Low Water Vapor Mixing Ratios Research Articles

Intercomparison of midlatitude tropospheric and lower-stratospheric water vapor measurements and comparison to ECMWF humidity data

Abstract. Accurate measurement of water vapor in the climate-sensitive region near the tropopause is very challenging. Unexplained systematic discrepancies between measurements at low water vapor mixing ratios made by different instruments on airborne platforms have limited our ability to adequately address a number of relevant scientific questions on the humidity distribution, cloud formation and climate impact in that region. Therefore, during the past decade, the scientific community has undertaken substantial efforts to understand these discrepancies and improve the quality of water vapor measurements. This study presents a comprehensive intercomparison of airborne state-of-the-art in situ hygrometers deployed on board the DLR (German Aerospace Center) research aircraft HALO (High Altitude and LOng Range Research Aircraft) during the Midlatitude CIRRUS (ML-CIRRUS) campaign conducted in 2014 over central Europe. The instrument intercomparison shows that the hygrometer measurements agree within their combined accuracy (±10 % to 15 %, depending on the humidity regime); total mean values agree within 2.5 %. However, systematic differences on the order of 10 % and up to a maximum of 15 % are found for mixing ratios below 10 parts per million (ppm) H2O. A comparison of relative humidity within cirrus clouds does not indicate a systematic instrument bias in either water vapor or temperature measurements in the upper troposphere. Furthermore, in situ measurements are compared to model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) which are interpolated along the ML-CIRRUS flight tracks. We find a mean agreement within ±10 % throughout the troposphere and a significant wet bias in the model on the order of 100 % to 150 % in the stratosphere close to the tropopause. Consistent with previous studies, this analysis indicates that the model deficit is mainly caused by too weak of a humidity gradient at the tropopause.

Deuterium excess in subtropical free troposphere water vapor: Continuous measurements from the Chajnantor Plateau, northern Chile

AbstractWater vapor measured continuously by cavity ring‐down spectroscopy from July 2012 to March 2013 on the hyperarid Chajnantor Plateau, northern Chile (elevation = 5080 m, pressure ≈ 550 hPa), has a mean deuterium excess (d‐excess = δD − 8*δ18O) of 46‰ ± 5‰ and frequently exceeds 100‰ at low water vapor mixing ratios (q ≤ 500 ppmv). These measurements provide empirical support for theoretical predictions of free troposphere d‐excess. The d‐excess measured at this site can be understood in terms of supersaturation with respect to ice at relative humidities between 100% and 130%, followed by mixing with moist midtropospheric or lower tropospheric air en route to the plateau. The d‐excess measured at Chajnantor is consistent with predictions for d‐excess in the upper troposphere from isotope‐enabled general circulation models and with high vapor saturation over ice in cloud‐resolving and microphysical models.

Comparison of Fast In situ Stratospheric Hygrometer (FISH) measurements of water vapor in the upper troposphere and lower stratosphere (UTLS) with ECMWF (re)analysis data

Abstract. An evaluation of water vapor in the upper troposphere and lower stratosphere (UTLS) of the ERA-Interim, the global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF), is presented. Water vapor measurements are derived from the Fast In situ Stratospheric Hygrometer (FISH) during a large set of airborne measurement campaigns from 2001 to 2011 in the tropics, midlatitudes and polar regions, covering isentropic layers from 300 to 400K (5–18km). The comparison shows around 87% of the reanalysis data are within a factor of 2 of the FISH water vapor measurements and around 30% have a nearly perfect agreement with an over- and underestimation lower than 10%. Nevertheless, strong over- and underestimations can occur both in the UT and LS, in particularly in the extratropical LS and in the tropical UT, where severe over- and underestimations up to 10 times can occur. The analysis data from the evolving ECMWF operational system is also evaluated, and the FISH measurements are divided into time periods representing different cycles of the Integrated Forecast System (IFS). The agreement with FISH improves over the time, in particular when comparing water vapor fields for time periods before 2004 and after 2010. It appears that influences of tropical tropospheric and extratropical UTLS processes, e.g., convective and quasi-isentropic exchange processes, are particularly challenging for the simulation of the UTLS water vapor distribution. Both the reanalysis and operational analysis data show the tendency of an overestimation of low water vapor mixing ratio (⪅10ppmv) in the LS and underestimation of high water vapor mixing ratio (⪆300ppmv) in the UT.

