AS3.3

Instantaneous clear-sky CO2 forcing is known to vary significantly over the globe, but the climate factors which control this are not well understood. Building upon the work of Wilson (2012), we build a first-principles, analytical model for CO2 forcing which requires as input only the temperatures at the surface and roughly 20 hPa, as well as column relative humidity. This model quantitatively captures global variations in clear-sky CO2 forcing, and shows that the meridional forcing gradient is predominantly due to the meridional surface temperature gradient, with modulation by water vapor. In particular, the Simpsonian behavior of water vapor emission implies an upper bound on CO2 forcing (with respect to surface temperature) which is realized in the present day tropics.

How to cite: Jeevanjee, N., Seeley, J., Paynter, D., and Fueglistaler, S.: An analytical model for spatially varying clear-sky CO2 forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-217, https://doi.org/10.5194/egusphere-egu21-217, 2021.

We present the first incorporation and evaluation of the Common Representative Intermediates version 2.2 chemistry mechanism, CRI v2.2, for use in the United Kingdom Earth System Model (UKESM1). Tuned against the MCM v3.3.1, the CRI v2.2 mechanism builds on the previous CRI version, CRI v2.1, in UKESM1 (Archer-Nicholls et al., 2020) by updating isoprene chemistry and offers a more comprehensive description of tropospheric chemistry than the standard chemistry mechanism STRAT-TROP (ST).

CRI v2.2 adds state-of-the-art isoprene chemistry with the introduction of HO_x-recycling via the isoprene peroxy radical isomerisation pathway, making UKESM1 one of the first CMIP6 models to include this important chemistry. HO_x-recycling has noticeable effects on oxidants in regions with large emissions of biogenic volatile organic compounds (BVOCs). Low altitude OH in tropical forested regions increases by 75-150% relative to ST, reducing the existing model low bias compared to observations. Consequently, isoprene surface mixing ratios decrease considerably (25-40%), significantly improving the model high bias relative to ST. Methane lifetime decreases by 2% and tropospheric ozone burden increases by 4%.

Aerosol processes also differ between CRI v2.2 and ST, resulting in changes to the size and number distributions. Relative to ST, CRI v2.2 simulates an 8% decrease in the sulphate aerosol burden with 20% decreases in the nucleation and Aitken modes. By contrast, the secondary organic aerosol (SOA) nucleation mode burden increases by 11%. Globally, the average nucleation and Aitken mode aerosol number concentrations decrease by 20%.

The differences in aerosol and gas phase chemistry between CRI v2.2 and ST are likely to have impacts on the radiation budget. We plan to use CRI v2.2 and ST to investigate the influence that the chemical mechanism has on the simulated chemistry-climate feedbacks from BVOCs. In addition, CRI v2.2 will serve as the basis for the addition of a scheme describing the formation of highly oxygenated organic molecules (HOMs) from BVOCs, facilitating a semi-explicit mechanism for new particle formation from organic species.

How to cite: Weber, J., Archer-Nicholls, S., Abraham, N. L., Shin, Y. M., Bannan, T., Schwantes, R., Jenkin, M., Khan, A., and Archibald, A. T.: Incorporation and evaluation of the CRI v2.2 chemical mechanism in UKESM1: An alternative mechanism with updated isoprene chemistry for investigating the influence of BVOCs on atmospheric composition and climate., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1236, https://doi.org/10.5194/egusphere-egu21-1236, 2021.

Aerosol forcing remains the most uncertain component of the total climate forcing on the Earth system. RFMIP and AerChemMIP contained experiments that allow us to determine time-slice present day (2014 minus 1850) from 17 CMIP6 models, and transient (1850 to 2014, or 2100) aerosol forcing from 11 models. In CMIP6, aerosol present-day aerosol forcing is -1.01 (full range -1.37 to -0.63) W m^-2, a range considerably narrower than comprehensive assessments of aerosol forcing from multiple lines of evidence such as AR5 (-1.9 to -0.1 W m^-2) or Bellouin et al. 2020 (-2.0 to -0.35 W m^-2). The transient experiments also show a diversity in time histories, with most models showing a peak negative aerosol forcing at some time between 1975 and 2010, and recent trends varying from strongly recovering to slightly strengthening aerosol forcing. Models that were run to 2100 under SSP2-4.5 all show a projected weakening aerosol forcing.

