Abstract
Abstract. In this work, we postulate, implement and evaluate modifications to the "population-splitting" concept, introduced by Nenes and Seinfeld (2003), for calculation of water-condensation rates in droplet-activation parameterizations. The population-splitting approximation consists of dividing the population of growing droplets into two categories: those that experience significant growth after exposed to a supersaturation larger than their critical supersaturation, and those that do not grow much larger than their critical diameter. The modifications introduced here lead to an improved accuracy and precision of the parameterization-derived maximum supersaturation, smax, and droplet-number concentration, Nd, as determined by comparing against those of detailed numerical simulations of the activation process. A numerical computation of the first-order derivatives ∂ Nd/∂ χj of the parameterized Nd to input variables χi was performed and compared against the corresponding parcel-model-derived sensitivities, providing a thorough evaluation of the impacts of the introduced modifications in the parameterization ability to respond to aerosol characteristics. An evaluation of the parameterization computation of Nd and smax against detailed numerical simulations of the activation process showed a relative error of −6.0% ± 6.2% for smax, and −2.7% ± 4.8% for Nd, which represents a considerable reduction in prediction bias when compared to earlier versions of the parameterization. The proposed modifications require only minor changes for their numerical implementation in existing codes based on the population-splitting concept.
Highlights
During the process of cloud formation, preexisting aerosol particles act as cloud condensation nuclei (CCN), upon which cloud droplets first form and subsequently grow
Calculation of droplet number in atmospheric models requires the computation of new droplet formation, which occurs at subgrid scales and its representation is computationally expensive if done explicitly using numerical parcel models
We present the results of an evaluation of the parameterization performance against predictions of smax and Nd computed with a detailed numerical parcel model of the condensation growth of droplets
Summary
During the process of cloud formation, preexisting aerosol particles act as cloud condensation nuclei (CCN), upon which cloud droplets first form and subsequently grow. Calculation of droplet number in atmospheric models requires the computation of new droplet formation (i.e., droplet activation), which occurs at subgrid scales and its representation is computationally expensive if done explicitly using numerical parcel models. For this reason, parameterizations of the activation process have been developed. In these formulations, the fraction of atmospheric aerosol that activates into cloud droplets is determined for an air parcel that ascends with an updraft velocity (w) These activation parameterizations use a Lagrangian-parcel-model approach to study the detailed process of water-vapor condensation on the population of growing droplets. The availability of CCN is determined as function of supersaturation (e.g., using Köhler’s theory or an adsorption-activation theory, together with aerosol-size distribution and chemical composition), and second, by approximately solving the water-vapor balance in the Published by Copernicus Publications on behalf of the European Geosciences Union
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