Calculation Of Land PAR Using MODIS Data

Photosynthetically Active Radiation (PAR) is the part of the solarirradiance between the wavelengths of 400nm to 700nm. PAR, anindispensable factor of the terrestrial ecosystem, plays an important rolein global change. PAR is helpful to studying carbon cycle and carbontrend, and its sensitivity is also important to global climate. Theevaluation of the PAR on a large scale in China by remote sensinginformation is useful to understand the carbon store, carbon cycle andcarbon balance on the land of China.Based on the information of the aerosol, water vapor, and the surfacealbedo by remote sensing inversion, the cloudless land PAR on the spatialscale of 1KM is calculated by a simple spectral atmospheric radiativetransfer model, considering the absorption by various aerosols, watervapor, ozone and the uniformly mixed gases and the multi-layersreflection between the atmosphere bottom and the surface.In this model, the solar radiance is divided into the solar spectraldirect and diffuse irradiance, and the spectral absorption and scatteringwithin the atmosphere and upward reflected spectral irradiance ordirectional radiance at the top of the atmosphere in a cloud-freeatmosphere are considered. The absorption is related to absorption of theaerosol, water vapor, ozone, and the mixed atmosphere, and the scatteringis related to the scattering of Raying, aerosol, the multi-layers reflectionbetween the surface and the bottom of the atmosphere.The Moderate spectral resolution atmospheric Transmittancealgorithm and computer model (MODTRAN) are used to simulate therelationship between PAR and the aerosol of optical depth, the mode ofthe aerosol, the mode of atmosphere and the surface albedo. Therelationship between the PAR and the aerosol optical depth is a goodlinear, and the amount of PAR decrease with the increase of the aerosoloptical thickness. With the same atmosphere and the aerosol optical depth,the decrease of PAR is different at the different mode of the aerosol.Under the aerosol model of the Rural-WinterFall, theRural-SummerSpring, the City-WinterFall and the City-SummerSpring,the PARs decrease more and more slowly. The influence of the differentatmosphere on the PAR is negligible. The PAR is increasing with theincrement of the surface albedo, but the degree of the increment is small.Based on the simulation, the Moderate-resolution imaging spectroradiometer (MODIS) data are used to monitor PAR on a large scale after the remote sensing information inversion. In this study, the model of Land PAR is modified by the index of Land aerosol and the multiple reflections between the surface and atmosphere based on the algorithm of NASA Ocean PAR. The PAR calculation model as follows, Where Id is the direct radiance, Ir is the Raying diffuse radiance, Ia is the aerosol diffuse radiance, rs is the albedo of the atmosphere, BSAshort 和WSAshort are the Black semi-hemisphere albedo and the White semi-hemisphere albedo, separately.Because the PAR by remote sensing data is instantaneous, and theused PAR in practice is at the scale of day or the longer, the instantaneousmodel must be extended to the time scale of day. The extended model asfollows,Where I0 is the constant of the solar, τis the length of Day , R is theaverage distance between the sun and solar , Φis the geographic latitude,ω0 is the time angle of sunset, δis the sun equator latitude.Based on the model and algorithms, the MODIS data are processed,...