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

Journal article
Authors P. Jonsson
Ingegärd Eliasson
Björn Holmer
C. S. B. Grimmond
Published in Theoretical and Applied Climatology
Volume 85
Issue 3-4
Pages 185-201
ISSN 0177-798X
Publication year 2006
Published at Department of Earth Sciences
Pages 185-201
Language en
Links dx.doi.org/10.1007/s00704-005-0178-...
Keywords sky-view factors, wave-radiation, clear skies, thermal-radiation, cloudless skies, urban areas, surface, parameterization, atmosphere, formula
Subject categories Earth and Related Environmental Sciences

Abstract

This study investigates differences in longwave incoming radiation (L down arrow) 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 down arrow, shortwave incoming radiation (K down arrow), 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 down arrow flux decreases with increasing latitude, and temperature becomes an increasingly important factor in governing L down arrow variations further from the Equator. Humidity, as well as particle loading, differs significantly between the three cities. Differences between observed and modelled epsilon(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 down arrow by around 25m(-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 down arrow) did not show any correlation with particulate pollution. However, the difference between observed and calculated epsilon(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.

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