Abstract
Abstract. The marine-cloud brightening geoengineering technique has been suggested as a possible means of counteracting the positive radiative forcing associated with anthropogenic atmospheric CO2 increases. The focus of this study is to quantify the albedo response to aerosols injected into marine stratocumulus cloud from a point source at different times of day. We use a cloud-resolving model to investigate both weakly precipitating and non-precipitating regimes. Injection into both regimes induces a first indirect aerosol effect. Additionally, the weakly precipitating regime shows evidence of liquid water path gain associated with a second indirect aerosol effect that contributes to a more negative radiative forcing, and cloud changes indicative of a regime change to more persistent cloud. This results in a cloud albedo increase up to six times larger than in the non-precipitating case. These indirect effects show considerable variation with injection at different times in the diurnal cycle. For the weakly precipitating case, aerosol injection results in domain average increases in cloud albedo of 0.28 and 0.17 in the early and mid morning (03:00:00 local time (LT) and 08:00:00 LT respectively) and 0.01 in the evening (18:00:00 LT). No cloud develops when injecting into the cloud-free early afternoon (13:00:00 LT). However, the all-sky albedo increases (which include both the indirect and direct aerosol effects) are highest for early morning injection (0.11). Mid-morning and daytime injections produce increases of 0.06, with the direct aerosol effect compensating for the lack of cloud albedo perturbation during the cloud-free early afternoon. Evening injection results in an increase of 0.04. For the weakly precipitating case considered, the optimal injection time for planetary albedo response is the early morning. Here, the cloud has more opportunity develop into a more persistent non-precipitating regime prior to the dissipative effects of solar heating. The effectiveness of the sea-spray injection method is highly sensitive to diurnal injection time and the direct aerosol effect of an intense aerosol point source. Studies which ignore these factors could overstate the effectiveness of the marine cloud brightening technique.
Highlights
Geoscientific Gtheeoaemngoiunneteroinf gMsoslcoahrdermeadelsiaDhtiaoevnevrbeeealeconhpinpmgroptehoesneEdtatrhtha’ts dseucrrfeaacsee, M cloud albedo increase up to six times larger than in the non- creating a cooling effect that could potentially ameliorate fuprecipitating case
This study investigates the efficacy of aerosol injection on both cloud albedo and all-sky albedo increases, with injection at different times in the diurnal cycle, and at three different aerosol injection rates
The variations in simulated cloud properties are consistent with characteristic changes in radiative and dynamical features of the stratocumulus topped boundary layer over the diurnal cycle, and follow the similar or smaller domain sizes that have previously been used to study the behaviour of marine stratocumulus clouds (e.g. Stevens et al, 2005; Ackerman et al, 2009) and their sensitivities to atmospheric variables over the diurnal cycle (Chen et al, 2011)
Summary
We use the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem) V3.3.1 (Skamarock et al, 2008) in the large-eddy simulation (LES) configuration. Wang and Feingold, 2009a, b; Feingold et al, 2010; Wang et al, 2011) Despite these exclusions, the variations in simulated cloud properties are consistent with characteristic changes in radiative and dynamical features of the stratocumulus topped boundary layer over the diurnal cycle (discussed further in the Results and Discussion sections), and follow the similar or smaller domain sizes that have previously been used to study the behaviour of marine stratocumulus clouds (e.g. Stevens et al, 2005; Ackerman et al, 2009) and their sensitivities to atmospheric variables over the diurnal cycle (Chen et al, 2011)
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