Ensemble daily simulations for elucidating cloud–aerosol interactions under a large spread of realistic environmental conditions

Abstract

<p><strong>Abstract.</strong> Aerosol effects on cloud properties and the atmospheric energy and radiation budgets are studied through ensemble simulations over two month-long periods during the NARVAL campaigns (December 2013 and August 2016). For each day, two simulations are conducted with low and high cloud droplet number concentrations (CDNC), representing low and high aerosol concentrations, respectively. This large data-set, which is based on a large spread of co-varying realistic initial conditions, enables robust identification of the effect of CDNC changes on cloud properties. We show that increases in CDNC drive a reduction in the top of atmosphere (TOA) net shortwave flux (more reflection) and a decrease in the lower tropospheric stability for all cases examined, while the TOA longwave flux and the liquid and ice water path changes are generally positive. However, changes in cloud fraction or precipitation, that could appear significant for a given day, are not as robustly affected, and, at least for the summer month, are not statistically distinguishable from zero. These results highlight the need for using large statistics of initial conditions for cloud–aerosol studies for identifying the significance of the response. In addition, we demonstrate the dependence of the aerosol effects on the season, as it is shown that the TOA net radiative effect is doubled during the winter month as compared to the summer month. By separating the simulations into different dominant cloud regimes, we show that the difference between the different months emerge due to the compensation of the longwave effect induced by an increase in ice content as compared to the shortwave effect of the liquid clouds. The CDNC effect on the longwave is stronger in the summer as the clouds are deeper and the atmosphere is more unstable.</p>

The Impact of Resolving Subkilometer Processes on Aerosol‐Cloud Interactions of Low‐Level Clouds in Global Model Simulations

“…A second implication of our study is that the seasonal cycle in the ACIs can differ between different model configurations (SPCAM and UPCAM). This result, which supports that of Dagan and Stier (2020), highlights the importance of covering a wide range of meteorological contexts and seasons when comparing ACIs, particularly in high‐resolution models where computational costs of running simulations constrains decisions about the variety and duration of simulations.…”

Sub‐Tropical Aerosols Enhance Tropical Cloudiness—A Remote Aerosol‐Cloud Lifetime Effect

“…A major source of uncertainty is related to the cloud response to anthropogenic aerosols (Bellouin et al., 2019). It has been shown that aerosol‐cloud‐interaction (ACI) mechanisms are cloud‐regime‐dependent (Altaratz et al., 2014; Christensen et al., 2016; Dagan & Stier, 2020b; Gryspeerdt & Stier, 2012), and most previous studies have evaluated them on a per‐regime basis. However, since the different cloud regimes in the atmosphere are connected by the global circulation, aerosol effects on one regime may also affect the properties of different regimes, as well as the transition between the different regimes (Christensen et al., 2020; Dagan & Chemke, 2016; Goren et al., 2019).…”

Strong control of effective radiative forcing by the spatial pattern of absorbing aerosol

“…However, understanding the link between AA, circulation and ERF remains challenging 28 . Aerosol-cloud interactions are also known to depend on both the cloud regime and meteorological conditions [31][32][33] . Thus, through the co-location of aerosols with different cloud types, we may also expect a location-dependence through microphysical changes (however, this pathway is not examined here).…”