Effects of aerosols on clear-sky solar radiation in the ALADIN-HIRLAM NWP system
The direct shortwave radiative effect of aerosols under clear-sky conditions in the Aire Limitee Adaptation dynamique Developpement InterNational – High Resolution Limited Area Model (ALADIN-HIRLAM) numerical weather prediction system was investigated using three shortwave radiation schemes in di...
Main Authors: | , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-05-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/5933/2016/acp-16-5933-2016.pdf |
Summary: | The direct shortwave radiative effect of aerosols under clear-sky
conditions in the Aire Limitee Adaptation dynamique Developpement InterNational – High Resolution Limited
Area Model (ALADIN-HIRLAM) numerical weather prediction system was
investigated using three shortwave radiation schemes in diagnostic
single-column experiments: the Integrated Forecast System (IFS), acraneb2 and the hlradia radiation
schemes. The multi-band IFS scheme was formerly used operationally by the European Centre for Medium Range Weather Forecasts (ECMWF)
whereas hlradia and acraneb2 are broadband schemes. The former is a new
version of the HIRLAM radiation scheme while acraneb2 is the radiation scheme
in the ALARO-1 physics package.
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The aim was to evaluate the strengths and weaknesses of the numerical weather
prediction (NWP)
system regarding aerosols and to prepare it for use of real-time aerosol information. The
experiments were run with particular focus on the August 2010 Russian
wildfire case. Each of the three radiation schemes accurately (within
±4 % at midday) simulates the direct shortwave aerosol effect when
observed aerosol optical properties are used. When the aerosols were
excluded from the simulations, errors of more than +15 % in global
shortwave irradiance were found at midday, with the error reduced to +10 % when standard climatological aerosols were used. An error of −11 %
was seen at midday if only observed aerosol optical depths at 550 nm, and
not observation-based spectral dependence of aerosol optical depth, single
scattering albedos and asymmetry factors, were included in the simulations.
This demonstrates the importance of using the correct aerosol optical
properties. The dependency of the direct radiative effect of aerosols on
relative humidity was tested and shown to be within ±6 % in this
case. By modifying the assumptions about the shape of the IFS climatological
vertical aerosol profile, the inherent uncertainties associated with
assuming fixed vertical profiles were investigated. The shortwave heating
rates in the boundary layer changed by up to a factor of 2 in response to
the aerosol vertical distribution without changing the total aerosol optical
depth. Finally, we tested the radiative transfer approximations used in the
three radiation schemes for typical aerosol optical properties compared to
the accurate DISORT model. These approximations are found to be accurate to
within ±13 % even for large aerosol loads. |
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ISSN: | 1680-7316 1680-7324 |