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Nineteen limb viewing (occultation and passive thermal) and two nadir humidity data sets are intercompared and also compared to frostpoint hygrometer balloon sondes. The upper troposphere considered here covers the pressure range from 300–100 hPa. Water vapor in this region is a challenging measurement because concentrations vary between 2–1000 parts per million volume with sharp changes in vertical gradients near the tropopause. The atmospheric temperature is also highly variable ranging from 180–250 K. The assessment of satellite measured humidity is based on coincident comparisons with frostpoint hygrometer sondes, multi month mapped comparisons, zonal mean time series comparisons and coincident satellite to satellite comparisons. While the satellite fields show similar features in maps and time series, quantitatively, they can differ by a factor of two in concentration, with strong dependencies on the amount of H2O. Additionally, time-lag response corrected Vaisala-RS92 radiosondes are compared to satellites and the frostpoint hygrometer measurements. In summary, most satellite data sets reviewed here show on average ~30 % agreement amongst themselves and frostpoint data but with an additional ~30 % variability about the mean. The Vaisala-RS92 sonde even with a time-lag correction shows poor behavior for pressure less than 200 hPa.

As part of the second SPARC (Stratosphere–troposphere Processes And their Role in Climate) water vapor assessment (WAVAS-II), we present measurements taken from or coincident with seven sites from which ground-based microwave instruments measure water vapor in the middle atmosphere. Six of the ground-based instruments are part of the Network for the Detection of Atmospheric Composition Change (NDACC) and provide datasets that can be used for drift and trend assessment. We compare measurements from these ground-based instruments with satellite datasets that have provided retrievals of water vapor in the lower mesosphere over extended periods since 1996. We first compare biases between the satellite and ground-based instruments from the upper stratosphere to the upper mesosphere. We then show a number of time series comparisons at 0.46 hPa, a level that is sensitive to changes in H2O and CH4 entering the stratosphere but, because almost all CH4 has been oxidized, is relatively insensitive to dynamical variations. Interannual variations and drifts are investigated with respect to both the Aura Microwave Limb Sounder (MLS; from 2004 onwards) and each instrument's climatological mean. We find that the variation in the interannual difference in the mean H2O measured by any two instruments is typically ∼ 1%. Most of the datasets start in or after 2004 and show annual increases in H2O of 0–1 % yr−1. In particular, MLS shows a trend of between 0.5 % yr−1 and 0.7 % yr−1 at the comparison sites. However, the two longest measurement datasets used here, with measurements back to 1996, show much smaller trends of +0.1 % yr−1 (at Mauna Loa, Hawaii) and −0.1 % yr−1 (at Lauder, New Zealand).

This study investigates the abrupt and severe water vapour decline in the stratosphere beginning in the year 2000 (the "millennium water vapour drop") and other similarly strong stratospheric water vapour reductions by means of various simulations with the state-of-the-art Chemistry-Climate Model (CCM) EMAC (ECHAM/MESSy Atmospheric Chemistry Model). The model simulations differ with respect to the prescribed sea surface temperatures (SSTs) and whether nudging is applied or not. The CCM EMAC is able to most closely reproduce the signature and pattern of the water vapour drop in agreement with those derived from satellite observations if the model is nudged. Model results confirm that this extraordinary water vapour decline is particularly obvious in the tropical lower stratosphere and is related to a large decrease in cold point temperature. The drop signal propagates under dilution to the higher stratosphere and to the poles via the Brewer–Dobson circulation (BDC). We found that the driving forces for this significant decline in water vapour mixing ratios are tropical sea surface temperature (SST) changes due to a coincidence with a preceding strong El Niño–Southern Oscillation event (1997/1998) followed by a strong La Niña event (1999/2000) and supported by the change of the westerly to the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) in 2000. Correct (observed) SSTs are important for triggering the strong decline in water vapour. There are indications that, at least partly, SSTs contribute to the long period of low water vapour values from 2001 to 2006. For this period, the specific dynamical state of the atmosphere (overall atmospheric large-scale wind and temperature distribution) is important as well, as it causes the observed persistent low cold point temperatures. These are induced by a period of increased upwelling, which, however, has no corresponding pronounced signature in SSTs anomalies in the tropics. Our free-running simulations do not capture the drop as observed, because a) the cold point temperature has a low bias and thus the water vapour variability is reduced and b) because they do not simulate the appropriate dynamical state. Large negative water vapour declines are also found in other years and seem to be a feature which can be found after strong combined El Niño/La Niña events if the QBO west phase during La Niña changes to the east phase.

