Abstract
Abstract. This study presents and evaluates an updated algorithm for quantification of absorbing aerosols above clouds (AACs) from passive satellite measurements. The focus is biomass burning in the south-eastern Atlantic Ocean during the 2016 and 2017 ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign deployments. The algorithm retrieves the above-cloud aerosol optical depth (AOD) and underlying liquid cloud optical depth and is applied to measurements from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Visible Infrared Imaging Radiometer Suite (VIIRS) from 1997 to 2017. Airborne NASA Ames Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) and NASA Langley High Spectral Resolution Lidar 2 (HSRL2) data collected during ORACLES provide important validation for spectral AOD for MODIS and VIIRS; as the SeaWiFS mission ended in 2010, it cannot be evaluated directly. The 4STAR and HSRL2 comparisons are complementary and reveal performance generally in line with uncertainty estimates provided by the optimal estimation retrieval framework used. At present the two MODIS-based data records seem the most reliable, although there are differences between the deployments, which may indicate that the available data are not yet sufficient to provide a robust regional validation. Spatiotemporal patterns in the data sets are similar, and the time series are very strongly correlated with each other (correlation coefficients from 0.95 to 0.99). Offsets between the satellite data sets are thought to be chiefly due to differences in absolute calibration between the sensors. The available validation data for this type of algorithm are limited to a small number of field campaigns, and it is strongly recommended that such airborne measurements continue to be made, both over the southern Atlantic Ocean and elsewhere.
Highlights
Spaceborne monitoring of absorbing aerosols above clouds (AACs), typically smoke or mineral dust aerosols above liquid-phase clouds, has been a topic of increasing research interest in recent years. Yu and Zhang (2013) provide a review of the field, and Kacenelenbogen et al (2019) a more recent list of approaches to their quantification
The aerosol optical depth (AOD) dependence of the size distribution in the aerosol optical model assumed in the retrieval (Sect. 2.2) results in the wavelength dependence of AOD being a function of aerosol loading
These data are compared with two other sources; the first is the AE calculated over the same wavelength range from the 2016 and 2017 4STAR deployments
Summary
Spaceborne monitoring of absorbing aerosols above clouds (AACs), typically smoke or mineral dust aerosols above liquid-phase clouds, has been a topic of increasing research interest in recent years. Yu and Zhang (2013) provide a review of the field, and Kacenelenbogen et al (2019) a more recent list of approaches to their quantification. Their direct radiative effects can be very different from those above cloud-free surfaces (Hsu et al, 2003; Meyer et al, 2013; Zhang et al, 2014; Feng and Christopher, 2015), and they can have indirect and semi-direct effects on cloud formation, life cycle, and precipitation (Wilcox, 2012; Zhou et al, 2017) Their presence can lead to biases in retrieval of cloud optical depth (COD) and cloud effective radius (CER) if they are not accounted for, as they alter the brightness and spectral shape of the top-of-atmosphere (TOA) signal observed by passive sensors in a systematic way (Haywood et al, 2004). Global aerosol and cloud fields tend to show similar regional and seasonal variations year after year, and AACs frequently occur downwind of some important aerosol source regions These include, for example, smoke outflow from south-eastern Asia or southern Africa, as well as dust from the Sahara, Arabian Peninsula, and deserts in northeastern Asia This interannual repeatability means that AOD data sets can have a persistent coverage gap in these regions, which biases estimates of the total atmospheric aerosol burden and hinders aerosol transport analyses
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