Recent divergences in stratospheric water vapor measurements by frost point hygrometers and the Aura Microwave Limb Sounder (original) (raw)
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Journal of Geophysical Research, 2008
1] We compare matched retrievals of upper tropospheric water vapor (UTWV) mixing ratios from the Microwave Limb Sounder (MLS) instrument on the Aura satellite, and the Atmospheric Infrared Sounder (AIRS) instrument on the Aqua satellite. Because each instrument's sampling is affected by tropical conditions, about half of mutually observed scenes in the tropics yield simultaneous successful retrievals from both systems. The fraction of mutually retrieved scenes drops to 30% at higher latitudes where clouds significantly inhibit AIRS sounding. Essentially all scenes observed by MLS in extratropical and polar regions yield successful retrievals. At 250 hPa in the tropics, measurements from the two instruments are highly correlated, the differences of their means ( D q ) are smaller than 10%, and the standard deviations of their differences (s q ) are 30% or less. At 300 hPa, MLS means are drier by 10-15%, and s q is 40-60%, indicating that responses of MLS and AIRS to UTWV perturbations are not one-to-one. Root mean square agreement is also poorer over the poles at 300 hPa and at 200 and 150 hPa at lower latitudes. In these regions, j D q j = 10% or more, and s q = 40-70%. Correlations between the two data sets are 0.7-0.9 at 300 and 250 hPa globally and at 200 hPa in the tropics. This high correlation indicates that s q of 50% or greater comes mainly from systematic differences in sensitivity of the two instruments, especially for small and large UTWV amounts; larger values of s q are generally not due to large random errors from either instrument. An AIRS low-end sensitivity threshold of 15-20 ppmv leads to poorer agreement under the driest conditions. Disagreement at 300 hPa likely comes from overestimation by MLS for the wettest conditions of >400 ppmv. While MLS is biased slightly dry overall at 300 hPa, it is biased wet in the wettest regions, particularly those associated with deep convection. These sensitivity differences explain nonunity slopes of linear fits to the two data sets. MLS everywhere has a greater dynamic range than AIRS, with larger maxima and smaller minima. Good agreement at 250 hPa suggests AIRS uncertainties of 25% up to the reported 250-200 hPa layer in the tropics and extratropics, consistent with previous comparisons with balloon-and aircraft-borne instruments. The agreement at 250 hPa also indicates that MLS is reliable from its reported 215-hPa level upward in altitude.
Journal of Geophysical Research, 2007
1] Here we present extensive observations of stratospheric and upper tropospheric water vapor using the balloon-borne Cryogenic Frost point Hygrometer (CFH) in support of the Aura Microwave Limb Sounder (MLS) satellite instrument. Coincident measurements were used for the validation of MLS version 1.5 and for a limited validation of MLS version 2.2 water vapor. The sensitivity of MLS is on average 30% lower than that of CFH, which is fully compensated by a constant offset at stratospheric levels but only partially compensated at tropospheric levels, leading to an upper tropospheric dry bias. The sensitivity of MLS observations may be adjusted using the correlation parameters provided here. For version 1.5 stratospheric observations at pressures of 68 hPa and smaller MLS retrievals and CFH in situ observations agree on average to within 2.3% ± 11.8%. At 100 hPa the agreement is to within 6.4% ± 22% and at upper tropospheric pressures to within 23% ± 37%. In the tropical stratosphere during the boreal winter the agreement is not as good. The ''tape recorder'' amplitude in MLS observations depends on the vertical profile of water vapor mixing ratio and shows a significant interannual variation. The agreement between stratospheric observations by MLS version 2.2 and CFH is comparable to the agreement using MLS version 1.5. The variability in the difference between observations by MLS version 2.2 and CFH at tropospheric levels is significantly reduced, but a tropospheric dry bias and a reduced sensitivity remain in this version. In the validation data set a dry bias at 177.8 hPa of À24.1% ± 16.0% is statistically significant. Citation: Vömel, H., et al. (2007), Validation of Aura Microwave Limb Sounder water vapor by balloon-borne Cryogenic Frost point Hygrometer measurements,
Journal of Geophysical Research, 2009
1] We present a detailed intercomparison of five ground-based 22 GHz microwave radiometers for stratospheric and mesospheric water vapor. Four of these instruments are members of the Network for the Detection of Atmospheric Composition Change (NDACC). The global measurements of middle atmospheric water vapor of the Microwave Limb Sounder (MLS) onboard the Aura satellite serve as reference and allow intercomparison of the ground-based systems that are located between 45°S and 57°N. The retrievals of water vapor profiles from the ground-based radiation measurements have been made consistent to a large extent: for the required temperature profiles, we used the global temperature measurements of MLS and we agreed on one common set of spectroscopic parameters. The agreement with the reference measurements is better than ±8% in the altitude range from 0.01 to 3 hPa. Strong correlation is found between the ground-based and the reference data in the mesosphere with respect to seasonal cycle and planetary waves. In the stratosphere the measurements are generally more noisy and become sensitive to instrumental instabilities toward lower levels (pressures greater than 3 hPa). We further present a compilation of a NDACC data set based on the retrieval parameters described herein but using a temperature climatology derived from the MLS record. This makes the ground-based measurements independent of additional information and allows extension of the data set for years in a homogeneous manner. (2009), Validation of ground-based microwave radiometers at 22 GHz for stratospheric and mesospheric water vapor,
