FAR INFRARED SPECTRAL MEASUREMENTS OF THE ATMOSPHERIC THERMAL RADIATION

A major limitation in our understanding of the climatic effects of water vapour and clouds is that, whilst we have broadband observations that can determine the overall radiative impact, our ability to unequivocally link this to the underlying components of the atmosphere is under- constrained because of the lack of spectrally resolved measurements over a significant portion of the emission, namely within the far infrared (FIR) between approximately 100 and 600 cm-1. Observations in the FIR can reduce the existing uncertainties about the role of water vapour and cirrus cloud in climate change because the FIR region is strongly sensitive to mid-upper level tropospheric humidity through the intense water vapour rotational band, especially between 200 and 400 cm-1, and hence contains vital information pertaining to both the water vapour concentration and vertical distribution. Here we present the analysis of the measurements performed by an uncooled Fourier transform spectroradiometer named REFIR-PAD (Radiation Explorer in the Far Infrared - Prototype for Applications and Development) prototype which performs measurements in a spectral range from 100 to 1400 cm-1 with 0.25 of maximum resolution. REFIR-PAD is a prototype developed as a field demonstrator of a space-borne instrument, designed with the requirements of reliability, light weight (55 kg) and low power consumption (about 50 W) to fly on board stratospheric balloons and for ground-based observations. The instrument uses two room temperature pyroelectric detectors and an optical design with two input ports/two output channels, which allows to access and control all the input sources of signal. Maximum resolution is 0.25 cm-1 and the NESR is about 1 mW/ (m2 sr cm-1) in 200-600 cm-1 spectral region. The accuracy reached in the spectral measurement of the thermal radiation is studied for the whole dataset acquired by REFIR-PAD in ground-based campaigns since 2007 from different high-altitude stations: Testa Grigia Station, Cervinia-Italy, (3480 m asl), Cerro Toco, Atacama- Chile, (5380 m asl), Concordia Base, Dome C-Antarctica (3230 m asl). These measurements allow to quantify the radiative contribution to the Earth's radiation budget due to water vapour and cirrus clouds in the long-wave part of the emission spectrum above 15 mm of wavelength. The availability of these spectral measurements has opened the possibility to address the main issues of the FIR spectral range, anyway an integrated approach with other validation measurements should be pursued in order to provide the means for an assessment of the importance of this region and to try to extend these kind of measurements to a global coverage scenario by designing a suitable space application.