Tropospheric Water Vapor Measurements Research Articles

GEWEX Water Vapor Assessment: Validation of AIRS Tropospheric Humidity Profiles With Characterized Radiosonde Soundings

AbstractThe tropospheric water vapor profile record from the Atmospheric Infrared Sounder (AIRS) now spans over a decade, making it a valuable resource for climate studies. To be considered as a Climate Data Record, it is key that the ultimate performance of these observations are understood. The Global Energy and Water cycle Exchanges (GEWEX) water vapor assessment has been tasked with characterizing the current state of the art in water vapor products currently available for climate analysis. Within the scope of this exercise, water vapor profiles from AIRS have been assessed using collocated characterized in situ measurements of tropospheric water vapor between 2007 and 2012. We first show how previously published methods for correcting radiosondes can be applied to global records, which show high correlations to Global Climate Observing System Reference Upper‐Air Network (GRUAN) performance at pressures between the surface and 250 hPa. We go further and show the first comparison of uncertainties from both the newly created Characterized Radiosonde Measurement (CRM) and GRUAN data sets. Global estimates of AIRS water vapor profile (wet/dry) biases relative to GRUAN and CRM are within 6 ± 0.3% ppmv and 15 ± 0.1% ppmv below 300 hPa, respectively. The CRM record allows latitudinal analysis for the first time, which when examined shows sensitivity to changes in absolute concentration due to large‐scale circulation in the International Tropical Convergence Zone. This paper advances the use of state‐of‐the‐art in situ records for characterizing absolute performance, recognizing that long‐term stability needs further research.

Quality Assessment of the First Measurements of Tropospheric Water Vapor and Temperature by the HAMSTRAD Radiometer Over Concordia Station, Antarctica

The HAMSTRAD microwave instrument operates at 60 and 183 GHz and measures temperature and water vapor, respectively, from 0- to 10-km altitude with a time resolution of 7 min. The radiometer has been successfully deployed at Dome C (Concordia Station), Antarctica (75°06' S, 123°21' E, 3233 m amsl) during the first summertime campaign for 12 days in January-February 2009. The radiometer has been continuously running since January 2010, hosted within a dedicated shelter. We have used the very first set of HAMSTRAD data, recorded when the instrument was outdoors, to assess its potential to sound the troposphere over Dome C, from the planetary boundary layer (PBL) up to the tropopause ( ~ 6 km above surface, ~ 9 km amsl). We have compared the HAMSTRAD measurements to several sets of measurements performed at the Dome-C station or in its vicinity: meteorological radiosondes, in situ PT100 and Humicap sondes along the vertical extent of a 45-m tower, meteorological sensor attached to the HAMSTRAD instrument, and the spaceborne Infrared Atmospheric Sounding Interferometer (IASI) instrument onboard the EUMETSAT MetOp-A satellite in polar orbit. The variability of integrated water vapor (IWV) observed by HAMSTRAD with extremely low values of 0.5 kg ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> was also measured by the radiosondes (very high HAMSTRAD versus radiosonde correlation of 0.98), whereas IASI cloud-free measurements did not reproduce well the HAMSTRAD IWV variation (weak HAMSTRAD versus IASI correlation of 0.58). The measurements of absolute humidity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) from HAMSTRAD at Dome C cover a large vertical extent from the surface to about 6 km above surface with a high sensitivity in the free troposphere. The strong diurnal variation of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O observed by the in situ sensors in the PBL is not well detected by the radiometer. In the free troposphere, the HAMSTRAD versus radiosonde H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O correlation can reach 0.8-0.9. Around the tropopause, HAMSTRAD shows the same variability as IASI and radiosondes but with a dry bias of 0.01 g ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . HAMSTRAD tends to show a wetter atmosphere by 0.1-0.3 g ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> compared with radiosondes from the surface to ~ 2-km altitude and a drier atmosphere above by ~ 0.1g ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The sensitivity of the temperature profiles from HAMSTRAD is very high in the PBL and in the free troposphere but degrades around the tropopause. The strong diurnal signal measured above the surface by HAMSTRAD (3-6 K) is consistent with all the other in situ data sets. The temporal evolution over the 12-day period in the PBL is also consistent with all other data sets (radiosondes, IASI, in situ sondes, and meteorological sensors). In the free troposphere and around the tropopause, the HAMSTRAD temporal evolution is consistent with that observed by radiosondes and IASI, although a cold bias exists compared with IASI and radiosondes around the tropopause. For heights less than 4 km above surface, HAMSTRAD correlates very well with radiosondes and in situ sensors (correlation better than 0.8) but less well with IASI (0.4). Below the tropopause, the IASI and HAMSTRAD correlation reaches 0.9, whereas above the tropopause, the correlation of IASI and radiosondes with HAMSTRAD is rather low (<; 0.5). Throughout the 12-day period (except on January 23), in the lowermost troposphere for heights less than 500 m above surface, the HAMSTRAD temperature profiles agree with the profiles measured by the radiosondes. From 500 m up to 5 km above the surface, the HAMSTRAD temperature profile has a cold bias from 1 to 5 K compared with the radiosondes, but for some dates (e.g., on January 25 and 29), the HAMSTRAD temperature is very close to the radiosonde temperature. HAMSTRAD generally measures a tropopause lower and warmer than the radiosondes except on some occasions, for instance, on January 23, 30, and 31. In the lower stratosphere, HAMSTRAD measurements of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O and temperature have little sensitivity. Based upon 5-day back trajectory analyses, the great variability of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O and temperature above Dome C as measured by the different instruments from the surface up to the tropopause over the 12-day period can be explained by the origin of air masses. The Dome-C site is found to be under the influence of the oceanic middle latitudes and the Antarctic coastal latitudes, but on some occasions, the air masses originated from the Antarctic continent are associated with colder and drier episodes.

