Abstract
Abstract. Aerosol effects on low-level clouds over the Nordic Countries are investigated by combining in situ ground-based aerosol measurements with remote sensing data of clouds and precipitation. Ten years of number size distribution data from two aerosol measurement stations (Vavihill, Sweden and Hyytiälä, Finland) provide aerosol number concentrations in the atmospheric boundary layer. This is combined with cloud satellite data from the Moderate Resolution Imaging Spectroradiometer and weather radar data from the Baltic Sea Experiment. Also, how the meteorological conditions affect the clouds is investigated using reanalysis data from the European Centre for Medium-Range Weather Forecasts. The cloud droplet effective radius is found to decrease when the aerosol number concentration increases, while the cloud optical thickness does not vary with boundary layer aerosol number concentrations. Furthermore, the aerosol–cloud interaction parameter (ACI), a measure of how the effective radius is influenced by the number concentration of cloud active particles, is found to be somewhere between 0.10 and 0.18 and the magnitude of the ACI is greatest when the number concentration of particles with a diameter larger than 130 nm is used. Lower precipitation intensity in the weather radar images is associated with higher aerosol number concentrations. In addition, at Hyytiälä the particle number concentrations is generally higher for non-precipitating cases than for precipitating cases. The apparent absence of the first indirect effect of aerosols on low-level clouds over land raises questions regarding the magnitude of the indirect aerosol radiative forcing.
Highlights
Aerosol particles are required for clouds to form under atmospheric conditions
At 0.1 % supersaturation the ratios are greater than one implying that N130 is higher than the number of aerosol particles that are activated by the cloud condensation nuclei (CCN) counter
Ten years of ground-based aerosol measurements have been combined with satellite cloud retrievals and weather radar precipitation data to provide insights on how aerosols affect continental low-level clouds over the Nordic countries
Summary
Aerosol particles are required for clouds to form under atmospheric conditions. The aerosol loading in the atmosphere has significantly increased since the beginning of the industrial revolution, which may have altered the microphysical properties of clouds (Boucher et al, 2013). Smaller cloud droplets have been hypothesized to lead to a reduction in drizzle from the clouds, which could lead to a prolonged cloud lifetime (Albrecht, 1989) These anthropogenic impacts that the aerosols have on clouds are named the first and second indirect aerosol effects. Low-level stratiform clouds mainly reflect incoming short-wave solar radiation but do not affect the outgoing long-wave radiation to any great degree (Boucher et al, 2013). These clouds are very common over subtropical oceans but are present over land and other parts of the ocean
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