More than a decade ago it was suggested that a cooling of stratospheric temperatures by 1 K or an increase of 1 ppmv of stratospheric water vapour could promote denitrification, the permanent removal of nitrogen species from the stratosphere by solid polar stratospheric cloud (PSC) particles. In fact, during the two Arctic winters 2009/10 and 2010/11 the strongest denitrification in the recent decade was observed. Sensitivity studies along air parcel trajectories are performed to test how a future stratospheric water vapour (H2O) increase of 1 ppmv or a temperature decrease of 1 K would affect PSC formation. We perform our study based on measurements made during the Arctic winter 2010/11. Air parcel trajectories were calculated 6 days backward in time based on PSCs detected by CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder satellite observations). The sensitivity study was performed on single trajectories as well as on a trajectory ensemble. The sensitivity study shows a clear prolongation of the potential for PSC formation and PSC existence when the temperature in the stratosphere is decreased by 1 K and water vapour is increased by 1 ppmv. Based on 15 years of satellite measurements (2000–2014) from UARS/HALOE, Envisat/MIPAS, Odin/SMR, Aura/MLS, Envisat/SCIAMACHY and SCISAT/ACE-FTS it is further investigated if there is a decrease in temperature and/or increase of water vapour (H2O) observed in the polar regions similar to that observed at midlatitudes and in the tropics. Performing linear regression analyses we derive from the Envisat/MIPAS (2002–2012) and Aura/MLS (2004–2014) observations predominantly positive changes in the potential temperature range 350 to 1000 K. The linear changes in water vapour derived from Envisat/MIPAS observations are largely insignificant, while those from Aura/MLS are mostly significant. For the temperature neither of the two instruments indicate any significant changes. Given the strong inter-annual variation observed in water vapour and particular temperature the severe denitrification observed in 2010/11 cannot be directly related to any changes in water vapour and temperature since the millennium. However, the observations indicate a clear correlation between cold winters and enhanced water vapour mixing ratios. This indicates a connection between dynamical and radiative processes that govern water vapour and temperature in the Arctic lower stratosphere.

We present the strongly revised IMK/IAA MIPAS-ENVISAT CH4 and N2O data products for the MIPAS full-resolution (versions V5H_CH4_21 and V5H_N2O_21) and for the reduced-resolution period (versions V5R_CH4_224, V5R_CH4_225, V5R_N2O_224 and V5R_N2O_225). These data sets cover both MIPAS measurement periods from June 2002 until March 2004 and from January 2005 to April 2012. Differences with older retrieval versions which are known to have a high bias are discussed. The usage of the HITRAN 2008 spectroscopic data set leads to lower values for both gases in the lower part of the profile. The improved correction of additive radiance offsets and handling of background radiance continua allows for aerosol contributions at altitudes in the upper stratosphere and above. These changes lead to more plausible values, both in the radiance offset and in the profiles of the continuum absorption coefficients. They also increase the fraction of converged retrievals. Some minor changes were applied to the constraint of the inverse problem, causing small differences in the retrieved profiles, mostly due to the relaxation of off-diagonal regularisation matrix elements for the calculation of jointly retrieved absorption coefficient profiles. Spectral microwindows have been adjusted to avoid areas with saturated spectral signatures. Jointly retrieving profiles of water vapour and nitric acid serves to compensate spectroscopic inconsistencies. We discuss the averaging kernels of the profiles and their vertical resolution. The latter ranges from 2.5 to 7 km for CH4, and from 2.5 to 6 km for N2O in the reduced-resolution period. For the full-resolution period, the vertical resolution is in the order of 3 to 6 km for both gases. We find the retrieval errors in the lower part of the profiles mostly to be around 15 % for CH4 and below 10 % for N2O. The errors above 25 or 30 km increase to values between 10 and 20 %, except for CH4 from the reduced-resolution period, where the estimated errors stay below 15 %.

