Not only anthropogenic greenhouse gases but also microscopic anthropogenic air pollution particles called aerosols influence Earth's climate. Despite strong research efforts, the aerosol forcing of Earth's climate is still poorly quantified compared to the greenhouse gas (GHG) induced forcing (Bellouin et al., 2020). Considering multiple lines of evidence, total aerosol effective radiative forcing is estimated to be from −2.0 to −0.4 W/m 2 with a 90% likelihood (Bellouin et al., 2020). The fact that aerosols offset a poorly quantified fraction of GHG-induced positive radiative forcing makes it challenging to estimate the sensitivity of the Earth's climate to anthropogenic radiative forcing (Stevens et al., 2016) and improve the reliability of climate projections.Besides the direct radiative interactions, aerosols act as cloud condensation nuclei (CCN) and modulate cloud properties. The climate forcing caused by aerosol impacts on clouds is especially poorly quantified (Bellouin et al., 2020). The first indirect effect of aerosols, also called the Twomey effect (Twomey, 1974), refers to the increased cloud droplet number concentration (CDNC) in clouds. The Twomey effect raises the cloud albedo and induces a cooling effect on the Earth's climate. The magnitude of the physically well understood Twomey effect is relatively uncertain (Quaas et al., 2020), but the cooling effect is confirmed by multiple lines of evidence (Bellouin et al., 2020).The second aerosol indirect effect concerns the liquid water path (LWP) and cloud fraction response to increased CDNC (Albrecht, 1989;IPCC, 2013). LWP can increase due to suppressed collision-coalescence efficiency leading to suppressed precipitation (Albrecht, 1989) and decrease due to aerosol-enhanced entrainment (Ackerman et al., 2004;Bretherton et al., 2007;Wood, 2007). However, the net cloud water response to aerosols remains relatively poorly constrained (Bellouin et al., 2020).
<p>Reducing uncertainty in aerosol-cloud interactions is necessary for more reliable climate projections. Understanding the effects of anthropogenic aerosols on clouds remains a challenge due to complex processes governing the cloud adjustments to increased cloud droplet numbers. Using SEVIRI data, we study the daily evolution of polluted cloud tracks induced by strong pollution sources in the European part of Russia. We use semi-automated cloud droplet effective radius based statistical classification algorithm to differentiate between polluted and nearby unpolluted pixels in the satellite images. We use the 15-minute resolution Cloud Physical Properties product by KNMI to study changes in polluted cloud properties during the daytime. In some cases, cloud water increases during the day and in some cases decreases in polluted clouds compared to the nearby unpolluted clouds. On average, the diurnal evolution of cloud water is very similar between polluted and unpolluted clouds. Interestingly, there is less cloud water in polluted clouds already in the morning, suggesting that cloud water decreases more in polluted clouds during the night. The relatively weak average decrease in cloud water agrees with MODIS-based estimate (Toll et al 2019, Nature, https://doi.org/10.1038/s41586-019-1423-9).</p>
Using geostationary satellite data, we show that strong cloud perturbations downwind of isolated continental anthropogenic aerosol sources can live for multiple days, and 7% of tracks live longer than 48 hr. The median lifetime for polluted cloud tracks in Eastern Europe studied using Spinning Enhanced Visible and Infrared Imager data is 18 hr but shows a strongly skewed distribution toward longer lifetimes. This suggests that ship‐track‐like cloud perturbations help infer causal relationships between aerosols and clouds. Strong anthropogenic cloud perturbations are usually visible until favorable liquid‐water low‐level clouds exist. Only in 12% of cases do liquid‐phase stratiform clouds persist, but tracks disappear. Longer‐lived tracks occur in anticyclonic conditions, and in case of statically very stable lower troposphere and low relative humidity above clouds.
<p>Anthropogenic aerosol particles affect clouds by serving as cloud condensation nuclei, thus significantly influencing Earth&#8217;s energy balance. The magnitude of aerosol-induced changes in cloud properties is still uncertain. This is primarily due to the meteorological covariability between aerosols and clouds, which hinders inferring causal relationships. Industrial air pollution sources serve as natural experiments to study strong anthropogenic cloud perturbations (Toll et al. 2019 <em>Nature </em>https://doi.org/10.1038/s41586-019-1423-9) and allow us to infer causal relationships between aerosols and clouds.</p> <p>&#160;</p> <p>We use geostationary satellite observations to study the temporal evolution of polluted clouds. Polluted clouds are usually thinner than nearby unpolluted clouds. But in some cases, the polluted clouds grow much thicker in the afternoon than the nearby unpolluted clouds. We find that continental polluted cloud tracks are relatively long-lived, with a median lifetime of 18 hours. Moreover, there are many cases where polluted cloud tracks are visible for multiple consecutive days. This means polluted cloud tracks live long enough for clouds to fully adjust to aerosol-increased cloud droplet numbers. Future work is needed to combine geostationary and polar orbiting satellite observations of polluted cloud tracks to develop stronger observational constraints for aerosol-cloud interactions.</p>
<p>The cooling of the Earth&#8217;s climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. We discuss how causal relationship between aerosols and clouds can be derived from contrast between clouds polluted by anthropogenic aerosols and nearby unpolluted clouds. Ship tracks have been long considered to be real-world laboratories of aerosol-cloud interactions. More recently, polluted cloud tracks induced by aerosols emitted from volcanoes and wildfires and various industrial sources - such as oil refineries, smelters, coal-fired power plants, and cities have been analysed (Toll et al. 2019; Nature, https://doi.org/10.1038/s41586-019-1423-9). In this research, we extend satellite observations of polluted cloud tracks from Toll et al. (2019) with analysis of smaller and larger scale polluted cloud areas detected in satellite images.</p><p>&#160;</p><p>Polluted clouds are detected in MODIS and SEVIRI satellite images as areas with strongly increased cloud droplet number concentrations. Polluted cloud tracks can be utilized to study frequency and magnitude of anthropogenic cloud droplet number perturbations and subsequent cloud adjustments. Anthropogenic aerosol perturbations on liquid-water clouds are detected in various major global industrial areas. Both tens of kilometres wide ship-track-like polluted cloud tracks and hundreds by hundreds of kilometres wide polluted cloud areas show that cloud water can both increase and decrease in response to aerosols depending on meteorological conditions. On average, there is relatively weak decrease in cloud water. Polluted cloud tracks also show that cloud fraction can both increase and decrease compared to nearby less polluted clouds. Applicability of pollution tracks to study impact of absorbing aerosols situated above clouds on below-lying clouds is discussed. We expect that utilization of real-world laboratories of aerosol impacts on clouds will lead to improved physical parameterizations in global climate models and more reliable projections of the future climate.</p>
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