Global Distribution Of Par As Observed From Satellites

Abstract

Information on the Photosynthetically Active Radiation (PAR) is needed for estimating Net Primary Productivity (NPP) over land and in oceans. The diurnal cycle of PAR is also of interest because the response of most ecosystems to the intensity of PAR is non-linear. On local scales, information on PAR is obtained from measurements in the 0.4-0.7 pm spectral interval; in absence af such measurements, PAR is estimated from total solar irradiance using predetermined "conversion constants". We have developed a spectral model for inferring surface solar irradiance from satellite observations [i] and [2]. The estimation of the surface solar irradiance is obtained by couplin satellite observed visible radiances at the top of the atmosphere with a radiative transfer model which allows to account for the extinction processes in the atmosphere. The absorption by ozone, water vapor and the extinction by molecules, aerosols and clouds are parameterized. Values of ozone amount , surf ace pressure , temperature , precipitable water and aerosol characteristics can be obtained from observations or from climatology. The boundary conditions in the model are represented by a global model of surface albedo which is wavelength and solar zenith angle dependent. The spectral intervals of the model are such that it is possible to compute the PAR (Fig. 1). The solar model was implemented with global satellite data that are now becoming available under the International Satellite Cloud Climatology Project (ISCCP) , to obtain the global distribution of PAR and of the 'conversion constants'. The ISCCP data are based on observations taken every three hours. Therefore, it is possible to study the diurnal cycle of PAR. These results and implications for the estimation of primary productivity will be discussed.