Lyman-alpha Hygrometer Research Articles

UV Absorption Hygrometer for Fast-Response Airborne Water Vapor Measurements

Abstract A next-generation vacuum-ultraviolet (Lyman-alpha) absorption hygrometer for high-rate research aircraft humidity measurements designed by the National Center for Atmospheric Research is described. It retains the high data rate, optical and mechanical simplicity, and low maintenance of previous Lyman-alpha hygrometers, while incorporating modern electronics and rugged, long-lived commercially available lamps and detectors. The mass of the sensing head is 2.0 kg in a volume of 3700 cm3, while the power supply is 1.3 kg mass in a volume of 1100 cm3. Power draw is 0.2 A at 120 V alternating current (AC). In bench and aircraft flight testing the prototype shows a bandwidth of 35 Hz and mixing ratio noise of ±0.5% over a water vapor mixing ratio range of 2–15 g kg−1. This range can be scaled to lower mixing ratios by increasing the pathlength. This performance enables measurements of water vapor concentration with high spatial resolution from research aircraft. The prototype instrument has flown over 380 h with minimal maintenance or repairs.

Hydration of the lower stratosphere by ice crystal geysers over land convective systems

Abstract. The possible impact of deep convective overshooting over land has been explored by six simultaneous soundings of water vapour, particles and ozone in the lower stratosphere next to Mesoscale Convective Systems (MCSs) during the monsoon season over West Africa in Niamey, Niger in August 2006. The water vapour measurements were carried out using a fast response FLASH-B Lyman-alpha hygrometer. The high vertical resolution observations of the instrument show the presence of accumulation of enhanced water vapour layers between the tropopause at 370 K and the 420 K level. Most of these moist layers are shown connected with overshooting events occurring upwind as identified from satellite IR images over which the air mass probed by the sondes passed during the three previous days. In the case of a local overshoot identified by echo top turrets above the tropopause by the MIT C-band radar also in Niamey, tight coincidence was found between enhanced water vapour, ice crystal and ozone dip layers indicative of fast uplift of tropospheric air across the tropopause. The water vapour mixing ratio in the enriched layers exceeds frequently by 1–3 ppmv the average 6 ppmv saturation ratio at the tropopause and by up to 7 ppmv in the extreme case of local storm in coincidence with the presence of ice crystals. The presence of such layers strongly suggests hydration of the lower stratosphere by geyser-like injection of ice particles over overshooting turrets. The pile-like increase of water vapour up to 19 km seen by the high-resolution hygrometer during the season of maximum temperature of the tropopause, suggests that the above hydration mechanism may contribute to the summer maximum moisture in the lower stratosphere. If this interpretation is correct, hydration by ice geysers across the tropopause might be an important contributor to the stratospheric water vapour budget.

Intercomparisons of Stratospheric Water Vapor Sensors: FLASH-B and NOAA/CMDL Frost-Point Hygrometer

Studies of global climate rely critically on accurate water vapor measurements. In this paper, a comparison of the NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL) frost-point hygrometer and the Fluorescent Advanced Stratospheric Hygrometer for Balloon (FLASH-B) Lyman-alpha hygrometer is reported. Both instruments were part of a small balloon payload that was launched multiple times at Sodankylä, Finland. The comparison shows agreement well within the instrumental uncertainties between both sensors in the Arctic stratospheric vortex. The mean deviation between both instruments in the range between 15 and 25 km is −2.4% ± 3.1% (one standard deviation). The comparison identified some instrumental issues, such as a low mirror-temperature calibration correction for the NOAA/CMDL frost-point hygrometer as well as a time lag. It was found that the FLASH-B hygrometer measures water vapor reliably above 7 km in the polar atmosphere. Comparisons in the upper troposphere are affected by the gain change of the NOAA/CMDL hygrometer, causing a lag and a wet bias in the tropospheric low gain setting under the dry conditions in the upper troposphere.

Stratospheric water vapour in the vicinity of the Arctic polar vortex

Abstract. The stratospheric water vapour mixing ratio inside, outside, and at the edge of the polar vortex has been accurately measured by the FLASH-B Lyman-Alpha hygrometer during the LAUTLOS campaign in Sodankylä, Finland, in January and February 2004. The retrieved H2O profiles reveal a detailed view on the Arctic lower stratospheric water vapour distribution, and provide a valuable dataset for the validation of model and satellite data. Analysing the measurements with the semi-lagrangian advection model MIMOSA, water vapour profiles typical for the polar vortex' interior and exterior have been identified, and laminae in the observed profiles have been correlated to filamentary structures in the potential vorticity field. Applying the validated MIMOSA transport scheme to specific humidity fields from operational ECMWF analyses, large discrepancies from the observed profiles arise. Although MIMOSA is able to reproduce weak water vapour filaments and improves the shape of the profiles compared to operational ECMWF analyses, both models reveal a dry bias of about 1 ppmv in the lower stratosphere above 400 K, accounting for a relative difference from the measurements in the order of 20%. The large dry bias in the analysis representation of stratospheric water vapour in the Arctic implies the need for future regular measurements of water vapour in the polar stratosphere to allow the validation and improvement of climate models.

