Abstract
This paper presents a simple semi-empirical model for estimating global photosynthetically active radiation (PAR) under all sky conditions. The model expresses PAR as a function of cloud index, aerosol optical depth, total ozone column, solar zenith angle, and air mass. The formulation of the model was based on a four-year period (2008–2011) of PAR data obtained from the measurements at four solar monitoring stations in a tropical environment of Thailand. These are Chiang Mai (18.78°N, 98.98°E), Ubon Ratchathani (15.25°N, 104.87°E), Nakhon Pathom (13.82°N, 100.04°E), and Songkhla (7.20°N, 100.60°E). The cloud index was derived from MTSAT-1R satellite, whereas the aerosol optical depth was obtained from MODIS/Terra satellite. For the total ozone column, it was retrieved from OMI/Aura satellite. The model was validated against independent data set from the four stations. It was found that hourly PAR estimated from the proposed model and that obtained from the measurements were in reasonable agreement, with the root mean square difference (RMSD) and mean bias difference (MBD) of 14.3% and −5.8%, respectively. In addition, for the case of monthly average hourly PAR, RMSD and MBD were reduced to 11.1% and −5.1%, respectively.
Highlights
Active radiation (PAR) is part of the solar radiation spectrum from 400 to 700 nm
We proposed a semi-empirical model to relate Photosynthetically active radiation (PAR) to the atmospheric parameters, namely, the cloud index, aerosol optical depth, total ozone column, and air mass, as follows: QP = a0QextE0 cos θZe−(a1AOD+a2OZ)ma (1 − a3n), (2)
When tested against an independent data set for hourly PAR, the proposed model gave the values of root mean square difference (RMSD) of 14.3% and mean bias difference (MBD) of −5.8%
Summary
Active radiation (PAR) is part of the solar radiation spectrum from 400 to 700 nm. PAR data need to be obtained from modeling approaches One of these approaches is to use a ratio of PAR to broadband solar radiation to estimate PAR from more widely measured broadband radiation. The use of PAR-to-global radiation ratio has an advantage in terms of the simplicity, but it can be effectively used only for the locations where broadband solar radiation data are available. Global PAR measured at four sites in Thailand and the corresponding cloud index, aerosol optical depth, and total ozone column obtained from satellites were used for the development of the model
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