Journal of Geophysical Research, 2009
1] Building on previously published details of the laboratory calibrations of the Harvard Lyman-a photofragment fluorescence hygrometer (HWV) on the NASA ER-2 and WB-57 aircraft, we describe here the validation process for HWV, which includes laboratory calibrations and intercomparisons with other Harvard water vapor instruments at water vapor mixing ratios from 0 to 10 ppmv, followed by in-flight intercomparisons with the same Harvard hygrometers. The observed agreement exhibited in the laboratory and during intercomparisons helps corroborate the accuracy of HWV. In light of the validated accuracy of HWV, we present and evaluate a series of intercomparisons with satellite and balloon borne water vapor instruments made from the upper troposphere to the lower stratosphere in the tropics and midlatitudes. Whether on the NASA ER-2 or WB-57 aircraft, HWV has consistently measured about 1-1.5 ppmv higher than the balloon-borne NOAA/ESRL/GMD frost point hygrometer (CMDL), the NOAA Cryogenic Frost point Hygrometer (CFH), and the Microwave Limb Sounder (MLS) on the Aura satellite in regions of the atmosphere where water vapor is <10 ppmv. Comparisons in the tropics with the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite show large variable differences near the tropopause that converge to $10% above 460 K, with HWV higher. Results we show from the Aqua Validation and Intercomparison Experiment (AquaVIT) at the AIDA chamber in Karlsruhe do not reflect the observed in-flight differences. We illustrate that the interpretation of the results of comparisons between modeled and measured representations of the seasonal cycle of water entering the lower tropical stratosphere is dictated by which data set is used. Citation: Weinstock, E. M., et al. (2009), Validation of the Harvard Lyman-a in situ water vapor instrument: Implications for the mechanisms that control stratospheric water vapor,
Journal of Geophysical Research, 1999
In an effort to better constrain atmospheric water vapor mixing ratios and to understand the discrepancies between different measurements of water vapor in the stratosphere and troposphere, we have carefully examined data from the Harvard Lyman-tz photofragment fluorescence hygrometer, which has flown on the NASA ER-2 aircraft from 1992 through 1998. The instrument is calibrated in the laboratory before and after each deployment, and the calibration is checked by direct absorption measurements in the troposphere. On certain flights, the ER-2 flew level tracks during which water vapor varied by up to 80 ppmv, under nearly constant atmospheric conditions, These flights provide a stringent test of our calibration via direct absorption and indicate agreement to within 3%. During the 1997 Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) mission, our Lyman-tz instrument was compared with a new diode laser hygrometer from the Jet Propulsion Laboratory. Overall agreement was 5% during the June/July deployment and 1 øk for potential temperatures of 490 to 540 K. The accuracy of our instrument is shown to be _+5%, with an additional offset of at most 0.1 ppmv. Data from this instrument, combined with simultaneous measurements of CH4 and H2, are therefore ideal for studies of the hydrogen budget of the lower stratosphere. 1. Introduction This paper addresses the question, What is the accuracy of the Harvard in situ Lyman-tz photofragment fluorescence hygrometer? We present evidence from laboratory calibrations, in-flight calibration checks with direct absorption measurements in the mid to upper troposphere, and a recent intercomparison with a new diode laser hygrometer [May, 1998], all of which indicate an accuracy of_+5%. Additional evidence based on an analysis of the hydrogen budget of the stratosphere is discussed in a companion paper [Hurst et al., this issue]. Atmospheric water vapor is critically important in chemistry, climate, and polar processing. For example, a 15% change in water vapor corresponds approximately to a 1 K change in ice saturation temperature or a 0.7 K change in the onset temperature of liquid HNO3/H2SO4/H20 particle growth in the polar vortex. The concentration of water is thus an important
Journal of Geophysical Research, 1997
We compare water vapor measurements from the Naval Research Laboratory groundbased Water Vapor Millimeter-wave Spectrometer (WVMS) instruments with measurements taken by five space-based instruments. For coincident measurements the retrievals from all of the instruments show qualitatively similar altitude profiles. The retrieved mixing ratios from most instruments generally differ from an average calculated using retrievals from all of the instruments by < i ppmv at most altitudes from 40 km to 80 km. Comparisons with the Microwave Limb Sounder (MLS) and the Halogen Occultation Experiment (HALOE) allow for the validation of observed temporal variations. The observed variations show similar annual and semiannual cycles. A comparison of several years of data from HALOE and WVMS also shows that the instruments are detecting similar interannual variations. A regression analysis of the WVMS and HALOE data sets shows that the observed variability is consistent within the estimated errors in the mesosphere and that in the upper stratosphere, where the natural variability is small, there is a positive correlation between the WVMS and the HALOE data. from two sites of the Network for the Detection of Stratospheric Change (NDSC). During these years there has been a tremendous increase in the available data on water vapor in the middle atmosphere. In addition to the WVMS measurements, contributions to this water vapor data set come from instruments aboard the Upper Atmosphere Research Satellite (UARS), including the Halogen Occultation In this paper we compare the WVMS measurements with several sets of space-based measurements.
