Seasonally transported aerosol layers over southeast Atlantic are closer to underlying clouds than previously reported

Rajapakshe, Chamara; Zhang, Zhibo; Yorks, John E.; Yu, Hongbin; Tan, Qian; Meyer, Kerry; Platnick, S.; Winker, David M.

doi:10.1002/2017gl073559

Cited by 62 publications

(80 citation statements)

References 48 publications

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“…While it has been well documented that emission injection heights of fires require better characterization (Pfister et al, ), characterizing their vertical profiles over the WNAO is of great importance with regard to vertical heating rates and stability (Taubman et al, ) and interactions with clouds. Studies in other regions such as the southeastern Atlantic (Rajapakshe et al, ) and northeastern Pacific Ocean (Mardi et al, ) have looked at vertical characteristics such as the distance between plumes and cloud top heights. Similar work is warranted for the WNAO in pursuit of better understanding of regional ACI and subsequent effects on radiative forcing.…”

Section: Synthesis Of Results and Future Outlookmentioning

confidence: 99%

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

et al. 2020

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Decades of atmospheric research have focused on the Western North Atlantic Ocean (WNAO) region because of its unique location that offers accessibility for airborne and ship measurements, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews these scientific investigations for the WNAO region, including the East Coast of North America and the island of Bermuda. Over 50 field campaigns and long‐term monitoring programs, in addition to 715 peer‐reviewed publications between 1946 and 2019, have provided a firm foundation of knowledge for these areas. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO atmospheric research into eight major categories, with some studies fitting into multiple categories (relative %): aerosols (25%); gases (24%); development/validation of techniques, models, and retrievals (18%); meteorology and transport (9%); air‐sea interactions (8%); clouds/storms (8%); atmospheric deposition (7%); and aerosol‐cloud interactions (2%). Recommendations for future research are provided in the categories highlighted above.

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Section: Synthesis Of Results and Future Outlookmentioning

confidence: 99%

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

et al. 2020

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“…Transport System (CATS) (Yorks et al, 2014), shows that the CALIOP algorithm probably overestimates the base of the aerosol layer by 500 m (Rajapakshe et al, 2017). Deaconu et al (2017) showed that the CALIOP operational algorithm underestimates the AOT above clouds with a factor of 2 to 4 depending on the aerosol type, when compared to other methods dedicated for aerosol above cloud retrievals -the POLDER polarisation method (Waquet et al, 2009) and the CALIOP depolarisation ratio method DRM (Deaconu et al, 2017;Hu et al, 2007).…”

Section: Radiative Transfer Calculation and Synergy Caliop / Poldermentioning

confidence: 99%

Satellite inference of water vapor and aerosol-above-cloud combined effect on radiative budget and cloud top processes in the Southeast Atlantic Ocean

Deaconu

Ferlay

Waquet

et al. 2019

Preprint

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Aerosols have a direct effect on the Earth's radiative budget and can also affect the cloud development and lifetime, and the aerosols above clouds (AAC) are particularly associated with high uncertainties in global climate models. Therefore, it is prerequisite to improve the description and understanding of these situations. During the austral winter, large loadings of biomass burning aerosols originating from fires in the southern African subcontinent are lifted and transported westwards, 15 across the Southeast Atlantic Ocean. The negligible wet scavenging of these absorbing aerosols leads to a near-persistent smoke layer above one of the largest stratocumulus cloud deck on the planet. Therefore, the Southeast Atlantic region is a very important area for studying the impact of above cloud absorbing aerosols, their radiative forcing and their possible effects on clouds.In this study we aim to analyse and quantify the effect of smoke loadings on cloud properties using a synergy of 20 different remote sensing technics from A-Train retrievals (methods based on the passive instruments POLDER and MODIS and the operational method of the spaceborne lidar CALIOP), collocated with ERA-Interim re-analysis meteorological profiles.To analyse the possible mechanisms of AACs effects on cloud properties, we developed a high and low aerosol loading approach, that consists in evaluating the change in radiative quantities (i.e. cloud top cooling, heating rate vertical profiles) and cloud properties with the smoke loading. During this analysis, we account for the variation in the meteorological conditions 25 over our sample area.The results show that the region we focus on is primarily under the energetic influence of absorbing aerosols, leading to a significant positive shortwave direct effect at the top of the atmosphere. For larger loads of AACs, clouds are optically thicker, with an increase in liquid water path of 20 g.m -2 and lower cloud top altitudes by 100 m. These results do not contradict the semi-direct effect of above cloud aerosols, explored in previous studies. Furthermore, we observe a strong correlation 30 between the aerosol and the water vapor loadings, which has to be accounted for. A detailed analysis of the heating rates shows that the absorbing aerosols are 90 % responsible for warming the ambient air where they reside, with approximately +5.7 K/day, while the accompanying water vapour above clouds has a longwave effect of +4.7 K/day (equivalent to 7% decrease) on the cloud top cooling. We infer that this decreased cloud top cooling in particular, in addition with the higher humidity above the clouds, might modify the cloud top entrainment rate and its effect, leading to thicker clouds. Therefore, smoke (the 35 Atmos. Chem. Phys. Discuss., https://doi.

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“…However, contact between the base of smoke layers and the cloud-topped MBL is highly variable and difficult to constrain with satellite remote sensing (e.g. (Rajapakshe et al, 2017)). At Ascension Island (ASI, details below), there is frequently heavy smoke intrusion into the 10 MBL earlier in the burning season (June-August) than expected given the later (September-October) peak in aerosol optical depth (Zuidema et al, 2018).…”

mentioning

confidence: 99%

Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

Pennypacker¹,

Diamond²,

Wood³

2019

Preprint

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We study forty-one days with daily median surface accumulation mode aerosol particle concentrations below 50 cm -3 (ultra-clean conditions) observed at Ascension Island (7.9°S, 14.4°W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol-cloud-radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June -October). That means that these critical months can feature both the highest surface aerosol numbers, 10 from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016 -April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at 15 Ascension Island follow similar isobaric back-trajectories, the variability in this entrainment is likely closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Finally, surface drizzle rates, frequencies and accumulation -as well as retrievals of liquid water path -all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the 20 backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol-cloud interactions in the region.

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Seasonally transported aerosol layers over southeast Atlantic are closer to underlying clouds than previously reported

Cited by 62 publications

References 48 publications

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

Satellite inference of water vapor and aerosol-above-cloud combined effect on radiative budget and cloud top processes in the Southeast Atlantic Ocean

Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

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