Long-term measurements of cloud droplet concentrations and aerosol–cloud interactions in continental boundary layer clouds

The effects of aerosol on cloud droplet effective radius (R_eff), cloud optical thickness and cloud droplet number concentration (N _d) are analysed both from long-term direct ground-based in situ measurements conducted at the Puijo measurement station in Eastern Finland and from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Terra and Aqua satellites. The mean in situ N _d during the period of study was 217 cm⁻³, while the MODIS-based N _d was 171 cm⁻³. The absolute values, and the dependence of both N _d observations on the measured aerosol number concentration in the accumulation mode (N _acc), are quite similar. In both data sets N _d is clearly dependent on N _acc, for N _acc values lower than approximately 450 cm⁻³. Also, the values of the aerosol–cloud-interaction parameter [ACI=(1/3)*d ln(N _d)/d ln(N _acc)] are quite similar for N _acc<400 cm⁻³ with values of 0.16 and 0.14 from in situ and MODIS measurements, respectively. With higher N _acc (>450 cm⁻³) N _d increases only slowly. Similarly, the effect of aerosol on MODIS-retrieved R _eff is visible only at low N _acc values. In a sub set of data, the cloud and aerosol properties were measured simultaneously. For that data the comparison between MODIS-derived N _d and directly measured N _d, or the cloud droplet number concentration estimated from N _acc values (N _d,p), shows a correlation, which is greatly improved after careful screening using a ceilometer to make sure that only single cloud layers existed. This suggests that such determination of the number of cloud layers is very important when trying to match ground-based measurements to MODIS measurements.