Stratospheric Water Vapor Concentrations Research Articles

First airborne water vapor lidar measurements in the tropical upper troposphere and mid-latitudes lower stratosphere: accuracy evaluation and intercomparisons with other instruments

Abstract. In the tropics, deep convection is the major source of uncertainty in water vapor transport to the upper troposphere and into the stratosphere. Although accurate measurements in this region would be of first order importance to better understand the processes that govern stratospheric water vapor concentrations and trends in the context of a changing climate, they are sparse because of instrumental shortcomings and observational challenges. Therefore, the Falcon research aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR) flew a zenith-viewing water vapor differential absorption lidar (DIAL) during the Tropical Convection, Cirrus and Nitrogen Oxides Experiment (TROCCINOX) in 2004 and 2005 in Brazil. The measurements were performed alternatively on three water vapor absorption lines of different strength around 940 nm. These are the first aircraft DIAL measurements in the tropical upper troposphere and in the mid-latitudes lower stratosphere. Sensitivity analyses reveal an accuracy of 5% between altitudes of 8 and 16 km. This is confirmed by intercomparisons with the Fast In-situ Stratospheric Hygrometer (FISH) and the Fluorescent Advanced Stratospheric Hygrometer (FLASH) onboard the Russian M-55 Geophysica research aircraft during five coordinated flights. The average relative differences between FISH and DIAL amount to −3%±8% and between FLASH and DIAL to −8%±14%, negative meaning DIAL is more humid. The average distance between the probed air masses was 129 km. The DIAL is found to have no altitude- or latitude-dependent bias. A comparison with the balloon ascent of a laser absorption spectrometer gives an average difference of 0%±19% at a distance of 75 km. Six tropical DIAL under-flights of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT reveal a mean difference of −8%±49% at an average distance of 315 km. While the comparison with MIPAS is somewhat less significant due to poorer comparison conditions, the agreement with the in-situ hygrometers provides evidence of the excellent quality of FISH, FLASH and DIAL. Most DIAL profiles exhibit a smooth exponential decrease of water vapor mixing ratio in the tropical upper troposphere to lower stratosphere transition. The hygropause with a minimum mixing ratio of 2.5 µmol/mol is found between 15 and 17 km. A high-resolution (2 km horizontal, 0.2 km vertical) DIAL cross section through the anvil outflow of tropical convection shows that the ambient humidity is increased by a factor of three across 100 km.

Testing the Fixed Anvil Temperature Hypothesis in a Cloud-Resolving Model

Abstract Using cloud-resolving simulations of tropical radiative–convective equilibrium, it is shown that the anvil temperature changes by less than 0.5 K with a 2-K change in SST, lending support to the fixed anvil temperature (FAT) hypothesis. The results suggest that for plausible ozone profiles, a decrease in the air’s emission capability instead of ozone heating shall remain the control on the detrainment level, and the FAT hypothesis should hold. The anvil temperature also remains unchanged with other changes in the system such as the doubled CO2 mixing ratio, doubled stratospheric water vapor concentration, and dynamical cooling due to the Brewer–Dobson circulations. The results are robust when a different microphysics scheme is used.

Control of interannual and longer‐term variability of stratospheric water vapor

We use trajectory calculations based on 40‐year European Reanalysis (ERA‐40) data to predict the water mixing ratio of air entering the stratosphere in the tropics ([H2O]e) and thereby to examine interannual and longer‐term changes. [H2O]e is determined from the saturation mixing ratio of the coldest point during ascent from the troposphere to the stratosphere (the Lagrangian cold point). These model predictions for the time variation of [H2O]e agree very well with a broad range of measurements (Stratospheric Aerosol and Gas Experiment (SAGE) II, Halogen Occultation Experiment (HALOE), Microwave Limb Sounder (MLS), and Atmospheric Trace Molecule Spectroscopy (ATMOS)). During periods when measurements are consistent among various sensors and ERA‐40 temperatures show good agreement with radiosondes (1995–2002), the correlation between model predictions and HALOE water vapor anomalies in the tropical lower stratosphere is r = 0.81, as high as that between HALOE and SAGE II (r = 0.8). The model predictions suggest that the stratospheric quasi‐biennial oscillation, El Niño–Southern Oscillation, and possibly volcanic eruptions all play a significant role in modulating [H2O]e, leading to interannual anomalies of order 0.5 ppmv with timescales of several months to years. Although the Lagrangian calculations show substantial interannual variability of transport pathways into the stratosphere, the results show that the interannual anomalies of [H2O]e are dominated by anomalies of the zonal mean temperature rather than by transport changes or localized temperature anomalies. This reinforces the paradox of apparently increasing stratospheric water vapor concentrations alongside, if anything, slightly decreasing temperatures at the tropical tropopause. The combination of measurement uncertainties and relatively strong interannual variability with periods of several months to years, on the one hand, limits our ability to detect, attribute, and verify long‐term trends and, on the other hand, raises the question as to whether the previously published estimates of long‐term trends are too large.