By fitting a simple relationship of how globally integrated emissions of black carbon, organic carbon and SO₂ relate to effective radiative forcing from aerosol-radiation interactions (ERFari) and aerosol-cloud interactions (ERFaci), an emissions to forcing relationship can be determined for these 11 RFMIP and AerChemMIP models. Using a 100,000 member Monte Carlo ensemble of historical aerosol time series, where coefficients are drawn from these model-derived distributions, and total 1850 to 2014 aerosol forcing is taken from the wider distributions of Bellouin et al. (2020), we create a best estimate historical time series for aerosol forcing (with uncertainty) that is constrained to historical warming and observed ocean heat uptake using a simple climate model. This method can also be used to predict aerosol forcing from future emissions scenarios, such as the SSPs and those derived from integrated assessment models, and provides estimates of the likely ranges for equilibrium climate sensitivity and transient climate response based on the historical aerosol forcing.

How to cite: Smith, C., Harris, G., Palmer, M., Bellouin, N., Collins, W., Schulz, M., Myhre, G., Golaz, J.-C., Ringer, M., Storelvmo, T., and Forster, P.: Past, present and future aerosol forcing derived from CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2863, https://doi.org/10.5194/egusphere-egu21-2863, 2021.

Near-term climate forcers (NTCFs), including aerosols and chemically reactive gases such as tropospheric ozone and methane, offer a potential way to mitigate climate change and improve air quality--so called "win-win" mitigation policies.   Prior studies support improved air quality under NTCF mitigation, but with conflicting climate impacts that range from a significant reduction in the rate of global warming to only a modest impact.  Here, we use state-of-the-art chemistry-climate model simulations conducted as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) to quantify the 21st-century impact of NTCF reductions, using a realistic future emission scenario with a consistent air quality policy.  Non-methane NTCF (NMNTCF; aerosols and ozone precursors) mitigation improves air quality, but leads to significant increases in global mean precipitation of 1.3% by mid-century and 1.4% by end-of-the-century, and corresponding surface warming of 0.23 and 0.21 K.  NTCF (all-NTCF; including methane) mitigation further improves air quality, with larger reductions of up to 45% for ozone pollution, while offsetting half of the wetting by mid-century (0.7% increase) and all the wetting by end-of-the-century (non-significant 0.1% increase) and leading to surface cooling of -0.15 K by mid-century and -0.50 K by end-of-the-century.  This suggests that methane mitigation offsets warming induced from reductions in NMNTCFs, while also leading to net improvements in air quality.

How to cite: Allen, R., Horowitz, L., Naik, V., Oshima, N., O'Connor, F., Turnock, S., Shim, S., Le Sager, P., van Noije, T., Tsigaridis, K., Bauer, S., Sentman, L., John, J., Broderick, C., Deushi, M., Folberth, G., Fujimori, S., and Collins, W.: Significant climate benefits from near-term climate forcer mitigation in spite of aerosol reductions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3162, https://doi.org/10.5194/egusphere-egu21-3162, 2021.

The effective radiative forcing (ERF) from aerosols and chemically reactive gases is calculated for several of the models that contributed to the CMIP6 project. The design of the experiments allowed for the calculation of the ERF due to each individual aerosol and gas, although where the aerosol and chemistry were modelled as interactive processes additional diagnostics were used to understand how these processes contributed to the overall ERF. The control used was the emission or concentration of the species in 1850 (considered the pre-industrial baseline for these experiments), with the perturbation using a specified emission or concentration of the species based on 2014 values as the present-day value.  The experiments were run over a period of 30 years, with sea-surface temperatures held constant, and the ERF was obtained as the net change in TOA radiative flux between the perturbed and control run.

The aerosols considered were black carbon (BC), organic carbon (OC), SO₂ and NH₃, and the combination of these constituents in was modelled in the ‘aer’ experiment.

The chemically-reactive gases included methane, nitrous oxide (N₂O), and the ozone precursors nitrogen oxides (NO_x) and volatile organic compounds (VOC) as well as ozone (O₃).