Summertime tropospheric ozone assessment over the Mediterranean region using the thermal infrared IASI/MetOp sounder and the WRF-Chem model

Abstract. Over the Mediterranean region, elevated tropospheric ozone (O3) values are recorded, especially in summer. We use the thermal Infrared Atmospheric Sounding Interferometer (IASI) and the Weather Research and Forecasting Model with Chemistry (WRF-Chem) to understand and interpret the factors and emission sources responsible for the high O3 concentrations observed in the Mediterranean troposphere. Six years (2008–2013) of IASI data have been analyzed and results show consistent maxima during summer, with an increase of up to 22% in the [0–8] km O3 column in the eastern part of the basin compared to the middle of the basin. We focus on summer 2010 to investigate the processes that contribute to these summer maxima. Using two modeled O3 tracers (inflow to the model domain and local anthropogenic emissions), we show that, between the surface and 2 km, O3 is mostly formed from anthropogenic emissions, while above 4 km it is mostly transported from outside the domain or from stratospheric origins. Evidence of stratosphere-to-troposphere exchange (STE) events in the eastern part of the basin is shown, and corresponds to a low water vapor mixing ratio and high potential vorticity.

Dry phase of tropical lower stratospheric water vapor: Role of BDC, convection and ozone variability

In the present paper, the relationship between dry phase of water vapor in the tropical lower stratosphere (TLS) and 100hPa temperatures (T100) has been examined. Role of various processes, such as Brewer–Dobson circulation (BDC), convective activities and seasonal minimum of ozone mixing ratio, has been quantified to explain the reason of low T100 over Indonesian-Australian western Pacific region (IAWPR). Aura MLS data show that low water vapor mixing ratio (WVMR) first appears over the western Pacific during northern hemisphere (NH) winters and directs the dry phase of TLS. Observations of low T100 (~188K), close association between WVMR and T100, slow transport of water vapor in TLS and saturation of air close to 82.5–100hPa over IAWPR indicate conditions favorable for 'freeze drying'. Present analysis has brought out several interesting features (a) in addition to BDC, convective activities and low ozone mixing ratio near the tropopause level seems to be contributing to the low T100 over IAWPR during NH winter/spring, (b) apart from the seasonal decrease, T100 over IAWPR is noted to be continually low throughout the year by ~1.2K than the zonal mean value where part of such decrease in T100 (i.e. ~0.7K) can be understood in terms of water vapor feedback process and (c) wave activity of different temporal scale and amplitude (~1–2K) also modulate T100 over IAWPR. MLS observations also provide an evidence of coupling between the surface and TLS processes, if the sea surface temperature over IAWPR is more than 301.7K.

Observations of Temperature, Wind, Cirrus, and Trace Gases in the Tropical Tropopause Transition Layer during the MJO*

Abstract Satellite observations of temperature, optically thin cirrus clouds, and trace gases derived from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and the Microwave Limb Sounder (MLS) are analyzed in combination with Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) wind and humidity fields in the tropical tropopause transition layer (TTL), using the Madden–Julian oscillation (MJO) as a carrier signal. MJO-related deep convection induces planetary-scale Kelvin and Rossby waves in the stably stratified TTL. Regions of ascent in these waves are associated with anomalously low temperatures, high radiative heating rates, enhanced cirrus occurrence, and high carbon monoxide and low ozone concentrations. Low water vapor mixing ratio anomalies lag the low temperature anomalies by about 1–2 weeks. The anomalies in all fields propagate eastward, circumnavigating the tropical belt over a roughly 40-day interval. Equatorial cross sections reveal that the anomalies tilt eastward with height in the TTL and propagate downward from the lower stratosphere into the upper troposphere. As MJO-related convection moves into the western Pacific and dissipates, a fast-moving Kelvin wave flanked by Rossby waves propagates eastward across South America and Africa into the western Indian Ocean. The region of equatorial westerly wind anomalies behind the Kelvin wave front lengthens until it encompasses most of the tropics at the 150-hPa level, giving rise to equatorially symmetric, anomalously low zonal-mean temperature and water vapor mixing ratio and enhanced cirrus above about 100 hPa.

Stratospheric dryness: model simulations and satellite observations

Abstract. The mechanisms responsible for the extreme dryness of the stratosphere have been debated for decades. A key difficulty has been the lack of comprehensive models which are able to reproduce the observations. Here we examine results from the coupled lower-middle atmosphere chemistry general circulation model ECHAM5/MESSy1 together with satellite observations. Our model results match observed temperatures in the tropical lower stratosphere and realistically represent the seasonal and inter-annual variability of water vapor. The model reproduces the very low water vapor mixing ratios (below 2 ppmv) periodically observed at the tropical tropopause near 100 hPa, as well as the characteristic tape recorder signal up to about 10 hPa, providing evidence that the dehydration mechanism is well-captured. Our results confirm that the entry of tropospheric air into the tropical stratosphere is forced by large-scale wave dynamics, whereas radiative cooling regionally decelerates upwelling and can even cause downwelling. Thin cirrus forms in the cold air above cumulonimbus clouds, and the associated sedimentation of ice particles between 100 and 200 hPa reduces water mass fluxes by nearly two orders of magnitude compared to air mass fluxes. Transport into the stratosphere is supported by regional net radiative heating, to a large extent in the outer tropics. During summer very deep monsoon convection over Southeast Asia, centered over Tibet, moistens the stratosphere.

