The need for accurate long‐term measurements of water vapor in the upper troposphere and lower stratosphere with global coverage
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Abstract Water vapor is the most important greenhouse gas in the atmosphere although changes in carbon dioxide constitute the “control knob” for surface temperatures. While the latter fact is well recognized, resulting in extensive space‐borne and ground‐based measurement programs for carbon dioxide as detailed in the studies by Keeling et al. (1996), Kuze et al. (2009), and Liu et al. (2014), the need for an accurate characterization of the long‐term changes in upper tropospheric and lower stratospheric ( UTLS ) water vapor has not yet resulted in sufficiently extensive long‐term international measurement programs (although first steps have been taken). Here, we argue for the implementation of a long‐term balloon‐borne measurement program for UTLS water vapor covering the entire globe that will likely have to be sustained for hundreds of years.Cite
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Sudden stratospheric warming
Quasi-biennial oscillation
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The behavior of air parcel transport between hemispheres and between the troposphere and stratosphere is studied by directly calculating 180-day trajectories based on observed data. It is shown that air parcels from the extra-tropical troposphere of one hemisphere are transported to the other hemisphere through the upper troposphere. The estimated inter-heimispheric exchange time is about one year. The main pathway from the troposphere to the stratosphere is the tropical tropopause. The flow from the equatorial lower stratosphere branches off in two directions, i, e., one is the return flow to the troposphere through the sub-tropical tropopause gaps and the other is the poleward flow in the lower stratosphere.
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There is experimental evidence that the stratospheric reservoir of Pb-210 aerosols is fed from the troposphere by injections into the stratosphere of its mother nuclide Rn-222. In steady state conditions, it is possible to calculate, for the tropospheric air penetrating into the stratosphere, that an Rn-222 concentration of 10 pCi kg-1 is necessary to balance the stratospheric Pb-210 losses. This concentration is not typical of the values usually measured in the upper troposphere. This supports the idea that the troposphere-to-stratosphere transfers are discontinuous and occur following a strong tropospheric upward transport.
Box model
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In situ water vapor measurements performed in the upper troposphere and the lower stratosphere, from an aircraft, in the North Atlantic region around 50°N, are analyzed. It is shown that the water vapor content observed in the region located slightly above the local tropopause is consistent with the transport from troposphere to stratosphere at middle and high latitudes. It appears that there is a seasonal variation with a higher water vapor content during summer. The source of this variation could be due to the difference in the relative contribution of the troposphere to stratosphere isentropic and convective transport as well as transport from the upper levels of the stratosphere due to the general circulation.
Tropopause
Sudden stratospheric warming
Middle latitudes
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Stratosphere-troposphere exchange (STE) and its effects on the stratospheric and tropospheric chemical compositions have been studied for the past two decades, but details on how mass is transported between the stratosphere and the troposphere are not well established. The goal of this study is to better describe global properties of cross tropopause trajectories, and to understand the processes related to transport of mass between the troposphere and the stratosphere. This understanding led us to build the simplest model which captures the most important properties of STE. To do this, nine-day extra-tropical stratosphere-troposphere exchange trajectories covering a period of 10 years, calculated using the ERA-15 re-analysis data, are investigated. The present study shows that the fraction of trajectories that reside in the stratosphere or in the troposphere does not depend on the direction of the exchange (stratosphere-to-troposphere transport, STT, or troposphere-to-stratosphere transport, TST). Trajectories are found to reside longer in the troposphere than in the stratosphere which suggests that they are driven down by asymmetric two-way motion. A random walk model is used to see whether this asymmetric transport is a result of a diffusive process. The transport of trajectories along isentropic coordinates is found to be compatible with a Brownian motion with higher probabilities to go downward. Since stratosphere-troposphere exchange reflects a differential motion of air masses and the tropopause, the potential temperature at the tropopause directly above or below the air mass is also investigated. The tropopause steps distributions are not stationary and they show some dynamical behaviors like the deformation of the tropopause at exchange time. Dispersion of trajectories in the atmosphere was furthermore investigated using several methods. They gave rise to three different transport mechanisms: diffusion, sub-diffusion and super-diffusion transports.%%%%Les echanges entre la stratosphere et la troposphere (STE) et leurs effets sur la composition chimique de la stratosphere et de la troposphere ont ete etudies intensivement, mais les details sur la maniere dont les masses d'air sont transportes entre la stratosphere et la troposphere ne sont pas bien etablis. Le but de cette etude est de decrire les proprietes globales des trajectoires qui traversent la tropopause, et de mieux comprendre les processus lies au transport de la matiere entre la troposphere et la stratosphere. Une fois ces details etablis, nous avons essaye de construire un modele simple qui capture les proprietes les plus importantes des STE. Pour ce faire, des trajectoires d'echanges entre la stratosphere et la troposphere (STE), couvrant les extra-tropiques durant une periode de 10 ans ont ete calculees en utilisant les donnees d'ERA-15. Dans cette etude on montre que la fraction de trajectoires qui resident dans la stratosphere ou dans la troposphere ne depend pas de la direction de l'echange (transport de la stratosphere a la …
