ship speeds make winds estimated using the course unreliable (direction errors exceeding 60°) for ship speeds less than 2.0 m s-1. The threshold where the direction difference between a course estimated and true wind reaches an acceptable level (±10°) depends upon the ship, winds, and currents in the vessel's region of operation.