Estimation of errors of the spectral method for measuring the concentration of water vapor and moisture reserves in the surface layer of the earth’s atmosphere

The paper discusses the specifics of measuring and monitoring moisture reserves in the atmosphere using radar methods, involving standard meteorological and satellite-based observation data, as well as methods of microwave sounding and optical spectroscopy. It is noted that among the above-listed methods, the most economically efficient are the optical methods (e.g., spectral LIDAR, active and passive, ground-based), which make it possible to promptly and adequately determine the atmospheric moisture reserves in the known greenhouse gas absorption lines, as well as replace and supplement radar data. The equipment that implements the optical methods includes inexpensive and readily available portable transceiver lenses, as well as compact and lightweight fiber-optic radiation sources and receivers, which are convenient for use when performing satellite-based measurements. A spectral method for measuring the concentration of water vapors in the atmosphere in the presence of solar radiation has been proposed and tested. The location of the measurements is the northeast of the Moscow region. When processing the results of measurements carried out on a single water absorption line, it was found that the experimental data match the results of direct meteorological measurements with an error of about 7.5%. It was established that on the water absorption line (approximately 1653 nm), the interfering factor is the nearby methane absorption line, which introduces a systematic error into the measurement results. It was suggested that the probable reason of the results deviation from the meteorological measurement data has to do with the absorption cross-section of methane molecules being approximately four orders of magnitude larger than that of water vapor molecules. As a result, emitted light is absorbed mostly by methane, thus reducing the fraction of water vapor absorption at a wavelength close to the methane absorption line. Compensatory corrections have been introduced, making it possible to determine the water vapor concentration and moisture reserves in the air column with an error of several percent at this wavelength. The proposed spectral method enables long-term monitoring of the water vapor concentration and moisture reserves in the atmosphere, and allows obtaining live data in real time. The experimental data, on average, agree with the measurement results obtained using radiometers that operate in the microwave range. The obtained results can be used to promptly estimate the moisture reserves and water vapor concentration in the entire surface layer of the atmosphere over large areas, to calibrate radiometers and radars, and to improve the accuracy of measuring the above parameters and forecasting dangerous natural phenomena, such as hurricanes, floods, and landslides.