Comparison of Measured and Modeled Nocturnal Clear Sky Longwave Downward Radiation at Payerne, Switzerland

Abstract

In this study, a comparison of measured and modeled nocturnal clear sky Longwave Downward Radiation (LDR) in Payerne, Switzerland, was performed. The Radiative Transfer Models (RTM) MODTRAN and libRadtran were applied to compute LDR irradiances. Absorption coefficients for libRadtran calculations were generated using the line‐by‐line model ARTS and the band model LOWTRAN. 39 clear nights were chosen to calculate LDR and to compare with broadband LDR measurements and LDR measurements taken in the wavelength range 8 to 14 μm. To run the models, two different types of vertical pressure, temperature and humidity profiles were implemented: (a) radiosonde profiles normalized to 2 meter ground values of pressure, temperature and humidity and scaled to the total Integrated Water Vapor (IWV) content; (b) seasonal standard profiles of the McClatchey midlatitude atmosphere normalized to ground measurements and scaled to the total IWV content. In the broadband range, MODTRAN revealed mean differences (measured minus calculated LDR) of −1.2 W m−2±2.5 W m−2 and +7.7 W m−2±5.2 W m−2 for profile (a) and (b) respectively. In the wavelength range 8 to 14 μm, mean differences were −1.4 W m−2±2.0 W m−2 and +2.1 W m−2±3.7 W m−2. LOWTRAN revealed mean biases of +6.0 W m−2±2.9 W m−2 and +14.2 W m−2±5.6 W m−2 in the broadband range. In the wavelength range 8 to 14 μm, the biases were +4.1 W m−2±2.4 W m−2 and +7.4 W m−2±4.1 W m−2 for profile (a) and (b) respectively. Line‐by‐line calculations using ARTS were only performed with profile (a) in the wavelength range 8 to 14 μm. Results showed a mean difference of −0.7 W m−2±2.0 W m−2 between measurements and computations.