Longwave incoming radiation in the Tropics: results from field work in three African cities

Summary This study investigates differences in longwave incoming radiation (L↓) within and between three African cities, Dar es Salaam (Tanzania), Ouagadougou (Burkina Faso), and Gaborone (Botswana), during the dry season, and evaluates the performance of a model to simulate these fluxes. In each city, direct observations of L↓, shortwave incoming radiation (K↓), air temperature, air humidity, and total suspended particle (TSP) concentration for three land uses (CBD, green residential, and traditional residential) were taken. The observed L↓ flux decreases with increasing latitude, and temperature becomes an increasingly important factor in governing L↓ variations further from the Equator. Humidity, as well as particle loading, differs significantly between the three cities. Differences between observed and modelled ɛ_sky for rural stations near all cities showed a clear diurnal variation, with maximum differences of 0.08 between day and night. This diurnal difference was incorporated in the model and, for urban areas the model overestimates L↓ by around 25 Wm⁻². However, this model performs equally well regardless of the land use considered in any of the cities. The residual (difference between observed and modelled urban L↓) did not show any correlation with particulate pollution. However, the difference between observed and calculated ɛ_sky is around 0.05 higher in Ouagadougou compared to the other cities, likely due to the heavy dust load observed here. It is concluded that tropical urban longwave radiation is not dramatically different from the mid latitudes.