Abstract
Abstract. Reliable reference measurements over the ocean are essential for the evaluation and improvement of satellite- and model-based aerosol datasets. Within the framework of the Maritime Aerosol Network, shipborne reference datasets have been collected over the Atlantic Ocean since 2004 with Microtops Sun photometers. These were recently complemented by measurements with the multi-spectral GUVis-3511 shadowband radiometer during five cruises with the research vessel Polarstern. The aerosol optical depth (AOD) uncertainty estimate of both shipborne instruments of ±0.02 can be confirmed if the GUVis instrument is cross calibrated to the Microtops instrument to account for differences in calibration, and if an empirical correction to account for the broad shadowband as well as the effects of forward scattering is introduced. Based on these two datasets, a comprehensive evaluation of aerosol products from the Moderate Resolution Imaging Spectroradiometer (MODIS) flown on NASA's Earth Observing System satellites, the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat satellite, and the Copernicus Atmosphere Monitoring Service reanalysis (CAMS RA) is presented. For this purpose, focus is given to the accuracy of the AOD at 630 nm in combination with the Ångström exponent (AE), discussed in the context of the ambient aerosol type. In general, the evaluation of MODIS AOD from the official level-2 aerosol products of C6.1 against the Microtops AOD product confirms that 76 % of data points fall into the expected error limits given by previous validation studies. The SEVIRI-based AOD product exhibits a 25 % larger scatter than the MODIS AOD products at the instrument's native spectral channels. Further, the comparison of CAMS RA and MODIS AOD versus the shipborne reference shows similar performance for both datasets, with some differences arising from the assimilation and model assumptions. When considering aerosol conditions, an overestimation of AE is found for scenes dominated by desert dust for MODIS and SEVIRI products versus the shipborne reference dataset. As the composition of the mixture of aerosol in satellite products is constrained by model assumptions, this highlights the importance of considering the aerosol type in evaluation studies for identifying problematic aspects.
Highlights
Aerosol particles directly influence the Earth’s radiation budget through their interaction with solar and terrestrial radiation, and indirectly by modifying the optical properties of clouds (Boucher et al, 2013)
This section gives an overview of the methods used for aerosol classification (Sect. 3.1) and collocation of satellite and shipborne measurements (Sect. 3.2), and presents the statistical measures used for evaluation (Sect. 3.3), as well as the correction approach adopted for the GUVis aerosol product for better comparability to MIC aerosol optical depth (AOD) (Sect. 3.4)
An evaluation of the AOD product of the GUVis shadowband radiometer compared to the Microtops Sun photometer as reference was previously described by Witthuhn et al (2017), considering one cruise of the research vessel (RV) Polarstern (PS83)
Summary
Aerosol particles directly influence the Earth’s radiation budget through their interaction with solar and terrestrial radiation, and indirectly by modifying the optical properties of clouds (Boucher et al, 2013). Studies of aerosol effects on the climate system are based on radiative transfer models. Tegen, 2003; Christopher and Jones, 2007; Nabat et al, 2015) are the major contributors to the direct radiative effect of aerosol. Observations of aerosol load and optical properties with global coverage are required to improve our understanding of climate-relevant aerosol processes. Satellite remote sensing provides global observations of aerosol properties and the radiation budget (Chen et al, 2011; Kahn, 2012). These observations are key to quantify direct radiative effects of aerosols, in particular over the ocean, where only limited surface observations, e.g. from ships, are available (Haywood et al, 1999). Due to the sensitivity of the retrievals to factors such as instrumental calibra-
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