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...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
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 |
| Summary: | 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. |
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| ISSN: | 1680-7316 1680-7324 |