Temperature Fields in the Tropical Tropopause Transition Layer

Abstract There has been increasing recognition of the role of the tropical tropopause layer (TTL) in determining stratospheric water vapor concentrations—the TTL being a layer of transition between air showing tropospheric properties below and stratospheric properties above. This study investigates the spatial structure of temperatures in the TTL. A dehydration index based on the atmospheric region with temperatures colder than a specific reference temperature was defined to examine the TTL temperature structure and possible influences on stratospheric water vapor. The results indicate that dehydration regions with cold temperatures (e.g., <190 K) occur mainly over the western Pacific and are about 1.5–2.0 km in depth during Northern Hemisphere winter. The dehydration index is mainly dependent on the annual cycle of the TTL temperatures, but is strongly affected by interannual variations associated with the quasi-biennial oscillation (QBO) and El Nino–Southern Oscillation (ENSO). Dehydration regions with ...

Results of an interactively coupled atmospheric chemistry – general circulation model: Comparison with observations

Abstract. The coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM is presented which enables a simultaneous treatment of meteorology and atmospheric chemistry and their feedbacks. This is the first model which interactively combines a general circulation model with a chemical model, employing most of the important reactions and species necessary to describe the stratospheric and upper tropospheric ozone chemistry, and which is computationally fast enough to allow long-term integrations with currently available computer resources. This is possible as the model time-step used for the chemistry can be chosen as large as the integration time-step for the dynamics. Vertically the atmosphere is discretized by 39 levels from the surface up to the top layer which is centred at 10 hPa, with a relatively high vertical resolution of approximately 700 m near the extra-tropical tropopause. We present the results of a control simulation representing recent conditions (1990) and compare it to available observations. The focus is on investigations of stratospheric dynamics and chemistry relevant to describe the stratospheric ozone layer. ECHAM4.L39(DLR)/CHEM reproduces main features of stratospheric dynamics in the arctic vortex region, including stratospheric warming events. This constitutes a major improvement compared to earlier model versions. However, apparent shortcomings in Antarctic circulation and temperatures persist. The seasonal and interannual variability of the ozone layer is simulated in accordance with observations. Activation and deactivation of chlorine in the polar stratospheric vortices and their inter-hemispheric differences are reproduced. Considering methane oxidation as part of the dynamic-chemistry feedback results in an improved representation of the spatial distribution of stratospheric water vapour concentrations. The current model constitutes a powerful tool to investigate, for instance, the combined direct and indirect effects of anthropogenic trace gas emissions.Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry) – Meteorology and atmospheric dynamics (general circulation; middle atmosphere dynamics)

The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado

Stratospheric water vapor concentrations measured at two midlatitude locations in the northern hemisphere show water vapor amounts have increased at a rate of 1–1.5% yr−1 (0.05–0.07 ppmv yr−1) for the past 35 years. At Washington, D.C., measurements were made from 1964–1976, and at Boulder, Colorado, observations began in 1980 and continue to the present. While these two data sets do not comprise a single time series, they individually show increases over their respective measurement periods. At Boulder the trends do not show strong seasonal differences; significant increases are found throughout the year in the altitude range 16–28 km. In winter these trends are significant down to about 13 km.

A latitudinal survey of mesospheric and upper stratospheric water vapor

As part of the LAtitudinal DIstribution of Middle Atmosphere Structure (LADIMAS) campaign, measurements of mesospheric and upper stratospheric water vapor concentration were made over a latitudinal range from 53 N to 63 S. The 22-GHz emission line of water vapor was observed by a new, portable, cryogenically cooled microwave radiometer that was carried on board the German research vessel Polarstern as it sailed from Bremerhaven, Germany, to the Antarctic during November and December, 1991. Water vapor profiles were obtained at approximately 5 deg latitude intervals for an altitude range of 40 to 80 km.

