Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part I: Miniaturizing the sampling volume

Abstract

An improved Lyman-alpha hygrometer has been developed in order to measure high-frequency small-scale fluctuations of water vapor concentration in the lower atmosphere and in tunnels (e.g., IMST large air–sea interaction simulation tunnel). The sampling volume is reduced to 3 mm in diameter by aerodynamic shaping; the signal-to-noise ratio is largely increased by designing special UV sources and using a photomultiplier connected to a low-noise electrometer; parasitic sensitivities are reduced by a precise control of the variable path length and by filtering the UV beam with an oxygen cell. This article describes the problems and improvements related to reducing the sampling volume and presents some characteristic spectral functions obtained in IMST tunnel, up to 200 Hz. A further article will describe the O2 filtering technique, the control of sensitivity, and the data reduction procedures.

Flow distortion in flux measurements with IMST/SA-CNRS Lyman-alpha hygrometer

“…But no explanation to this evolution has yet been found. This behaviour is even more puzzling when considering that the spectral coherency between the signals from the hygrometer and the thermometer is practically unaffected by this evolution (Mestayer et al 1986). …”

“…The Institut de M6canique Statistique de la Turbulence and the Service d'A6ronomie du Centre National de la Recherche Scientifique have cooperated to develop a line of improved Lyman-alpha hygrometers (Mestayer et al 1985(Mestayer et al , 1986(Mestayer et al , 1987. In order to measure the small-scale fast fluctuations of laboratory humidity turbulent fields, the sampling volume has been reduced to a cylinder of air 3 mm in diameter and a few mm in length.…”

“…The corresponding frequency is of the order of 100 Hz (U = 5 ms-l). Mestayer et al (1986) have shown that the hygrometer effective frequency limit N e, the frequency for which the signal and noise spectra are equal (E, (Ne) = E (Ne)) is about 200 Hz. Therefore this hygrometer seems able not only to correctly measure a large part of the intertial sub-range of the spectrum (Mestayer et al 1986) but also to estimate the local flux of water vapour.…”

“…2) resembles the shape usually observed for scalar flux cospectra (Mestayer et al 1983 a), but it seems to approach zero at high frequencies faster than expected (about 40 Hz instead of 100-140 Hz). This could be partly due to the phase lag between the two probes (Mestayer et al 1986). On the other hand, even if the cospectrum aspect appeared normal, it would be hard to assess its validity and that of its integral because it is very difficult to dismiss the flow distortion effects.…”

“…For instance the variation of its sensitivity with y is not perfectly known. But more importantly, this probe is, by far, the best available hygrometer for small scale measurements in tunnel turbulence (Mestayer et al 1986). It is therefore impossible to deduce its performances and limitations from comparisons with any "reference hygrometer".…”