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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-449
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Jun 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Unveiling aerosol-cloud interactions Part 2: Minimizing the effects of aerosol swelling and wet scavenging in ECHAM6-HAM2 for comparison to satellite data
David Neubauer1, Matthew W. Christensen2,3, Caroline Poulsen2, and Ulrike Lohmann1 1Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, 8092, Switzerland
2RAL Space, STFC Rutherford Appleton Lab, Harwell, OX11 0QX, United Kingdom
3Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom
Abstract. Aerosol-cloud interactions (ACI) are uncertain and the estimates of the ACI effective radiative forcing (ERFaci) magnitude show a large variability. Within the Aerosol_cci project the susceptibility of cloud properties to changes in aerosol properties are derived from the high resolution AATSR dataset using the Cloud-Aerosol Pairing Algorithm (CAPA) (as described in our companion paper) and compared to susceptibilities from the global aerosol climate model ECHAM6-HAM2 and MODIS-CERES data. For ECHAM6-HAM2 the dry aerosol is analysed to mimic the effect of CAPA. Furthermore the analysis is done for different environmental regimes.

The aerosol-liquid water path relationship in ECHAM6-HAM2 is systematically stronger than in AATSR-CAPA data and cannot be explained by an overestimation of autoconversion when using diagnostic precipitation but rather by aerosol swelling in regions where humidity is high and clouds are present. When aerosol water is removed from the analysis in ECHAM6-HAM2 the strength of the susceptibilities of liquid water path, cloud droplet number concentration and cloud albedo as well as ERFaci agree much better with the ones of AATSR-CAPA or MODIS-CERES. For comparing satellite derived to model derived susceptibilities this study finds it more appropriate to use dry aerosol in the computation of model susceptibilities.

We further find that while the observed relationships of different satellite sensors (AATSR-CAPA vs. MODIS-CERES) are not always consistent for tested environmental conditions the relationships in ECHAM6-HAM2 are missing a strong dependence on environmental conditions which is an indication that feedback processes like cloud top entrainment are missing or not well represented in the model.

Next to aerosol swelling, also wet scavenging and aerosol processing have an impact on liquid water path, cloud albedo and cloud droplet number susceptibilities. Aerosol processing leads to negative liquid water path susceptibilities to changes in aerosol index (AI) in ECHAM6-HAM2, likely due to aerosol size changes by aerosol processing. This is an indication that AI is not necessarily a better proxy for cloud condensation nuclei than the less size dependent aerosol optical depth.

Our results indicate that for statistical analysis of aerosol-cloud interactions the unwanted effects of aerosol swelling, wet scavenging and aerosol processing need to be minimized when computing susceptibilities of cloud variables to changes in aerosol.


Citation: Neubauer, D., Christensen, M. W., Poulsen, C., and Lohmann, U.: Unveiling aerosol-cloud interactions Part 2: Minimizing the effects of aerosol swelling and wet scavenging in ECHAM6-HAM2 for comparison to satellite data, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-449, in review, 2017.
David Neubauer et al.
David Neubauer et al.
David Neubauer et al.

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Short summary
When aerosol particles take up water their number may seem increased optically. On the other hand if aerosol particles are removed by precipitation (formation) their number will decrease. We applied methods to account for such effects in model and satellite data to analyse the change in cloud properties by changes in aerosol particles number. The agreement of model and satellite data improves when these effects are accounted for.
When aerosol particles take up water their number may seem increased optically. On the other...
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