Abstract. International shipping emissions (ISE), particularly sulfur dioxide, can influence the global radiation budget by interacting with clouds and radiation after being oxidized into sulfate aerosols. A better understanding of the uncertainties in estimating the cloud radiative effects (CREs) of ISE is of great importance in climate science. Many international shipping tracks cover oceans with substantial natural dimethyl sulfide (DMS) emissions. The interplay between these two major aerosol sources on CREs over vast oceanic regions with a relatively low aerosol concentration is an intriguing yet poorly addressed issue confounding estimation of the CREs of ISE. Using an Earth system model including two aerosol modules with different aerosol mixing configurations, we derive a significant global net CRE of ISE (−0.153 W m−2 with a standard error of ±0.004 W m−2) when using emissions consistent with current ship emission regulations. This global net CRE would become much weaker and actually insignificant (−0.001 W m−2 standard error of ±0.007 W m−2) if a more stringent regulation were adopted. We then reveal that the ISE-induced CRE would achieve a significant enhancement when a lower DMS emission is prescribed in the simulations, owing to the sublinear relationship between aerosol concentration and cloud response. In addition, this study also demonstrates that the representation of certain aerosol processes, such as mixing states, can influence the magnitude and pattern of the ISE-induced CRE. These findings suggest a reevaluation of the ISE-induced CRE with consideration of DMS variability.
The evolution of chemical species during the life cycle of a severe local storm has been simulated using our three‐dimensional cloud chemistry model (Wang and Chang, 1993a, b). The importance of dynamical transport was found to be different for different gases, depending on their solubility. For insoluble gases with a comparatively long chemical lifetime, dynamical transport was a major factor in the evolution of in‐cloud gas phase concentration. The three‐dimensional divergence (convergence) and eddy mixing accounted for nearly 100% of the total variation of the in‐cloud O3 amount. For the highly soluble gases such as HNO3 and H2O2 the dominant sources and sinks were dissolution and evaporation. The dissolution of HNO3 and H2O2 accounted for more than 44% of the total change. The extensive condensation occurring in selected parts of the storm caused sharp decreases in the HNO3 and H2O2 mixing ratios. The dissolution of SO2 can exceed the influence of dynamical processes for a short time period, corresponding to a newly born convective cell. For other times, dynamics was the major factor influencing the variation of the SO2 in‐cloud amount. The in‐cloud amounts of the gas phase pollutants were reduced significantly during the development of the storm. The net decrease of SO2 is approximately equal to 4 ppt on the average throughout the entire cloud volume during the simulation, or approximately −3 ppt/h in rate. For O3 the total change is approximately equal to −1.1 ppb/h on the average. The net decrease of HNO3 is equal to 20 ppt on the average, or −15 ppt/h in rate. The H2O2 decreasing rate is approximately equal to −5.9 ppb/h on the average, while the average net change is about 7.8 ppb in the cloud volume and averaged through the duration of the simulation.
Abstract. In order to simulate an aerosol indirect effect, most global aerosol-climate models utilize an activation scheme to physically relate the ambient aerosol burden to the droplet number nucleated in newly-formed clouds. While successful in this role, activation schemes are becoming frequently called upon to handle chemically-diverse aerosol populations of ever-increasing complexity. As a result, there is a need to evaluate the performance of existing schemes when interfacing with these complex aerosol populations and to consider ways to incorporate additional processes within them. We describe an emulator of a detailed cloud parcel model which can be used to assess aerosol activation, and compare it with two activation parameterizations used in global aerosol models. The emulator is constructed using a sensitivity analysis approach (polynomial chaos expansion) which reproduces the behavior of the parent parcel model across the full range of aerosol properties simulated by an aerosol-climate model. Using offline, iterative calculations with aerosol fields from the Community Earth System Model/Model of Aerosols for Research of Climate (CESM/MARC), we identify subsets of aerosol parameters to which diagnosed aerosol activation is most sensitive, and use these to train metamodels including and excluding the influence of giant CCN for coupling with the model. Across the large parameter space used to train them, the metamodels estimate droplet number concentration with a mean relative error of 9.2 % for aerosol populations without giant CCN, and 6.9 % when including them. Using offline activation calculations with CESM/MARC aerosol fields, the best-performing metamodel has a mean relative error of 4.6 %, which is comparable with the two widely-used activation schemes considered here (which have mean relative errors of 2.9 % and 6.7 %, respectively). We identify the potential for regional biases to arise when estimating droplet number using different activation schemes, particularly in oceanic regimes where our best-performing emulator tends to over-predict by 7 %, whereas the reference activation schemes range in mean relative error from −3 % to 7 %. In these offline calculations, the metamodels which include the effects of giant CCN are accurate in continental regimes (mean relative error of 0.3 %), but strongly over-estimate droplet number in oceanic regimes by up to 22 %, particularly in the Southern Ocean. The biases in cloud droplet number resulting from the subjective choice of activation scheme could potentially influence the magnitude of the indirect effect diagnosed from the model incorporating it.
<p><strong>Abstract.</strong> Open-burning fires play an important role in the Earth's climate system. In addition to contributing a substantial fraction of global emissions of carbon dioxide, they are also a major source of atmospheric aerosols such as organic carbon, black carbon, and sulphate. These "fire aerosols" can influence the climate via both direct and indirect radiative effects. In this study, we investigate these radiative effects and the hydrological fast response using the Community Atmosphere Model version 5 (CAM5). Emissions of fire aerosols exert a global mean net radiative effect of &#8722;1.0&#8201;W&#8201;m<sup>&#8722;2</sup>, dominated by the cloud shortwave response to organic carbon aerosol. The net radiative effect is particularly strong over boreal regions. By comparing simulations using inter-annually varying versus inter-annually invariant emissions, we find that ignoring the inter-annual variability of the emissions can lead to systematic overestimation of the strength of the net radiative effect of the fire aerosols. Globally, the overestimation is +23&#8201;% (&#8722;0.2&#8201;W&#8201;m<sup>&#8722;2</sup>). Regionally, the overestimation can be substantially larger. For example, over Australia and New Zealand the overestimation is +58&#8201;% (&#8722;1.2&#8201;W&#8201;m<sup>&#8722;2</sup>), while over Boreal Asia the overestimation is +43&#8201;% (&#8722;1.9&#8201;W&#8201;m<sup>&#8722;2</sup>). The systematic overestimation of the net radiative effect of the fire aerosols is likely due to the non-linear influence of aerosols on clouds. However, ignoring inter-annual variability in the emissions does not appear to significantly impact the hydrological fast response. In order to improve understanding of the climate system, we need to more accurately quantify the effects of aerosols, taking into account important characteristics such as inter-annual variability.</p>
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