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La météorologie tropicale est un élément majeur pour le fonctionnement de l’atmosphère et pour le climat terrestre. Le satellite Megha-Tropiques regroupe des instruments de télédétection utilisant des algorithmes de restitution complexes. Cette thèse participe à la mise au point de stratégies de validation de ces algorithmes par l’acquisition d’une meilleure connaissance de la phase glacée des systèmes convectifs de méso-échelle (MCS) tropicaux, en s’appuyant sur la campagne de mesures réalisée à Niamey au Niger à l’été 2010. De nombreux MCS à fort contenu en glace (IWC, Ice Water Content) ont été documentés à la fois par une instrumentation aéroportée, et par des radars au sol. Les informations obtenues grâce aux sondes aéroportées, et l’utilisation d’une loi masse-diamètre permettent de calculer une valeur de réflectivité Zin-situ. Le développement d’une méthode de colocalisation des mesures réalisées par les radars sol sur la trajectoire de l’avion a abouti à la validation du calcul de Zin-situ. La relation entre la réflectivité et l’IWC n’a pas été clairement observée pour le radar-précipitation du MIT. De plus, l’IWC est mieux documenté avec un radar-nuage qu’avec un radar-précipitation car ce dernier est trop sensible aux cristaux de grande taille. Les mesures in-situ s’avèrent donc indispensables pour obtenir l’information microphysique utile à la validation des algorithmes de restitution satellites et elles ne peuvent pas être remplacées par des mesures de réflectivités effectuées depuis le sol. L’utilisation du modèle numérique WRF (Weather Research and Forecasting) pourrait également permettre de connaître au mieux les MCS. Pour le cas d’étude analysé dans cette thèse, la modélisation a généré une ligne de grains mais n’a pas reproduit correctement toutes les caractéristiques du MCS réellement observé. Des différences dynamiques et microphysiques sont apparues. L’analyse du champ de réflectivité simulé grâce aux CFAD (Contoured Frequency by Altitude Diagrams) a montré une sous-estimation de la réflectivité par rapport aux observations. L’utilisation du schéma microphysique de Morrison, plus complexe que celui de Thompson initialement employé, n’a pas permis d’améliorer les résultats. Les performances du modèle WRF ne sont pas encore suffisantes pour aider à la validation des algorithmes de restitution satellites. / Tropical meteorology is a major issue for atmospheric physics and earth’s climate. The Megha-Tropiques satellite combines several teledetection instruments which need complex restitution algorithms. This work contributes to the development of validation’s strategies for these algorithms. This requires a better knowledge of the tropical mesoscale convective systems’ (MCS) ice phase. In this thesis, we use data from the Niamey’s (Niger) campaign, which took place during summer 2010. Numerous MCS with high Ice Water Content (IWC) were analyzed with an airborne instrumentation and ground radars. Reflectivity Zin-situ is calculated using airborne microphysic probes’ information and a mass-diameter relationship. A spatial and temporal interpolation technique is developed to colocalize the aircraft position with ground radar measurements. This method leads to the validation of Zin-situ calculation. The relationship between reflectivity and IWC is not satisfactory for the MIT precipitation radar. Moreover, the cloud radar gives better informations about the IWC than the precipitation radar. Indeed, precipitation radars are too sensitive to large ice crystals. Therefore, in-situ measurements are essential to get microphysic information in order to validate restitution algorithms used by satellites. They can’t be replaced by ground based reflectivity measurements. The WRF (Weather Research and Forecasting) model was used in order to get a better knowledge of MCS. In this work, we analyzed one case study. For this case, WRF generates a typical squall line but it doesn’t correctly reproduce every observed characteristics. Several dynamical and microphysical differences appear between simulation and observations. The simulated reflectivity field is analyzed by CFAD (Contoured Frequency by Altitude Diagrams) and it shows a general underestimated reflectivity compared to the observations. The Thompson microphysic scheme is replaced by the more complex Morrison scheme, but this modification doesn’t improve the results of the simulation. Consequently, the WRF model isn’t yet efficient enough to help with the restitution algorithms’ validation.

Honeywell Technology Solutions Lab, India / Weather is a major contributing factor in aviation accidents, incidents and delays. Doppler weather radar has emerged as a potent tool to observe weather. Aircraft carry onboard radars but their range and angular resolution are limited. Networks of ground-based weather radars provide extensive coverage of weather over large geographic regions. It would be helpful if these data can be transmitted to the pilot. However, these data are highly voluminous and the bandwidth of the ground-air communication links is limited and expensive. Hence, these data have to be compressed to an extent where they are suitable for transmission over low-bandwidth links. Several methods have been developed to compress pictorial data. General-purpose schemes do not take into account the nature of data and hence do not yield high compression ratios. A scheme for extreme compression of weather radar data is developed in this thesis that does not significantly degrade the meteorological information contained in these data. The method is based on contour encoding. It approximates a contour by a set of systematically chosen ‘control points’ that preserve its fine structure up to a certain level. The contours may be obtained using a thresholding process based on NWS or custom reflectivity levels. This process may result in region and hole contours, enclosing `high' or `low' areas, which may be nested. A tag bit is used to label region and hole contours. The control point extraction method first obtains a smoothed reference contour by averaging the original contour. Then the points on the original contour with maximum deviation from the smoothed contour between the crossings of these contours are identified and are designated as control points. Additional control points are added midway between the control point and the crossing points on either side of it, if the length of the segment between the crossing points exceeds a certain length. The control points, referenced with respect to the top-left corner of each contour for compact quantification, are transmitted to the receiving end. The contour is retrieved from the control points at the receiving end using spline interpolation. The region and hole contours are identified using the tag bit. The pixels between the region and hole contours at a given threshold level are filled using the color corresponding to it. This method is repeated till all the contours for a given threshold level are exhausted, and the process is carried out for all other thresholds, thereby resulting in a composite picture of the reconstructed field. Extensive studies have been conducted by using metrics such as compression ratio, fidelity of reconstruction and visual perception. In particular the effect of the smoothing factor, the choice of the degree of spline interpolation and the choice of thresholds are studied. It has been shown that a smoothing percentage of about 10% is optimal for most data. A degree 2 of spline interpolation is found to be best suited for smooth contour reconstruction. Augmenting NWS thresholds has resulted in improved visual perception, but at the expense of a decrease in the compression ratio. Two enhancements to the basic method that include adjustments to the control points to achieve better reconstruction and bit manipulations on the control points to obtain higher compression are proposed. The spline interpolation inherently tends to move the reconstructed contour away from the control points. This has been somewhat compensated by stretching the control points away from the smoothed reference contour. The amount and direction of stretch are optimized with respect to actual data fields to yield better reconstruction. In the bit manipulation study, the effects of discarding the least significant bits of the control point addresses are analyzed in detail. Simple bit truncation introduces a bias in the contour description and reconstruction, which is removed to a great extent by employing a bias compensation mechanism. The results obtained are compared with other methods devised for encoding weather radar contours.

