Intercomparison of global PAR data sets

Abstract

Global data sets of surface‐incident photosynthetically active radiation (PAR, 400–700 nm) from three satellite‐based methodologies are compared with each other and with ground data. Two are based on C1 data from the International Satellite Cloud Climatology Project (ISCCP): ISCCP‐BR was produced from an existing total shortwave irradiance data set by assuming 50% PAR; ISCCP‐PL is an existing PAR‐specific data set. TOMS PAR is based on 370 nm data from the Nimbus‐7 Total Ozone Mapping Spectrometer (TOMS). ISCCP‐BR and ISCCP‐PL are typically 12–16% and 8–12% greater than TOMS PAR, respectively. Comparison of ISCCP‐BR, ISCCP‐PL, and TOMS PAR with ground data at a mid‐latitude site for non‐winter (snow/ice‐free) months yields RMS differences of 28.1%, 13.7%, and 7.2%, and biases of +25.9%, +12.0%, and +2.8%, respectively. The comparison indicates TOMS PAR provides the most reliable estimates of PAR for snow/ice‐free, growing season months, i.e. when the energy is effective for photosynthesis. Differences among the data sets constitute a significant source of variation in results from biospheric process models that include PAR as a variable.