Estimation of the aerosol radiative forcing at ground level, over land, and in cloudless atmosphere, from METEOSAT-7 observation: method and case study

[摘要] A new method is proposed to estimate the spatial and temporal variability ofthe solar radiative flux reaching the surface over land (DSSF), as well as theAerosol Radiative Forcing (ARF), in cloud-free atmosphere.The objective ofregional applications of the method is attainable by using the visible broadband ofMETEOSAT-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 andradiative transfer computations.

The validation of DSSF is performed comparing retrievals with ground-based measurementsacquired in two contrasted environments: an urban site near Paris and acontinental background site located South East of France.The study isconcentrated on aerosol episodes occurring around the 2003 summer heatwave, providing 42 cases of comparison for variable solar zenith angle(from 59° to 69°), variable aerosol type (biomass burningemissions and urban pollution), and variable aerosol optical thickness (a factor 6 in magnitude).The method reproduces measurements of DSSF within an accuracy assessment of20 W m⁻² (5% in relative) in 70% of the situations, and within40 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 radianceat the top of the atmosphere, DSSF temporal variability is assumed to be causedonly by aerosols, and consequently ARF at ground level and over land is alsoretrieved: ARF is computed as the difference between DSSF and a parameterisedaerosol-free reference level.Retrievals are linearly correlated with theground-based measurements of the aerosol optical thickness (AOT): sensitivityis included between 120 and 160 W m⁻² perunity of AOT at 440 nm.AOT being an instantaneous measure indicative of theaerosol columnar amount, we prove the feasibility to infer instantaneous aerosolradiative impact at the ground level over land with METEOSAT-7 visible channel.

[学科分类] 大气科学