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

Abstract. 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 = 0.78), the AOT retrieved by SEVIRI is 20.3 % 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 is in good agreement (R = 0.90). The cloud droplet effective radius (CER) retrieved by SEVIRI is consistently smaller than MODIS by 2.2 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.

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

“…The retrieved cloud properties are only weakly sensitive to assumptions on the properties of the absorbing aerosol, with biases lower than 6% in τ and 3% in r eff . The retrieved cloud properties match well MODIS retrievals and in‐situ measurements from the CLARIFY field campaign (Peers et al., 2021 ).…”

“…( 2019 ) algorithm accounts for absorbing aerosols located above clouds. The presence of absorbing aerosol above clouds has a small effect on retrieved cloud drop effective radius r eff , but cloud optical depth τ is underestimated by 35% when ignoring above‐cloud aerosol (Peers et al., 2021 ). The retrieved cloud properties are only weakly sensitive to assumptions on the properties of the absorbing aerosol, with biases lower than 6% in τ and 3% in r eff .…”

Sunlight-absorbing aerosol amplifies the seasonal cycle in low-cloud fraction over the southeast Atlantic

“…2). Comparisons to aircraft-derived above-cloud aerosol optical depths reveal a genuinely high bias to the satellite ACAOD estimates which is thought to reflect too high a single-scattering albedo assumption within the retrieval (Peers et al, 2021).…”

Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

“…Other examples are documented in Diamond et al (2018), Abel et al (2020). Pennypacker et al (2020) also document that ultra-clean days at Ascension are most common during September, although they only partially attribute these to a Southern Ocean origin.…”