Spectrally Resolved Measurement of the Downwelling Longwave Radiance from an High-Altitude Station: Spectroscopic Issues in the Data Analysis

During the ECOWAR (Earth Cooling by Water vapor Radiation) measurement campaign, held in March, 2007 from the Testa Grigia station on the italian-swiss Alps, the REFIR-PAD (Radiation Explorer in the Far-InfraRed - Prototype for Applications and Development) spectroradiometer performed spectrally resolved measurements of the downwelling atmospheric radiance in the 100 _ 1100 cm_1 spectral range, with a resolution of 0.5 cm_1 . Measurements were performed both in clear sky conditions and in presence of cirrus clouds, with total columnar water vapor values ranging from 0.5 to 3 mm. The measured spectra were analyzed through a least _ 2 data fitting procedure based on the MINUIT minimization routine and the LBLRTM version 11.3 forward model, providing integrated water vapor content, cloud optical density and information on vertical water vapor and temperature profiles. Fitting residuals from the data analysis process show significant features in the 300 _ 600 cm_1 region, that are well above measurement uncertainty, giving a hint on possible refinements both in the water vapor continuum model and spectroscopy.