A Lyman-Alpha Hygrometer with Variable Pathlength

Abstract A novel design of a Lyman-alpha hygrometer for surface layer flux measurements is presented using a variable pathlength. The benefit of this device is the capability for in situ calibration during a field experiment with only a short interruption of the measurements of 5 min or less. A description of the device, its operating specifications, and an example of the calibration process are included.

Conditional analysis of temperature and humidity microfronts and ejection/sweep motions within and above a deciduous forest

A conditional sampling technique using a multilevel scheme was applied to the detection of temperature and humidity microfronts and organized ejection/sweep motions under different atmospheric stabilities. Data were obtained with seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers within and above a deciduous forest. Both temperature and humidity microfronts were identified in unstable cases, but only humidity microfronts could be detected under neutral conditions. Inverted temperature ramps occurred under slightly stable conditions. Occasionally, wave-like patterns appeared within the canopy, seemingly coupled with inverse ramps occurring above the forest. The frequency of occurrence of scalar microfronts appears to have no clear dependence on atmospheric stability, and averages 74–84 s per cycle with a mode of about 50 s per cycle. However, the strength of ejections and sweeps, shown by the vertical velocity averaged within structures, was reduced by increasing atmospheric stability. Structures identified under different stabilities show many similarities in their patterns of scalar ramps, and associated velocity and surface pressure. Profiles of short-term averaged longitudinal velocity at different times during the microfront passage show that the air within the canopy was retarded and an intensified shear above the canopy occurred prior to the passage of the microfront. Results from the present conditional analysis strongly suggest an important role of shear instability in the formation of canopy coherent structure.

A New Fast Response Instrument for Measuring Total Water Content from Aircraft

Abstract A device for measuring the total water content of a parcel of air from an aircraft has been developed. The total water of a parcel of air is a conserved quantity, independent of phase changes, provided there is no transport of water through the parcel boundaries. Current airborne hygrometers normally attempt to measure the water content in individual phases and the presence of other phases invariably influences the quality of the data. However, any liquid water or ice entering this new probe is efficiently evaporated and the resultant water vapor measured using a Lyman-alpha hygrometer. In airborne trials the device was calibrated against a cooled-mirror dewpoint device. Runs were conducted in warm stratocumulus tops, through small cumulus, in mixed-phase precipitation and cirrus cloud. In all cases the device was found to produce high quality, fast response data.

Improved Lyman-alpha hygrometer. Part III: Performance of a field version for small scale atmospheric turbulence measurements over the sea

An all-weather ruggedized and simplified version of the IMST/SA–CNRS improved Lyman-alpha hygrometer has been developed in cooperation with Riso/ National Laboratory to measure the small-scale, high-frequency fluctuations of humidity at an open-sea research platform. This version is characterized by the integration of the photomultiplier detection electronics inside the support and by a casings design especially adapted to operation in adverse conditions. Comparisons with other fast hygrometers during the HEXOS Main Experiment indicate that this instrument has probably, to date, the best sensitivity, signal-to-noise ratio, and effective frequency limit. It is able to measure the entire inertioconvective subrange of scales and probably, under some conditions, a part of the dissipation range.

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

A Lyman-alpha hygrometer with a reduced sampling volume has been developed in order to measure small-scale, fast fluctuations of humidity in laboratory. It is combined with an X-wire anemometer probe to measure the local value of the turbulent flux or with a resistant wire temperature probe to measure humidity-temperature mixed statistics. In order to determine the influence of the flow distortion by the probe itself on these measurements, the structure of the flow is investigated experimentally both inside the hygrometer sampling volume and at the location of the velocity or temperature probe. This investigation includes flow visualizations, measurements with a single hot wire in a calibration tunnel and measurements with a X-wire probe and a temperature resistance wire probe in a large turbulent boundary layer.

Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part II: The O2 filtering technique

The detector of the IMST/SA-CNRS improved Lyman-alpha hygrometer is a solar-blind photomultiplier tube that ensures large signal-to-noise ratio and allows a miniaturization of the sampling volume. This PM tube has a relatively large spectral response and can detect, in addition to the Lyman-alpha line, hydrogen molecular transitions emitted by the discharge source between 120 and 180 nm. In order to avoid a spurious sensitivity to oxygen density fluctuations, an active filtering cell is added to absorb the undesired lines. The instrument sensitivity is calculated with the help of a two-gas, three-spectral-line UV-beam absorption model, fitted to calibration measurements. The technique of the active filtering associated with the three-line absorption model produces the following advantages: (1) reduced sensitivity to oxygen fluctuations, (2) a clear physical meaning to the model terms allowing the sensitivity computation in most conditions, (3) increased signal-to-noise ratio, and (4) in most conditions, improved sensitivity to water vapor compared to the monochromatic Lyman-alpha line.

