Radiation relationship over natural surface in Tanzania, with special attention to Longwave radiation

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University of Dar es Salaam
Measurement techniques of incident solar radiation are well established. Reliable measurements of shortwave radiation are presently made on a research and routine basis in Tanzania. Measurements of other components of the radiation balance or their combinations, such as net short-wave, net long wave and net all wave radiation, and long wave radiation from the atmosphere and from specified surfaces are, however, generally not available in the tropics. Empirical or semi-theoretical estimations of these parameters are made on the basis of studies conducted mainly in temperate climates. Longwave emissions from a surface or the atmosphere depend upon their effective (or apparent) temperature which in turn depends on the energy balance of the surface or the atmosphere under prevailing climatic factors. Hence long wave measurements are important in determining the state of a surface or the atmosphere. In this study, various components of the radiation balance have been measured at two places with different climates, (dry, semi-arid at Dodoma; and humid Dar es Salaam) in Tanzania, above three natural surfaces: natural grass, short (meteorological station) grass, and bare soil under clear skies and a few cloudy sky conditions. Various radiation relationships with respect to net all wave radiation have been presented. These have been used to evaluate heating coefficients for the natural surfaces. This parameter, first proposed by Monteith and Szeicz (1961), quantifies the long wave property of a surface with respect to net all wave radiation of that surface, depending on the prevailing environmental conditions above and within the surface. This parameter has a good theoretical basis and would allow the long wave properties of various surfaces to be calculated on a routine basis. However various studies conducted have been inconclusive with regards to its consistency, such that even its validity has been questioned. Instrument related problems and variations in downward long wave radiation have been cited to explain the inconsistency of measured values of heating coefficients. From hourly averages of radiation balance components, our evaluations of heating coefficients indicate that they behave with explainable consistency over the various natural surfaces. More conclusions about this parameter could not be drawn as we did not collect data over longer periods. Hourly means of downward long wave radiation, derived as a residual from measurements using an adapted Swissteco radiometer, were found to have a systematic, instrument related error, proportional to incident shortwave radiation, which we corrected by checking the internal consistency of our data set. The corrected diurnal variation of hourly means of downward long wave radiation under clear sky conditions in the semi-arid climate of Dodoma was found to have an anomalous minimum around midday, which we could confirm by examining data from other studies. An overall, Swanbank-type daily atmospheric emissivity relationship (e.g. equation 4.14) has been established for estimating daily or longer term mean downward long wave radiation (equation 4.17). When this equation was compared with several other empirical Swanbank-type formulae from the literature, it was found that most of these relationships fall within a narrow band on a plot of (calculated) downward long wave radiation versus equivalent black body radiation at screen level air temperature (figure 4.17). Since the various Swanbank-type formulae had been established using data taken over different locations and climates, this result implies that daily or longer term average atmospheric emissivities may have a rather limited range of variation. From our limited set of cloudy sky measurements, and other published data, the conurbation of clouds to total downward long wave radiation from the atmosphere has been determined and was found to be close to some other values found in the literature. Using monthly mean climatologically data for Dar Es Salaam and Dodoma, several empirical atmospheric long wave radiation formulae (including the “Penman” long wave formula, equation 2.35) have been compared in order to rationalize the net long wave term in the Penman evaporation formula (equation 4.27) for use under our tropical conditions. It is proposed that “Penman” long wave formula should be replaced, in the tropics, by the Brustsaert formula (equation 2.36) which has been derived using sound theoretical principles. For a minimum data input approach to evaporation calculations, the Brutsaert formula may be used as a general calibration equation; from which a Swanbank-type formula may be established for a particular location using accurate values of air temperature and vapour pressure at screen level for that location. Solar radiation, Tanzania
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Solar radiation, Tanzania
Jiwaji, N. T. (1990) Radiation relationship over natural surface in Tanzania, with special attention to Longwave radiation, Doctoral dissertation, University of Dar es Salaam. Available at (