Surface Radiative Fluxes in Sub-Sahel Africa

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Abstract
This paper reports results of observations of radiative fluxes measured in sub-Sahel Africa during a 2-yr period (1992–94). Shortwave radiation in the solar spectrum (0.2–4.0 μm), photosynthetically active radiation (0.4–0.7 μm), and longwave radiation (4.0–50.0 μm) were observed. In this study, the annual variability and the effects of dust on these fluxes (in particular, on the ratio of photosynthetically active radiation to the total shortwave radiation) were characterized. This ratio, known as the conversion factor, is important in modeling net primary productivity and the total CO2 budget. In the past, this ratio was assumed to be constant. The authors’ observations indicate that the daily average conversion factor has a strong annual cycle with a minimum of 0.41 in the middle of the dry season and a maximum of 0.55 in the first half of the rainy season, which is consistent with theoretical computations. The 2-yr mean is 0.49, only about 2% less than the nominal value of 0.50. The 2-yr mean values of the daily (24 h) averages of shortwave, photosynthetically active, and longwave downward fluxes are 200, 98, and 398 W m−2, respectively. The average shortwave transmittance is 0.482. The two years of observations differed in the mean value of the downward shortwave component by about 7 W m−2, the second year having lower values, with a similar decrease in photosynthetically active radiation and an increase in the longwave component by about 5 W m−2, which would point to an increased cloudiness during the second year. The long-term objective of this study is to have high-quality ground truth in this climatic region for validating satellite-inferred surface radiative fluxes.

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