AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Droplet Activation Parameterization for Climate Models: The Population Splitting Concept Revisited
RICARDO MORALES BETANCOURT, Athanasios Nenes, Georgia Institute of Technology
Abstract Number: 338 Working Group: Aerosols, Clouds, and Climate
Abstract Aerosol-cloud interactions constitute one of the most uncertain aspects of anthropogenic climate change estimates. The magnitude of these interactions as represented in climate models strongly depends on the process of aerosol activation. This process is the most direct physical link between aerosols and clouds properties. Calculation of droplet number in GCMs requires the computation of new droplet formation (i.e., droplet activation), through physically based activation parameterizations. Accurately determining the size of the droplets when exposed to the maximum supersaturation is crucial to compute the condensation rate onto the growing droplets and ultimately, to determine the number of activated droplets, Nd. Given that aerosol particles larger than about 200 nm do not grow in equilibrium with the ambient supersaturation, computation of its condensation rate is particularly challenging. In this work we postulate, implement and evaluate modifications to the ``population splitting'' concept introduced in the activation parameterization of Nenes and Seinfeld (2003) for calculation of water condensation rates in droplet activation parameterizations. These changes aim to achieve a more physically consistent representation of the condensation rate on the largest aerosol particles during the activation process. The modifications introduced here lead to an improved accuracy and precision of the parameterization-derived Smax and Nd as determined by comparing against those of detailed numerical simulations of the activation process. A numerical computation of the first-order derivatives of the parameterized Nd to input variables 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. The proposed modifications require only minor changes for their numerical implementation in existing codes based on the population splitting concept.