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Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?

Lebo, Z. J. and Morrison, H. and Seinfeld, J. H. (2012) Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment? Atmospheric Chemistry and Physics, 12 (20). pp. 9941-9964. ISSN 1680-7316. doi:10.5194/acp-12-9941-2012.

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Three configurations of a bulk microphysics scheme in conjunction with a detailed bin scheme are implemented in the Weather Research and Forecasting (WRF) model to specifically address the role of the saturation adjustment assumption (i.e., condensing/evaporating the surplus/deficit water vapor relative to saturation in one time step) on aerosol-induced invigoration of deep convective clouds. The bulk model configurations are designed to treat cloud droplet condensation/evaporation using either saturation adjustment, as employed in most bulk models, or an explicit representation of supersaturation over a time step, as used in bin models. Results demonstrate that the use of saturation adjustment artificially enhances condensation and latent heating at low levels and limits the potential for an increase in aerosol concentration to increase buoyancy at mid to upper levels. This leads to a small weakening of the time- and domain-averaged convective mass flux (~-3%) in polluted compared to clean conditions. In contrast, the bin model and bulk scheme with explicit prediction of supersaturation simulate an increase in latent heating aloft and the convective updraft mass flux is weakly invigorated (~5%). The bin model also produces a large increase in domain-mean cumulative surface precipitation in polluted conditions (~18%), while all of the bulk model configurations simulate little change in precipitation. Finally, it is shown that the cold pool weakens substantially with increased aerosol loading when saturation adjustment is applied, which acts to reduce the low-level convergence and weaken the convective dynamics. With an explicit treatment of supersaturation in the bulk and bin models there is little change in cold pool strength, so that the convective response to polluted conditions is influenced more by changes in latent heating aloft. It is concluded that the use of saturation adjustment can explain differences in the response of cold pool evolution and convective dynamics with aerosol loading simulated by the bulk and bin models, but cannot explain large differences in the response of surface precipitation between these models.

Item Type:Article
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URLURL TypeDescription
Lebo, Z. J.0000-0002-1064-4833
Morrison, H.0000-0001-8480-9787
Seinfeld, J. H.0000-0003-1344-4068
Additional Information:© 2012 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 18 March 2012; Published in Atmos. Chem. Phys. Discuss: 19 April 2012; Revised: 21 September 2012; Accepted: 10 October 2012; Published: 30 October 2012. This work was supported by the Office of Naval Research grant N00014-10-1-0200. Computations were carried out on the CITerra cluster of the Geological and Planetary Sciences Division at Caltech. HM acknowledges partial support from NOAA grant NA08OAR4310543, US DOE ARM DE-FG02- 08ER64574, and the NSF Science and Technology Center for Multiscale Modeling of Atmospheric Processes (CMMAP), managed by Colorado State University under cooperative agreement ATM-0425247.
Funding AgencyGrant Number
Office of Naval Research (ONR)N00014-10-1-0200
National Oceanic and Atmospheric Administration (NOAA)NA08OAR4310543
Department of Energy (DOE)DE-FG02-08ER64574
Issue or Number:20
Record Number:CaltechAUTHORS:20121129-135155685
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Official Citation:Lebo, Z. J., Morrison, H., and Seinfeld, J. H.: Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?, Atmos. Chem. Phys., 12, 9941-9964, doi:10.5194/acp-12-9941-2012, 2012.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35735
Deposited By: Jason Perez
Deposited On:29 Nov 2012 22:19
Last Modified:09 Nov 2021 23:17

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