Investigation of short-term effective radiative forcing of fire aerosols over North America using nudged hindcast ensembles

Liu, Yawen; Zhang, Kai; Qian, Yun; Wang, Yuhang; Zou, Yufei; Song, Yongjia; Wan, Hui; Liu, Xiaohong; Yang, Xiu‐Qun

doi:10.5194/acp-18-31-2018

Cited by 18 publications

(16 citation statements)

References 54 publications

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“…of large-scale clouds are parameterized using the prognostic cloud fraction and prognostic condensate (PC2) scheme (Wilson et al, 2008a, b) with modifications described in Morcrette (2012). The atmospheric boundary layer is parameterized using the turbulence closure scheme of Lock et al (2000) with modifications described in Lock (2001) and Brown et al (2008).…”

Section: Model Detailsmentioning

confidence: 99%

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The decomposition of cloud–aerosol forcing in the UK Earth System Model (UKESM1)

Grosvenor

Carslaw

2020

Atmos. Chem. Phys.

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Abstract. Climate variability in the North Atlantic influences processes such as hurricane activity and droughts. Global model simulations have identified aerosol–cloud interactions (ACIs) as an important driver of sea surface temperature variability via surface aerosol forcing. However, ACIs are a major cause of uncertainty in climate forcing; therefore, caution is needed in interpreting the results from coarse-resolution, highly parameterized global models. Here, we separate and quantify the components of the surface shortwave effective radiative forcing (ERF) due to aerosol in the atmosphere-only version of the UK Earth System Model (UKESM1) and evaluate the cloud properties and their radiative effects against observations. We focus on a northern region of the North Atlantic (NA) where stratocumulus clouds dominate (denoted the northern NA region) and a southern region where trade cumulus and broken stratocumulus dominate (southern NA region). Aerosol forcing was diagnosed using a pair of simulations in which the meteorology is approximately fixed via nudging to analysis; one simulation has pre-industrial (PI) and one has present-day (PD) aerosol emissions. This model does not include aerosol effects within the convective parameterization (but aerosol does affect the clouds associated with detrainment) and so it should be noted that the representation of aerosol forcing for convection is incomplete. Contributions to the surface ERF from changes in cloud fraction (fc), in-cloud liquid water path (LWPic) and droplet number concentration (Nd) were quantified. Over the northern NA region, increases in Nd and LWPic dominate the forcing. This is likely because the already-high fc there reduces the chances of further large increases in fc and allows cloud brightening to act over a larger region. Over the southern NA region, increases in fc dominate due to the suppression of rain by the additional aerosols. Aerosol-driven increases in macrophysical cloud properties (LWPic and fc) will rely on the response of the boundary layer parameterization, along with input from the cloud microphysics scheme, which are highly uncertain processes. Model grid boxes with low-altitude clouds present in both the PI and PD dominate the forcing in both regions. In the northern NA, the brightening of completely overcast low cloud scenes (100 % cloud cover, likely stratocumulus) contributes the most, whereas in the southern NA the creation of clouds with fc of around 20 % from clear skies in the PI was the largest single contributor, suggesting that trade cumulus clouds are created in response to increases in aerosol. The creation of near-overcast clouds was also important there. The correct spatial pattern, coverage and properties of clouds are important for determining the magnitude of aerosol forcing, so we also assess the realism of the modelled PD clouds against satellite observations. We find that the model reproduces the spatial pattern of all the observed cloud variables well but that there are biases. The shortwave top-of-the-atmosphere (SWTOA) flux is overestimated by 5.8 % in the northern NA region and 1.7 % in the southern NA, which we attribute mainly to positive biases in low-altitude fc. Nd is too low by −20.6 % in the northern NA and too high by 21.5 % in the southern NA but does not contribute greatly to the main SWTOA biases. Cloudy-sky liquid water path mainly shows biases north of Scandinavia that reach between 50 % and 100 % and dominate the SWTOA bias in that region. The large contribution to aerosol forcing in the UKESM1 model from highly uncertain macrophysical adjustments suggests that further targeted observations are needed to assess rain formation processes, how they depend on aerosols and the model response to precipitation in order to reduce uncertainty in climate projections.

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Section: Model Detailsmentioning

confidence: 99%

“…In order to avoid issues regarding the representativeness of a limited number of years of meteorology, it would be useful to do future work such as that performed here using multiple years of data. Noise in some of the fields shown here may also be reduced by employing an ensemble approach as in Liu et al (2018).…”

Section: Recommendationsmentioning

confidence: 99%

The decomposition of cloud–aerosol forcing in the UK Earth System Model (UKESM1)

Grosvenor

Carslaw

2020

Atmos. Chem. Phys.

