Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1107–1116 www.elsevier.com/locate/jastp Downward atmospheric longwave irradiance under clear and cloudy skies: Measurement and parameterization M.G. Iziomon a ; ∗ , H. Mayer b , A. Matzarakis b a DepartmentofPhysicsandAtmosphericScience,DalhousieUniversity,HalifaxNSB3H3JS,Canada b MeteorologicalInstitute,UniversityofFreiburg,Freiburg,Germany Received 22 July 2002; received in revised form 14 May 2003; accepted 15 July 2003 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 I ↓ 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 I↓ 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 I↓ under variable sky conditions. c 2003 Elsevier Ltd. All rights reserved. Keywords: Downward atmospheric radiation models; Lowland; Mountain; REKLIP; ARM; Sky conditions 1. Introduction Downward longwave atmospheric irradiance I↓ incident at the earth surface is a very signicant constituent of the global radiation budget. Its knowledge is required for (a) the forecast of nocturnal frosts, fogs, temperature variation and cloudiness; (b) energy balance studies; (c) the design of radiant cooling systems as well as (d) calculations on cli- mate variability and global warming (Crawford and Duchon, 1999; IPCC, 2001). However, current attempts to investigate energy balance at the earth’s surface are still being hindered by lack of data on longwave uxes (Garratt and Prata, 1996; ∗ Corresponding author. Tel.: +1-902-494-1820; fax: +1-902- 494-5191. E-mail addresses: iziomon@mathstat.dal.ca (M.G. Iziomon), helmut.mayer@meteo.uni-freiburg.de (H. Mayer), andreas.matzarakis@meteo.uni-freiburg.de (A. Matzarakis). Kessler and Jaeger, 1999). In view of this, there is a grow- ing interest in alternative techniques for the estimation of longwave uxes at the surface. If the atmosphere is understood formally as a grey body, the amount of downwelling longwave irradiance is deter- mined by the bulk emissivity atm and eective tempera- ture Tatm of the overlying atmosphere according to I↓ = atm(Tatm) 4 , where denotes the Stefan-Boltzmann con- stant (5:67 × 10 -8 Wm -2 K 4 ). Since it is dicult to spec- ify atm and Tatm for a vertical column of the atmosphere (Crawford and Duchon, 1999), I↓ could be parameterized from air temperature Ta (K) and/or vapor pressure e (hPa) measured close to the ground, such that during clear-sky (with cloud cover N = 0 okta), I↓(0) = 0(Ta ;e)T 4 a : (1) In Eq. (1), 0 (dimensionless) represents the eective clear-sky atmospheric emissivity. 1364-6826/$ - see front matter c 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2003.07.007