Abstract
Abstract. For the first time, a liquid-water cloud study of the aerosol–cloud-dynamics relationship, solely based on lidar, was conducted. Twenty-nine cases of pure liquid-water altocumulus layers were observed with a novel dual-field-of-view Raman lidar over the polluted central European site of Leipzig, Germany, between September 2010 and September 2012. By means of the novel Raman lidar technique, cloud properties such as the droplet effective radius and cloud droplet number concentration (CDNC) in the lower part of altocumulus layers are obtained. The conventional aerosol Raman lidar technique provides the aerosol extinction coefficient (used as aerosol proxy) below cloud base. A collocated Doppler lidar measures the vertical velocity at cloud base and thus updraft and downdraft occurrence. Here, we present the key results of our statistical analysis of the 2010–2012 observations. Besides a clear aerosol effect on cloud droplet number concentration in the lower part of the altocumulus layers during updraft periods, turbulent mixing and entrainment of dry air is assumed to be the main reason for the found weak correlation between aerosol proxy and CDNC higher up in the cloud. The corresponding aerosol–cloud interaction parameter based on changes in cloud droplet number concentration with aerosol loading was found to be close to 0.8 at 30–70 m above cloud base during updraft periods and below 0.4 when ignoring vertical-wind information in the analysis. Our findings are extensively compared with literature values and agree well with airborne observations.
Highlights
The indirect aerosol effect on climate results from two cloudinfluencing aspects
In the framework of a feasibility study from 2008 to 2012, we investigated the potential of a novel cloud lidar (Schmidt et al, 2013; Schmidt, 2014) combined with a Doppler lidar to provide new insight into the influence of aerosol particles on the evolution of pure liquid-water altocumulus layers (Schmidt et al, 2014)
Disregarding this potential bias, the aerosol–cloud interaction effect is smallest in the cloud layer from 70 to 120 m with ACIN = 0 and strongest just above cloud base (ACIN = 0.38)
Summary
The indirect aerosol effect on climate results from two cloudinfluencing aspects. Atmospheric aerosol particles act as cloud condensation nuclei (CCN) in liquid-water droplet formation and as ice nuclei in processes of heterogeneous ice nucleation. 4) in which aerosol observations (at ground or far below cloud base) were correlated with remote-sensing products such as the cloud-column-averaged effective radius or cloud mean droplet number concentration to describe the impact of a given aerosol load on the evolution and microphysical properties of a cloud layer. In our opinion, such experimental approaches do not allow a proper quantification of ACI because the effects of aerosol and cloud dynamics cannot be resolved and separated. In our opinion ACI parameters should only be used to guide modeling groups to develop realistic microphysical parameterization schemes for the consideration of aerosol effects in the complex evolution of liquid-water clouds
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