On the variability and modelling of surface albedo and long-wave radiation components

Abstract

Variability and modelling of surface albedo and long-wave radiation components are presented for grassland sites whose elevation ranged from 212 to 1489 m a.s.l. Data sets utilised for the study were recorded in south-west Germany during REKLIP (REgio KLIma Projekt), which extended from 1991 to 1996. Inter-annual variability of albedo α and outgoing long-wave radiation R l↑ at the study sites was less than 9%, thus, rendering the measured data quite suitable for diagnostic and modelling purposes. Annually, α extended from 22 (at 212 m a.s.l.) to 36% (at 1489 m a.s.l.), while R l↑ declined from 367 (at 212 m a.s.l.) to 326 W m −2 (at 1489 m a.s.l.). Consequently, radiation efficiency at the lowest elevated grassland site was approximately 1.3 times that of the highest elevated site. The observed decline in incoming long-wave radiation R l↓ with altitude relates physically to the vertical distribution of air temperature and green house gases (particularly atmospheric water vapour and CO 2). Ratio of annual R l↓ to R l↑ at the sites was about 0.87. While albedo dropped with cloud cover N at both low and high altitude sites in summer, there was, however, a noteworthy discrepancy in winter, when α declined with N at the low altitude location but increased with it at the high altitude location. Mean daily sums of R l↑ fell with N in summer, but rose with it in winter for both low and high altitude sites. In view of the major role played by altitude in regulating climate and radiative budget at the surface, we have extended Dong et al. model to include seasonal altitude terms for estimating grassland albedo. In addition, outgoing long-wave radiation under all sky conditions has been expressed here as a function of air temperature, absorbed short-wave radiation and altitude.