The potential impacts of pollution on a nondrizzling stratus deck: Does aerosol number matter more than type?

Abstract

[1] In this paper results from a cloud-resolving model that can efficiently examine the impact of aerosol on nondrizzling stratus clouds will be shown. Because the model tracks aerosol and cloud droplets in a Lagrangian framework, it does not suffer from numerical errors associated with advection, and unlike most Eulerian approaches, the method can track cloud boundaries as they move across a grid cell. After illustrating the capability of the model to reproduce various observed cloud statistics such as the cloud water mixing ratio and the mean cloud droplet radius from the DYCOMS-II field program, the ability of the model to assess the impact of changes in aerosol number and composition on a stratus deck will be highlighted. Specifically, by using activation curves appropriate for soluble, insoluble, or a mixture of both types of aerosol and for certain extreme aerosol regimes, i.e., a majority of the aerosol are hydrophobic carbon aerosol, limiting situations were examined to bound their impact on clouds. However, though these situations may be somewhat extreme, they could occasionally occur in the atmosphere, e.g., an oceanic stratus field downwind of a large ship or an urban area. Not unexpectedly, results from these simulations support previous ship track observations that for increasing aerosol numbers, cloud droplet number concentrations increase, whereas cloud droplet radii decrease. However, these simulations also suggest that the correlation between cloud droplet number concentration and aerosol number concentration may be not only a function of aerosol number concentration but also aerosol types and/or cloud dynamics.

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

“…The mechanism of CDSD broadening in this study requires the model to consider both solute and curvature effects all the time (i.e., before and after activation, deactivation and reactivation). Our results suggest the importance of solute and curvature effects to the deactivation and reactivation processes, which are consistent with previous studies (e.g., Andrejczuk et al, 2008;Hoffmann et al, 2015;Hoffmann, 2017;Chen et al, 2018). However the results are counter to some other studies where details of activation and deactivation are argued to be unimportant in the cloud simulation (e.g., Srivastava, 1991;Chuang et al, 1997;Grabowski et al, 2018).…”

“…Therefore, the growth of the large droplet here is dominated by its in-cloud lifetime. Previous studies show that although the mean lifetime of cloud droplets is usually less than half an hour, the residence time for some lucky cloud droplets can be longer than 1 h (e.g., Feingold et al, 1996;Kogan, 2006;Andrejczuk et al, 2008). Those long-lifetime cloud droplets might contribute to large droplets in the cloud, similar to long-lifetime ice particles in mixed-phase clouds (Yang et al, 2015).…”

“…The latter can cause cloud droplet evaporation, deactivation and reactivation (Korolev et al, 2013;Yang et al, 2016). In addition, the lifetime of the cloud parcel is usually less than 1 h (Andrejczuk et al, 2008). Therefore, one should be aware that results in this study are based on a very idealized state.…”

Droplet growth in warm turbulent clouds

“…Recently, a different Lagrangian approach was developed, with the focus not on air parcels but on particles carried by the flow such as cloud droplets. In such an approach, the Eulerian fluid flow model is coupled to the Lagrangian representation of the thermodynamics (see, for example, Andrejczuk et al, 2008Andrejczuk et al, , 2010Shima et al, 2009). This approach is in the spirit of the pseudo-LES simulations presented in Lanotte et al (2009) and discussed in section 4.1.3.…”

Meeting Summaries

“…Cloud-resolving models, which treat aerosol-cloud processes in detail, are used to incorporate laboratory and field observations to develop parameterizations for coarse global climate models (Tao et al 2007). A new comprehensive cloud-resolving model that explicitly treats aerosol activation processes in detail was used to successfully reproduce field observations, which previous models had failed to do (Andrejczuk et al 2008). These new methods will stimulate improvements in treatment of cloud processes in next-generation climate models.…”