Phased array weather radar antennas with beam steering capabilities are suitable alternatives to weather radars with mechanically scanning reflector antennas. Dual-polarized phased-array weather radar antennas, however, demand careful assessment of the x-polar characteristics. The low x-pol radiation of polarimetric weather radar antennas is of significant importance for the proper classification and qualitative estimation of hydrometeors in illuminate volumes. Unfortunately, array antennas display changing x-pol contributions during the electronical beam steering process. Typically, the x-pol radiation will be substantially increased in the co-polar main beam direction but also in other angular directions. Consequently, it is a vital challenge to design arrays with low x-pol contribution during beam steering. In this dissertation a new phased-array weather radar concept is developed. The phased array system configuration can be used to substitute state-of-the-art weather radars with reflector antennas. Furthermore, a dense network of these phased-array radars can be used to substitute a network of high power weather radars, which are used nowadays. The research focus of this work is the development of a dual-polarized microstrip patch antenna with phased-array capability and very high polarization purity. In this regard, new graphical techniques are developed to investigate the causes and the reduction of the x-pol radiation of isolated (stand-alone) microstrip patch antennas. To further reduce the x-pol contribution of antennas, optimization methods have been investigated, evaluated and developed. For the first time in literature, differential-feed antenna arrays are compared to excitation optimized single-feed antenna arrays in their x-pol contribution in the boresight direction and during beam steering. In particular, two dual-polarized 4x8 antenna arrays have been developed and simulated by CST MWS, produced as multilayer PCB and verified at the compact antenna test range at RWTH Aachen. The results show that the x-pol contributions of arrays are significantly reduced for differentially-feed antenna arrays, even when beam steering is performed. During the azimuth scan of 120_ a record setting x-pol suppression of -45 dB and -36 dB could be measured for the horizontal and vertical polarization channels, respectively. / Wetterradarsysteme mit phasengesteuerten Antennen stellen eine echte Alternative zu Wetterradarsystemen mit mechanisch drehenden Reflektorantennen dar. Dual-polarisierte phasengesteuerte Antennen müssen jedoch sehr genau in ihrem Kreuzpolarisationsverhalten verifiziert werden, um für den Wetterradarbereich von Nutzen zu sein. Die Unterdrückung der kreuzpolaren Anteile von Radarantennen ist von fundamentaler Bedeutung, um Hydrometeore mit Hilfe von polarimetrischen Wetterradarsystemen klassifizieren und qualitativ bestimmen zu können. Die hohe Anforderung an Polarisationsreinheit ist mit aktuell erhältlichen Arraydesigns nur schwierig zu realisieren, da sich die Kreuzpolarisationsunterdrückung während des elektronischen Schwenks der Hauptkeule signifikant verschlechtert. Diese Dissertation stellt ein Wetterradar Systemkonzept mit phasengesteuerter Gruppenantenne vor, welches die aktuell genutzten Wetterradare mit Reflektorantennen ablösen könnte. Der Fokus der Arbeit wurde auf die Entwicklung einer Dual-polarimetrischen, polarisationsreinen und phasengesteuerten Mikrostreifenleiterantennen gelegt. Hierbei wurden neue grafische Verfahren entwickelt, die es ermöglichen, die Generierung der kreuzpolaren Anteile von isolierten Patchantennen (Einzelpatche) zu erklären und zu minimieren. Um die kreuzpolaren Anteile weiter herabzusetzen wurden Optimierungsverfahren für Arrayantennen erforscht, bewertet und neu entwickelt. Zum ersten Mal wurden differentiell gespeiste mit einzeln gespeisten Antennenarrays in ihrem Kreuzpolarisationsverhalten während des elektronischen Schwenks der Hauptkeule verglichen. Zwei Dual- polarimetrische 4x8 Antennenarrays (differentiell gespeist und mit optimierter Phasenansteuerung) wurden zu diesem Zweck mittels CST MWS entworfen, simuliert, als Multilagenplatine gefertigt und an der Antennentestanlage der RWTH Aachen vermessen. Die Resultate zeigen, dass die Kreuzpolarisationsanteile bei differentiell gespeisten Mikrostreifenleiterantennen in Gruppenkonfiguration, selbst beim elektronischen Schwenk der Hauptkeule, signifikant minimiert werden konnten. Für einen azimutalen Scanbereich von 120_ konnte eine exzellente Kreuzpolarisationsunterdrückung zwischen -45 dB und -36 dB messtechnisch für den horizontalen und vertikalen Polarisationskanal nachgewiesen werden.