An alternative mechanism explaining the hygropause formation in tropical regions

Radiosonde data obtained during CEPEX and TOGA‐COARE experiments show that a very simple mechanism can explain the existence, in tropical regions, of a local hygropause with a very low water vapor mixing ratio, less than 2 ppmv. This mechanism explains also why this hygropause is generally located well above the average tropopause in the lower stratosphere, and why it can temporarily coincide with the local tropopause. This interpretation is based on the existence, during periods of intense convective phenomena, of an uplift of the tropopause inducing a cooling of its temperature. We indeed find in these circumstances, when temperature and water vapor measurements are simultaneously available, very low water vapor values at the same level as the temperature minimum. After the end of the intense convection episode, the minimum temperature level is reestablished at a lower altitude while the hygropause remains positioned in the lower stratosphere but with a low value for the water vapor mixing ratio.

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.

Mechanisms controlling water vapor in the lower stratosphere: “A tale of two stratospheres”

We present an analysis of the mechanisms controlling stratospheric water vapor based on in situ profiles made at 37.4°N and at altitudes up to 20 km. The stratosphere can be conveniently divided into two air masses: the overworld (potential temperature θ>380 K) and the lowermost stratosphere (θ<380). Our data support the canonical theory that air primarily enters the overworld by passing through the tropical tropopause. The low water vapor mixing ratios in the overworld, a few parts per million by volume (ppmv), are determined by the low temperatures encountered at the tropical tropopause, as well as oxidation of methane and molecular hydrogen. Air enters the lowermost stratosphere both by diabatically descending from the overworld across the 380‐K potential temperature surface and by passing through the extratropical tropopause. Air parcels crossing the extratropical tropopause experience higher temperatures than air crossing the tropical tropopause, allowing higher water vapor in the lowermost stratosphere (tens of ppmv) than in the overworld. Our data are consistent with the pathway for air crossing the extratropical tropopause being isentropic advection from lower latitudes, although we cannot exclude contributions from other paths.

A comparison of observed (Haloe) and modeled (CCM2) methane and stratospheric water vapor

Recent measurements (21 September–15 October 1992) of methane and water vapor by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) are compared with model results for the same season from a troposphere‐middle atmosphere version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2). Several important features of the two constituent fields are well reproduced by the CCM2, despite the use of simplified methane photochemistry in the CCM2 and some notable differences between the model's zonal mean circulation and climatology. Observed features simulated by the model include the following: 1) subsidence over a deep layer in the Southern Hemisphere polar vortex; 2) widespread dehydration in the polar vortex; 3) existence of a region of low water vapor mixing ratios extending from the Antarctic into the Northern Hemisphere tropics, which suggests that Antarctic dehydration contributes to midlatitude and tropical dryness in the stratosphere.

The southern hemisphere lower stratosphere during August and September 1987: Analyses based on the United Kingdom Meteorological Office global model

The main object of this paper is to present in one place a series of meteorological analyses to aid the interpretation of the in situ Airborne Antarctic Ozone Experiment (AAOE) observations. Maps and sections of meteorological variables derived from the United Kingdom Meteorological Office global model are presented for ER‐2 and DC‐8 flight days. Analyzed temperatures and winds are generally in good agreement with AAOE observations at all levels. Minor discrepancies in winds and temperatures are evident, particularly at DC‐8 altitudes, and are discussed in the paper. Model analyses show temperatures at 6O°W cold enough to saturate water at 1 part per million by volume as late as September 14 and that following that date a sudden increase in the temperature at all levels within the vortex precludes further saturation at low water vapor mixing ratios. Maps of potential vorticity are presented on the 428‐K potential temperature surface for the AAOE flight days. These show that the vortex is essentially circumpolar although there are periods when major distortions are apparent. At the 428‐K potential temperature level, the area of the vortex, as defined by the areas enclosed by selected potential vorticity contours, remains approximately constant throughout the AAOE.

