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Photosynthetically Active Radiation (PAR) as Observed from Satellites

Geophysical Parameter for Improving Modeling of the Hydrological Cycle and Net Primary Productivity

Earth's climate depends on its radiative balance, controlled by solar input, surface properties, and distribution of radiatively active gases, clouds, and aerosols in the atmosphere. Of special interest is the solar radiation in the visible part of the spectrum, namely, in the interval of 400-700nm, known as Photosynthetically Active Radiation (PAR). By their control of the evapotranspiration process, information on the spatial and temporal distribution of  PAR and related parameters, such as PAR albedo,  are required for modeling the hydrological cycle and for estimating global oceanic and terrestrial net primary productivity (NPP). The diurnal variation of PAR is also of interest because the response of most environmental systems to the intensity of PAR is non-linear. Stomatal resistance, which controls evapotranspiration, as well as the net flux of CO2, have distinct diurnal and seasonal variations. The use of climate models for simulating plausible climate change scenarios, requires improved capabilities in respect to hydrologic modeling and in assessing the effects of increased greenhouse gases. Because large scale information on PAR was not available in the past, the total shortwave (SW) radiation (200- 4000nm), reduced by a constant factor of one half, was used as a proxy to PAR. Prospects to improve current modeling capabilities are improving, as information on parameters like PAR is becoming available.