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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
23 Nov 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
The relative importance of macrophysical and cloud albedo changes for aerosol induced radiative effects in stratocumulus
Daniel P. Grosvenor1, Paul R. Field1,2, Adrian A. Hill2, and Benjamin J. Shipway2 1School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
2Met Office, Exeter, UK
Abstract. Aerosol-cloud interactions are explored using 1 km resolution simulations of SE Pacific stratocumulus clouds that include realistic meteorology along with newly implemented cloud microphysics and sub-grid cloud schemes. The model was critically assessed against observations of Liquid Water Path (LWP), broadband fluxes, cloud fraction (fc), droplet number concentrations (Nd) and radar reflectivities.

Aerosol loading sensitivity tests showed that at low aerosol loadings, changes to aerosol affected shortwave fluxes equally through changes to cloud macrophysical charateristics (LWP, fc) and cloud albedo changes due solely to Nd changes. However, at high aerosol loadings, only the Nd albedo change was important. Evidence was also provided to show that a treatment of sub-grid clouds is as important as order of magnitude changes in aerosol loading for the accurate simulation of stratocumulus at this grid resolution.

Overall, the control model demonstrated a credible ability to reproduce observations suggesting that many of the important physical processes for accurately simulating these clouds are represented within the model and giving some confidence in the predictions of the model concerning stratocumulus and the impact of aerosol. For example, the control run was able to reproduce the shape and magnitude of the observed diurnal cycle of domain mean LWP to within ~ 10 g m−2 for the nighttime, but with an overestimate for the daytime of up to 30 g m−2. The latter was attributed to the uniform aerosol fields imposed on the model, which meant that the model failed to include the low Nd mode that was observed further offshore, preventing the LWP removal through precipitation that likely occurred in reality. The boundary layer was too low by around 260 m, which was attributed to the driving global model analysis. The shapes and sizes of the observed bands of clouds and open-cell-like regions of low areal cloud cover were qualitatively captured. The daytime fc frequency distribution was reproduced to within fc = 0.04 for fc > ~ 0.7 as was the domain mean nighttime fc (at a single time) to within fc = 0.02. Frequency distributions of shortwave top-of-the-atmosphere (TOA) fluxes from satellite were well represented by the model with only a slight underestimate of the mean by 15 %; this was attributed to near--shore aerosol concentrations that were too low for the particular times of the satellite overpasses. TOA longwave flux distributions were close to those from satellite with agreement of the mean value to within 0.4 %. From comparisons of Nd distributions to those from satellite it was found that the Nd mode from the model agreed with the higher of the two observed modes to within ~ 15 %.

Citation: Grosvenor, D. P., Field, P. R., Hill, A. A., and Shipway, B. J.: The relative importance of macrophysical and cloud albedo changes for aerosol induced radiative effects in stratocumulus, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1017, in review, 2016.
Daniel P. Grosvenor et al.
Daniel P. Grosvenor et al.


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Latest update: 26 Mar 2017
Publications Copernicus
Short summary
We used a weather model to simulate low level layer clouds that lie off the coast of Chile and tested how they would be affected by airbourne particulate matter (aerosols) according to the model. We found that as aerosols were increased the clouds reflected more and more of the sun’s incoming energy due to the combined effects of the cloud droplets becoming smaller, the thickening of clouds and increased areal coverage. However, the latter two effects were only important at low aerosol levels.
We used a weather model to simulate low level layer clouds that lie off the coast of Chile and...