Global solar radiation in a southern African savanna environment

Global solar radiation (K↓) was measured from April through December 2000 at Lusaka, Zambia under varying meteorological conditions. Extraterrestrial radiation and Hadley cell movement determined seasonality in both K↓ and the clearness index (ratio of solar to extraterrestrial radiation). Atmospheric controls caused distinct diurnal, monthly and seasonal variability in all-sky K↓ and the clearness index. Under subtropical anticyclones (April - November), clear skies and low precipitable water enhanced the clearness index values. Widespread biomass burning in southern Africa contributed to a doubling in the atmospheric turbidity in the hot-dry season, and attenuated cloudless-sky K↓ by an additional 10%. Increased cloud cover and precipitable water, associated with the ITCZ, yielded a decreasing trend and more day-to-day variability in both K↓ and the clearness index in the warm-wet season.

Employing a Houghton model variant, errors in hourly cloudless-sky K↓ estimations varied closely with monthly atmospheric turbidity. Widespread biomass burning throughout southern Africa contributed significantly to large differences between the modelled and observed cloudless-sky values. A modified model that simulated biomass burning effects on seasonal turbidity, calculated large shortfalls of all-sky K↓ with high variability as a consequence of cloud effects. This variability was strongly linked to Hadley cell movement. The measurement and model results highlight the linkages between atmospheric (extraterrestrial radiation, cloud, etc.) and surface (biomass burning) control mechanisms on the receipt of solar radiation in southern Africa.