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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L19809, doi:10.1029/2007GL030021, 2007

Impact on modeled cloud characteristics due to simplified treatment of uniform cloud condensation nuclei during NEAQS 2004

William I. Gustafson Jr.

Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA


Elaine G. Chapman

Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA


Steven J. Ghan

Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA


Richard C. Easter

Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA


Jerome D. Fast

Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA


Abstract

Subgrid-scale cloud condensation nuclei (CCN) heterogeneity is not represented in global climate models (GCM) and potentially contributes systematic errors to simulated cloud effects. High-resolution WRF-Chem model simulations were performed to investigate the impact of assuming a uniform CCN distribution on cloud properties and surface radiation over a region the size of a GCM grid column. Results indicate that a prescribed CCN distribution allowing for vertical and temporal fluctuations does substantially better in simulating cloud properties and radiative effects than does a prescribed uniform and constant CCN distribution. Spatially and temporally averaged net effects on downwelling shortwave radiation are between −3 and −11 W m−2 for the fluctuating and uniform distributions, respectively, versus a control simulation with fully interactive aerosols. Both prescribed CCN distributions produce optically thicker clouds more often than the control, with the mean cloud optical depth increasing by over 25% when using the uniform and constant CCN distribution.

Received 14 March 2007; accepted 13 September 2007; published 12 October 2007.

Keywords: aerosol-cloud effects; cloud physics; radiation.

Index Terms: 3311 Atmospheric Processes: Clouds and aerosols; 3310 Atmospheric Processes: Clouds and cloud feedbacks; 0321 Atmospheric Composition and Structure: Cloud/radiation interaction; 3355 Atmospheric Processes: Regional modeling; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906).

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Citation: Gustafson, W. I., Jr., E. G. Chapman, S. J. Ghan, R. C. Easter, and J. D. Fast (2007), Impact on modeled cloud characteristics due to simplified treatment of uniform cloud condensation nuclei during NEAQS 2004, Geophys. Res. Lett., 34, L19809, doi:10.1029/2007GL030021.