An efficient algorithm for calculating photosynthetically active radiation with MODIS products

Abstract

Photosynthetically active radiation (PAR) is the critical forcing data in ecological and agricultural fields. Remote sensing can be utilized to derive spatiotemporally continuous PAR. Empirical algorithms can be used to quickly retrieve surface PAR data sets, but their accuracy cannot be guaranteed in regions without local calibration. Physical algorithms generally incorporate all relevant physical processes and can be used globally, but their computational efficiency is often low. In this paper, an efficient algorithm is developed to calculate surface PAR by combining a clear-sky PAR model and the parameterizations for cloud transmittances. In the algorithm, the transmittances for water vapor, ozone, Rayleigh, aerosol, and cloud are each handled across the whole PAR band (400–700 nm). In addition, the contribution of the multiple reflections between surface ground and the atmosphere are also expressly considered. The new algorithm is applied to estimate instantaneous PAR with inputs from Moderate Resolution Imaging Spectroradiometer (MODIS) products onboard both Terra and Aqua platforms. The daily PAR is estimated from these two instantaneous values by an upscaling method. The instantaneous and daily PAR estimates were validated with in situ data collected in the USA and China. The results indicate that the new algorithm, based on MODIS products, can effectively retrieve PAR with root mean square errors (RMSE) of about 40 W m− 2 and 15 W m− 2 at instantaneous and daily-mean time scales, respectively. These performances are generally better than those of previous studies.