Climate processes

Abstract:

The radiative forcing of anthropogenic aerosol remains a key uncertainty in the understanding of climate change. This study quantifies the model spread in aerosol forcing associated with (i) variability internal to the atmosphere and (ii) differences in the model representation of weather. We do so by performing ensembles of atmosphere-only simulations with four state-of-the-art Earth system models, three of which will be used in the sixth coupled model inter-comparison project (CMIP6, Eyring et al., 2016). In those models we reduce the complexity of the anthropogenic aerosol by prescribing the same annually-repeating patterns of the anthropogenic aerosol optical properties and associated effects on the cloud reflectivity. We quantify a comparably small model spread in the long-term averaged ERF compared to the overall possible range in annual ERF estimates associated with model-internal variability. This implies that identifying the true model spread in ERF associated with differences in the representation of meteorological processes and natural aerosol requires averaging over a sufficiently large number of annual estimates. We characterize the model diversity in clouds and use satellite products as benchmarks. Despite major inter-model differences in natural aerosol and clouds, all models show only a small change in the global-mean ERF due to the substantial change in the global anthropogenic aerosol distribution between the mid-1970s and mid-2000s, the ensemble mean ERF being −0.47Wm−2 for the mid-1970s and −0.51Wm−2 for the mid-2000s. This result suggests that inter-comparing ERF changes between two periods rather than absolute magnitudes relative to pre-industrial might provide a more stringent test for a model's ability for representing climate evolutions.

Abstract:

Abstract. The impact of aerosols on ice- and mixed-phase processes in deep convective clouds remains highly uncertain and the wide range of interacting microphysical processes are still poorly understood. To understand these processes, we analyse diagnostic output of all individual microphysical process rates for two cloud microphysics schemes in the Weather and Research Forecasting model (WRF). We investigate the response of individual processes to changes in aerosol conditions and the propagation of perturbations through the microphysics all the way to the macrophysical development of the convective clouds. We perform simulations for two different cases of idealised supercells using two double-moment bulk microphysics schemes and a bin microphysics scheme. We use simulations with a comprehensive range of values for cloud droplet number concentration (CDNC) and cloud condensation nuclei (CCN) concentration as a proxy for aerosol effects on convective clouds. We have developed a new cloud tracking algorithm to analyse the morphology and time evolution of individually tracked convective cells in the simulations and their response to the aerosol perturbations. This analysis confirms an expected decrease in warm rain formation processes due to autoconversion and accretion for polluted conditions. The height at which the freezing occurs increases with increasing CDNC. However, there is no evidence of a significant increase in the total amount of latent heat release from freezing and riming. The cloud mass and the altitude of the cloud centre of gravity show contrasting responses to changes in proxies for aerosol number concentration between the different microphysics schemes.

Abstract:

The cloud droplet number concentration (Nd) is of central interest to improve the understanding of cloud physics and for quantifying the effective radiative forcing by aerosol‐cloud interactions. Current standard satellite retrievals do not operationally provide Nd, but it can be inferred from retrievals of cloud optical depth (τc) cloud droplet effective radius (re) and cloud top temperature. This review summarizes issues with this approach and quantifies uncertainties. A total relative uncertainty of 78 % is inferred for pixel‐level retrievals for relatively homogeneous, optically thick and unobscured stratiform clouds with favorable viewing geometry. The uncertainty is even greater if these conditions are not met. For averages over 1o×1o regions the uncertainty is reduced to 54 % assuming random errors for instrument uncertainties. In contrast, the few evaluation studies against reference in‐situ observations suggest much better accuracy with little variability in the bias. More such studies are required for a better error characterization. Nd uncertainty is dominated by errors in re and, therefore, improvements in re retrievals would greatly improve the quality of the Nd retrievals. Recommendations are made for how this might be achieved. Some existing Nd datasets are compared and discussed, and best practices for the use of Nd data from current passive instruments (e.g., filtering criteria) are recommended. Emerging alternative Nd estimates are also considered. Firstly, new ideas to use additional information from existing and upcoming spaceborne instruments are discussed, and secondly, approaches using high‐quality ground‐based observations are examined.

Abstract:

The aging of refractory black carbon (rBC) aerosol by sulfate-driven chemistry has been constrained in coal-fired power-plant plumes using the NOAA WP-3D research aircraft during the Southern Nexus (SENEX) study, which took place in the Southeastern US in June and July of 2013. A Single Particle Soot Photometer (SP2) determined the microphysical properties of rBC-containing particles including single-particle rBC mass and the presence and amount of internally-mixed non-rBC material, hereafter referred to as “coatings”. Most power-plant influenced air was associated with very slightly increased amounts of non-refractory material, likely sulfate internally mixed with the rBC, however this increase was statistically insignificant even after semi-Lagrangian exposure for up to 5 h. On average, the increase in coating thickness was 2 ± 4 nm for particles containing 3–5 fg rBC. Similarly, the number fraction of rBC-containing particles that could be identified as internally mixed was increased by plume chemistry by only 1.3 ± 1.3%. These direct measurements of microphysical aging of rBC-containing aerosol by power plant emissions constrain the enhancement of sulfate chemistry on both rBC’s column-integrated absorption optical depth, and rBC-containing aerosol’s ability to act as cloud condensation nuclei. Appling Mie and k-Köhler theories to the SP2 observations, permits the resulting effect on rBC ambient light-absorption to be capped at the 2–6% level.