A merged time series of stratospheric water vapour built from the Halogen Occultation Instrument (HALOE) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data between 60° S and 60° N and 15 to 30 km and covering the years 1992 to 2012 was analysed by multivariate linear regression, including an 11-year solar cycle proxy. Lower stratospheric water vapour was found to reveal a phase-shifted anti-correlation with the solar cycle, with lowest water vapour after solar maximum. The phase shift is composed of an inherent constant time lag of about 2 years and a second component following the stratospheric age of air. The amplitudes of the water vapour response are largest close to the tropical tropopause (up to 0.35 ppmv) and decrease with altitude and latitude. Including the solar cycle proxy in the regression results in linear trends of water vapour being negative over the full altitude/latitude range, while without the solar proxy, positive water vapour trends in the lower stratosphere were found. We conclude from these results that a solar signal seems to be generated at the tropical tropopause which is most likely imprinted on the stratospheric water vapour abundances and transported to higher altitudes and latitudes via the Brewer–Dobson circulation. Hence it is concluded that the tropical tropopause temperature at the final dehydration point of air may also be governed to some degree by the solar cycle. The negative water vapour trends obtained when considering the solar cycle impact on water vapour abundances can possibly solve the "water vapour conundrum" of increasing stratospheric water vapour abundances despite constant or even decreasing tropopause temperatures.

The Hygrosonde-2 campaign took place on 16 December 2001 at Esrange/Sweden (68° N, 21° E) with the aim to investigate the small scale distribution of water vapour in the middle atmosphere in the vicinity of the Arctic polar vortex. In situ balloon and rocket-borne measurements of water vapour were performed by means of OH fluorescence hygrometry. The combined measurements yielded a high resolution water vapour profile up to an altitude of 75 km. Using the characteristic of water vapour being a dynamical tracer it was possible to directly relate the water vapour data to the location of the polar vortex edge, which separates air masses of different character inside and outside the polar vortex. The measurements probed extra-vortex air in the altitude range between 45 km and 60 km and vortex air elsewhere. Transitions between vortex and extra-vortex usually coincided with wind shears caused by gravity waves which advect air masses with different water vapour volume mixing ratios. From the combination of the results from the Hygrosonde-2 campaign and the first flight of the optical hygrometer in 1994 (Hygrosonde-1) a clear picture of the characteristic water vapour distribution inside and outside the polar vortex can be drawn. Systematic differences in the water vapour concentration between the inside and outside of the polar vortex can be observed all the way up into the mesosphere. It is also evident that in situ measurements with high spatial resolution are needed to fully account for the small-scale exchange processes in the polar winter middle atmosphere.

The air induced broadening coefficients of the pure rotational transitions of H 2 O at 556.936 GHz ( 1 10 ← 1 01 ) , and 752.033 GHz ( 2 11 ← 2 02 ) were measured by terahertz time-domain spectroscopy. The air broadening coefficient was determined to be 4.012 ( 30 ) MHz Torr - 1 for the 556.936 GHz line and 3.964 ( 40 ) MHz Torr - 1 for the 752.033 GHz line, respectively. The present broadening coefficients for the 556.936 GHz water line are significantly smaller than those of Markov and Krupnov [Measurements of the pressure shift of the 1(10)–1(01) water line at 556.936 GHz produced by mixtures of gases. J Mol Spect 1995:172;211–4] but relatively close to the values of the HITRAN database. The measured data may improve the accuracy of the abundance of water vapor retrieved from spectra obtained by the Odin/SMR satellite instrument. The effect on the satellite retrieval processing is discussed.

