The single-scattering properties of six non-spherical ice crystals, droxtals, plates, solid columns, hollow columns, aggregates and 6-branch bullet rosettes are simulated. The anomalous diffraction theory (ADT) is applied to the simulation of the extinction efficiency and the absorption efficiency. Because the first order reflection is considered, the accuracy of the absorption efficiency increases with the increasing of the size parameter. Compared with the reference single-scattering properties from an improved geometric optics method (IGOM), the errors in the extinction and absorption efficiencies are small. In addition, the asymmetry factor is formulated within the framework of diffraction and external reflection. The asymmetry factor based on the ADT matches very well with the IGOM counterpart when the absorption is strong, but needs an improvement in the solar region. The errors in conjunction with the application of the ADT-based optical properties to the computation of atmospheric fluxes and heating rates, based on the Fu-Liou model also are investigated. Two cases, one for tropical cirrus clouds and the other for mid-latitude cirrus clouds, are designed. It is found that the errors of bulk asymmetry factor between ADT-based and IGOM-based result in an overestimation of downward infrared (IR) fluxes and upward solar fluxes, and an underestimation of upward IR fluxes and downward solar fluxes. The errors of the fluxes and heating rates based on two sets of single-scattering properties are caused mainly by the underestimation of the bulk absorption efficiency based on ADT. It is also shown that ADT-based optical properties generate more accurate radiative properties for tropical cirrus clouds than for the mid-latitude cirrus clouds. In conclusion, the ADT-based method can generate reasonably accurate single-scattering properties of ice crystals, and can result in reasonable upward IR and solar fluxes at top of atmosphere (TOA), downward IR fluxes at the surface, and net heating rates.

Thin cirrus clouds play an important radiative role in the earth's atmosphere and climate system, yet are one of the least understood components of the climate system. With the use of data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), thin cirrus and thick cloud distributions in the tropics are analyzed at 121, 100, and 82 hPa. Observations obtained between December 2006 and November 2007 show that thin cirrus between 30°N and 30°S occur in close proximity to regions of intense convection and are positively correlated with low values of outgoing longwave radiation (OLR). In conjunction with the CALIPSO data, water vapor data from the Earth Observing System (EOS) Microwave Limb Sounder (MLS), OLR data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/, and linearly interpolated NCEP reanalysis temperature data were used. These data were used to examine how thick and thin cirrus cloud fractions at 121-hPa and 100-hPa are related to relative humidity with respect to ice (RHI), temperature, and OLR. Our observations show that both RHI and convection play important roles in the development and maintenance of thick and thin cirrus clouds at the pressure levels of interest. The highest fractions of clouds are almost always seen within OLR values representative of convection and at relatively high values of RHI. However, when peaks in cloud fraction are found above the convective threshold, higher RHI values are needed than are needed when convection is responsible for the formation and maintenance of these clouds.

Zugl.: München, Universiẗat, Diss., 2006. / ISRN DLR-FB-2006-10.

Zugl.: Berlin, Freie University, Diss., 2006.