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“…This allows for a direct, time evolving calculation of ERF based on differences in top-of-atmosphere radiative imbalance between the simulations (Chen and Gettelman, 2016;Liu et al, 2018). Although they may not capture the full range of atmospheric adjustments (Forster et al, 2016), nudged ERF calculations are sufficiently comparable to fSST based calculations that they will be used in CovidMIP provided they have prescribed the same emissions as described above.…”

Section: Erf Calculationsmentioning

confidence: 99%

Modifying emission scenario projections to account for the effects of COVID-19: protocol for Covid-MIP

Lamboll¹,

Jones²,

Skeie³

et al. 2020

Preprint

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Abstract. Lockdowns to avoid the spread of COVID-19 have created an unprecedented reduction in human emissions. While the country-level scale of emissions changes can be estimated in near-real-time, the more detailed, gridded emissions estimates that are required to run General Circulation Models (GCM) of the climate will take longer to collect. In this paper we use recorded and projected country-and-sector activity levels to modify gridded predictions from the MESSAGE-GLOBIOM SSP2-4.5 scenario. We provide updated projections for concentrations of greenhouse gases, emissions fields for aerosols and precursors, and the ozone and optical properties that result from this. The codebase to perform similar modifications to other scenarios is also provided. We outline the means by which these results may be used in a model intercomparison project (CovidMIP) to investigate the impact of national lockdown measures on climate. This includes three strands: an assessment of short-term effects (5-year period), of longer-term effects (30 years) and an investigation into the separate effects of changes in emissions of greenhouse gases and aerosols. This last strand supports possible attribution of observed changes in the climate system, hence these simulations will also form part of the Detection and Attribution Model Intercomparison Project (DAMIP).

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“…Fire imposes great impacts on global climate through effects on radiative forcing, terrestrial ecosystems, and biogeochemical cycling (Li and Lawrence 2017). The feedback mechanisms between fire and climate interactions are still a great challenge and remain fundamentally uncertain (Liu et al 2014;Hantson et al 2016).…”

Section: Introductionmentioning

confidence: 99%

“…With a regional model, Lu and Sokolik (2013) found that fire aerosols lead to an increase of cloud water path during Canadian boreal wildfires in the summer of 2007. Liu et al (2018) found that fire aerosols have significant impacts on both liquid and ice clouds over southern Mexico and the central United States during the fire events in April 2009.…”

Section: Introductionmentioning

confidence: 99%

Impacts of Wildfire Aerosols on Global Energy Budget and Climate: The Role of Climate Feedbacks

et al. 2020

Self Cite

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Aerosols emitted from wildfires could significantly affect global climate through perturbing global radiation balance. In this study, the Community Earth System Model with prescribed daily fire aerosol emissions is used to investigate fire aerosols’ impacts on global climate with emphasis on the role of climate feedbacks. The total global fire aerosol radiative effect (RE) is estimated to be −0.78 ± 0.29 W m−2, which is mostly from shortwave RE due to aerosol–cloud interactions (REaci; −0.70 ± 0.20 W m−2). The associated global annual-mean surface air temperature (SAT) change ∆T is −0.64 ± 0.16 K with the largest reduction in the Arctic regions where the shortwave REaci is strong. Associated with the cooling, the Arctic sea ice is increased, which acts to reamplify the Arctic cooling through a positive ice-albedo feedback. The fast response (irrelevant to ∆T) tends to decrease surface latent heat flux into atmosphere in the tropics to balance strong atmospheric fire black carbon absorption, which reduces the precipitation in the tropical land regions (southern Africa and South America). The climate feedback processes (associated with ∆T) lead to a significant surface latent heat flux reduction over global ocean areas, which could explain most (~80%) of the global precipitation reduction. The precipitation significantly decreases in deep tropical regions (5°N) but increases in the Southern Hemisphere tropical ocean, which is associated with the southward shift of the intertropical convergence zone and the weakening of Southern Hemisphere Hadley cell. Such changes could partly compensate the interhemispheric temperature asymmetry induced by boreal forest fire aerosol indirect effects, through intensifying the cross-equator atmospheric heat transport.

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Investigation of short-term effective radiative forcing of fire aerosols over North America using nudged hindcast ensembles

Cited by 18 publications

References 54 publications

The decomposition of cloud–aerosol forcing in the UK Earth System Model (UKESM1)

The decomposition of cloud–aerosol forcing in the UK Earth System Model (UKESM1)

Modifying emission scenario projections to account for the effects of COVID-19: protocol for Covid-MIP

Impacts of Wildfire Aerosols on Global Energy Budget and Climate: The Role of Climate Feedbacks

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