Evidence of a threshold wind speed in tower-mounted scatterometer data

The normalized radar backscatter (σ^o) 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. There is additional evidence for a threshold wind speed in tower-mounted scatterometer data in an uncontrolled marine environment. In situ wind measurements and corresponding σ^o values obtained from YSCAT, an ultra-wideband scatterometer deployed on the Canada Centre for Inland Waters research tower at Lake Ontario, are used to detect and estimate the threshold wind speed. There is evidence for a detectable threshold in approximately half of the observations. The observed threshold wind speeds correspond well to theoretical threshold wind speeds.