The impact of aerosols on non-precipitating marine stratocumulus. I: Model description and prediction of the indirect effect

✍ Scribed by A. A. Hill; S. Dobbie; Y. Yin

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 269 KB
• Volume: 134
• Category: Article
• ISSN: 0035-9009
• DOI: 10.1002/qj.278

✦ Synopsis

Abstract

In this paper, the first of a two‐part study, we use the UK Met Office large‐eddy simulation model (LEM) with fully integrated size‐bin‐resolved cloud microphysics (BR‐LEM) to investigate the effects of increasing cloud condensation nuclei (CCN) concentrations on non‐precipitating marine stratocumulus. It is shown, as expected, that increasing CCN concentrations produces an increase in cloud‐drop number concentration and a decrease in cloud‐drop effective radius. However, for the case presented, we demonstrate that increasing CCN concentrations causes an increase in the rate of evaporative cooling at the cloud top, which drives stronger boundary‐layer dynamics, leading to more cloud‐top entrainment, which results in a reduction in boundary‐layer relative humidity and a reduction in LWP with increasing CCN. Comparison of the BR‐LEM simulations with LEM simulations that employ a simpler single‐moment bulk scheme (Bulk‐LEM) show that the bulk microphysics scheme fails to simulate this CCN‐‐entrainment feedback and the associated reduction in liquid water path. It is shown that, for a very polluted case, the failure of the bulk microphysics to capture this evaporation‐‐entrainment feedback results in a 60% overestimation of the indirect forcing estimate compared to the BR‐LEM. We conclude that it is necessary to realistically simulate the dynamic feedbacks associated with increased CCN, otherwise the indirect will be overestimated. Copyright © 2008 Royal Meteorological Society