Downward atmospheric longwave irradiance under clear and cloudy skies: Measurement and parameterization

This paper evaluates models for the estimation of downward longwave atmospheric irradiance at a lowland location and a mountain location under clear and cloudy skies. The multiyear (1992-1995) data sets utilized for the study were recorded in southwest Germany during the REgio KLIma Projekt (REKLIP). Annual mean of downward atmospheric irradiance Odown arrow ranged from 315 to 328 W m(-2) at the lowland site and from 282 to 290 W m(-2) at the mountain site. Inter-annual variability of Odown arrow at the sites was less than 2%. Six existing downward longwave clear-sky irradiance models were assessed in this study. In addition, this study proposes a new parameterization for estimating downward longwave clear-sky irradiance at the surface. The new parameterization, which is validated with data from the Oklahoma-based U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program, performed better than the other six models. It produced estimates, which agree with measurements more closely (within 5% for lowland and 7% for mountain locations). The incorporation of quadratic cloud terms in the parameterization allows for the estimation of Odown arrow under variable sky conditions. (C) 2003 Elsevier Ltd. All rights reserved.