The Down-welling Short-wave Radiation (DSR) flux (W.m-2) at the
surface level represents the short-wave fraction of the solar
irradiance (0.3-4 µm) reaching the soil background. DSR primarily
determines the proportion of solar energy that can be potentially absorbed
by the surface. DSR depends essentially on solar geometry and cloudiness.
The DSR flux is a major input for
ONC
to quantify the atmospheric forcing in
the carbon cycle model.
OFM
uses the DSR flux in productivity modelling.
EARS and
CNRM/Météo-France
assess DSR from geo-stationary sensors data using different approaches.
DSR product from METEOSAT images
over Euro-Mediterranean region, first 10-day period of August 2004.
The EWBMS
(Energy and Water Balance Monitoring System) database provided
by EARS contains a DSR flux product derived from METEOSAT visible
[0.3 - 1.05µm] images at 0.04° of space resolution. Since only noon
images are used, a Fourier analysis of the daily solar cycle allows to
relate the noon value to daily average. For a clear pixel, a modified
version of the Kondratyev model (1969), which takes into account the
reflection and absorption in the atmosphere, relates the DSR flux at
noon to the intensity of solar radiation at the edge of the atmosphere
(1355 W.m-2) and to the solar radiation transmission through
the cloud free atmosphere. To obtain a daily DSR flux product, the
conversion factor is determined by integration of the daily solar cycle
and is a function of latitude and day of year. When a pixel is cloudy,
the solar radiation transmission through the clouds is calculated from
the observed cloud albedo according to a relationship derived from the
Kubelka-Munk theory. Hereafter, the calculation of the DSR is similar
as in the case of cloud-free pixels, except that the atmospheric
transmission factor is replaced by the cloud transmission factor. The
DSR flux is calculated on a daily basis and then for longer time periods
by averaging (see figure above). This DSR flux has been validated by
comparison with field measurements.
CNRM/Météo-France has the objective to generate a global DSR product
from the fusion of geo-stationary meteorological satellites data (METEOSAT-7
and METEOSAT-5, GMS-5, GOES-10 and GOES-8, INSAT and FY-2) and
polar sensor systems observations(NOAA/AVHRR). The physical algorithm is split
up in two scenarii corresponding to clear and cloudy sky cases. An innovative
method has been developed for the clear-sky case based on the generation
of a look-up-table (LUT) to simulate the up-welling transmitted
radiation as well as the DSR in varying atmospheric compounds
(water vapor, ozone, aerosols). The LUT being global, the generation
must be optimized in order to reduce the computing time for operational
purposes: the resolution on geometry of radiative transfer calculations
is degraded and analytical interpolation is applied to cover all
possible cases. The LUT is adapted to the cloudy sky case, using a cloud
mask provided by the
University of
Karlsrhue (IMK). This product will be provided on a 2-hour basis at 0.5°
of space resolution,
according to the Observatories requirements. As users are concerned
with the quality of the products, the DSR is delivered with a root mean
sqare error (rmse) for the accuracy assessment, and a quality flag on
a pixel-per-pixel basis. The final DSR product will be compared to the
ground-based measurements of the BSRN (Baseline Surface Radiation Network).
References
Kondratyev, K. Y., Radiation in the atmosphere, New York, London:
Academic Press, 1969.
