Observation of absorbing aerosols above clouds over the South-East Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

Abstract

<p><strong>Abstract.</strong> To evaluate the SEVIRI retrieval for aerosols above clouds presented in Part 1 of the companion paper, the algorithm is applied over the South East Atlantic Ocean during the CLARIFY-2017 field campaign period. The first step of our analysis compares the retrieved aerosol and cloud properties against equivalent products from the MODIS MOD06ACAERO retrieval (Meyer et al., 2015). While the correlation between the two satellite retrievals of the above-cloud Aerosol Optical Thickness (AOT) is good (R = 0.75), the AOT retrieved by SEVIRI is 16.5 % smaller than that obtained from the MODIS retrieval. This difference in AOT is attributed mainly to the more absorbing aerosol model assumed for the SEVIRI retrieval compared to MODIS. The underlying Cloud Optical Thickness (COT) derived from the two satellites are in good agreement (R = 0.90). The Cloud droplet Effective Radius (CER) retrieved by SEVIRI is consistently smaller than MODIS by ~2 µm, which is mainly caused by the use of different spectral bands of the satellite instruments. In the second part of our analysis, we compare the forecast water vapour profiles used for the SEVIRI atmospheric correction as well as the aforementioned aerosol and cloud products with in situ measurements made from the Facility for Airborne Atmospheric Measurements (FAAM) aircraft platform during the CLARIFY-2017 campaign. Around Ascension Island, the column water vapour used to correct the SEVIRI signal is overestimated by 3.1 mm in the forecast compared to that measured by dropsondes. However, the evidence suggests that the accuracy of the atmospheric correction improves closer to the African coast. Consistency is observed between the SEVIRI above-cloud AOT and in situ measurements (from cavity ring-down spectroscopy instruments) when the measured single scattering albedo is close to that assumed in the retrieval algorithm. On the other hand, the satellite retrieval overestimates the AOT when the assumed aerosol model is not absorbing enough. Consistency is also found between the cloud properties retrieved by SEVIRI and the CER measured by a cloud droplet probe and the liquid water path derived from a microwave radiometer. Despite the instrumental limitations of the geostationary satellite, the consistency obtained between SEVIRI, MODIS and the aircraft measurements demonstrates the ability of the retrieval in providing additional information on the temporal evolution of the aerosol properties above clouds.</p>

Empirically derived parameterizations of the direct aerosol radiative effect based on ORACLES aircraft observations

“…Our paper is focused on instantaneous DARE and stops short of providing an "all-ORACLES" (diurnally integrated) DARE estimate. A promising approach in this regard is to use geostationary satellite retrievals of cloud and aerosol properties (Peers et al, 2020) in conjunction with in situ aircraft data and radiative transfer calculations. Alternatively, one can use the satellite radiances to extrapolate from the spatially and temporally limited aircraft observations to obtain regional estimates of the diurnally integrated DARE, circumventing the satellite retrievals.…”

Spatiotemporal Heterogeneity of Aerosol and Cloud Properties Over the Southeast Atlantic: An Observational Analysis

“…SEVI-RI shows the largest bias due to significantly underestimated COD retrievals, particularly for larger SSFR CODs. Peers et al (2021) also found a lower mean COD from SEVIRI than MODIS Meyer at a 0.1° spatial resolution comparison over the southeast Atlantic in August and September 2017, especially for larger COD values.…”

“…These shortcomings have motivated the algorithm development of ACAOD products (e.g., Jethva et al, 2013;Meyer et al, 2013Meyer et al, , 2015Sayer et al, 2016) and ACAOD intercomparisons among active and passive satellite platforms (e.g., Deaconu et al, 2017;Jethva et al, 2014). Airborne measurements useful for satellite ACAOD evaluations were limited prior to the recent field deployments over the southeast Atlantic (e.g., Jethva et al, 2016;Sayer et al, 2016), but the ORACLES and the CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017 (CLARIFY-2017) field experiments have led to robust quantitative evaluations of satellite ACAOD (Jethva et al, 2018;LeBlanc et al, 2020;Peers et al, 2021;.…”

Realism of Lagrangian Large Eddy Simulations Driven by Reanalysis Meteorology: Tracking a Pocket of Open Cells Under a Biomass Burning Aerosol Layer

“…The ORAC retrieval gives systematically lower τ values than the Peers retrieval (Figures 4b, 4d, and 4f). This low bias is small on the first day and larger on the second day, when it assumes values that are by and large consistent with an underestimation of 35% caused by ignoring above‐cloud aerosol (Peers et al., 2021).…”