Lennart Olsson

Incoming Photosynthetic Active Radiation (PAR) is an essential variable for modelling aboveground primary
production of ecosystems through the light-use efficiency approach. A method is presented where daily classifications
of cloud cover (CLAVR) from the NOAA AVHRR satellite sensor is used to estimate surface incident short wave (SW)
flux from which PAR can be assessed. The study area is the Inner Mongolia Autonomous Region (IMAR) of northern
China. Daily time steps of calculated theoretical incoming global radiation outside the atmosphere, is adjusted
according to the clear, mixed or cloudy classification in the NOAA Pathfinder data set at 8x8 km grid-cells. For the
different CLAVR classifications, empirical relationships to atmospheric transparency were established against
ground measurements of SW flux. Clear pixels corresponded to an average 61% penetration of the theoretical
radiation at the top of the atmosphere and mixed and cloudy pixels to 47% and 40% respectively. The CLAVR time
series is evaluated regarding consistency and diurnal precision against measured SW flux and hours of bright
sunshine. Modelled monthly fluxes over the growing season were acceptable compared to measured (NRMSE = 6.
6%) and about as good as deriving fluxes from measurements of bright sunshine hours. The global NOAA Pathfinder
archive provides an opportunity to assess PAR over the past 20 years at a considerably higher spatial resolution than
with methods based on geo-stationary meteorological satellite data sets and without interpolations from scarce
measurements of bright sunshine hours.