For some models we were able to use radiative kernels to find the relative importance of rapid adjustments such as cloud changes, atmospheric temperature and water vapour in the overall value of the ERF. We also used double-calls, where the effect of the aerosol or gas was removed from the radiative transfer calculations in the model, to examine the relative contributions of aerosol-cloud interactions and direct radiative effects.

The spread of the results between models is also considered, and the differences attributed to how the models represent different processes, e.g. aerosol-cloud interactions, and the complexity of the atmospheric chemistry modelling. The multi-model means are given below.

Table 1 Multi-model means for ERF for aerosols

Table 2 Multi-model means for ERF for chemically reactive gases

We find the overall aerosol ERF (aer in Table 1) is consistent with previous work, although the results for black carbon (BC) show adjustments that are generally weaker than in previous studies. We also found that cloud effects  can have a large impact on the overall ERF, including for the chemically-reactive gases.

The impact of other processes, especially atmospheric chemistry on the overall results is also discussed.

How to cite: Thornhill, G., Collins, W., Kramer, R., Olivie, D., and Skeie, R.: Using the AerChemMIP experiments to calculate radiative forcing from aerosols and chemically reactive gases, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7757, https://doi.org/10.5194/egusphere-egu21-7757, 2021.

Short lived climate forcers (SLCFs) are important atmospheric components as they can influence climate, through interactions with the Earth’s radiative balance, but also impact regional air quality. Two important SLCFs are tropospheric O₃ and fine particulate matter (with a diameter less than 2.5 microns – PM_2.5). Future policy measures aim to provide co-benefits to both climate and air quality via mitigation of SLCFs. However, it is still uncertain how future reductions in SLCFs will impact both climate and air quality. Sensitivity experiments conducted as part of the 6^th Coupled Model Intercomparison Project (CMIP6) provide an opportunity to assess the climate and air quality impacts of different mitigation scenarios.

Here we use results from UKESM1 (an Earth system model with interactive chemistry and aerosols) for the future climate and emission scenario ssp370SST, an atmosphere only simulation that assumes low mitigation of climate and air pollutants. We then compare results from this scenario to different sensitivity experiments to assess the impact on climate, through effective radiative forcing (ERF), and air quality, by changes in surface concentrations of O₃ and PM_2.5. The sensitivity experiments consider reductions to CH₄ concentrations and emissions of O₃ and aerosol precursors, individually and combined. Additional sensitivity experiments also consider the individual impact from land-use change, climate change and all emissions.

Compared to ssp370SST, scenarios that strongly mitigate both aerosol and O₃ precursors, including CH₄, produce the largest benefits in 2100 to global air quality, a 10-25% reduction in global annual mean O₃ and PM_2.5 concentrations, and climate, a change in ERF of up to -1.2 Wm^-2. If CH₄ concentrations are not reduced but other aerosols and O₃ precursors are, then there are still benefits to air quality in 2100, relative to ssp370SST, but the change in the ERF becomes positive (up to +0.4 Wm^-2), mainly due to aerosols reductions. The impact of solely reducing CH₄ concentrations results in a large change in ERF (-1.4 Wm^-2) and large reductions in surface O₃ (-15%) in 2100 but has negligible impacts on surface PM_2.5. Reducing only aerosol precursors decreases PM_2.5 concentrations but results in a positive change in O₃ concentrations and ERF in 2100. Reducing only tropospheric O₃ precursors decreases surface O₃ concentrations but has minimal impact on PM_2.5 concentrations and the ERF in 2100. Implementing different land-use policies has a small impact on the ERF in 2100 but slightly increases both global surface O₃ and PM_2.5.

The results from this single model study show the importance of considering the different impacts of SLCFs on future air quality and climate metrics.

How to cite: Turnock, S., Davli, M., Keeble, J., Robertson, E., and O'Connor, F.: The Climate and Air Quality response under different future ssp370 pathways in UKESM1, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8216, https://doi.org/10.5194/egusphere-egu21-8216, 2021.

How to cite: Hardacre, C., Mulcahy, J. P., Pope, R., Li, C., Rumbold, S., and Jones, C.: Challenging UKESM1 with SO2 and sulphate observational data to evaluate the aerosol sulphur cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10736, https://doi.org/10.5194/egusphere-egu21-10736, 2021.