Limb measurements at 557 GHz by the Sub-millimetre and Millimetre Radiometer (SMR) onboard the Odin satellite provide water vapour and temperature information throughout the mesosphere. A sensitivity study for these retrievals has been performed, in order to identify the most critical parameters affecting the retrievals, with focus on the polar summer mesosphere conditions. Uncertainties in the instrument characterisation and the a priori value chosen for the pointing offset were found to have significant effects on the retrieved profiles. It is also shown that spectroscopic parameters play a role. An obvious improvement of the water and temperature retrieval can be obtained when we account for some of these critical parameters. However, some further improvement of the instrument behaviour characterisation is still needed.

This modelling study aims at an improved understanding of the processes that determine the water vapour budget in the stratosphere by means of the investigation of water isotope ratios. An additional (and separate from the actual) hydrological cycle has been introduced into the chemistry–climate model EMAC, including the water isotopologues HDO and H218O and their physical fractionation processes. Additionally an explicit computation of the contribution of methane oxidation to H2O and HDO has been incorporated. The model expansions allow detailed analyses of water vapour and its isotope ratio with respect to deuterium throughout the stratosphere and in the transition region to the troposphere. In order to assure the correct representation of the water isotopologues in the model's hydrological cycle, the expanded system has been evaluated in several steps. The physical fractionation effects have been evaluated by comparison of the simulated isotopic composition of precipitation with measurements from a ground-based network (GNIP) and with the results from the isotopologue-enabled general circulation model ECHAM5-wiso. The model's representation of the chemical HDO precursor CH3D in the stratosphere has been confirmed by a comparison with chemical transport models (1-D, CHEM2D) and measurements from radiosonde flights. Finally, the simulated stratospheric HDO and the isotopic composition of water vapour have been evaluated, with respect to retrievals from three different satellite instruments (MIPAS, ACE-FTS, SMR). Discrepancies in stratospheric water vapour isotope ratios between two of the three satellite retrievals can now partly be explained.

Studying the isotopic composition of water vapour in the lower stratosphere can reveal the driving mechanisms of changes in the stratospheric water vapour budget and therefore help to explain the trends and variations of stratospheric water vapour during the recent decades. We equipped a global chemistry climate model with a description of the water isotopologue HDO, comprising its physical and chemical fractionation effects throughout the hydrological cycle. We use this model to improve our understanding of the processes, which determine the patterns in the stratospheric water isotope composition and in the water vapour budget, itself. The link between the water vapour budget and its isotopic composition in the tropical stratosphere is presented through their correlation in a simulated 21 year time series. The two quantities depend on the same processes, however, are influenced with different strengths. A sensitivity experiment shows that fractionation effects during the oxidation of methane has a damping effect on the stratospheric tape recorder signal in the water isotope ratio. Moreover, the chemically produced high water isotope ratios overshadow the tape recorder in the upper stratosphere. Investigating the origin of the boreal summer tape recorder signal in the lower stratosphere reveals isotopically enriched water vapour crossing the tropopause over the subtropical Western Pacific. A correlation analysis confirms this link, which identifies the Asian Summer Monsoon as the major contributor for the intrusion of isotopically enriched water vapour into the stratosphere during boreal summer. Furthermore, convective ice lofting is shown to have a substantial impact on the isotope ratios of water vapour in the upper troposphere and lower stratosphere.

The space shuttle launched for the last time on 8 July 2011. As with most shuttle launches, the three main engines injected about 350 t of water vapor between 100 and 115 km off the east coast of the United States during its ascent to orbit. We follow the motion of this exhaust with a variety of satellite and ground-based data sets and find that (1) the shuttle water vapor plume spread out horizontally in all directions over a distance of 3000 to 4000 km in 18 h, (2) a portion of the plume reached northern Europe in 21 h to form polar mesospheric clouds (PMCs) that are brighter than over 99% of all PMCs observed in that region, and (3) the observed altitude dependence of the particle size is reversed with larger particles above smaller particles. We use a one- dimensional cloud formation model initialized with predictions of a plume diffusion model to simulate the unusually bright PMCs. We find that eddy mixing can move the plume water vapor down to the mesopause near 90 km where ice particles can form. If the eddy diffusion coefficient is 400 to 1000 m(2)/s, the predicted integrated cloud brightness is in agreement with both satellite and ground-based observations of the shuttle PMCs. The propellant mass of the shuttle is about 20% of that from all vehicles launched during the northern 2011 PMC season. We suggest that the brightest PMC population near 70 degrees N is formed by space traffic exhaust.