Journal of Geophysical Research, 2007
1] The validation of version 2.2 (v2.2) H 2 O measurements from the Earth Observing System (EOS) Microwave Limb Sounder (Aura MLS) on the Aura satellite are presented. Results from comparisons made with Aqua Atmospheric Infrared Sounder (AIRS), Vaisala radiosondes, frost point hygrometer, and WB57 aircraft hygrometers are presented. Comparisons with the Aura MLS v1.5 H 2 O, Goddard global modeling and assimilation office Earth Observing System analyses are also discussed. For H 2 O mixing ratios less than 500 ppmv, the MLS v2.2 has an accuracy better than 25% between 316 and 147 hPa. The precision is 65% at 316 hPa that reduces to 25% at 147 hPa. This performance is better than expected from MLS measurement systematic error analyses. MLS overestimates H 2 O for mixing ratios greater than 500 ppmv which is consistent with a scaling error in either the calibrated or calculated MLS radiances. The validation of the accuracy of MLS v2.2 H 2 O from 121 to 83 hPa which is expected to be better than 15% cannot be confirmed at this time because of large disagreements among the hygrometers used in the AVE campaigns. The precision of the v2.2 H 2 O from 121 to 83 hPa is 10-20%. The vertical resolution is 1.5-3.5 km depending on height. The horizontal resolution is 210 Â 7 km 2 along and perpendicular to the Aura orbit track, respectively. Relative humidity is calculated from H 2 O and temperature. The precision, accuracy, and spatial resolution are worse than for H 2 O. Citation: Read, W. G., et al. (2007), Aura Microwave Limb Sounder upper tropospheric and lower stratospheric H 2 O and relative humidity with respect to ice validation,
Advancements, measurement uncertainties, and recent comparisons of the NOAA frostpoint hygrometer
Atmospheric Measurement Techniques Discussions, 2016
The NOAA frostpoint hygrometer (FPH) is a balloon borne instrument flown monthly at three sites to measure water vapor profiles up to 28 km. The FPH record from Boulder, Colorado is the longest continuous stratospheric water vapor record. The instrument has an uncertainty in the stratosphere that is < 6 % and up to 12 % in the troposphere. A digital microcontroller version of the instrument improved upon the older versions in 2008 with sunlight filtering, better frost control, and resistance to radio frequency interference (RFI). A new thermistor calibration technique was implemented in 2014, decreasing the uncertainty in the thermistor calibration fit to less than 0.01 °C over the full range of frostpoint temperatures (−93 °C to +20 °C) measured during a profile. Results from multiple water vapor intercomparisons are presented, including the excellent agreement during AquaVIT-2 chamber experiments over six days that provides...
Journal of the Atmospheric Sciences, 2014
By analyzing the almost-decade-long record of water vapor measurements from the Microwave Limb Sounder (MLS) instrument on the NASA Aura satellite and by detailed diagnostic analysis of the results from state-of-the art climate model simulations, this study confirmed the conceptual picture of the interannual variation in equatorial stratospheric water vapor discussed in earlier papers (e.g., Geller et al.). The interannual anomalies in water vapor are strongly related to the dynamical quasi-biennial oscillation (QBO), and this study presents the first QBO composite of the time-height structure of the equatorial water vapor anomalies. The anomalies display upward propagation below about 10 hPa in a manner analogous to the annual ''tape recorder'' effect, but at higher levels they show clear downward propagation. This study examined these variations in the Model for Interdisciplinary Research on Climate (MIROC)-AGCM and in four models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that simulate realistic QBOs. Diagnostic budget analysis of the MIROC-AGCM data and comparisons among the CMIP5 model results demonstrate (i) the importance of temperature anomalies at the tropopause induced by the QBO for lowerstratospheric water vapor variations and (ii) that upper-stratospheric water vapor anomalies are largely driven by advection of the mean vertical gradient of water content by the QBO interannual fluctuations in the vertical wind.