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Metamodeling of Droplet Activation for Global Climate Models

Author(s)
Rothenberg, Daniel Alexander; Wang, Chien
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Abstract
The nucleation of cloud droplets from the ambient aerosol is a critical physical process that must be resolved for global models to faithfully predict aerosol–cloud interactions and aerosol indirect effects on climate. To better represent droplet nucleation from a complex, multimodal, and multicomponent aerosol population within the context of a global model, a new metamodeling framework is applied to derive an efficient and accurate activation parameterization. The framework applies polynomial chaos expansion to a detailed parcel model in order to derive an emulator that maps thermodynamic and aerosol parameters to the supersaturation maximum achieved in an adiabatically ascending parcel and can be used to diagnose droplet number from a single lognormal aerosol mode. The emulator requires much less computational time to build, store, and evaluate than a high-dimensional lookup table. Compared to large sample sets from the detailed parcel model, the relative error in the predicted supersaturation maximum and activated droplet number computed with the best emulator is -0.6% ± 9.9% and 0.8% ± 17.8% (one standard deviation), respectively. On average, the emulators constructed here are as accurate and between 10 and 17 times faster than a leading physically based activation parameterization. Because the underlying parcel model being emulated resolves size-dependent droplet growth factors, the emulator captures kinetic limitations on activation. The results discussed in this work suggest that this metamodeling framework can be extended to accurately account for the detailed activation of a complex aerosol population in an arbitrary coupled global aerosol–climate model.
Date issued
2016-02
URI
http://hdl.handle.net/1721.1/108357
Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Journal
Journal of the Atmospheric Sciences
Publisher
American Meteorological Society
Citation
Rothenberg, Daniel and Wang, Chien. “Metamodeling of Droplet Activation for Global Climate Models.” Journal of the Atmospheric Sciences 73, no. 3 (March 2016): 1255–1272. © American Meteorological Society
Version: Final published version
ISSN
0022-4928
1520-0469

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