Evidence of a Threshold Wind Speed in Tower-mounted Scatterometer Data

The normalized radar backscatter (σ) in scatterometer measurements over water is theorized to go to zero below a threshold wind speed due to insufficient friction between the wind and water to create capillary waves from which the radar signal scatters. Evidence of the threshold wind speed and a hysteresis effect have been observed in airship and wave tank data. The threshold wind speed is additionally evidenced in tower-mounted scatterometer data in an uncontrolled marine environment. In situ wind measurements and corresponding σ values obtained from YSCAT, an ultra-wideband scatterometer deployed on the Canada Centre for InlandWaters research tower at lake Ontario, are used to detect and estimate the threshold wind speed. A detectable threshold is evidenced in approximately 1/2 of the observations. The observed threshold wind speeds correspond well to theoretical threshold wind speeds.