Micropulse water vapor differential absorption lidar: transmitter design and performance

An all diode-laser-based micropulse differential absorption lidar (DIAL) laser transmitter for tropospheric water vapor and aerosol profiling is presented. The micropulse DIAL (MPD) transmitter utilizes two continuous wave (cw) external cavity diode lasers (ECDL) to seed an actively pulsed, overdriven tapered semiconductor optical amplifier (TSOA). The MPD laser produces up to 7 watts of peak power over a 1 µs pulse duration (7 µJ) and a 10 kHz pulse repetition frequency. Spectral switching between the online and offline seed lasers is achieved on a 1Hz basis using a fiber optic switch to allow for more accurate sampling of the atmospheric volume between the online and offline laser shots. The high laser spectral purity of greater than 0.9996 coupled with the broad tunability of the laser transmitter will allow for accurate measurements of tropospheric water vapor in a wide range of geographic locations under varying atmospheric conditions. This paper describes the design and performance characteristics of a third generation MPD laser transmitter with enhanced laser performance over the previous generation DIAL system.

Convenient method for calibrating system constant of scanning water vapor Raman lidar

Lower tropospheric water vapor measurements are performed at nighttime using the mobile atmosphere monitoring lidar-2 (AML-2) which is operated by the Anhui Institute of Optics and Fine Mechanics. In this lidar system, a 354.7-nm light from a Nd:YAG laser is used as stimulating source, whose Raman shifted center wavelengths are at 386.7 and 407.5 nm for nitrogen and water vapor, respectively. We present a novel and convenient method for determining the Raman lidar calibration constant according to the scanning performance of this lidar. We are likewise able to realize the measurement of water vapor profile in the low troposphere. The error induced by the uncertainty of calibrated constants is within 7% for the Raman lidar system. Experimental results from two months of study indicate that the method of calibrating the lidar system constant is feasible, and the Raman lidar performance is stable and reliable.

Absolute accuracy of water vapor measurements from six operational radiosonde types launched during AWEX‐G and implications for AIRS validation

