Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean

Retrieved aerosol optical thickness (AOT) based on data obtained by the Sea-viewing Wide Field Sensor (SeaWiFS) is combined with surface wind speed, obtained at the European Centre for Medium-Range Weather Forecasts (ECMWF), over the North Pacific for September 2001. In this study a cloud-screening approach is introduced in an attempt to exclude pixels partly or fully covered by clouds. The relatively broad swath width through which the nadir-viewing SeaWiFS scanned over the North Pacific means that the AOT can be estimated according to a relatively large range of wind speeds for each of the scenes analyzed. The sensitivity in AOT due to sea salt and hygroscopic growth of the marine aerosols was also investigated. The validation approach is based on previous parameterization in combination with the environmental quantities wind speed, RH and boundary layer height (BLH), estimated at the ECMWF. In this study a factor of 2 higher AOT is obtained between the highest wind speed (12 m s− 1) and the lowest wind speed range (0–4 m s− 1) for September 2001 over remote ocean areas. This is supported by the validation of the results. The enhancement in AOT is explained by a combination of hygroscopic growth of the marine aerosols (~ 40%) and an increase in the sea salt particle mass concentrations (~ 60%), caused by a wind-driven water vapor and sea salt flux respectively. Reasonable agreement (within 1 to 52%) occurs also between satellite-retrieved aerosol optical thickness and AOT observed at two AERONET (AErosol RObotic NETwork) ground-based remote sensing stations. The overall variability is also observed by this comparison. Finally, possible reasons why relatively large standard deviations occur around the mean values of AOT, when all data is taken into consideration in the analyses for September 2001, are discussed.