La présente étude porte sur la simulation numérique de la croissance des cristaux de glace dans le sillage proche d'une aile rectangulaire munie de deux injecteurs qui modélisent les deux moteurs. Dans cette configuration, les phénomènes microphysiques interviennent lors de l'interaction du jet, issu du moteur, et du tourbillon marginal qui se développe à chaque bout d'aile. Cet écoulement, très turbulent, perturbe fortement l'air environnant. Les jets diffusent dans l'atmosphère et s'enroulent autour des deux tourbillons de bout d'aile. Ces jets contiennent de la vapeur d'eau, des suies, des gaz mais également des aérosols et particules chargées. Le modèle microphysique utilisé dans cette étude repose sur l'hypothèse que la vapeur d'eau condense uniquement sur les particules de suie. Les simulations numériques sont effectuées à l’aide du code CEDRE développé à l’ONERA. Les méthodes numériques sont basées sur une approche volume finie pour des maillages non structurés généralisés. La résolution des équations de Navier stokes compressibles pour des fluides multi-espèces se fait selon une approche de type RANS et seul le champ stationnaire, jusqu'à huit envergures en aval de la maquette, est calculé. La turbulence de l'écoulement est modélisée au moyen du système de fermeture à deux équations k-l . Cette approche permet d'obtenir une description spatiale plus réaliste de l'interaction entre le jet et le tourbillon marginal. Le champ aérodynamique du sillage est ainsi comparé aux données expérimentales existantes. Le jet est correctement enroulé autour du tourbillon à huit envergures, et la dilution du panache est bien décrite par les simulations. Le modèle microphysique est ensuite couplé au modèle aérodynamique. Une première simulation porte sur les phénomènes microphysiques intervenant dans le sillage de la maquette dans des conditions particulières, représentatives d'un avion commercial en vol de croisière dans une atmosphère saturée par rapport à la glace. L'influence de la taille initiale des particules de suies émises par les moteurs ainsi que l'humidité relative de l'atmosphère, sur les propriétés de la traînée de condensation, sont ensuite étudiées et discutées. Ce travail, de part la stratégie de calcul mise en place et notamment l'utilisation de maillages non structurés généralisés, permettra d'appréhender le rôle de certains paramètres clés liés à l'avion comme la géométrie des ailes ou bien encore la position des nacelles sur les propriétés microphysiques de la traînée de condensation. / Numerical simulations of ice particles growth, in the near wake of a rectangular wing with two injectors, are presented in this study. In this configuration, microphysical processes occur during the interaction between the engines jets and the marginal vortices developping at each wing tip. This strong turbulent flow disturbs highly the environmental flow. The jets diffuse in the atmosphere and are wrapped around the two wing tip vortices. They contain water vapour, soots, gas, aerosols and charged particles as well. One of the hypothesis of the microphysical modeling, used in this study, is that water vapour condenses on soot particles only. Numerical simulations are performed with the code CEDRE developed at ONERA. The numerical methods are based on a cell-centered finite volume approach for general unstructured grids. A Navier-Stokes solver for turbulent, compressible and multi-species flows with a RANS approach, based on the k-l turbulence model, is used. Only stationary states of the flow, until eight spans downstream the setup, are computed. This approach enables to get a better spatial description of the interaction between the jet and the marginal vortex. The numerical flow field is then compared to existing experimental data. The jet is correctly wrapped around the wing tip vortex at eight spans and the dilution of the effluents is well described by our simulations. The microphysical model is then coupled to the aerodynamics. The microphysical processes occurring in the wake of the setup in specific conditions, representative of a cruising civil aircraft in an ice-saturated atmosphere, are firstly simulated. The role, on the contrail’s properties, of soot particles initial size and of the atmospheric humidity is studied and discussed. This work, through its computational strategy, with the use of unstructured grids, will enable to understand the potential role of some key parameters such as the wings geometry or the engines position on the contrail properties.

