Estimation of the aerosol radiative forcing at ground level, over land, and in cloudless atmosphere, from METEOSAT-7 observation: method and case study T. Elias1 and J.-L. Roujean2 1LMD, Ecole Polytechnique, 91128 Palaiseau cedex, France 2CNRM/GMME/MATIS, Meteo-France, 42, avenue G. Coriolis, 31057 Toulouse cedex, France
Abstract. A new method is proposed to estimate the spatial and temporal variability of
the solar radiative flux reaching the surface over land (DSSF), as well as the
Aerosol Radiative Forcing (ARF), in cloud-free atmosphere. The objective of
regional applications of the method is attainable by using the visible broadband of
METEOSAT-7 satellite instrument which scans Europe and Africa on a half-hourly basis.
The method relies on a selection of best correspondence between METEOSAT-7 radiance and
radiative transfer computations.
The validation of DSSF is performed comparing retrievals with ground-based measurements
acquired in two contrasted environments: an urban site near Paris and a
continental background site located South East of France. The study is
concentrated on aerosol episodes occurring around the 2003 summer heat
wave, providing 42 cases of comparison for variable solar zenith angle
(from 59° to 69°), variable aerosol type (biomass burning
emissions and urban pollution), and variable aerosol optical thickness (a factor 6 in magnitude).
The method reproduces measurements of DSSF within an accuracy assessment of
20 W m−2 (5% in relative) in 70% of the situations, and within
40 W m−2 in 90% of the situations, for the two case studies considered here.
Considering aerosol is the main contributor in changing the measured radiance
at the top of the atmosphere, DSSF temporal variability is assumed to be caused
only by aerosols, and consequently ARF at ground level and over land is also
retrieved: ARF is computed as the difference between DSSF and a parameterised
aerosol-free reference level. Retrievals are linearly correlated with the
ground-based measurements of the aerosol optical thickness (AOT): sensitivity
is included between 120 and 160 W m−2 per
unity of AOT at 440 nm. AOT being an instantaneous measure indicative of the
aerosol columnar amount, we prove the feasibility to infer instantaneous aerosol
radiative impact at the ground level over land with METEOSAT-7 visible channel.
Citation: Elias, T. and Roujean, J.-L.: Estimation of the aerosol radiative forcing at ground level, over land, and in cloudless atmosphere, from METEOSAT-7 observation: method and case study, Atmos. Chem. Phys., 8, 625-636, doi:10.5194/acp-8-625-2008, 2008.