Adjustment of Visually Observed Ship Winds (Beaufort Winds) in ICOADS

ABSTRACT The bias adjustment of visually estimated ship winds in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) is addressed through the comparison to the QuickSCAT scatterometer equivalent neutral winds. We assume that visually estimated winds and satellite scatterometer winds share similar characteristics, which are a function of stress rather than wind speed, and treat the estimated ship winds as equivalent neutral winds. Under such an assumption, we use statistical analyses to calculate the bias correction for estimated ship winds. Because observation practices vary by country and data provider, ICOADS identifies datasets by "deck" which is a number that allows for differentiating the source of the records (different deck numbers indicate different data collections provided to ICOADS, each which may contain one or more sources/countries). Three ICOADS decks 792, 926, and 992 contain the vast majority (~90%) of collocated visually estimated ship winds covering the time period November 1999-October 2009. The Root-Mean-Square difference between these visually estimated ship winds and scatterometer winds are 3.0ms-1, 2.8ms-1 and 2.9ms-1 for each major deck respectively. Following the methodology of Freilich (1997) and Freilich and Dunbar (1999), we numerically show that for lower wind speeds (0ms-1-5ms-1 in this case) that the random error in the component of the visually estimated ship winds causes an artificial appearance of an overestimation relative to satellite scatterometer winds. We also extend this statistical artifact test to test higher wind speeds (12ms-1-18ms-1 in this case) through a Monte Carlo approach. An apparent slight drop of the conditional sample means relative to reference line is shown to be a statistical artifact. These artificial biases are properly accounted in this study. A new bias correction, LMS correction, is calculated and also compared to prior corrections such as Lindau (1995). This new bias correction is available for wind speeds ranging from 0ms-1 to 17ms-1, because there are too few spatial and temporal collocated matches at wind speed greater than 17ms-1. We are limited in our ability to perform the adjustments required for intercallibration because when comparing visual winds to scatterometer winds the necessary wind speed observations are rare and small in magnitude.