Aerial observations of stratospheric descent in a Gulf of Genoa cyclone

The structure of a cyclone in the Gulf of Genoa on April 24, 25, 1982 is examined using an enhanced rawinsonde network, data from four ALPEX research flights, and satellite ozone and water vapor observations. This particular cyclone had, as a precursor, a mesoscale upper level vortex which approached the Alps from the north at a rate of about 400 km/6 hours. This small cyclone had a roughly axisymmetric tropopause fold. Cold air below the cyclone prevented it from having a surface pressure signature. Upon encountering the Alps, the cold air appeared to be deflected, allowing the cyclone reaching the Gulf of Genoa to extend to the surface. Aircraft data at thep≈447 mb level reveal a small volume of stratospheric air slightly ahead of the vortex center. This air is marked by high ozone concentration, low water vapor mixing ratio, and high relative vorticity. The boundaries of this region are very well defined with a horizontal scale of about 20 km, but with some evidence of advection and mixing processes. Satellite derived maps of vertically integrated ozone (TOMS) agree with the aircraft data regarding the position of the ozone maximum. Satellite images in the water vapor band /6.7 μm) also agree with the aircraft data, and provide some information on the time evolution of the stratospheric intrusion.

The global distribution and variability of stratospheric constituents measured by LIMS

The purpose of the Nimbus 7 LIMS experiment was to sound the composition and structure of the upper atmosphere and provide data for study of photochemistry, radiation, and dynamics processes. Vertical profiles were measured of temperature and ozone (O 3) over the 10-km to 65-km range and water vapor (H 2O), nitrogen dioxide (NO 2), and nitric acid (HNO 3) over the 10-km to ∼50-km range. Latitude coverage extended from 64°S to 84°N. Several general features of the atmosphere have emerged from data analyses thus far. Nitrogen dioxide exhibits rapid latitudinal variations in winter and shows hemispheric asymmetry with generally higher vertical column amount in the summer hemisphere. HNO 3 data show that this gas is highly variable with altitude, latitude, and season. Smallest mixing ratios occur in the tropics, and the largest values occur in the high latitude winter hemisphere. The results show that O 3, NO 2, and HNO 3 are strongly affected during a stratospheric warming. There is a persistently low water vapor mixing ratio in the tropical lower stratosphere (∼2–3 ppmv), a poleward gradient at all times in the mission, and evidence of increasing mixing ratio with altitude at tropical and middle latitudes.

Water vapor budget in the lower stratosphere over Japan.

The water vapor budget in the lower stratosphere over Japan was studied. At Kagoshima and Tateno the observed water vapor mass mixing ratio is explained by a modified Brewer circulation model adding the water vapor source of CH4 oxidation except in spring at Kagoshima. The vertical eddy diffusion coefficient at an altitude of 20.7 km was estimated to be 1.2-1.4×103cm2sec-1 from the vertical profiles of water vapor and the production rate of water vapor by the oxidation of CH4. Seasonal variation of the residence time of water vapor in the lower stratosphere was obtained; minimum in winter-spring (0.3 year) and maximum in fall (0.95 year). Mean vertical velocity of the return Hadley circulation (the descending branch of the Hadley circulation) was estimated to be 0.010-0.035cm sec-1. Water vapor in the lower stratosphere at Sapporo cannot be explained by the same model as Tateno and Kagoshima except in spring. A very low water vapor mixing ratio is observed in spring from Kagoshima through Sapporo and in winter at Sapporo, suggesting some water vapor removal processes. Aerosols in the lower stratosphere make negligible contribution to the water vapor budget except after an extensive volcanic eruption.

Thunderstorms as possible micrometeorological sink for stratospheric water

The dryness of the stratosphere has been explained, in general terms, as water condensation from the rising branch of the Hadley cell at the tropical tropopause (Brewer, 1949), but Ellsaesser (1974) suggested that the mean tropical tropopause is not cold enough to account for the observed water vapor mixing ratios. During intense thunderstorms that in part penetrate the tropopause, investigators have observed (1) an increase in local stratospheric water vapor and (2) the temporary presence of air parcels substantially colder than and higher than the tropopause. These cold parcels are calculated to have extremely low water vapor mixing ratios, and their occurrence in the stratosphere suggests a mechanism whereby the effective condensation temperature could be systematically colder than the tropopause. Ice crystals from the cloud, evaporating in the warmer stratosphere, presumably cause the observed increase in water vapor, but mixing of cold desiccated air parcels with lower stratospheric air would tend to decrease its water content. Thus there are opposing factors concerning the role of severe cumulonimbus storms on stratospheric water, and it may require detailed, microphysical analysis to see which effect is larger.