Abstract
Abstract. 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 (Next-generation Aircraft Remote-Sensing for Validation Studies, December 2013 and August 2016). For each day, two simulations are conducted with low and high cloud droplet number concentrations (CDNCs), 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 a large sample 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 emerges 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 flux is stronger in the summer as the clouds are deeper and the atmosphere is more unstable.
Highlights
Cloud droplets form on suitable aerosols which can serve as cloud condensation nuclei
The different cloud droplet number concentrations (CDNCs) scenarios serve as proxy for different aerosol concentration conditions and are chosen as they represent the range typically observed over the ocean (Rosenfeld et al, 2019; Gryspeerdt et al, 2019)
We previously showed that an increase in CDNC drives an increase in the ice content at the upper troposphere and a reduction in the outgoing LW radiation (Dagan et al, 2020); here we show that this trend is statistically significant (Fig. 5c)
Summary
Cloud droplets form on suitable aerosols which can serve as cloud condensation nuclei. It was shown that the total column atmospheric radiative warming (QR = (FSTWOA − FSSWFC) + (FLTWOA − FLSWFC), defined as the rate of net atmospheric diabatic warming due to radiative shortwave (SW) and longwave (LW) fluxes at the surface (SFC) and top of the atmosphere (TOA), with all fluxes positive downwards) is substantially increased with CDNC in a deep-cloud-dominated case (by ∼ 10 W m−2), while a much smaller increase (∼ 1.6 W m−2) is shown in a shallow-cloud-dominated case This trend is caused by an increase in the upward mass flux of ice and water vapour to the upper troposphere that leads to reduced outgoing longwave radiation (Fan et al, 2012). Barbados is located north of the mean Intertropical Convergence Zone (ITCZ) location, in a way that samples both the trade region, dominated by shallow cumulus during the boreal winter, and the transition to deep convection as the ITCZ migrates northward during boreal summer (Stevens et al, 2016) This location enables the investigation of different cloud regimes and different meteorological conditions. The clouds near Barbados have been shown to be representative of clouds across the trade winds region (Medeiros and Nuijens, 2016)
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