Heterogeneous physicochemistry of the polar ozone hole

The heterogeneous physical and chemical processes that occur in the presence of and involve polar stratospheric clouds (PSCs) are investigated. The theory developed here is guided by, and compared for consistency with, the extensive observations from the Airborne Antarctic Ozone Experiment. We first describe the characteristics of PSCs that affect chemical processes, such as particle composition, cloud surface area and mass, and aerosol mechanical time constants. The vapor pressures of trace compounds measured over ice in laboratory settings are discussed and shown to be consistent with in situ observations and simple thermodynamics. The mechanism for the formation of nitric acid haze (type I PSC) is elucidated. To estimate key chemical time constants, we derive expressions for the rates of mass transfer to PSC particles and reaction rates on surfaces; here, laboratory measurements of “sticking coefficients” are related to the fundamental parameters of surface physics and chemistry. By comparing relative rates of physical and chemical processes, together with data and simulations on the seasonal evolution of polar ice clouds (in a companion paper (Toon et al., 1989)), we reach several important conclusions. The HCl + ClONO2, ClONO2 + H2O, N2O5 + HCl, and N2O5 + H2O reactions can occur on early forming type I PSC (haze) particles, converting inert chlorine to active chlorine (Cl2, HOCl, and ClNO2) and active nitrogen to HNO3 relatively quickly. Denitrification occurs somewhat later in the winter season with the formation of type II PSC (ice cirrus) clouds, which can absorb HNO3 in solid solution and remove the HNO3 by sedimentation; the degree of denitrification is sensitive to the cooling rate and the time constant for condensation of nitric acid haze. Dechlorination does not occur as efficiently as denitrification because the HCl reservoir is effectively depleted by conversion into active chlorine before the onset of type II cloud formation and denitrification. If HF is soluble in ice, defluorination is expected at the same time as denitrification, although the extent of defluorination would be effectively limited by a more pronounced vertical separation between HF and PSCs. The reactions listed above provide pathways to deplete the inert chlorine reservoir (HCl, ClONO2) on a time scale of days to weeks in the presence of type I clouds (although photochemical recycling of chlorine can occur on this time scale as well). Type II PSCs and, to a lesser extent, type III PSCs (lee wave clouds) contribute to chemical processing late in the winter that sustains the chemical imbalance of the polar stratosphere. We also discuss nonlinearities in the combined heterogeneous/homogeneous chemical system and show, using a simple model, that the decadal evolution of the Antarctic ozone hole may be understood, in connection with the accumulation of fluorocarbons in the atmosphere, through nonlinearities in the heterogeneous chemistry, with possible contributing effects of variations in stratospheric temperatures and water vapor concentrations, which appear to have caused an increase in PSC frequency, extent, and duration in recent years.

Measurement of stratospheric water vapor by cryogenic collection

Cryogenic collection of water vapor and subsequent volumetric measurement in the laboratory were used to measure stratospheric water vapor concentrations. Also measured were the deuterium and tritium content of the water samples. The isotopic measurements provide information not only on contamination during sampling, but also on the origin or previous history of the water vapor. The stratospheric H2O profiles show an increase with altitude which is weak for the high latitudes and steeper around 30°N. Profiles of total hydrogen (the sum of H2O, CH4, and H2) show no significant vertical gradients indicating that the H2O increase was essentially caused by the vertical decrease in the CH4 mixing ratio. Isotopic measurements from one flight over Alaska indicate the same.

Effect of changes in stratospheric water vapor on ozone reduction estimates

Previous studies of the effect of water vapor on the destruction of ozone in the stratosphere perturbed by NOx or ClX pollutants did not include temperature feedback effects (i.e., they assumed a fixed temperature profile). These studies indicated that if an increase in stratospheric water vapor concentration were induced (perhaps as a result of an increase in the temperature of the tropical tropopause caused by a change in stratospheric ozone), there would be a significant further reduction in total ozone. Our results from using a one‐dimensional transport kinetics model with a fixed temperature profile confirm these earlier findings. However, when the temperature profile is allowed to change as the species concentrations change, the sensitivity of total ozone to changes in water vapor abundance is greatly reduced. In the case of an NOx injection at 20 km there is even a reversal of sign such that an increase in stratospheric water vapor leads to an ozone increase relative to the case with no change in water vapor.

Effect of Chlorofluoromethane Infrared Radiation on Zonal Atmospheric Temperatures

Abstract A GCM sensitivity experiment has been carried out to estimate the change of atmospheric climate due to the radiative effects of 10 ppb of chlorofluoromethanes (CFM's). This was done with NCAR's old 12-layer stratosphere GCM and with an appropriate specified change of ocean temperature. Two maxima in zonal average temperature change were found, both in the tropics, one in the upper troposphere and one centered at the tropopause. In these regions the temperature change exceeds the surface temperature change by greater than a factor of 2. If this temperature change is scaled down to correspond approximately to that inferred for indefinite continuation of the 1975 CFM emission, the model calculation indicates that the temperature of the tropical tropopause would still increase by about 2 K. If stratospheric water vapor concentrations are controlled by the saturation vapor pressure at the trapopause, an increase of tropical tropopause temperature would imply an increase in stratospheric water vapor co...

The Variability of Water Vapor in the Stratosphere

Abstract Monthly measurements of stratospheric water vapor concentration, initiated as an IQSY program, have continued from a site near Washington, D.C., and now constitute a six-year time series of homogeneous data which may be examined for evidence of stratospheric water vapor variability. A regression analysis was performed for six pressure levels (100–mb) to examine the variability of water vapor concentration in terms of two components, a linear trend and an annual cycle. A highly significant trend of increasing mixing ratio from 2 to 3 ppm is found for all pressure levels over the six-year interval. A significant annual cycle is indicated for the higher pressure levels. Study of the literature indicates that similar annual cycles and trends have been observed in the height of the tropical tropopause and in temperatures at or near the tropopause. It is suggested that variability in the drying potential of the tropical tropopause region leads to corresponding variability in stratospheric mixing ratio ...