Stationary wave patterns in middle atmospheric ozone (O3) and water vapour (H2O) are an important factor in the atmospheric circulation, but there is a strong gap in diagnosing and understanding their configuration and origin. Based on Odin satellite data from 2001 to 2010 we investigate the stationary wave patterns in O3 and H2O as indicated by the seasonal long-term means of the zonally asymmetric components O3* = O3-[O3] and H2O* = H2O-[H2O] ([O3], [H2O]: zonal means). At mid- and polar latitudes we find a pronounced wave one pattern in both constituents. In the Northern Hemisphere, the wave patterns increase during autumn, maintain their strength during winter and decay during spring, with maximum amplitudes of about 10–20 % of the zonal mean values. During winter, the wave one in O3* shows a maximum over the North Pacific/Aleutians and a minimum over the North Atlantic/Northern Europe and a double-peak structure with enhanced amplitude in the lower and in the upper stratosphere. The wave one in H2O* extends from the lower stratosphere to the upper mesosphere with a westward shift in phase with increasing height including a jump in phase at upper stratosphere altitudes. In the Southern Hemisphere, similar wave patterns occur mainly during southern spring. By comparing the observed wave patterns in O3* and H2O* with a linear solution of a steady-state transport equation for a zonally asymmetric tracer component we find that these wave patterns are primarily due to zonally asymmetric transport by geostrophically balanced winds, which are derived from observed temperature profiles. In addition temperature-dependent photochemistry contributes substantially to the spatial structure of the wave pattern in O3* . Further influences, e.g., zonal asymmetries in eddy mixing processes, are discussed.

In 2000 a sudden severe drop in stratospheric water vapor levels interrupted the supposed long-term increase of this greenhouse gas, an important contributor to global warming and climate variability. Satellite sensors observed a recovery in the following years, hidden behind a large variability. More recently, during 2011 and 2012, measurements revealed another severe drop in stratospheric water vapor concentrations.

Using newly analyzed mesospheric water vapor and temperature observations from the Sub-Millimeter Radiometer instrument aboard the Odin research satellite over the period 2001-2009, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the winters of 2004, 2006, and 2009. In the three cases, the descent follows the recovery of the upper stratospheric polar vortex from a major midwinter stratospheric sudden warming. It is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, i.e., an elevated polar stratopause, near 75 km, and its slower descent to prewarming level (near 1 hPa) over 1.5-2 months. These three winters stand out in the current record of Odin/Sub-Millimeter Radiometer observations started in July 2001.

In this paper we present Odin submillimeter radiometer (Odin/SMR) water vapor measurements in the upper mesosphere and lower thermosphere with focus on the polar latitudes in winter. Measurements since 2003 have been compiled to provide a first overview of the water vapor distribution in this altitude range. Our observations show a distinct seasonal increase of the water vapor concentration during winter at a given altitude above 90 km. Above 95 km the observations exhibit the annual water vapor maximum during wintertime. Model simulations from the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) and the Whole Atmosphere Community Climate Model version 3 (WACCM3) show results that are very similar to the observations. We suggest that the observed increase in water vapor during winter is mainly caused by a combination of upwelling of moister air from lower altitudes and diffusion processes. Distinct interhemispheric differences in the winter water vapor distribution in the upper mesosphere and lower thermosphere can be observed, both in the observations and the model results. The seasonal water vapor increase in the polar regions is much more pronounced in the Southern Hemisphere winter where higher concentrations can be observed. This observation is most likely due to interhemispheric differences in the underlying dynamics and diffusion processes. [References: 50]

