Abstract
Abstract. Large cloud condensation nuclei (CCN) (e.g., aged dust particles and seasalt) cannot attain their equilibrium size during the typical timescale of cloud droplet activation. Cloud activation parameterizations applied to aerosol with a large fraction of large CCN often do not account for this limitation adequately and can give biased predictions of cloud droplet number concentration (CDNC). Here we present a simple approach to address this problem that can easily be incorporated into cloud activation parameterizations. This method is demonstrated with activation parameterizations based on the "population splitting" concept of Nenes and Seinfeld (2003); it is shown that accounting for large CCN effects eliminates a positive bias in CDNC where the aerosol dry geometric diameter is greater than 0.5 μm. The method proposed here can also be extended to include the water vapor depletion from pre-existing droplets and ice crystals in global and regional atmospheric models.
Highlights
Cloud droplet activation is the direct microphysical link between aerosol and clouds, and its accurate description is essential for studying aerosol indirect climate effects
In accordance with Kohler theory, cloud condensation nuclei (CCN) activate into cloud droplets when the ambient supersaturation is above the global maximum of their equilibrium curve and sufficient time is allowed for their wet size to exceed their critical diameter (Seinfeld and Pandis, 1998)
Every physically-based droplet formation parameterization conceptually consists of two steps, one involving the determination of the “CCN spectrum” and one determining the maximum supersaturation, smax, that develops in the ascending parcel
Summary
Cloud droplet activation is the direct microphysical link between aerosol and clouds, and its accurate description is essential for studying aerosol indirect climate effects. Droplet growth may be subject to a variety of kinetic limitations, one of which is the so-called “inertial mechanism” (Nenes et al, 2001) Due to their large dry size, inertially-limited particles, having very low critical supersaturation, cannot attain their critical size within the timescale typically associated with activation in clouds. The approach of Twomey (1959) is used for the latter, which works for most atmospheric aerosol and presumes the size of droplets at cloud base is negligible compared to the growth experienced up to the level of maximum supersaturation in the cloud (e.g., Twomey, 1959; Nenes and Seinfeld, 2003) This assumption is subject to increasingly-large error as the dry particle size increases and can lead to significant underestimation in droplet size and surface area if giant CCN are present. The application of this method does not require reformulation of a parameterization, and is illustrated using the parameterizations of Nenes and Seinfeld (2003) and Fountoukis and Nenes (2005)
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