Smoke and Clouds above the Southeast Atlantic: Upcoming Field Campaigns Probe Absorbing Aerosol’s Impact on Climate

Zuidema, Paquita; Redemann, Jens; Haywood, Jim M.; Wood, Robert; Piketh, S.; Hipondoka, Martin; Formenti, Paola

doi:10.1175/bams-d-15-00082.1

Cited by 196 publications

(217 citation statements)

References 2 publications

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“…Airborne measurements are extremely useful in providing information on aerosols' above-cloud properties. Several ongoing and planned airborne field campaigns will attempt to characterize the properties of biomass-burning aerosols over the South Atlantic Ocean (Zuidema et al, 2016). Planned measurements from the French Falcon 20 aircraft, equipped with a high-resolution lidar, an airborne sun photometer and a POLDER-like sensor, will notably be considered for a future validation of CALIOP DRM and POLDER abovecloud aerosol products.…”

Section: Discussionmentioning

confidence: 99%

Consistency of aerosols above clouds characterization from A-Train active and passive measurements

et al. 2017

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Abstract. This study presents a comparison between the retrieval of optical properties of aerosol above clouds (AAC) from different techniques developed for the A-Train sensors CALIOP/CALIPSO and POLDER/PARASOL. The main objective is to analyse the consistency between the results derived from the active and the passive measurements. We compare the aerosol optical thickness (AOT) above optically thick clouds (cloud optical thickness (COT) larger than 3) and their Ångström exponent (AE). These parameters are retrieved with the CALIOP operational method, the POLDER operational polarization method and the CALIOP-based depolarization ratio method (DRM) -for which we also propose a calibrated version (denominated DRM SODA , where SODA is the Synergized Optical Depth of Aerosols). We analyse 6 months of data over three distinctive regions characterized by different types of aerosols and clouds. Additionally, for these regions, we select three case studies: a biomass-burning event over the South Atlantic Ocean, a Saharan dust case over the North Atlantic Ocean and a Siberian biomass-burning event over the North Pacific Ocean. Four and a half years of data are studied over the entire globe for distinct situations where aerosol and cloud layers are in contact or vertically separated. Overall, the regional analysis shows a good correlation between the POLDER and the DRM SODA AOTs when the microphysics of aerosols is dominated by fine-mode particles of biomass-burning aerosols from southern Africa (correlation coefficient (R 2 ) of 0.83) or coarse-mode aerosols of Saharan dust (R 2 of 0.82). A good correlation between these methods (R 2 of 0.68) is also observed in the global treatment, when the aerosol and cloud layers are separated well. The analysis of detached layers also shows a mean difference in AOT of 0.07 at 532 nm between POLDER and DRM SODA at a global scale. The correlation between the retrievals decreases when a complex mixture of aerosols is expected (R 2 of 0.37) -as in the East Asia region -and when the aerosol-cloud layers are in contact (R 2 of 0.36). The correlation coefficient between the CALIOP operational method and POLDER is found to be low, as the CALIOP method largely underestimates the aerosol loading above clouds by a factor that ranges from 2 to 4.Potential biases on the retrieved AOT as a function of cloud properties are also investigated. For different types of scenes, the retrieval of above-cloud AOT from POLDER and from DRM are compared for different underlying cloud properties (droplet effective radius (r eff ) and COT retrieved with MODIS). The results reveal that DRM AOT vary with r eff . When accounting for r eff in the DRM algorithm, the consistency between the methods increases. The sensitivity study shows that an additional polarized signal coming from aerosols located within the cloud could affect the polarization method, which leads to an overestimation of the AOT retrieved with POLDER algorithm. In addition, the aerosols attached to or within the cloud can potentially impact the DRM...

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Section: Discussionmentioning

confidence: 99%

Consistency of aerosols above clouds characterization from A-Train active and passive measurements

et al. 2017

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“…These images are representative of the northeast Atlantic (Zuidema et al 2012;Nuijens et al 2017). The (Zuidema et al 2016). Radiosonde winds indicate west-northwestward boundary layer flow (yellow vector) deeper clouds spawn downdrafts capable of bringing down air that is drier than the nearsurface air, contributing to a lowering of the equivalent potential temperature (Zuidema et al 2012).…”

