SURFACE RADIATIVE FLUXES IN SUB-SAHEL AFRICA

This paper reports results:of observations of radiative fluxes measure d in sub-Sahel Africa during a 2-yr period (1992-94). Shortwave radiat ion in the solar spectrum (0.2-4.0 mu m), photosynthetically active ra diation (0.4-0.7 mu m), and longwave radiation (4.0-50.0 mu m) were ob served. In this study, the annual variability and the effects of dust on these fluxes (in particular, on the ratio of photosynthetically act ive radiation to the total shortwave radiation) were characterized. Th is ratio, known as the conversion factor, is important in modeling net primary productivity and the total CO2 budget. In the past, this rati o was assumed to be constant. The authors' observations indicate that the daily average conversion factor has a strong annual cycle with a m inimum of 0.41 in the middle of the dry season and a maximum of 0.55 i n the first half of the rainy season, which is consistent with theoret ical 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) averag es of shortwave, photosynthetically active, and longwave downward flux es are 200, 98, and 398 W m(-2), respectively. The average shortwave t ransmittance is 0.482. The two years of observations differed in the m ean value of the downward shortwave component by about 7 W m(-2) the s econd year having lower values, with a similar decrease in photosynthe tically active radiation and an increase in the longwave component by about 5 W m(-2), which would point to an increased cloudiness during t he 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.