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

Abstract

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 Ø↓ 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 Ø↓ 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 Ø↓ under variable sky conditions.