Calibration of an Airborne Lyman-Alpha Hygrometer and Measurement of Water Vapor Flux Using a Thermoelectric Hygrometer

Abstract An improved calibration technique for an airborne Lyman-alpha hygrometer is presented. Like previous methods, it relies upon simultaneous measurement of absolute humidity determined from a slower response hygrometer. We show that a substantial improvement in the Lyman-alpha calibration is obtained by accounting for the time lag of the slower instrument. To show our technique we use data from Lyman-alpha and thermoelectric devices on the NCAR Electra during an investigation of the nearly neutral boundary layer over the Arabian Sea as part of the WMO/ICSU Summer Monsoon Experiment. We also show that for near-neutral conditions the eddy-correlation water vapor flux can be adequately estimated using the fast response vertical velocity data from a gust probe and slower response data from the thermoelectric device, which has been properly advanced to account for the time lag.

Temperature Sensitivity of Lyman-Alpha Hygrometers

The response of an E.R.C. Lyman-alpha hygrometer to humidity and temperature variations has been experimentally determined in a humidity calibration tunnel. The data are compared to a simple two-gas/two-line model to determine the errors made in atmospheric measurements of humidity variance and flux due to turbulent fluctuations of temperature. The model also was used to study the influence of path length on the hygrometer response. The emission by current Lyman-alpha sources of H2 molecular lines, even in small quantity, can cause temperature sensitivity errors much larger than expected in spectral functions of humidity or mixed humidity-temperature statistics. These errors largely depend on the quality of spectral emission, path length and mean humidity and temperature.

Turbulence spectra of CO2, water vapor, temperature and wind velocity fluctuations over a crop surface

A prototype rapid-response CO2 sensor was used in conjunction with a Lyman-alpha hygrometer, fine-wire thermocouples and a three-dimensional drag anemometer to measure CO2, humidity, temperature and wind velocity fluctuations. Measurements were made over a soybean crop grown on relatively flat terrain near Mead, Nebraska.

Eddy correlation measurements of CO2, latent heat, and sensible heat fluxes over a crop surface

Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO2 sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities. Diurnal fluxes of sensible heat, latent heat and CO2 were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO2 fluxes were near 1.0 mg m−2 (ground area) s−1. The eddy correlation technique was also employed in this study to measure nocturnal CO2 fluxes caused by respiration from plants, soil, and roots. These CO2 fluxes ranged from - 0.1 to - 0.25 mg m−2s−1. From the data collected over mature soybeans, a relationship between CO2 flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 ΜEi m−2 s−1. The light compensation point was found to be about 160 ΜEi m−2 s−1.

The Effects of Volume Averaging on Spectra Measured with a Lyman-Alpha Hygrometer

Abstract Because the Lyman-alpha hygrometer averages turbulent fluctuations in humidity over a right circular cylinder, the spectral response of the instrument degrades at higher wavenumbers. This paper contains a derivation of the three-dimensional spectral averaging function and uses this function, with a new model for the scalar spectrum, to numerically evaluate how this spatial averaging affects measured humidity spectra and humidity variance dissipation rates. In general, hygrometer parameters can be chosen that allow spectral measurements to moderately high wavenumbers; but with the size of source and detector tubes currently in use, an accurate measurement of the humidity variance dissipation rate appears impossible.

Humidity Fluctuations over a Vegetated Surface Measured with a Lyman-Alpha Hygrometer and a Fine-Wire Thermocouple Psychrometer

Simultaneous measurements of humidity fluctuations over a crop made with a specially modified Lyman-alpha hygrometer and a fine-wire thermocouple psychrometer are compared. Standard deviations of the two sets of data are comparable except occasionally when wind speeds were low. The psychrometer appears to underestimate the vertical flux of water vapor due to its slow response. Analysis of humidity spectra and moisture flux cospectra shows that the Lyman-alpha hygrometer is superior to the psychrometer in response at high frequencies and low wind speeds.

The Variable-Path Lyman-Alpha Hygrometer and Its Operating Characteristics

Abstract A hygrometer capable of making in situ humidity measurements with good accuracy and excellent time-space resolution is described. Based on the absorption of Lyman-alpha radiation by water vapor, the instrument uses a variable-path technique for achieving long-term stability and is characterized by 12 ms response time, 5% accuracy, 1 cm typical sensing path, and an operating range of −35° to +30°C dew point. The hygrometer is now operational on NCAR aircraft.