Time-dependent, non-monotonic response of warm convective cloud fields to changes in aerosol loading

Guy Dagan,Ilan Koren,Reuven H Heiblum,Orit Altaratz

doi:10.5194/acp-17-7435-2017

Guy Dagan, Ilan Koren + Show 2 more

Open Access

https://doi.org/10.5194/acp-17-7435-2017

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Abstract

Abstract. Large eddy simulations (LESs) with bin microphysics are used here to study cloud fields' sensitivity to changes in aerosol loading and the time evolution of this response. Similarly to the known response of a single cloud, we show that the mean field properties change in a non-monotonic trend, with an optimum aerosol concentration for which the field reaches its maximal water mass or rain yield. This trend is a result of competition between processes that encourage cloud development versus those that suppress it. However, another layer of complexity is added when considering clouds' impact on the field's thermodynamic properties and how this is dependent on aerosol loading. Under polluted conditions, rain is suppressed and the non-precipitating clouds act to increase atmospheric instability. This results in warming of the lower part of the cloudy layer (in which there is net condensation) and cooling of the upper part (net evaporation). Evaporation at the upper part of the cloudy layer in the polluted simulations raises humidity at these levels and thus amplifies the development of the next generation of clouds (preconditioning effect). On the other hand, under clean conditions, the precipitating clouds drive net warming of the cloudy layer and net cooling of the sub-cloud layer due to rain evaporation. These two effects act to stabilize the atmospheric boundary layer with time (consumption of the instability). The evolution of the field's thermodynamic properties affects the cloud properties in return, as shown by the migration of the optimal aerosol concentration toward higher values.

Highlights

Despite the extensive research conducted in the last few decades and the fact that clouds have an important role in the Earth’s energy balance (Trenberth et al, 2009), clouds are still considered to be one of the largest sources of uncertainty in the study of climate and climate change (Forster et al, 2007; Boucher et al, 2013).Warm-cloud formation depends on the availability of water vapor and aerosols acting as cloud condensation nuclei (CCN)
The total water mass as a function of aerosol concentration shows a clear reversal in the trend (Fig. 1a)
This difference can be explained by the link to the cloud fraction (CF) that decreases above an aerosol loading of 25 cm−3

Summary

Introduction

Despite the extensive research conducted in the last few decades and the fact that clouds have an important role in the Earth’s energy balance (Trenberth et al, 2009), clouds are still considered to be one of the largest sources of uncertainty in the study of climate and climate change (Forster et al, 2007; Boucher et al, 2013).Warm-cloud (containing liquid water only) formation depends on the availability of water vapor and aerosols acting as cloud condensation nuclei (CCN). Changes in aerosol concentration modulate the cloud droplet size distribution and total number. Polluted clouds (forming under high aerosol loading) initially have smaller and more numerous droplets, with a narrower size distribution compared to clean clouds (Squires, 1958; Squires and Twomey, 1960; Warner and Twomey, 1967; Fitzgerald and Spyers-Duran, 1973). The initial droplet size distribution affects key cloud processes such as condensation–evaporation, collision– coalescence and sedimentation. The condensation– evaporation process is proportional to the total droplet surface area, which increases with the droplet number concentration (for a given total liquid water mass). The condensation in polluted clouds is more efficient (higher condensation rate or shorter consumption time of the supersaturation – Pinsky et al, 2013; Seiki and Nakajima, 2014; Koren et al, 2014; Kogan and Martin, 1994; Dagan et al, 2015a). The evaporation induces downdrafts and stronger vorticity and can lead to stronger mixing of the cloud with its environment in polluted conditions (Xue and Feingold, 2006; Jiang et al, 2006; Small et al, 2009)

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