On the importance of cumulus penetration on the microphysical and optical properties of stratocumulus clouds

[摘要] Owing to their extensive spatial coverage, stratocumulus clouds play acrucial role in the radiation budget of the earth. Climate models need anaccurate characterisation of stratocumulus in order to provide an accurateforecast. However, remote sensing as well as in-situ observations reveal thaton several occasions, cumulus clouds present below the stratocumulus, often have a significantimpact on the main stratocumulus microphysical properties. Thiswas observed during the ACE-2 (Aerosol Characterisation Experiment-2) campaigndesigned 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 theextent of this cumulus-stratocumuls coupling. In addition, we made extensive use ofmicrophysical observations from the C-130 aircraft that was operated during ACE-2.For the ACE-2 case studies considered in this paper, our analysis revealedthat the chemical, microphysical and optical characteristics of the main stratocumulus cloud deck hadsignificant contributions from cumulus clouds that often penetrated the stratocumulus deck.The amount of fine mode ionic species, the averagedroplet number concentrations, the effective radii and the optical depths during the flightA562 (when cumulus clouds interacted with the main stratocumulus) were estimated and model runs thatincluded this effect yielded microphysical and optical properties which compared more favourably withthe observations than the runs which did not. This study highlights the importance of including thesecumulus effects in stratocumulus related modelling studies.

[学科分类] 大气科学