Impact of the representation of marine stratocumulus clouds on the anthropogenic aerosol effect

D Neubauer,U Lohmann,M G Frontoso,C Hoose

doi:10.5194/acp-14-11997-2014

D Neubauer, U Lohmann + Show 2 more

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https://doi.org/10.5194/acp-14-11997-2014

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Abstract

Abstract. Stratocumulus clouds are important for climate as they reflect large amounts of solar radiation back into space. However they are difficult to simulate in global climate models because they form under a sharp inversion and are thin. A comparison of model simulations with the ECHAM6-HAM2 global aerosol climate model to observations, reanalysis and literature data revealed too strong turbulent mixing at the top of stratocumulus clouds and a lack of vertical resolution. Further reasons for cloud biases in stratocumulus regions are the too "active" shallow convection scheme, the cloud cover scheme and possibly too low subsidence rates. To address some of these issues and improve the representation of stratocumulus clouds, we made three distinct changes to ECHAM6-HAM2. With a "sharp" stability function in the turbulent mixing scheme we have observed, similar to previous studies, increases in stratocumulus cloud cover and liquid water path. With an increased vertical resolution in the lower troposphere in ECHAM6-HAM2 the stratocumulus clouds form higher up in the atmosphere and their vertical extent agrees better with reanalysis data. The recently implemented in-cloud aerosol processing in stratiform clouds is used to improve the aerosol representation in the model. Including the improvements also affects the anthropogenic aerosol effect. In-cloud aerosol processing in ECHAM6-HAM2 leads to a decrease in the anthropogenic aerosol effect in the global annual mean from −1.19 Wm−2 in the reference simulation to −1.08 Wm−2, while using a "sharp" stability function leads to an increase to −1.34 Wm−2. The results from the simulations with increased vertical resolution are diverse but increase the anthropogenic aerosol effect to −2.08 Wm−2 at 47 levels and −2.30 Wm−2 at 95 levels.

Highlights

Stratocumulus clouds are important for future climate predictions as they have a strong cooling effect (Bretherthon et al, 2004; Williams and Webb, 2009)
The stratocumulus conditions are met in ECHAM6-HAM2 in similar areas to those in ERA-Interim but less frequently (Fig. 4). This is because large values of lower troposhperic stability (LTS) occur 12 % less often in ECHAM6-HAM2 than in the reanalysis data in areas where both stratocumulus conditions are met
As the conditions of strong LTS and subsidence together are less frequently met in ECHAM6-HAM2, stratocumulus clouds form less often than in ERA-Interim

Summary

Introduction

Stratocumulus clouds are important for future climate predictions as they have a strong cooling effect (Bretherthon et al, 2004; Williams and Webb, 2009). It is challenging to represent the complex interaction between aerosol and clouds in a global climate model. Recent high-resolution large eddy simulation studies showed that the liquid water path may either increase or decrease with increased cloud droplet number concentrations (Nd) in contrast to the thickening from reduced precipitation efficiency (Ackerman et al, 2004; Bretherton et al, 2007; Hill et al, 2008; Sandu et al, 2008; Ackerman et al, 2009; Petters et al, 2013). The increased entrainment is explained either by increased evaporative cooling at cloud top due to stronger turbulence (Ackerman et al, 2004; Hill et al, 2008; Ackerman et al, 2009) or a stronger evaporative cooling efficiency (Bretherton, 2007). The increase in entrainment is substantially reduced when cloud water sedimentation is included in the simulation (Bretherthon et al, 2007; Ackerman et al, 2009). Global climate models typically only represent the reduced precipitation efficiency via an autoconversion pa-

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