Deriving the effect of wind speed on clean marine aerosol optical properties using the A-Train satellites

Abstract

The relationship between "clean marine" aerosol optical properties and ocean surface wind speed is explored using remotely sensed data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the CALIPSO satellite and the Advanced Microwave Scanning Radiometer (AMSR-E) on board the AQUA satellite. Detailed data analyses are carried out over 15 regions selected to be representative of different areas of the global ocean for the time period from June 2006 to April 2011. Based on remotely sensed optical properties the CALIPSO algorithm is capable of discriminating "clean marine" aerosols from other types often present over the ocean (such as urban/industrial pollution, desert dust and biomass burning). The global mean optical depth of "clean marine" aerosol at 532 nm (AOD₅₃₂) is found to be 0.052 ± 0.038 (mean plus or minus standard deviation). The mean layer integrated particulate depolarization ratio of marine aerosols is 0.02 ± 0.016. Integrated attenuated backscatter and color ratio of marine aerosols at 532 nm were found to be 0.003 ± 0.002 sr⁻¹ and 0.530 ± 0.149, respectively. A logistic regression between AOD₅₃₂ and 10-m surface wind speed (<i>U</i>₁₀) revealed three distinct regimes. For <i>U</i>₁₀ ≤ 4 m s⁻¹ the mean CALIPSO-derived AOD₅₃₂ is found to be 0.02 ± 0.003 with little dependency on the surface wind speed. For 4 < <i>U</i>₁₀ ≤ 12 m s⁻¹, representing the dominant fraction of all available data, marine aerosol optical depth is linearly correlated with the surface wind speed values, with a slope of 0.006 s m⁻¹. In this intermediate wind speed region, the AOD₅₃₂ vs. <i>U</i>₁₀ regression slope derived here is comparable to previously reported values. At very high wind speed values (<i>U</i>₁₀ > 18 m s^&minus;1), the AOD₅₃₂-wind speed relationship showed a tendency toward leveling off, asymptotically approaching value of 0.15. The conclusions of this study regarding the aerosol extinction vs. wind speed relationship may have been influenced by the constant lidar ratio used for CALIPSO-derived AOD₅₃₂. Nevertheless, active satellite sensor used in this study that allows separation of maritime wind induced component of AOD from the total AOD over the ocean could lead to improvements in optical properties of sea spray aerosols and their production mechanisms.