A detailed assessment of radiosonde water vapor measurement accuracy throughout the tropospheric column is needed for assessing the impact of observational error on applications that use the radiosonde data as input, such as forecast modeling, radiative transfer calculations, remote sensor retrieval validation, climate trend studies, and development of climatologies and cloud and radiation parameterizations. Six operational radiosonde types were flown together in various combinations with a reference‐quality hygrometer during the Atmospheric Infrared Sounder (AIRS) Water Vapor Experiment‐Ground (AWEX‐G), while simultaneous measurements were acquired from Raman lidar and microwave radiometers. This study determines the mean accuracy and variability of the radiosonde water vapor measurements relative to simultaneous measurements from the University of Colorado (CU) Cryogenic Frostpoint Hygrometer (CFH), a reference‐quality standard of known absolute accuracy. The accuracy and performance characteristics of the following radiosonde types are evaluated: Vaisala RS80‐H, RS90, and RS92; Sippican Mark IIa; Modem GL98; and the Meteolabor Snow White hygrometer. A validated correction for sensor time lag error is found to improve the accuracy and reduce the variability of upper tropospheric water vapor measurements from the Vaisala radiosondes. The AWEX data set is also used to derive and validate a new empirical correction that improves the mean calibration accuracy of Vaisala measurements by an amount that depends on the temperature, relative humidity, and sensor type. Fully corrected Vaisala radiosonde measurements are found to be suitably accurate for AIRS validation throughout the troposphere, whereas the other radiosonde types are suitably accurate under only a subset of tropospheric conditions. Although this study focuses on the accuracy of nighttime radiosonde measurements, comparison of Vaisala RS90 measurements to water vapor retrievals from a microwave radiometer reveals a 6–8% dry bias in daytime RS90 measurements that is caused by solar heating of the sensor. An AWEX‐like data set of daytime measurements is highly desirable to complete the accuracy assessment, ideally from a tropical location where the full range of tropospheric temperatures can be sampled.

Water vapor heterogeneity related to tropopause folds over the North Atlantic revealed by airborne water vapor differential absorption lidar

Airborne differential absorption lidar (DIAL) measurements of tropospheric water vapor and aerosol/clouds are presented from transfers across the North Atlantic on 13–15 May and 16–18 June 2002. The intense dynamical activity over the Atlantic is reflected in complex structures like deep tropopause folds, extended dry layers, and tilted aerosol filaments. Intrusions with H2O mixing ratios below 0.03 g kg−1 regularly develop along the storm track over intense cyclones and correspond to analyzed (European Centre for Medium‐Range Weather Forecasts (ECMWF)) or simulated (MM5) potential vorticity anomalies. Sloping folds are typically 1 km thick in vertical profiles but on horizontal sections resemble long filaments with widths of a few hundred kilometers. Filaments of solid, roughly micron‐sized particles occur inside and along the axes of the tropopause folds. The particles most likely originate in forest fires, from where they are injected to the lower stratosphere by deep convection and later on reenter the troposphere in developing baroclinic disturbances. Their laminar appearance indicates that these intrusions have not yet been mixed but are eroded on timescales significantly larger than a few days by stretching‐thinning, stirring, and only finally, turbulent diffusion. MM5 mesoscale model simulations and, with less accuracy, also ECMWF analyses are capable of reproducing the dynamical structures associated with the observed dry intrusions. However, deep inside the intrusions where scales are small, even the MM5‐simulated water vapor mixing ratios are more than twice those observed. The large dynamical range of water vapor mixing ratios (3 orders of magnitude) and large gradients (&gt;2 g kg−1 km−1 at the intrusions' boundaries) constitute a challenge for future space‐borne water vapor DIAL systems.

Lidar measurements of tropospheric water vapor and aerosol profiles over southeastern Italy

A combined Rayleigh‐Raman lidar based on a XeF excimer laser (351 nm) has been used for the regular monitoring of tropospheric aerosols and water vapor profiles over southern Italy (40°20′N, 18°6′E) in the framework of the European Aerosol Research Lidar Network. Coincident measurements of vertical profiles of the aerosol backscattering ratio R(z), the aerosol extinction coefficient αλoaer(z), and the water vapor mixing ratio w(z) have been performed during several months to investigate the correlation of R(z) and αλoaer(z) with water vapor and to characterize the aerosol optical properties. A strong correlation has been found between the spatial and temporal evolution of R(z) and w(z) both in summer and in autumn regime. The experimental results have revealed that R(z) increases with w(z) and that the aerosol and water vapor burdens are lower in autumn than in the summer. The analysis of the data by using the 4‐day analytical backward trajectories has revealed that the dependence of αλoaer(z) and R(z) on w(z) is quite affected by the typical advection patterns over the lidar site. Aerosols providing a larger contribution to backscattering ratio and extinction coefficient as w(z) increases have been monitored when air masses from northern and eastern European countries were advected over Lecce. The findings in terms of the lidar ratio S have revealed that aerosols with average S values ranging from 50 to 63 sr have been monitored when air masses from northern and eastern Europe were advected over Lecce. On the contrary, average S values ranging from 48 to 74 have been retrieved for air masses advected from North Africa.

