Effect of clouds on the photosynthetically active radiation received in Kampala, Uganda.

Abstract

Photosynthetically active radiation (PAR) plays an important part in climate change and ecological processes. Few PAR measurements are available some in countries such as Uganda. Thus it is important to investigate the temporal characteristics of PAR in Uganda for further radiation budget and ecosystem studies. The temporal characteristics of PAR were analysed for different sky conditions for a period of 13 years from January 2005 to January 2018 using data recorded by the NILU-UV instrument installed at the roof top of the Departent of Physics, Makerere University located in Kampala, Uganda. Under clear-sky scenarios, the expected annual cycle was observed for PAR, with higher values during the months of March, September and October, and minimum values during the months of January, June and December due to the different course of the solar zenith angle along the year. This same behaviour is also roughly found for PAR under all-sky situations, where the cloud presence slightly mod- ified the monthly mean radiative field. Thus, a maximum mean value of 8.30 MJ m−2 day−1 was found for all-skies in March and a value of 8.99 MJ m−2 day−1 for clear-skies in the same month; while a minimum mean value of 6.51 MJ m−2 day−1 was found for all-skies in July and 7.44 MJ m−2 day−1 for clear-skies in the same month. The inter-annual daily irradiances of the measured PAR values showed variations, with a maximum value of 7.79 MJ m−2 day−1 observed in 2010 and the lowest values of 7.58 MJ m−2 day−1 observed in 2011. The monthly variations for the ratios of the PAR/GHI varied from 0.442 in November to 0.468 in March, resulting in an annual mean value of 0.455. Linear models for predicting PAR were developed for each month with high positive correlation coefficients between PAR and global solar radiation.