Abstract
Abstract. Aerosol–cloud interaction continues to be one of the leading uncertain components of climate models, primarily due to the lack of adequate knowledge of the complex microphysical and radiative processes of the aerosol–cloud system. Situations when light-absorbing aerosols such as carbonaceous particles and windblown dust overlay low-level cloud decks are commonly found in several regions of the world. Contrary to the known cooling effects of these aerosols in cloud-free scenario over darker surfaces, an overlapping situation of the absorbing aerosols over the cloud can lead to a significant level of atmospheric absorption exerting a positive radiative forcing (warming) at the top of the atmosphere. We contribute to this topic by introducing a new global product of above-cloud aerosol optical depth (ACAOD) of absorbing aerosols retrieved from the near-UV observations made by the Ozone Monitoring Instrument (OMI) onboard NASA's Aura platform. Physically based on an unambiguous “color ratio” effect in the near-UV caused by the aerosol absorption above the cloud, the OMACA (OMI above-cloud aerosols) algorithm simultaneously retrieves the optical depths of aerosols and clouds under a prescribed state of the atmosphere. The OMACA algorithm shares many similarities with the two-channel cloud-free OMAERUV algorithm, including the use of AIRS carbon monoxide for aerosol type identification, CALIOP-based aerosol layer height dataset, and an OMI-based surface albedo database. We present the algorithm architecture, inversion procedure, retrieval quality flags, initial validation results, and results from a 12-year long OMI record (2005–2016) including global climatology of the frequency of occurrence, ACAOD, and aerosol-corrected cloud optical depth. A comparative analysis of the OMACA-retrieved ACAOD, collocated with equivalent accurate measurements from the HSRL-2 lidar for the ORACLES Phase I operation (August–September 2016), revealed a good agreement (R = 0.77, RMSE = 0.10). The long-term OMACA record reveals several important regions of the world, where the carbonaceous aerosols from the seasonal biomass burning and mineral dust originated over the continents are found to overlie low-level cloud decks with moderate (0.3 < ACAOD < 0.5, away from the sources) to higher levels of ACAOD (> 0.8 in the proximity to the sources), including the southeastern Atlantic Ocean, southern Indian Ocean, Southeast Asia, the tropical Atlantic Ocean off the coast of western Africa, and northern Arabian sea. No significant long-term trend in the frequency of occurrence of aerosols above the clouds and ACAOD is noticed when OMI observations that are free from the “row anomaly” throughout the operation are considered. If not accounted for, the effects of aerosol absorption above the clouds introduce low bias in the retrieval of cloud optical depth with a profound impact on increasing ACAOD and cloud brightness. The OMACA aerosol product from OMI presented in this paper offers a crucial missing piece of information from the aerosol loading above cloud that will help us to quantify the radiative effects of clouds when overlaid with aerosols and their resultant impact on cloud properties and climate.
Highlights
Aerosol–cloud interactions continue to be the most significant source of uncertainty in estimating the role of aerosols and clouds in Earth’s changing radiation budget (IPCC, 2013)
Radiative transfer simulations show that while Lambert equivalent reflector (LER) is directly proportional to COD, the layers of absorbing aerosols above cloud produces higher magnitudes of UV aerosol index (UVAI) that depend on the above-cloud aerosol loading (AOD), aerosol model, and cloud brightness (COD)
All OMACA Level 2 orbital data (2005–2016) for the respective regions and the two aerosol types were accumulated separately and subsequently averaged as a function of corresponding aerosol absorption optical depth (AAOD) bins of a sampling size of 5000 retrievals. For both aerosol types, increasing the magnitude of negative bias in the retrieval of apparent COD is related to AAOD, suggesting the impact of aerosol absorption on the retrieved COD when the presence of absorbing aerosols is ignored in the inversion
Summary
Aerosol–cloud interactions continue to be the most significant source of uncertainty in estimating the role of aerosols and clouds in Earth’s changing radiation budget (IPCC, 2013). An important aspect of the problem is when the light-absorbing aerosols such as biomass burning generated carbonaceous particles and windblown mineral dust overlay low-level cloud decks Such situations are commonly observed from satellites over several oceanic and continental regions of the world on daily to seasonal scales (Devasthale and Thomas, 2011; AlfaroContreras et al, 2016). The development of several independent algorithms that quantify aerosol loading above cloud from satellite-based active, as well passive, sensors has been a breakthrough These techniques have shown the potential to retrieve above-cloud aerosol optical depth (ACAOD) using measurements from different A-train sensors (Jethva et al, 2014). Using an iterative optimal estimation approach applied to the MODIS observations, Meyer et al (2015) and Sayer et al (2016) have developed multispectral algorithms to simultaneously retrieve ACAOD, COD, and cloud effective radius for scenes with absorbing aerosols over clouds.
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