ABSTRACT
A large number of simple formulae for the estimation of total longwave atmospheric irradiance has been compared to both measured data and to output from the spectral model MODTRAN. MODTRAN predictions under cloudless sky agree well with the average of 38 profile based models, and observed irradiances on the average agree with MODTRAN predictions within some 5 Wm2. It was also found that for “normally” stratified cloudfree atmospheres, the formulae of Berdahl & Fromberg and of Frank & Püntener, both expressing the emittance as a linear function of dew point temperature, adequately reflect the radiation physics over a wide range of temperature and humidity. Besides, the formulae of Swinbank and of Czeplak & Kasten, which both expresses the emittance as a quadratic function of dry bulb temperature, turned out to reflect the radiation physics of “normally” stratified not too dry atmospheres. For “abnormally” stratified atmospheres, a correction to empirical formulae was found. For overcast situations, the increase in atmospheric emittance beyond its cloudless value showed up to be a function of the fractional cloud cover, the cloud emittance, and the surface minus cloud base temperature difference, as phrased by Martin & Berdahl. Except for a coarser spectral resolution and a slight underestimation the simple spectral model of Berger conforms well with spectral irradiances derived from observed spectral radiances.
