Abstract
Abstract. Empirical relationships that link cloud droplet number (CDN) to aerosol number or mass are commonly used to calculate global fields of CDN for climate forcing assessments. In this work we use a sectional global model of sulfate and sea-salt aerosol coupled to a mechanistic aerosol activation scheme to explore the limitations of this approach. We find that a given aerosol number concentration produces a wide range of CDN concentrations due to variations in the shape of the aerosol size distribution. On a global scale, the dependence of CDN on the size distribution results in regional biases in predicted CDN (for a given aerosol number). Empirical relationships between aerosol number and CDN are often derived from regional data but applied to the entire globe. In an analogous process, we derive regional "correlation-relations" between aerosol number and CDN and apply these regional relations to calculations of CDN on the global scale. The global mean percentage error in CDN caused by using regionally derived CDN-aerosol relations is 20 to 26%, which is about half the global mean percentage change in CDN caused by doubling the updraft velocity. However, the error is as much as 25–75% in the Southern Ocean, the Arctic and regions of persistent stratocumulus when an aerosol-CDN correlation relation from the North Atlantic is used. These regions produce much higher CDN concentrations (for a given aerosol number) than predicted by the globally uniform empirical relations. CDN-aerosol number relations from different regions also show very different sensitivity to changing aerosol. The magnitude of the rate of change of CDN with particle number, a measure of the aerosol efficacy, varies by a factor 4. CDN in cloud processed regions of persistent stratocumulus is particularly sensitive to changing aerosol number. It is therefore likely that the indirect effect will be underestimated in these important regions.
Highlights
The prediction of cloud droplet number (CDN) in a global aerosol model is a challenging task, but is vital if we are to reduce the uncertainty surrounding the quantification of the aerosol indirect effect
We show that none of the frequently used aerosol-CDN relations is able to capture regional variations in CDN caused by systematic variations in the aerosol size distribution
We find that the probability of a given aerosol number concentration producing a relatively high/low CDN concentration has a distinct global pattern that can be explained in terms of predictable variations in the aerosol size distribution
Summary
The prediction of cloud droplet number (CDN) in a global aerosol model is a challenging task, but is vital if we are to reduce the uncertainty surrounding the quantification of the aerosol indirect effect. The alternative to an empirical relation is to calculate cloud drop number in a physically based or mechanistic way (e.g., Abdul-Razzak and Ghan, 2000; Nenes and Seinfeld, 2003; Fountoukis and Nenes, 2005; Ming et al, 2006). In this approach the modeled aerosol particle size distribution is used to calculate an activation diameter for a particular cloud parcel updraft speed. We restrict the study to variations in CDN at cloud base for an assumed updraft velocity and do not attempt to calculate prognostic droplet number taking into account collision-coalescence and other cloud microphysical processes.
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