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Some impact of pollutants on the development and optical properties of stratocumulus clouds
Author(s) -
Ghosh S.,
Jonas P. R.,
Wood R.
Publication year - 2000
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712656912
Subject(s) - parametrization (atmospheric modeling) , radiative transfer , aerosol , environmental science , large eddy simulation , meteorology , liquid water content , atmospheric sciences , marine stratocumulus , cloud computing , physics , computer science , turbulence , quantum mechanics , operating system
Two major recent field programmes—the European Cloud Radiation Experiment (EUCREX) and the Aerosol Characterization Experiment II (ACE‐2)‐have extensively analysed the dynamical, microphysical and radiative attributes of stratocumulus clouds contaminated by continental air. Although an extensive set of dynamical and microphysical data are now available, there are no accounts of any matching theoretical modelling studies. To fully understand and numerically model the interplay between the dynamics, microphysics, radiative and chemical properties of the two clouds chosen for our case‐studies would require a full three‐dimensional large‐eddy simulation (LES) model coupled to a full‐size resolving microphysical model where the computational costs would be prohibitive. In this study we have ‘optimized’ the classic Kessler parametrization scheme so that it is effectively able to distinguish between clean and contaminated clouds. We perform LES runs with the optimized scheme to study the morphology and the dynamics of the clouds and then use a second one‐dimensional microphysical parcel model run with identical environmental conditions to study the effects of pollution on the clouds as well as the droplet spectral evolution. This procedure yields extremely good agreement with observations at modest computational expense. It is shown that nitric acid (HNO 3 ) vapour in the parts per billion by volume (p.p.b.v.) range affects cloud formation by increasing the number of cloud droplets and decreasing the mean size compared to an acid‐free simulation. The effects of HNO 3 contamination on the EUCREX case‐study is evident owing to the proximity of this cloud to sources of air pollutants. With 10 p.p.b.v. of HNO 3 , we are able to achieve good agreement with the observations of the droplet effective radii as well as with observations of the optical‐depth variation. For the ACE‐2 cloud which formed further away from sources of pollutants, even on a typical ‘polluted’ day when the ambient HNO 3 was ∼5 parts per trillion by volume, the drop concentration was found to be insensitive to changes in the HNO 3 .