First-Principles Calibrated Water Vapor Raman Lidar for Atmospheric Humidity Profiling

Water vapor plays a fundamental role in the radiative energy transfer, hydrological cycle, and atmospheric chemistry processes that determine weather and climate. Because of its essential role in the atmospheric processes, the water vapor spatial distribution and its temporal evolution are among the most important parameters in numerical weather prediction and climate models.

The operational humidity profiling in meteorology is carried out mostly by balloon-borne radio sondes. Capacitive, or less often, chilled mirrors are used as sensors. Both types of sensors rely on empirical or semi empirical relations between the measurand and the relative humidity or the due (frost) point and the measurement process can be affected by external processes.  Therefore sonde measurements are not traceable to primary standards, are prone to important systematic errors, and require careful corrections.  The use of sondes from different producers and their arbitrary replacement introduces incoherence in the global humidity database and discontinuities in long-term data that impedes climate studies. The microwave radiometers and GPS humidity observations lack spatial and time resolution and rely on number of assumptions which limit their accuracy. Differential Absorption Lidars (DIAL) are able of providing humidity profiles with high spatial and temporal resolution from air or space born platforms but their range and accuracy for ground-based monitoring is limited by the natural vertical humidity gradient.

 Recently, some national meteorological services (MeteoSwiss, DWD) have introduced Raman lidars for operational humidity profiling. Raman lidars exploit the proportionality between the intensity of scattered by Raman process laser radiation and the number density of scattering molecules. Water vapor/air mixing ratio is proportional to the ratio of the measured intensities of Raman scattering from water vapor and nitrogen molecules.  The coefficient of proportionality, denoted as calibration constant, is primary factor defining lidar measurement accuracy. The calibration constant is a complex function of instrumental parameters and spectroscopic parameters of the scattering molecules.  Derivation of the calibration constant using above mentioned parameters, known as “instrumental” calibration, is possible but leads to high uncertainty due to the uncertainties of the input parameters. Therefore Raman lidars are generally calibrated against reference instrument- most often radio sonde. The accuracy and the long-term coherence of such calibration is thus defined by the accuracy and coherence of the reference instrument. Furthermore, the lidar and the reference instrument nearly always sample different air masses with different time and space resolutions, leading to additional systematic errors.

A new, first-principles calibration method, will be presented. In this method, the calibration constant is derived from backscatter signals measured with the lidar receiver in a calibration cell filled with a reference water vapor/air mixture. The new calibration method eliminates the unavoidable in the cross-calibration methods errors and uncertainties induced by the reference instrument and by the calibration procedure. The reference mixture is prepared gravimetrically and allows for calibration constant uncertainties lower. Since the preparation relays on fundamental principles and basic unit of measurements, the reference mixture can be regarded as primary humidity standard. The obtained by the new method lidar calibration constant has the potential to be traceable to national standards of mass. The method allows deriving of the calibration constant, with uncertainty lower than 0.1%.

The traceability of the calibration will ensure long-term consistency and coherence of the lidar data. The calibrated in such way lidar has the potential to become a reference in vertical humidity profiling and radio-sonde, microwave radiometer and GPS water vapor measurements.

The calibration setup and calibration  procedure  as well some atmospheric measurement results taken with experimental Raman lidars calibrated by the new method will be presented.