Absorbing aerosol has the capacity to warm the climate, but the amount of warming is highly uncertain. AAOD (Absorptive Aeorosl Optical Depth) is an optical measure of the abundance of this absorbing aerosol, comprising mineral dust, black and brown carbon and can be retrieved from satellite measurements providing an almost global view on absorbing aerosol.

In this study we evaluate AEROCOM models with satellite observations of AAOD and SSA (Single Scattering Albedo) and interpret the discrepancies. Over source regions, diversity in model AAOD is mostly due to emissions even though models employ different assumptions regarding the imaginary refractive index. On the one hand this suggests emissions to be a major error source, on the other hand it suggests that the AEROCOM ensemble as a whole may have a bias with regards to MAC (Mass Absorption Coefficient). We show that in the models AAOD scales almost linearly with emissions (either black carbon or dust) and this allows the use of observations as a constraint.  In contrast, model diversity in AOD is shown to depend in almost equal measure on emissions, lifetimes and MECs (Mass Extinction Coefficient). We also analyse mineral dust and black carbon lifetimes by considering the contrast in AAOD over source regions and over outflow regions, and again provide observations constraints.

While the older Phase II models generally underestimate AAOD, Phase III models tend to straddle the observations, with some models over-estimating and other models underestimating AAOD. Emissions seem to be the driving factor in this difference. The amount of diversity is larger in the Phase III than Phase II models.

This study was conducted using four satellite datasets of AAOD and SSA. These datasets were extensively evaluated with AERONET. Dearth of observations prevents global assesment of the satellite retrievals. However, we show that model evaluation is relatively independent of the chosen dataset, even though we identify significant biases between the datasets.

How to cite: Schutgens, N. and Zhong, Q. and the AEROCOM/AEROSAT teams: AEROCOM/AEROSAT: an intercomparison of AAOD & SSA in model and satellite data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11992, https://doi.org/10.5194/egusphere-egu21-11992, 2021.

The CMIP6 Shared Socioeconomic Pathway (SSP) scenarios include projections of future changes in anthropogenic biomass-burning.  Globally, they assume a decrease in total fire emissions over the next century under all scenarios.  However, fire regimes and emissions are expected to additionally change with future climate, and the methodology used to project fire emissions in the SSP scenarios is opaque.

We aim to provide a more traceable estimate of future fire emissions under CMIP6 scenarios and evaluate the impacts for aerosol radiative forcing.  We utilise interactive wildfire emissions from four independent land-surface models (CLM5, JSBACH3.2, LPJ-GUESS, and ISBA-CTRIP) used within CMIP6 ESMs, and two different machine-learning methods (a random forest, and a generalised additive model) trained on historical data, to predict year 2100 biomass-burning aerosol emissions consistent with the CMIP6-modelled climate for three different scenarios: SSP126, SSP370, and SSP585.  This multi-method approach provides future fire emissions integrating information from observations, projections of climate, socioeconomic parameters and changes in vegetation distribution and fuel loads.

Our analysis shows a robust increase in fire emissions for large areas of the extra-tropics until the end of this century for all methods.  Although this pattern was present to an extent in the original SSP projections, both the interactive fire models and machine-learning methods predict substantially higher increases in extra-tropical emissions in 2100 than the corresponding SSP datasets.  Within the tropics the signal is mixed. Increases in emissions are largely driven by the temperature changes, while in some tropical areas reductions in fire emissions are driven by human factors and changes in precipitation, with the largest reductions in Africa. The machine-learning methods show a stronger reduction in the tropics than the interactive fire models, however overall there is strong agreement between both the models and the machine-learning methods.

We then use additional nudged atmospheric simulations with two state-of-the-art composition-climate models, UKESM1 and CESM2, to diagnose the impact of these updated fire emissions on aerosol burden and radiative forcing, compared with the original SSP prescribed emissions.  We provide estimates of future fire radiative forcing, compared to modern-day, under these CMIP6 scenarios which span both the severity of climate change in 2100, and the rate of reduction of other aerosol species.

How to cite: Kasoar, M., Hamilton, D., Dalmonech, D., Hantson, S., Lasslop, G., Voulgarakis, A., and Wells, C.: Improved estimates of future fire emissions under CMIP6 scenarios and implications for aerosol radiative forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13822, https://doi.org/10.5194/egusphere-egu21-13822, 2021.