Tropopause
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The tropopause is the boundary between the troposphere and the stratosphere, so its height determines the nature of processes in the atmosphere and depends on them. The criterion for determining the height of the tropopause is a decrease in the vertical temperature gradient in a layer above 5 km and at least 2 km thick. Consequently, the uncertainty in determining the height of the section between the troposphere and the stratosphere is 2 km or more. In addition, the temperature profile changes due to the influence of many reasons, so sometimes the height of the tropopause cannot even be determined. The optical properties of air in the troposphere and stratosphere differ, which makes it possible to measure the height of the section with high accuracy using a radiosonde with an optical sensor. The launches of radiosondes with optical sensors made it possible to measure the height of the tropopause by changing the attenuation coefficient of visible light. It turned out that the lower edge of the tropopause corresponds to a sharp change in the attenuation coefficient. The repeatability of the results was confirmed by the simultaneous launch of radiosondes. The results of the work will be useful for the tasks of weather forecasting and climate research.
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Structure and Composition of the Lower and Middle Atmosphere.- Radiative Processes in the Lower and Middle Atmosphere.- Dynamics of the Troposphere and Stratosphere.- Waves in the Troposphere and Stratosphere.- Chemical Processes in the Stratosphere and Troposphere.- Stratospheric Ozone Depletion and Antarctic Ozone Hole.- Transport Processes in the Stratosphere and Troposphere.- Stratosphere-Troposphere Exchange.- Stratospheric Influence on Tropospheric Weather and Climate.
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Abstract. This paper presents an evaluation of a new linear parameterization valid for the troposphere and the stratosphere, based on a first order approximation of the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM) MOCAGE of Météo-France. On the one hand, a one and a half years of LINCO simulation has been compared to output obtained from a detailed chemical scheme output. In spite of small differences, the seasonal and global CO distributions obtained by both schemes present similar general characteristics. The mean differences between both schemes remain small within about ±25 ppbv (part per billion by volume) in the troposphere and ±15 ppbv in the stratosphere. On the other hand, LINCO has been compared to diverse observations from satellite instruments covering the troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the stratosphere (Microwave Limb Sounder: MLS) and also from aircraft (Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC programme) mostly flying in the upper troposphere and lower stratosphere. A good agreement is generally found in the troposphere and the lower stratosphere. In the troposphere, the LINCO seasonal variations as well as the vertical and horizontal distributions are quite close to MOPITT CO observations. However, a bias of ~−40 ppbv is observed at 700 hPa between LINCO and MOPITT which is probably caused by too low emission values. In the stratosphere, MLS and LINCO present similar large-scale patterns, except over the poles where the CO concentration is underestimated by the model. We suggest that the underestimation of CO at polar latitudes is not related to the linear scheme but is induced by a too rapid transport by the meridional circulation. In the UTLS (Upper Troposphere Lower Stratosphere), LINCO tends to slightly overestimate the MOZAIC aircraft observations, with general small biases less than 2%. LINCO is a simple parameterization compared to a detailed chemical scheme, allowing very fast calculations and thus making possible long reanalyses of MOPITT CO data. The computational cost just corresponds to the transport of an additional passive tracer. For this, we used a variational 3-D-FGAT (First Guess at Appropriate Time) method in conjunction with MOCAGE for a long run of one and a half years. The data assimilation greatly improves the vertical CO distribution in the troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At 146 hPa, the assimilated CO 2-D distribution is improved compared to MLS observations by reducing the bias up to a factor of 2 in the tropics. At extratropical latitudes, the assimilated fields tend to underestimate the CO concentrations resulting from an excessive equator to pole circulation. This study confirms that the linear scheme is able to simulate reasonably well the CO distribution in the troposphere and in the lower stratosphere. Therefore, the low computing cost of the linear scheme opens new perspectives to make free runs and CO data assimilation runs at high resolution and over periods of several years.
Microwave Limb Sounder
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