Abstract
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 (U10) revealed three distinct regimes. For U10 ≤ 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 < U10 ≤ 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. U10 regression slope derived here is comparable to previously reported values. At very high wind speed values (U10 > 18 m s−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.
Highlights
Marine aerosols play significant role in global energy budget; they influence the planetary radiation balance directly by scattering and absorbing sunlight and indirectly by modifying cloud microphysical properties (Clarke and Kapustin, 2003; Murphy et al, 1998; Pierce and Adams, 2006)
Calculated global mean AOD532 is in a good agreement with AOD500 = 0.06, the most frequently occurring value of aerosol optical depth over the Central Pacific Ocean (Smirnov et al, 2003a, b) and within the range 0.02 < AOD550 < 0.067 of sixteen global models participated in the Aerosol Comparisons between Observations and Models (AeroCom) Experiment-A (Rind et al, 2009)
It was found that approximately 99.5 % of clean marine aerosol layers had AOD532 < 0.2, with AOD532 = 0.03 being the most frequently occurring value
Summary
Marine aerosols play significant role in global energy budget; they influence the planetary radiation balance directly by scattering and absorbing sunlight and indirectly by modifying cloud microphysical properties (Clarke and Kapustin, 2003; Murphy et al, 1998; Pierce and Adams, 2006). Based on the measurements from Minicoy Island in the Arabian Sea, Moorthy and Satheesh (2000) derived an exponential relationship between aerosol optical depth (AOD) and surface wind speed. Smirnov et al (2003a) studied the effects of surface wind speed on aerosol optical properties using ground based AErosol RObotic NETwork (AERONET) (Holben et al, 1998) located on Midway Island in the Pacific Ocean. Based on the measurements of aerosol properties at the Mace Head atmospheric research station under moderately windy conditions, Mulcahy et al (2008) established a power-law relationship between marine AOD and surface wind speed. The effect of wind speed on marine aerosol optical properties was thoroughly studied over the last several decades, comprehensive quantification of marine aerosols using remotely-sensed and ground-based measurement data suffered from a number of difficulties. Ties, such as accurate determination of the vertical location of aerosols and the ability to retrieve aerosol properties during the night as well as the day, CALIPSO can give new insight into the marine aerosol wind speed relationship
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