Rapid adjustments play an important role for the climate effect of drivers of climate change. These adjustments to temperature, water vapour and clouds occur before changes to the surface temperature. We show results from CMIP6 models and results of various climate drivers in Precipitation Driver and Model Intercomparison Project (PDRMIP). Rapid adjustments are particularly important for the climate effect of black carbon. The magnitude and sign of the black carbon rapid adjustments depend strongly on the vertical profile of black carbon. Black carbon in the upper troposphere causes a strong reduction of clouds. On the other hand, CO₂ increases upper-level clouds resulting in cloud rapid adjustment in some climate models is a substantial fraction of the cloud feedback

How to cite: Myhre, G.: Cloud rapid adjustments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14396, https://doi.org/10.5194/egusphere-egu21-14396, 2021.

Aerosols are a major climate forcer, but their historical effect has the largest uncertainty of any forcing; their mechanisms and impacts are not well understood. Due to their short lifetime, aerosols have large impacts near their emission region, but they also have effects on the climate in remote locations. In recent years, studies have investigated the influences of regional aerosols on global and regional climate, and the mechanisms that lead to remote responses to their inhomogeneous forcing. Using the Shared Socioeconomic Pathway scenarios (SSPs), transient future experiments were performed in UKESM1, testing the effect of African emissions following the SSP3-RCP7.0 scenario as the rest of the world follows SSP1-RCP1.9, relative to a global SSP1-RCP1.9 control. SSP3 sees higher direct anthropogenic aerosol emissions, but lower biomass burning emissions, over Africa. Experiments were performed changing each of these sets of emissions, and both. A further set of experiments additionally accounted for changing future CO₂ concentrations, to investigate the impact of CO₂ on the responses to aerosol perturbations. Impacts on radiation fluxes, temperature, circulation and precipitation are investigated, both over the emission region (Africa), where microphysical effects dominate, and remotely, where dynamical influences become more relevant. 

How to cite: Wells, C. and Voulgarakis, A.: The local and remote atmospheric impacts of Africa’s 21st century aerosol emission trajectory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14417, https://doi.org/10.5194/egusphere-egu21-14417, 2021.

We investigate a novel use of model nudging to interrogate radiative rapid adjustment mechanisms and their magnitudes in response to aerosol emission perturbations in an earth system model. The radiative effects of a forcing agent can be quantified using the effective radiative forcing (ERF). ERF is the sum of the instantaneous radiative forcing, and radiative adjustments – changes in the atmosphere’s state in response to the initial forcing agent that cause a further radiative forcing. Radiative adjustments are particularly important for aerosols, which affect clouds both via microphysical interactions and changes in circulation, stratification and convection. Understanding the different adjustment mechanisms and their contribution to the total ERF of different aerosol emissions is necessary to better understand how their ERF may change with future changes in anthropogenic aerosol emissions. In this work we investigate radiative adjustments resulting from changes in atmospheric temperature (and the resulting changes in stratification and convection) due to anthropogenic sulphate and black carbon aerosol forcing.

We have conducted multiple global atmosphere-only time-slice experiments using the UK Earth System Model (UKESM1). Each experiment has either control, black carbon perturbed, or sulphur dioxide perturbed emissions; and either no nudging, nudged horizontal winds (uv), or nudged horizontal winds and potential temperature (uvθ). The difference between nudged uvθ minus nudged uv simulations determines the atmospheric temperature related adjustments arising from the aerosol perturbation. We have also conducted repeats of each simulation, varying the nudging setup to test sensitivity to different nudging parameters.

We find that nudging horizontal winds affects the resulting ERF very little, whereas nudging potential temperature as well can cause a significant difference from the non-nudged experiments, primarily in the cloud radiative effect. However, this difference is sensitive to the strength of the nudging applied, for which we consider the most appropriate value.

How to cite: Coleman, M., Collins, W., Shine, K., Bellouin, N., and O'Connor, F.: Investigating model nudging as a novel technique to isolate aerosol radiative adjustment mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15065, https://doi.org/10.5194/egusphere-egu21-15065, 2021.

