Articles | Volume 18, issue 12
https://doi.org/10.5194/acp-18-8589-2018
https://doi.org/10.5194/acp-18-8589-2018
Research article
 | 
19 Jun 2018
Research article |  | 19 Jun 2018

Subgrid-scale variability in 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

Abstract. Atmosphere models with resolutions of several tens of kilometres take subgrid-scale variability in 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. While previous studies based on limited-area models and also a global model suggest that subgrid-scale variability in RH should be taken into account for estimating radiative forcing due to aerosol–radiation interactions (RFari), here we present the first estimate of RFari using a global atmospheric model with a parameterization for subgrid-scale variability in RH that is consistent with the assumptions in the model. For this, we sample the subsaturated part of the uniform RH-PDF from the cloud cover scheme for its application in 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. Due to the increase in AOD, net top of the atmosphere clear-sky solar radiation, SWnet, cls, decreases by −0.22 W m−2 ( ∼ −0.08 %). Finally, the RFari changes from −0.15 to −0.19 W m−2  by about 31 %. The reason for this very disproportionate effect is that anthropogenic aerosols are disproportionally hygroscopic.

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Short summary
Our study presents the first estimate of RFari using a global atmospheric model with a parameterization for subgrid-scale variability in RH that is consistent with the assumptions in the model. We find that the revision has a strong influence on the simulated radiative forcing (~ 31 %). In addition, we examine its effects on optical properties of the atmosphere and find an increase in AOD by about 7.8 %.
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