Spatial distributions of upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder

We characterize measurements of upper tropospheric (∼300–150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on board the Upper Atmosphere Research Satellite (UARS) in terms of their spatial and temporal variability. We present the climatology of water vapor mixing ratio for October 1991‐June 1997, which includes the seasonal means, the root‐mean‐square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upper tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the Walker circulation. We quantify the frequency of dry tropical observations in seasonal maps. The most prominent dry regions are located in the subtropics. On the 316 hPa surface there is some occurrence of low values of humidity throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern Hemisphere winter. The seasonal cycle of MLS water vapor is compared with Stratospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agreement is found between these two data sets for measurements near the extratropical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or tropospheric air. Frequency distributions display distinct characteristics and seasonal dependence for these two regions.

Tropospheric water vapor measurements over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX)

Fast‐response tunable diode laser measurements of water vapor were made over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) conducted in the fall of 1997. Resulting water vapor mixing ratios, in conjunction with air temperature and pressure measurements obtained by the DC‐8's Data Acquisition and Distribution System (DADS), were used to calculate the prevalence of ice‐saturated conditions (relative humidity with respect to ice &gt;100% or frost saturation) in the upper troposphere over the SONEX sampling region. Additionally, Appleman theory was applied to the data to determine the subset of ice‐saturated regions which would support formation of contrails. Results suggest that studies to determine the potential climate or radiative impact of contrails and aviation‐induced cirrus clouds should not only consider the climatology of frost‐saturated regions, but should also address whether ambient conditions can support contrail formation within these regions. A separate discussion within the paper describes an in‐flight intercomparison between the SONEX diode laser hygrometer and the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2) cryogenic hygrometer and shows that the instruments perform to within their stated accuracies (∼10%) over a range of mixing ratios and altitudes.

Baroclinic wave variations observed in MLS upper tropospheric water vapor

Upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder are used to investigate the structure and evolution of eastward traveling medium‐scale wave features in Southern Hemisphere summertime. The extratropical Southern Hemisphere summer circulation pattern is frequently dominated by medium scale waves which exhibit life cycles of baroclinic growth and barotropic decay. The water vapor field during such life cycles is examined here and found to be well correlated with meteorological fields derived from European Centre for Medium Range Weather Forecasts global analyses. From mid January to mid February 1992 several episodes of growth and decay in the amplitude of eastward traveling waves are found in the water vapor and meteorological data at levels of the upper troposphere, with zonal waves four, five and six being predominant modes. The water vapor data are compared with derived potential vorticity (PV) fields, with strong anticorrelation observed in middle and high latitudes. The results are consistent with model paradigms for the structure and evolution of baroclinic disturbances, coupled with the known characteristics of high PV and low water vapor mixing ratios in lower stratospheric air parcels and the reverse for upper tropospheric air.

Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar

A major improvement of a differential absorption lidar (DIAL) system for measurements of tropospheric water vapor and temperature is introduced. A Q-switched unidirectional alexandrite ring laser is injection seeded by a cw Ti:sapphire ring laser. Using an especially developed single-mode electronic, one starts the Q switch when the slave resonator is in resonance with the frequency of the Ti:sapphire laser. Long-term single-mode operation of the alexandrite laser is achieved. A single-shot spectral linewidth of <40 MHz and a frequency stability of 15 MHz rms can be specified. Thus what is to our knowledge the first single-mode DIAL system in the near infrared is presented.

Injection-seeded alexandrite ring laser: performance and application in a water-vapor differential absorption lidar

A new laser system for use of differential absorption lidar (DIAL) in measurements of tropospheric water vapor and temperature is introduced. This system operates in the 720-780-nm region and is configured as an alexandrite ring laser injection seeded by a cw Ti:sapphire ring laser. This combination provides for the necessary narrow-bandwidth, high-frequency stability and excellent spectral purity. A bandwidth of <5.0 x 10(-3) cm(-1), a frequency stability of 2.1 x 10(-3) cm(-1) rms, and a spectral purity of 99.995% at 726 nm have been achieved during extended periods of operation. A comparison of a DIAL water-vapor measurement with a radiosonde in the boundary layer between 500 and 2000 m was performed. The maximum deviation between the humidity profiles is 15%, the standard deviation 1.6%, and the difference between the mean values 1%.