A new version of the UKCA chemistry-climate model with highly reactive volatile organic compounds (VOCs) is used to investigate the ozone (O₃) responses in historical (2004-2014) and future (2045-2055) shared socio-economic pathways (SSPs) scenarios of CMIP6 AerChemMIP experiments. Significant increases in surface O₃ levels in South and East Asia are simulated in the new version compared with the standard UKCA model. The O₃ production and the O₃ burden averaged over the troposphere increase slightly by 6 % as a result of more highly reactive VOCs, but the O₃ lifetime is quite similar. Comparing the different SSP scenarios using this new model version we find the averaged surface O₃ concentrations are higher in the scenario with high emissions than for historical conditions. O₃ concentrations are much lower than historical O₃ concentrations when O₃ precursor concentrations are low. However, regional O₃ increases occur in East Asia in the future scenario with low emissions of short-lived climate forcers due to strong VOC limited regimes. Decreases in surface O₃ concentrations occur globally in the future scenario that has lower methane (CH₄) concentrations. We construct O₃ and O₃ production isopleths. These both suggest that the threshold of NO_x/VOCs shifting from NO_x limited to VOC limited regimes is approximately 0.8. More areas become VOC limited in South Asia in all future scenarios, but there is little change for East Asia. The hydroxyl radical (OH) concentrations generally increase in regions with high O₃ precursor abundances in the future scenario, but the high OH levels are offset by lower CH₄ concentrations in the future low CH₄ scenario. We find that there are small changes in O₃ production efficiency in continental regions in all future scenarios. Relative O₃ burden changes between the future SSP and historical scenarios are larger in the troposphere than in the planetary boundary layer (PBL), illustrating that O₃ burdens are less sensitive in the PBL under emission and climate change. The O₃ lifetime in the troposphere decreases in all future scenarios as compared to the historical period. We find that the decreases in O₃ precursors and CH₄ concentrations play important roles in reducing O₃ burdens in the future.

How to cite: Liu, Z., M. Doherty, R., Wild, O., M. O’Connor, F., and T. Turnock, S.: O3 responses in CMIP6 AerChemMIP experiments: different roles of O3 precursors, CH4 concentrations and climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15685, https://doi.org/10.5194/egusphere-egu21-15685, 2021.

A grand challenge in the field of chemistry-climate modelling is to understand the connection between anthropogenic emissions, atmospheric composition and the radiative forcing of trace gases and aerosols.   The AerChemMIP model intercomparison project, part of CMIP6, focuses on calculating the radiative forcing of gases and aerosol particles over the period 1850 to 2100.  We present an analysis of the trends in tropospheric ozone budget in the UKESM1 and other models from CMIP6 experiments. We discuss these trends in terms of chemical production and loss of ozone as well as physical processes such as transport and deposition.  Where possible, AerChemMIP attribution experiments such as histSST-piCH4, will be used to quantify the effect of individual emissions and forcing changes on the historical ozone burden and budget.  For future experiments, we focus on analogous experiments from the SSP3-70 scenario, a ‘regional rivalry’ shared socioeconomic pathway involving significant emissions changes.

How to cite: Griffiths, P., Guang, Z., Shim, S., Mulcahy, J., Murray, L., O'Connor, F., Archibald, A., and Pyle, J.: Analysis of ozone budget in CMIP6 experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16097, https://doi.org/10.5194/egusphere-egu21-16097, 2021.

The pre-industrial (PI; Year 1850) to present-day (PD; Year 2014) increase in methane concentration leads to a global mean effective radiative forcing (ERF) of 0.97 ± 0.04 W m^-2 in the UK’s Earth System Model, UKESM1. In comparison with the multi-model estimate of 0.75 ± 0.10 W m^-2 from the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP), UKESM1 has the highest methane ERF and lies outside the 1-sigma range. This is, in part, due to UKESM1 including interactive chemistry and positive indirect effects, such as methane-driven changes in tropospheric ozone. However, UKESM1 is the only model within AerChemMIP whose contribution to the methane ERF from tropospheric adjustments is positive – this is largely driven by the strong positive cloud adjustment in UKESM1, in contrast to other models. In this work, we apportion the total methane ERF between direct and indirect effects (including adjustments) and provide a process-based understanding of what is driving the positive cloud adjustment in UKESM1.