Section: Cold Pools From Convection Reaching the Mid-tropospherementioning

confidence: 99%

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

et al. 2017

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Pools of air cooled by partial rain evaporation span up to several hundreds of kilometers in nature and typically last less than 1 day, ultimately losing their identity to the large-scale flow. These fundamentally differ in character from the radiatively-driven dry pools defining convective aggregation. Advancement in remote sensing and in computer capabilities has promoted exploration of how precipitation-induced cold pool processes modify the convective spectrum and life cycle. This contribution surveys current understanding of such cold pools over the tropical and subtropical oceans. In shallow convection with low rain rates, the cold pools moisten, preserving the near-surface equivalent potential temperature or increasing it if the surface moisture fluxes cannot ventilate beyond the new surface layer; both conditions indicate downdraft origin air from within the boundary layer. When rain rates exceed $ 2 mm h À1 , convective-scale downdrafts can bring down drier air of lower equivalent potential temperature from above the boundary layer. The resulting density currents facilitate the lifting of locally thermodynamically favorable air and can impose an arc-shaped mesoscale cloud organization. This organization allows clouds capable of reaching 4-5 km within otherwise dry environments. These are more commonly observed in the northern hemisphere trade wind regime, where the flow to the intertropical 123Surv Geophys (2017) 38:1283-1305 https://doi.org/10.1007/s10712-017-9447-x convergence zone is unimpeded by the equator. Their near-surface air properties share much with those shown from cold pools sampled in the equatorial Indian Ocean. Cold pools are most effective at influencing the mesoscale organization when the atmosphere is moist in the lower free troposphere and dry above, suggesting an optimal range of water vapor paths. Outstanding questions on the relationship between cold pools, their accompanying moisture distribution and cloud cover are detailed further. Near-surface water vapor rings are documented in one model inside but near the cold pool edge; these are not consistent with observations, but do improve with smaller horizontal grid spacings.

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“…To start to remedy these difficulties, four major campaigns in 2016-7 may yield greatly improved understanding of this complex environment (Zuidema et al, 2016). Modelling the stratocumulus-to-cumulus transition (SCT) is challenging because the structure of the aerosols and clouds is often complex, with multiple thin layers of aerosol and cloud which may or may not be coupled together.…”

Section: Introductionmentioning

confidence: 99%

Large simulated radiative effects of smoke in the south-east Atlantic

Gordon¹,

Field²,

Abel³

et al. 2018

Preprint

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Abstract. A 1200 km-square area of the tropical south Atlantic Ocean near Ascension Island is studied with the HadGEM climate model at convection-permitting and global resolutions for a ten-day case study period in August 2016. During the simulation period, a plume of biomass burning smoke from Africa moves into the area and mixes into the clouds. We examine the interaction of the smoke with clouds and find it has substantial instantaneous direct, indirect and semi-direct radiative effects, which vary in magnitude between model configurations. The smoke aerosol is simulated realistically, and is found to affect dynamical, microphysical and radiative properties of the 10 atmosphere across the region. The model captures the large-scale horizontal transport of the aerosol adequately, approximately reproducing a transition from pristine to polluted conditions. However, for some of the simulation, the smoke is around 1km too low in altitude and therefore mixes into the clouds earlier than observed. Fire emissions increase the total aerosol burden by a factor 3.7 and cloud droplet number concentrations by a factor of 3, which is consistent with MODIS observations. Strong localised perturbations to heating and cooling rates due to the smoke affect the dynamics: in the regional model, the inversion relative to a simulation without smoke aerosol, when averaged over the polluted period. This increase is uncertain, and smaller in the global model. It is mostly due to radiatively driven dynamical changes: the reduced entrainment of dry air from above the cloud layer, rather than precipitation suppression by aerosol. 20The smoke has substantial direct radiative effects of +11.4 W m . However, the radiative effects are sensitive to the model set-up: the semi-direct effect is smaller in the global model, and also in a simulation with the Kogan (2013) parameterisation of autoconversion and accretion instead of the default, from Khairoutdinov & Kogan (2002). Furthermore, we simulate a liquid water path that is biased high compared to 25 satellite observations by 22% on average, and this leads to high estimates of the domain-averaged aerosol direct effect and the effect of the aerosol on cloud albedo. With these caveats, we simulate a large net cooling across the region, of −27.6 W m

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Smoke and Clouds above the Southeast Atlantic: Upcoming Field Campaigns Probe Absorbing Aerosol’s Impact on Climate

Cited by 196 publications

References 2 publications

Consistency of aerosols above clouds characterization from A-Train active and passive measurements

Consistency of aerosols above clouds characterization from A-Train active and passive measurements

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

Large simulated radiative effects of smoke in the south-east Atlantic

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