Abstract
Abstract. Aerosol-cloud interactions constitute a major uncertainty in future climate predictions. This study combines 10 years of ground-based aerosol particle measurements from two Nordic background stations (Vavihill and Hyytiälä) with MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data of convective clouds. The merged data are used to examine how aerosols affect cloud droplet sizes and precipitation from convective clouds over the Nordic countries. From the satellite scenes, vertical profiles of cloud droplet effective radius (re) are created by plotting retrieved cloud top re against cloud top temperature for the clouds in a given satellite scene. The profiles have been divided according to aerosol number concentrations but also meteorological reanalysis parameters from the ECMWF (European Centre for Medium-Range Forecasts). Furthermore, weather radar data from the BALTEX (Baltic Sea Experiment) and precipitation data from several ground-based meteorological measurement stations have been investigated to determine whether aerosols affect precipitation intensity and amount. Small re throughout the entire cloud profiles is associated with high aerosol number concentrations at both stations. However, aerosol number concentrations seem to affect neither the cloud optical thickness nor the vertical extent of the clouds in this study. Cloud profiles with no or little precipitation have smaller droplets than those with more precipitation. Moreover, the amount of precipitation that reaches the ground is affected by meteorological conditions such as the vertical extent of the clouds, the atmospheric instability and the relative humidity in the lower atmosphere rather than the aerosol number concentration. However, lower precipitation rates are associated with higher aerosol number concentrations for clouds with similar vertical extent. The combination of these ground-based and remote-sensing datasets provides a unique long-term study of the effects of aerosols on convective clouds over the Nordic countries.
Highlights
That aerosol particles have the ability to affect clouds has been known for several decades (Twomey, 1974)
The aim of this study was to investigate how convective clouds over Scandinavia are affected by boundary layer aerosol particles
Varying particle number concentration affects the clouds from the base throughout the vertical extent of the profiles since the profiles line up nicely according to N80 at all levels (Fig. 4a–c)
Summary
That aerosol particles have the ability to affect clouds has been known for several decades (Twomey, 1974). Aerosols in the atmosphere can affect the climate through scattering and absorption of incoming solar radiation and to a minor degree by influencing outgoing thermal radiation. This is called the direct aerosol effect, while anthropogenic aerosol effects on climate through interactions with clouds has been named the indirect effect (Lohmann and Feichter, 2005). The latter includes increased cloud albedo due to smaller but more numerous droplets (Twomey, 1974), suppression of drizzle, increased cloud lifetime (Albrecht, 1989), increased cloud height (Pincus and Baker, 1994) and semi-direct effects (Hansen et al, 1997). Previous studies have revealed various aerosol effects on clouds depending on cloud type and meteorological conditions (e.g. Khain et al, 2008; Fan et al, 2009; Lee et al, 2010)
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