A comparison between trajectory ensemble and adiabatic parcel modeled cloud properties and evaluation against airborne measurements

Abstract

A trajectory ensemble model (TEM) and an adiabatic air parcel model have been used to study the sensitivity of cloud droplet number concentration on aerosol chemical composition and vertical wind speed variations in the case of stratocumulus clouds. Cloud droplet number concentrations predicted using the TEM approach, in which trajectories are produced by large eddy simulation, and those derived from an air parcel model fed with a probability density function of measured updraft velocities, are compared to cloud droplet measurements conducted during the Cloud Processing of Regional Air Pollution Advecting Over Land and Sea (CLOPAP) campaign. It was found that much better agreement can be achieved using TEM than with the adiabatic air parcel model. The reason for this is the ripening process decreasing the cloud droplet number concentration in the air parcels having long in-cloud residence time. It was also found that the sensitivity of cloud droplet number concentration to aerosol properties is much smaller with TEM than expected from the adiabatic air parcel model simulations. As current parameterizations used to estimate cloud droplet number concentration in many large-scale models are based on adiabatic air parcel models, it is possible that the aerosol indirect effect for stratocumulus and stratus clouds with low vertical wind speeds is overestimated unless the sink resulting from ripening is taken into account. Copyright 2009 by the American Geophysical Union.