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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-839
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
13 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Subgrid-scale variability of clear-sky relative humidity and forcing by aerosol-radiation interactions in an atmosphere model
Paul Petersik, Marc Salzmann, Jan Kretzschmar, Ribu Cherian, Daniel Mewes, and Johannes Quaas Leipzig Institute for Meteorology, Universität Leipzig, Germany
Abstract. Atmosphere models with resolutions of several tens of kilometres take subgrid-scale variability of the total specific humidity qt into account by using a uniform probability density function (PDF) to predict fractional cloud cover. However, usually only mean relative humidity RH or mean clear-sky relative humidity RHcls is used to compute hygroscopic growth of soluble aerosol particles. In this study, a stochastic parameterization of subgrid-scale variability of RHcls is applied. For this, we sample the subsaturated part of the uniform RH-PDF from the cloud cover scheme for application in association with the hygroscopic growth parameterization in the ECHAM6-HAM2 atmosphere model. Due to the non-linear dependence of the hygroscopic growth on RH, this causes an increase in aerosol hygroscopic growth. Aerosol optical depth (AOD) increases by a global mean of 0.009 (∼ 7.8 % in comparison to the control run). Especially over the tropics AOD is enhanced with a mean of about 0.013. The ability of the model to simulate AOD is slightly improved with respect to satellite data from MODIS-Aqua. Due to the increase in AOD, net top of the atmosphere clear-sky solar radiation decreases by −0.22 W m−2 (∼ −0.08 %). Finally, the effective radiative forcing due to aerosol-radiation interactions under clear-sky conditions (ERFaricls) changes from −0.29 W m−2 to −0.45 W m−2 by about 57 %. The reason for this very disproportionate effect is that anthropogenic aerosols are disproportionally hygroscopic.

Citation: Petersik, P., Salzmann, M., Kretzschmar, J., Cherian, R., Mewes, D., and Quaas, J.: Subgrid-scale variability of clear-sky relative humidity and forcing by aerosol-radiation interactions in an atmosphere model, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-839, in review, 2017.
Paul Petersik et al.
Paul Petersik et al.
Paul Petersik et al.

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Short summary
Our study proposes a stochastic parametrization for the subgrid-scale variability of clear-sky relative humidity in a global circulation model. The parametrization is easy to implement and consistent with the assumptions in the cloud cover scheme. We examine its effects on optical and radiative properties of the atmosphere. Most importantly we find, that revision has a strong influence on the simulated radiative forcing.
Our study proposes a stochastic parametrization for the subgrid-scale variability of clear-sky...
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