Estimating down- and up-welling thermal radiation for use in mean radiant temperature

Thermal radiation significantly impacts the energy balance of human beings, and is usually assessed via mean radiant temperature Tmrt. The longwave down-welling radiation (LDR) and the up-welling from ground (LUR) are rarely measured, but are in the same order of magnitude as daily means of shortwave irradiation. This study evaluates simple, but theoretically based algorithms for LDR and LUR by comparison with measurements in regionally differing climates of Germany. LDR is modelled via the clear-sky algorithm (LDRc) of Prata in a version converging against a realistic value for a completely dry atmosphere. Its advantage is an improved predicted variance. To account for cloud impact, a shortwave cloud modification factor (CMFsol) derived from frequently measured solar global irradiation is applied for daylight hours and observed total cloudiness at night-time, respectively. It is accounted for the diurnal course in the effective temperature via relative humidity, whose impact is modulated dependent on cloud cover and altitude. LUR is modelled using the parameterisation of the Klima-Michel-Model. The selected algorithms show a universal applicability. A CMFsol is more closely related to hemispherical cloud base emittance than any cloud correction scheme requiring synoptic cloud observations. The LUR model accounts for 90 − 97 % of variance in observations. The uncertainty in Tmrt due to estimated LDR and LUR is comparable to that of the shortwave radiation components.