On the importance of cumulus penetration on the microphysical and optical properties of stratocumulus clouds S. Ghosh1, S. Osborne2, and M. H. Smith1 1Institute for Atmospheric Science, School of the Environment, University of Leeds, Leeds, LS2 9JT, UK 2Met Office, Exeter, Devon, EX1 3PB, UK
Abstract. Owing to their extensive spatial coverage, stratocumulus clouds play a
crucial role in the radiation budget of the earth. Climate models need an
accurate characterisation of stratocumulus in order to provide an accurate
forecast. However, remote sensing as well as in-situ observations reveal that
on several occasions, cumulus clouds present below the stratocumulus, often have a significant
impact on the main stratocumulus microphysical properties. This
was observed during the ACE-2 (Aerosol Characterisation Experiment-2) campaign
designed to study the impact of polluted continental air on stratocumulus formation.
In this paper we used a detailed micro-physical chemical parcel model to quantify the
extent of this cumulus-stratocumuls coupling. In addition, we made extensive use of
microphysical observations from the C-130 aircraft that was operated during ACE-2.
For the ACE-2 case studies considered in this paper, our analysis revealed
that the chemical, microphysical and optical characteristics of the main stratocumulus cloud deck had
significant contributions from cumulus clouds that often penetrated the stratocumulus deck.
The amount of fine mode ionic species, the average
droplet number concentrations, the effective radii and the optical depths during the flight
A562 (when cumulus clouds interacted with the main stratocumulus) were estimated and model runs that
included this effect yielded microphysical and optical properties which compared more favourably with
the observations than the runs which did not. This study highlights the importance of including these
cumulus effects in stratocumulus related modelling studies.
Citation: Ghosh, S., Osborne, S., and Smith, M. H.: On the importance of cumulus penetration on the microphysical and optical properties of stratocumulus clouds, Atmos. Chem. Phys., 5, 755-765, doi:10.5194/acp-5-755-2005, 2005.