Tellus: Series B, Chemical and Physical Meteorology (Oct 2013)

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

  • Irshad Ahmad,
  • Tero Mielonen,
  • Daniel P. Grosvenor,
  • Harri J. Portin,
  • Antti Arola,
  • Santtu Mikkonen,
  • Thomas Kühn,
  • Ari Leskinen,
  • Jorma Joutsensaari,
  • Mika Komppula,
  • Kari E. J. Lehtinen,
  • Ari Laaksonen,
  • Sami Romakkaniemi

DOI
https://doi.org/10.3402/tellusb.v65i0.20138
Journal volume & issue
Vol. 65, no. 0
pp. 1 – 10

Abstract

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The effects of aerosol on cloud droplet effective radius (Reff), cloud optical thickness and cloud droplet number concentration (Nd) 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 Nd during the period of study was 217 cm−3, while the MODIS-based Nd was 171 cm−3. The absolute values, and the dependence of both Nd observations on the measured aerosol number concentration in the accumulation mode (Nacc), are quite similar. In both data sets Nd is clearly dependent on N acc, for Nacc values lower than approximately 450 cm−3. Also, the values of the aerosol–cloud-interaction parameter [ACI=(1/3)*d ln(Nd)/d ln(Nacc)] are quite similar for Nacc<400 cm−3 with values of 0.16 and 0.14 from in situ and MODIS measurements, respectively. With higher Nacc (>450 cm−3) Nd increases only slowly. Similarly, the effect of aerosol on MODIS-retrieved Reff is visible only at low Nacc values. In a sub set of data, the cloud and aerosol properties were measured simultaneously. For that data the comparison between MODIS-derived Nd and directly measured N d, or the cloud droplet number concentration estimated from Nacc values (Nd,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.

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