Stratospheric Water Vapour in the Tropics: Observations by Ground-Based Microwave Radiometry

This thesis reports on observations of tropical stratospheric water vapour by the ground-based microwave radiometer/spectrometer WaRAM2 in 2007. The 22GHz receiver is set up at Mérida Atmospheric Research Station on top of Pico Espejo, Venezuela (8°32'N, 71°03'W, 4765m above sea level). It is the only such sensor that continuously operates at tropical latitudes. The high altitude site is ideally suitable for microwave observations, because most tropospheric water vapour is located below the sensor. Water vapour plays a key role in middle atmospheric processes. Because of its large infrared resonance, it strongly participates in the radiative budget, both in terms of a greenhouse effect at lower altitudes and radiative cooling at higher altitudes. It is a source gas for the highly reactive hydroxyl radical, and exerts indirect effects on ozone destruction in the formation of polar stratospheric clouds. Due to its long lifetime, water vapour also serves as a dynamical tracer

Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations

AbstractGlobal positioning system (GPS) radio occultation (RO) observations, first made of Earth's atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of RO observations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer

A new dynamic approach for statistical optimisation of GNSS radio occultation bending angles

Climate change has become a serious issue for our society. It is of great importance to accurately monitor climate change and provide reliable information to the society so that proper actions can be taken to alleviate the significant change of climate. Global Navigation Satellite Systems (GNSS) based radio occultation (RO) is a new satellite remote sensing technique that can provide high vertical resolution, long-term stable and global coverage atmospheric profiles of the Earth’s atmosphere. However, the quality of the retrieved atmospheric profiles decreases above about 30 km due to a low signal-to-noise ratio of GNSS signals at these high altitudes, since errors in bending angle profiles are propagated to refractivity profiles through an Abel integral and subsequently propagated to other atmospheric profiles through the hydrostatic integral. It is therefore important to carefully initialise the bending angles at high altitudes to minimise these error propagation effects and thereby optimise the climate monitoring utility of the retrieved profiles. Statistical optimisation is a commonly used method for this purpose. This method combines the observed bending angle profile and background bending angle profile based on their error covariance matrices to determine “optimised” bending angle profile. The focus of this thesis is to investigate an advanced statistical optimisation algorithm, which dynamically estimates both background and observation error covariance matrices, for the best determination of RO optimised bending angle profile. In this new algorithm, background bending angle profiles and their associated error covariance matrices are estimated using bending angles from multiple days of the European Centre for Medium-range Weather Forecasts (ECMWF) short-term (24h) forecast and analysis fields as well as the averaged observed bending angle. The background error matrices are constructed with geographically varying background error estimates on a daily-updated basis. The observation error covariance matrices are estimated using multiple days of RO data with geographically varying observation errors for an occultation event. The most distinctive advantage of the new algorithm is that both background and observation error covariance matrices are realistically estimated using large ensemble of climatological and observed data, while existing algorithms use crude formulations to estimate both error matrices. The new algorithm developed is evaluated against the algorithm used by the Wegener Center Occultation Processing System version 5.4 (OPSv5.4) by calculating statistical errors of retrieved atmospheric profiles relative to their reference profiles. Since the background errors at different heights are highly correlated and their covariance matrix is critical for the resulting optimised bending angles, the dynamically estimated background error covariance matrix is first used in statistical optimisation to retrieve atmospheric profiles from simulated MetOp as well as observed CHAMP and COSMIC RO events on single days. The dynamically estimated observation error covariance matrix is then used in the statistical optimisation together with the estimated background error covariance matrix to retrieve atmospheric profiles using the same test data. It can be concluded from the evaluation that if the estimated background error covariance matrix is solely used for the statistical optimisation, it can significantly reduce random errors and generate less or similar residual systematic errors (biases) in the optimised bending angles. The subsequent refractivity profiles and atmospheric (dry temperature) profiles retrieved are benefitted from the improved error characteristics of bending angles. If both observation and background error covariance matrices estimated from the new approach are used, the standard deviations of the optimised bending angles are only further reduced for simulated MetOp data, while for the observed CHAMP and COSMIC data, large random errors of bending angles are found at higher altitudes (e.g. > 50 km). This is likely to be that the observation errors are underestimated at high altitudes, where bending angles are largely affected by ionospheric effects and observation errors, and more weights are given to the noisy observed bending angles in the estimation of the optimised bending angles. Errors in CHAMP and COSMIC observed bending angles are further transferred downwards to their subsequently retrieved refractivity and dry temperature profiles, the quality of which is also degraded. The effects of the estimated background and observation error correlations on the atmospheric retrievals are investigated using simulated MetOp data. It is found that these realistically estimated correlations alone can reduce the random errors of the optimised bending angles significantly and improve the quality of the subsequent refractivities and temperatures. The performance of the new approach that uses only the new background matrix in the statistical optimisation on monthly occultation data is evaluated. The results show that the monthly errors are similar to those from single days, but in a smoother manner

Comparison of water vapor and temperature results from GPS radio occultation aboard CHAMP with MOZAIC aircraft measurements

