Diurnal Evolution of Cloud Water Responses to Aerosols

Abstract

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).

Evaluation of Aerosol-Cloud Interactions in E3SM using a Lagrangian Framework

“…Another potential caveat is the influence of the diurnal cycle on the cloud properties. LWPs are more decreased in polluted clouds during the afternoon and evening hours, a result that broadly agrees with observations of polluted cloud tracks (Rahu et al, 2022). The cloud fraction is decreased in polluted clouds the most during the night into early morning hours in the E3SM model.…”

Strong Anthropogenic Cloud Perturbations Can Persist for Multiple Days

“…We successfully detected polluted cloud tracks using geostationary SEVIRI data, but there were also multiple challenges, similar to previous work by Rahu et al (2022). Although there is a noticeable contrast between polluted and nearby unpolluted clouds both in the MODIS snapshots and SEVIRI data (Figure 1), the contrast is often substantially weaker in SEVIRI data.…”

“…Our results can be used to evaluate the quality of the machine learning methods (Watson-Parris et al, 2022;Yuan et al, 2019Yuan et al, , 2022 in sampling the lifespan of polluted cloud tracks. In the future, we plan to use geostationary satellite data to study the temporal evolution of cloud properties downwind of the aerosol point sources (Gryspeerdt et al, 2021;Rahu et al, 2022).…”

“… The number of polluted cloud tracks sampled for each studied site/city. The steel production site in Cherepovets, Russia, is previously known to frequently lead to anthropogenic cloud perturbations (Rahu et al., 2022; Trofimov et al., 2022). The pollution tracks were sampled in the autumns of 2006–2021.…”

“…The study focused on polluted cloud tracks in Eastern Europe (Figure 4), as ship‐track‐like polluted cloud tracks frequently occur in this region (Rahu et al., 2022; Toll et al., 2019). We sample the tracks in autumn (September, October, and November) from 2006 to 2021 (Figure 5a), as tracks most frequently occur in autumn in the study area (Rahu et al., 2022). The sampled anthropogenic aerosol sources inducing the pollution tracks include oil refineries, metallurgical, chemical and other industries (Table 1, Figure 6).…”