Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model
Atmosphere models with resolutions of several tens of kilometres take subgrid-scale variability in the total specific humidity <i>q</i><sub>t</sub> into account by using a uniform probability density function (PDF) to predict fractional cloud cover. However, usually only m...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2018-06-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/8589/2018/acp-18-8589-2018.pdf |
| _version_ | 1818291106194915328 |
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| author | P. Petersik M. Salzmann J. Kretzschmar R. Cherian D. Mewes J. Quaas |
| author_facet | P. Petersik M. Salzmann J. Kretzschmar R. Cherian D. Mewes J. Quaas |
| author_sort | P. Petersik |
| collection | DOAJ |
| description | Atmosphere models with resolutions of several tens of kilometres take
subgrid-scale variability in the total specific humidity <i>q</i><sub>t</sub> into account
by using a uniform probability density function (PDF) to predict fractional
cloud cover. However, usually only mean relative humidity,
<span style="text-decoration: overline;">RH</span>, or mean clear-sky relative humidity,
<span style="text-decoration: overline;">RH</span><sub>cls</sub>, 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 <i>%</i> 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,
SW<sub>net, cls</sub>, decreases by −0.22 W m<sup>−2</sup>
( ∼ −0.08 <i>%</i>). Finally, the RFari changes from −0.15 to
−0.19 W m<sup>−2</sup> by about 31 %. The reason for this very
disproportionate effect is that anthropogenic aerosols are disproportionally
hygroscopic. |
| first_indexed | 2024-12-13T02:38:47Z |
| format | Article |
| id | doaj.art-ec7a09b18a954ae49dc123724d6e42c2 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-13T02:38:47Z |
| publishDate | 2018-06-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-ec7a09b18a954ae49dc123724d6e42c22022-12-22T00:02:21ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242018-06-01188589859910.5194/acp-18-8589-2018Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere modelP. Petersik0M. Salzmann1J. Kretzschmar2R. Cherian3D. Mewes4J. Quaas5Leipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyLeipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyLeipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyLeipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyLeipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyLeipzig Institute for Meteorology, Universität Leipzig, Leipzig, GermanyAtmosphere models with resolutions of several tens of kilometres take subgrid-scale variability in the total specific humidity <i>q</i><sub>t</sub> into account by using a uniform probability density function (PDF) to predict fractional cloud cover. However, usually only mean relative humidity, <span style="text-decoration: overline;">RH</span>, or mean clear-sky relative humidity, <span style="text-decoration: overline;">RH</span><sub>cls</sub>, 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 <i>%</i> 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, SW<sub>net, cls</sub>, decreases by −0.22 W m<sup>−2</sup> ( ∼ −0.08 <i>%</i>). Finally, the RFari changes from −0.15 to −0.19 W m<sup>−2</sup> by about 31 %. The reason for this very disproportionate effect is that anthropogenic aerosols are disproportionally hygroscopic.https://www.atmos-chem-phys.net/18/8589/2018/acp-18-8589-2018.pdf |
| spellingShingle | P. Petersik M. Salzmann J. Kretzschmar R. Cherian D. Mewes J. Quaas Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model Atmospheric Chemistry and Physics |
| title | Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model |
| title_full | Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model |
| title_fullStr | Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model |
| title_full_unstemmed | Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model |
| title_short | Subgrid-scale variability in clear-sky relative humidity and forcing by aerosol–radiation interactions in an atmosphere model |
| title_sort | subgrid scale variability in clear sky relative humidity and forcing by aerosol radiation interactions in an atmosphere model |
| url | https://www.atmos-chem-phys.net/18/8589/2018/acp-18-8589-2018.pdf |
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