Global Positioning System (GPS) radio occultation (RO) observations aboard Low Earth Orbiting (LEO) satellites provide a powerful tool for global atmospheric sounding. Almost continuously activated since mid-2001, the CHAllenging Minisatellite Payload (CHAMP) GPS RO experiment provides up to 200 vertical atmospheric profiles per day. In this paper, we intercompare CHAMP RO humidity results and analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) with coinciding Measurement of OZone and water vapor by AIrbus in-service airCraft (MOZAIC) data collected during aircraft ascents and descents. About 320 coinciding profiles with CHAMP were found from 2001 to 2006 (coincidence radius: 3 h, 300 km). Between about 650 and 300 hPa, the CHAMP-MOZAIC humidity bias is smaller than the ECMWF-MOZAIC bias. On the other hand, the standard deviation between MOZAIC and CHAMP humidity is slightly higher than that between MOZAIC and ECMWF through the entire altitude range. Apart from the water vapor validation (ascent and descent data), we also compare MOZAIC cruise data at an altitude of typically 10-11 km with CHAMP refractivity and temperature results (dry retrieval), and corresponding ECMWF analysis data. Whereas refractivity data from MOZAIC, CHAMP, and ECMWF show excellent agreement, the CHAMP temperature exhibits a cold bias of about 0.9 K in comparison to MOZAIC and ECMWE

Étude de la variabilité et la tendance de l'ozone stratosphérique au-dessus des tropiques et subtropiques sud

Ozone plays an important role on photochemical equilibrium of atmosphere and participate on radiative balance process between hemispheres (Mecke, 1931). In the troposphere, ozone determines the oxidizing capacity of major species and absorbs continuously in the stratosphere the harmful ultraviolet radiation (McMichael et al, 2003). Based on the above facts, it is important to monitor ozone continuously with consistency and accuracy. Global total column ozone (TCO) has depleted gradually since 1980 with an increase of chlorofluorocarbon concentrations in the stratosphere due to anthropogenic activities. In 1987, the Montreal protocol was formulated in order to regulate the emissions of substances that deplete ozone. Concentrations of these substances are observed to decrease ten years after the Montreal protocol. Thus we have been expecting an increase in ozone by now (UNEP/PNUE, 2009; WMO, 2010 and 2014). The current needs are to achieve consistent and reliable measurements in which their exploitation on adapted methods/models can help scientists to follow the ozone evolution and to estimate long term ozone trend. In this work, a variety of ozone products from different instruments was combined in order to create reliable and homogenous dataset to study the ozone variability and trend over the southern tropics and subtropics. The dataset application on wavelets method allowed to identify the dynamic parameters that control ozone variability and their periodicities. These include seasonal variations of climate, the quasi-biennial oscillations, the El-Niño Southern Oscillation and the 11-years solar cycle. The behavior of each parameter and its influence on ozone variability were analysed based on statistical method and the Trend-Run model. The contribution and response of each variable on ozone variability were quantified from the model. The obtained trends results exhibit an increase of total ozone from 1998 to 2012 with a rate varying between 0 and 2.78% par decade (depending of the site and region). The ozone increase was observed mainly above 22 km and it is more important over the subtropical region with respect to equatorial zone.L'ozone joue un rôle primordial sur l'équilibre photochimique de l'atmosphère et participe au processus d'équilibrage radiatif entre les deux hémisphères (Mecke, 1931). Dans la troposphère, l'ozone détermine la capacité oxydante de la majorité des gaz et absorbe continuellement dans la stratosphère les radiations ultraviolettes nocives (McMicheal et al., 2003). D'où l'intérêt de surveiller la variation de la couche d'ozone de façon régulière. Il a été constaté au début des années 80, une diminution inquiétante et progressive de la colonne totale de l'ozone dûe aux émissions anthropiques des substances riches en chlore, brome et fluor. Ce constat a conduit au Protocole de Montréal en 1987 dont l'objectif est de mettre en place une politique internationale visant à réduire les émissions des substances appauvrissant l'ozone. Dix ans après la signature du dit Protocole, la concentration de ces substances commence à diminuer dans l'atmosphère et la prospection d'un recouvrement progressif de la couche d'ozone demeure aujourd'hui un sujet d'actualité (UNEP/PNUE, 2009 ; OMM, 2010 et 2014). Les besoins d'aujourd'hui sont de réaliser des mesures continues et fiables de l'ozone dont leurs exploitation dans des méthodes et/ ou des modèles bien adaptés à la problématique aideront la communauté à suivre l'évolution de l'ozone et d'estimer les tendances à long terme. Dans ce travail, une variété de produits d'ozone issue de différents instruments a été combinée pour construire des bases des données fiables et homogènes afin d'étudier sa variabilité et d'estimer la tendance de l'ozone dans les régions tropicale et subtropicale sud. L'application de ces bases de données sur les ondelettes a permis d'identifier les principaux forçages qui contrôlent la variabilité de l'ozone et la période de retour associée à chaque forçage. Il s'agit des variations saisonnières du climat, les oscillations quasi-biennales, les oscillations australes El-Niño et l'activité solaire dont le cycle moyen est évalué à 11ans. Le comportement et l'influence de chacun de ces paramètres sur la viabilité de l'ozone sont étudiés. Cette étude est faite en s'appuyant sur des méthodes statistiques et sur le modèle Trend-Run. Avec ce modèle, la part de contribution et la réponse de chaque paramètre sur la variabilité de l'ozone sont quantifiées. Les résultats sur les tendances montrent une augmentation de la couche d'ozone avec un taux variant entre 0 et 2.78% par décade (selon la région et le site) sur la période 1998-2012. Cette amélioration est bien observée au-dessus de 22km, surtout aux subtropiques par rapport à la région équatoriale

Climate change 2013: the physical science basis

This report argues that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. This is an an unedited version of the Intergovernmental Panel on Climate Change\u27s Working Group I contribution to the Fifth Assessment Report following the release of its Summary for Policymakers on 27 September 2013.  The full Report is posted in the version distributed to governments on 7 June 2013 and accepted by Working Group I and the Panel on 27 September 2013. It includes the Technical Summary, 14 chapters and an Atlas of Global and Regional Climate Projections. Following copy-editing, layout, final checks for errors and adjustments for changes in the Summary for Policymakers, the full Report will be published online in January 2014 and in book form by Cambridge University Press a few months later