Using additional UKESM1 paired simulations, we apportion the total methane ERF between its direct methane contribution and indirect contributions from ozone, water vapour, and aerosols. This approach offers the advantage that linearity is not assumed and it distinguishes between cloud effects that are dynamically-driven via changes in temperature and those that are aerosol-mediated. By analysing the chemistry-aerosol budgets and the cloud responses, we find that the PI to PD increase in methane leads to an indirect positive aerosol ERF of up to 0.3 ± 0.06 W m^-2, with a near-zero contribution from the instantaneous radiative forcing from aerosol-radiation interactions. Methane-driven changes in oxidants alter the relative contributions of the different sulphur dioxide oxidation pathways, causing a change in new particle formation rates and a shift in the aerosol size distribution towards fewer but larger particles. There is a resulting decrease in cloud droplet number concentration, an increase in cloud droplet effective radius, and a decrease in liquid water path in marine stratocumulus regions from aerosol-cloud interactions (mainly through the cloud lifetime effect). There is a subsequent change in the cloud radiative effect, with the positive change in the shortwave dominating over the negative change in the longwave. However, when aerosol-cloud interactions are disabled, the change in the cloud radiative effect is negative and is dominated by the reduction of cirrus clouds in the tropics, thus making UKESM1 more consistent with the other AerChemMIP models.

These results can explain some of the diversity in multi-model estimates of methane forcing and highlight the importance of chemistry-aerosol-cloud interactions when quantifying climate forcing by reactive greenhouse gases.

How to cite: O'Connor, F., Jamil, O., Andrews, T., Johnson, B., Mulcahy, J., and Manners, J.: Apportionment of the present-day forcing by methane using UKESM1: The role of chemistry-aerosol-cloud interactions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16375, https://doi.org/10.5194/egusphere-egu21-16375, 2021.

Atmospheric brown clouds (ABCs) are a dense and extensive pollution layer and have significant implications on air quality, agriculture, water cycle, and regional climate. The objective of the present study is to observe seasonal and spatial variations in the occurrence of ABCs and its radiative effects. The Indo-Gangetic plain (IGP) is the most populated region of India, which is an extended region in the foothills of the Himalayas. The IGP is one of the ABCs hotspots over the globe. The frequency of ABCs occurrences and radiative forcing were calculated using data from seven ground-based remote sensors situated across the IGP. We have used total ~ 5000 days of Level-2 aerosol measurements from seven AERosol Robotic NETwork (AERONET) stations (Karachi, Lahore, Jaipur, New Delhi, Kanpur, Gandhi college and Dhaka University) for three seasons (Pre-monsoon, Post-monsoon, and Winter) during 2000-2019. An algorithm based on the optical properties of aerosols is used to defined extreme pollution events (ABCs days) for each site. Our results show more frequent occurrences of ABCs over the region in the pre-monsoon out of all three seasons. However, spatial variation is found in all seasons, like maximum frequency of ABCs over western IGP region in post-monsoon and minimum is at eastern IGP region in the winter season. Further, we have used the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model to calculate radiative forcing during ABCs days on all sites of study. Aerosol optical depth (AOD) and absorption optical depth (AAOD) was used to calculate radiative forcing over the IGP region. Radiative forcing of ABCs is negative at both the surface (SRF) and top of the atmosphere (TOA), whereas it is positive in the atmosphere (ATM). In magnitude, it was found minimum in the pre-monsoon season at TOA. However, other seasons have specific features over specific locations, for example, in the winter season, radiative forcing is maximum over Kolkata at TOA, SRF, and ATM, which are -13.81 W/m², -50.90 W/m², and +37.09 W/m² respectively. In the pre-monsoon season, radiative forcing is maximum at Delhi (-9.59 W/m²) at TOA. In post-monsoon season radiative forcing maximum at Gandhi-college (-11.30 W/m²) at TOA. This ground observation is also compared with Modern Era Retrospective analysis and Research and Applications-2 (MEERA 2) modal data. These results indicate the cooling effect of ABCs at the surface and TOA over the IGP region throughout the period.

How to cite: Jangid, M. and Mishra, A.: Radiative forcing of atmospheric brown clouds over the Indo-Gangetic Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16481, https://doi.org/10.5194/egusphere-egu21-16481, 2021.