The Atmospheric Chemistry Experiment (ACE) mission was launched in August 2003 to sound the atmosphere by solar occultation. Water vapour (H2O), one of the most important molecules for climate and atmospheric chemistry, is one of the key species provided by the two principal instruments, the infrared Fourier Transform Spectrometer (ACE-FTS) and the MAESTRO UV-Visible spectrometer (ACE-MAESTRO). The first instrument performs measurements on several lines in the 1362–2137 cm−1 range, from which vertically resolved H2O concentration profiles are retrieved, from 7 to 90 km altitude. ACE-MAESTRO measures profiles using the water absorption band in the near infrared part of the spectrum at 926.0–969.7 nm. This paper presents a comprehensive validation of the ACE-FTS profiles. We have compared the H2O volume mixing ratio profiles with space-borne (SAGE II, HALOE, POAM III, MIPAS, SMR) observations and measurements from balloon-borne frostpoint hygrometers and a ground based lidar. We show that the ACE-FTS measurements provide H2O profiles with small retrieval uncertainties in the stratosphere (better than 5% from 15 to 70 km, gradually increasing above). The situation is unclear in the upper troposphere, due mainly to the high variability of the water vapour volume mixing ratio in this region. A new water vapour data product from the ACE-MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is also presented and initial comparisons with ACE-FTS are discussed.

Water vapour plays an important role for the chemistry and dynamics of the atmosphere. It is a strong greenhouse gas in the troposphere and contributes to cooling in the stratosphere. As the main source of chemically active HOx radicals, it is linked to many photo-chemical cycles controlling the composition of the middle atmosphere. In order to improve our knowledge of the amount and variability of water in the middle atmosphere, the Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, observes several thermal emission lines of water vapour in the 486–581 GHz spectral range from the Earth's limb. Bands centred at 488.9 and 490.4 GHz are used to study water vapour and its isotopes, on the basis of four observation days per month. Vertical profiles of View the MathML source, View the MathML source, and HDO are retrieved between roughly 20 and 70 km in the stratosphere and mesosphere. A strong water vapour line at 556.9 GHz is simultaneously measured in a second band, providing information in the mesosphere and lower thermosphere between about 40 and 100 km. Measurements of View the MathML source at 552.0 GHz in monthly intervals complete the picture of middle atmospheric water vapour provided by Odin/SMR. The measurements of the isotope HDO in the 20–70 km altitude range allow to study the isotopic ratio of deuterium in stratospheric water vapour (D/H), potentially supplying information on the origin of stratospheric water vapour: transport of tropospheric air through the tropical tropopause layer (TTL) versus in situ chemical production such as from methane oxidation. The unique measurements of the molecules View the MathML source and View the MathML source containing heavy isotopes of oxygen may provide a crucial test for our understanding of the complex chemical reaction mechanisms controlling the exchange of oxygen between water vapour and ozone.

Mesospheric water vapour measurements taken by the SMR instrument aboard the Odin satellite between 2002 and 2006 have been analysed with focus on the mesospheric semi-annual circulation in the tropical and subtropical region. This analysis provides the first complete picture of mesospheric SAO in water vapour, covering altitudes above 80 km where previous studies were limited. Our analysis shows a clear semi-annual variation in the water vapour distribution in the entire altitude range between 65 km and 100 km in the equatorial area. Maxima occur near the equinoxes below 75 km and around the solstices above 80 km. The phase reversal occurs in the small layer in-between, consistent with the downward propagation of the mesospheric SAO in the zonal wind in this altitude range. The SAO amplitude exhibits a double peak structure in the equatorial region, with maxima at about 75 km and 81 km. The observed amplitudes show higher values than an earlier analysis based on UARS/HALOE data. The upper peak amplitude remains relatively constant with latitude. The lower peak amplitude decreases towards higher latitudes, but recovers in the Southern Hemisphere subtropics. On the other hand, the annual variation is much more prominent in the Northern Hemisphere subtropics. Furthermore, higher volume mixing ratios during summer and lower values during winter are observed in the Northern Hemisphere subtropics, as compared to the corresponding latitude range in the Southern Hemisphere.