The water vapour continuum in near-infrared windows – current understanding and prospects for its inclusion in spectroscopic databases

Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption

Modification of the experimental setup of the FTIR spectrometer and thirty-meter optical cell for measurements of weak selective and nonselective absorptions

The improvement of the experimental setup based on a Fourier spectrometer Bruker IFS-125 and a 30-meter multipass optical cell is described. The improvement includes the cell equipment with a system of automated adjustment of the number of beam passes without cell depressurization and ensures the cell work at high temperatures

Transformation of carbide phases in a heat-resistant Fe-Cr-Ni alloys at operating temperatures

Dispersed phase quantity and morphology changes in the structure of superalloy HP40Nb series during a prolonged exposure at the temperature of 1150 °C have been obtained by the quantitative analysis method of the structure with carbide hardening alloys and using the experimental data of scanning electron microscopy and X-ray microanalysis. It has been shown that the carbide phase increase during the first hours of the cast alloy exposure at operating temperatures is caused by the consistent transformation of eutectic chromium carbides. An analysis of carbide phase transformation based on carbon balance has been carried out.Методом количественного анализа структуры сплавов с карбидным упрочнением с использованием экспериментальных данных сканирующей электронной микроскопии и рентгеноспекрального микроанализа получены зависимости изменения объемной доли и морфологии дисперсных фаз в структуре жаропрочного сплава серии HP40Nb в процессе длительной выдержки при температуре 1150 °С. Показано, что увеличение содержания карбидной фазы в течение первых часов выдержки литого сплава при температурах эксплуатации связано с последовательной трансформацией эвтектических карбидов хрома. Проведен анализ трасформации карбидных фаз на основе баланса содержания углерода

Tractional Electric Drive with Non-Sensing Element Vector Control System

The purpose of the paper is a structure formation and an analysis of non-sensing element vector control system developed for tractional electric drive with the help of mathematical simulation method. The paper presents a functional diagram of the electric drive with non-sensing element vector control system  operated by an asynchronous short-circuited electric motor.  Main expressions used for evaluation of variables of system conditions and parameters are cited in the paper. The paper provides results of mathematical simulation method for electric drive system taking into consideration various parameter values which confirm serviceability of the developed control system within the whole range of possible parameter chnges

Laboratory measurements of water vapour continuum absorption in spectral region 5000-5600 cm(-1): Evidence for water dimers

Paper associated with the CWVC (Clouds, Water Vapour and Climate) dataset held on the CEDA archive

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

For a long time, it has been believed that atmospheric absorption of radiation within wavelength regions of relatively high infrared transmittance (so-called ‘windows’) was dominated by the water vapour self-continuum, that is, spectrally smooth absorption caused by H2O−H2O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H2O−N2 bimolecular absorption in the Earth's atmosphere) was considered to be negligible in the windows. We report new retrievals of the water vapour foreign continuum from high-resolution laboratory measurements at temperatures between 350 and 430 K in four near-infrared windows between 1.1 and 5 μm (9000–2000 cm−1). Our results indicate that the foreign continuum in these windows has a very weak temperature dependence and is typically between one and two orders of magnitude stronger than that given in representations of the continuum currently used in many climate and weather prediction models. This indicates that absorption owing to the foreign continuum may be comparable to the self-continuum under atmospheric conditions in the investigated windows. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by approximately 0.46 W m−2 (or 0.6% of the total clear-sky absorption) by using these new measurements when compared with calculations applying the widely used MTCKD (Mlawer–Tobin–Clough–Kneizys–Davies) foreign-continuum model