Recent Lidar Technology Developments and Their Influence on Measurements of Tropospheric Water Vapor

In this paper the influences of recent technology developments in the areas of lasers, detectors, and optical filters of a differential absorption lidar (DIAL) system on the measurement of tropospheric water vapor (H20) profiles are discussed. The lidar parameters selected are based upon a diode-seeded Ti:sapphire laser that is locked to an H20 line in the 820- or 930-nm band of H20. To assess the influence of the mode of deployment on the measurement of tropospheric H20, DIAL performance is evaluated for operation from a medium-altitude (12 km) aircraft the ground, and space-based systems. It is found that incorporation of these developments could greatly enhance DIAL measurement capability.

Satellite‐derived water vapor corrections for Geosat altimetry

The absence of a radiometer on Geosat to provide simultaneous measurements of tropospheric water vapor makes it necessary to have the best possible estimates of coincident water vapor corrections from other satellite‐borne sensor systems. In this study, tropospheric water vapor contents are computed from two different satellite sensor systems. These water vapor estimates are intercompared and are compared with other coincident estimates of tropospheric water vapor from radiosondes and from the standard weather analyses of the Fleet Numerical Oceanography Center (FNOC). Statistical comparisons are also made with a climatological water vapor data base computed from the scanning multichannel microwave radiometer (SMMR). The primary data sources for this study are the TIROS operational vertical sounder (TOVS) flying on the NOAA series of polar orbiter weather satellites and the special sensor microwave imager (SSM/I) flying since July 1987 on Defense Meteorological Satellite Program (DMSP) satellite number 8. A period of overlapping TOVS and SSM/I data from the summer of 1987 is used for statistical comparisons to determine the utility and reliability of altimetric path length corrections computed from these data. Because of its all weather sensing capability the SSM/I is shown to be capable of providing reliable global water vapor maps over periods as short as 4 days, while the TOVS system can be reliably used only for compositing periods of a week or more because of cloud contamination. In terms of magnitudes, both systems provide water vapor estimates consistent with a global set of island‐ship radiosonde to within 0.5–0.9 g/cm2. The SSM/I provides the most detailed global water vapor maps, closely approximated by the 8‐day TOVS composites. The larger spatial resolution of the FNOC analysis and the SMMR climatology fails to properly resolve many of the important spatial details of the global water vapor distribution. The FNOC analysis systematically underestimates tropical water vapor contents. Seasonal means and standard deviations of both the TOVS and SSM/I global water vapor maps clearly demonstrate the importance of the annual cycle in atmospheric water vapor. The relative success of the SMMR climatology in matching some of the statistics emphasizes the importance of the annual cycle. Converted to altimetric path‐length corrections for typical satellite tracks in the North Atlantic and North Pacific, the water vapor errors are maximum just north of the equator in both oceans. SSM/I‐ and TOVS‐derived path length corrections are both significantly higher in the equatorial zone than those derived from the other water vapor maps.

Airborne and spaceborne lidar measurements of water vapor profiles: a sensitivity analysis

This paper presents an evaluation of the random and systematic error sources associated with differential absorption lidar (DIAL) measurements of tropospheric water vapor (H(2)O) profiles from airborne and spaceborne platforms. The results of this analysis are used in the development and performance evaluation of the Lidar Atmospheric Sensing Experiment (LASE) H(2)O DIAL system presently under development at the NASA Langley Research Center for operation on a high altitude ER-2 (advanced U-2) aircraft. The analysis shows that a <10% H(2)O profile measurement accuracy is possible for the LASE system with a vertical and horizontal resolution of 200 m and 10 km, respectively, at night and 300 m and 20 km during the day. Global measurements of H(2)O profiles from spaceborne DIAL systems can be made to a similar accuracy with a vertical resolution of 500 m and a horizontal resolution of 100 km.