Predicting Cloud Droplet Number Concentration in Community Atmosphere Model (CAM)-Oslo
A continuity equation for cloud droplet number concentration is implemented in an extended version of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 2.0.1 (CAM-2.0.1). The new continuity equation for cloud droplet number concentration consists of a nucleation term and several microphysical sink terms. The nucleation term is calculated based on a parameterization of activation of cloud condensation nuclei. A sub-grid distribution of vertical velocity is used to determine the range of supersaturations found within each model grid box. This supersaturation combined with the hygroscopicity of the aerosols present will determine the number of Cloud Condensation Nuclei (CCN) activated into cloud droplets. The aerosol types considered in this study are sea salt, sulfate, black carbon, organic carbon and mineral dust. The horizontal and vertical distributions of sulfate and carbonaceous aerosols are calculated based on AEROCOM (http://nansen.ipsl.jussieu.fr/AEROCOM) sources. These are combined with the background aerosols, which are a combination of sea salt, mineral dust and sulfate dependent on soil type, wind speed and location (Arctic, Antarctic, maritime, desert or continental). The resulting aerosol size distributions are multimodal, allowing sulfate, black carbon and organic carbon to be both internally and externally mixed with the background aerosols. Microphysical sink terms for cloud droplets are obtained from a prognostic cloud water scheme, assuming a direct proportionality between loss of cloud water and loss of cloud droplets. Based on the framework described above, the cloud droplet number concentration and cloud droplet effective radius can be determined. The resulting cloud radiative forcings (CRF) can hereafter be calculated. By comparing the CRF for two different model runs, one with pre-industrial aerosol sources and the other with sources corresponding to present day, the indirect effect of aerosols can be calculated. A significantly smaller aerosol indirect effect is found in this study compared to most other comparable studies. This is largely due to the introduction of microphysical sinks for cloud droplets and a cloud droplet activation scheme which accounts for the so called competition effect among CCN.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 899813
- Report Number(s):
- PNNL-SA-45899; KP1205010; TRN: US200708%%544
- Journal Information:
- Journal of Geophysical Research. D. (Atmospheres), 111(D24):D24208, Journal Name: Journal of Geophysical Research. D. (Atmospheres), 111(D24):D24208
- Country of Publication:
- United States
- Language:
- English
Similar Records
MIRAGE: Model Description and Evaluation of Aerosols and Trace Gases
Parameterizations of Cloud Microphysics and Indirect Aerosol Effects