Cette thèse s’intéresse aux ondes de gravité et à la turbulence dans la région de la tropopause tropicale (TTL pour tropical tropopause layer, entre 14 et 18 km d’altitude), et à leurs impacts sur les cirrus.Dans un premier temps, les fluctuations de température et de vent vertical induites dans la TTL par les ondes de gravité sont quantifiées et caractérisées à partir de mesures provenant de vols de ballons stratosphériques longue durée. Les perturbations observées sont comparées aux champs de fluctuations résolues par différents modèles atmosphériques globaux. À la lumière des observations, différentes méthodes de paramétrisation des fluctuations de température sont discutées.Dans un second temps, l’influence des ondes équatoriales et de gravité sur la microphysique des cirrus est étudiée. On considère d’abord l’impact des ondes de gravité de haute fréquence sur la nucléation des cristaux de glace. La question du rôle des anomalies de vent vertical induites par les ondes de basse fréquence sur le transport de la glace est ensuite abordée et son impact quantifié à l’aide d’observations in situ. Enfin, on étudie la formation et l’évolution d’un cirrus de grande échelle à l’aide de simulations numériques. Parmi les différents processus en jeu (radiatifs,...), on montre l’importance d’une onde équatoriale de grande échelle dans la structuration et l’évolution du champ nuageux.Dans une dernière partie, les fluctuations de vents de petite échelle dans la TTL, interprétées comme de la turbulence, sont étudiées à partirdes observations avion de la campagne ATTREX au-dessus de l’océan Pacifique. Leur impact sur le transport vertical de différents traceurs est quantifié. Il est inférieur à l’impact de l’upwelling équatorial de grande échelle mais néanmoins significatif. / Atmospheric waves and turbulence and their impacts on cirrus clouds in the Tropical Tropopause Layer (TTL, 14-18 km altitude) are studied using in situ observations, numerical simulations and theoretical approaches.First, long-duration stratospheric balloon measurements are used to analyze Lagrangian temperature and vertical wind fluctuations induced by gravity waves at the tropical tropopause. The amplitude and intermittency of wave fluctuations are assessed, and the observations are compared with resolved wave fluctuations in atmospheric models. Methods to parameterize Lagrangian temperature fluctuations are then discussed.Then, some impacts of waves on cirrus clouds microphysics are examined. We first consider the influence of high frequency gravity waves on the ice nucleation process. Next, we explore the interplay between ice crystal sedimentation and advection by the wind perturbations induced by low frequency waves. At last, we use numerical simulations to investigate the formation of a large-scale cirrus in the TTL. We demonstrate the role of large-scale equatorial waves and quantify the relevance of different processes (dynamics, radiative heating,...) in the cloud evolution.Finally, small-scale wind fluctuations, interpreted as turbulent bursts, are characterized using aircraft measurements from the ATTREX campaign in the tropical Pacific. The impact of the fluctuations on vertical mixing and on the TTL tracer budget is quantified. The vertical transport induced by turbulent mixing is found to be smaller than that induced by mean tropical upwelling, but nonetheless significant.

Les cirrus constituent une des principales sources d'incertitude dans les projections relatives au climat puisqu'ils peuvent contribuer au réchauffement comme au refroidissement de la planète. Cette thèse a consisté en l'exploitation de la base de données lidar de l'OPAR. La combinaison de méthodes de restitution ont permis d'améliorer la détermination de l'épaisseur optique. L'outil permet d'obtenir le rapport lidar effectif, qui dépend des propriétés microphysiques. L'exploitation de la base de données a permis de mettre en évidence le cycle saisonnier d'occurrence des cirrus. Une étude de cas portant sur l'étude de la variabilité temporelle d'un cirrus observé a été réalisée. Le couplage de l'exploitation des restitutions et de la modélisation à méso-échelle à partir du modèle Méso-NH a permis de montrer que les processus radiatifs et microphysiques interviennent et interagissent dans l'évolution du cirrus et dans sa durée de vie.

An investigation is presented into methods of characterising cirrus ice crystals from in-situ light scattering data. A database of scattering patterns from modelled crystals was created using the Ray Tracing with Diffraction on Facets (RTDF) model from the University of Hertfordshire, to which experimental and modelled data was fitted. Experimental data was gathered in the form of scattering patterns from ice analogue crystals with similar optical properties and hexagonal symmetry to ice, yet stable at room temperature. A laboratory rig is described which images scattering patterns from single particles while allowing precise control over the orientation of the particle with respect to the incident beam. Images of scattering patterns were captured and compared to patterns from modelled crystals with similar geometry. Methods for introducing particles en-masse and individually to the Small Ice Detector (SID) instruments are discussed, with particular emphasis on the calibration of the gain of the SID-2 instrument. The variation in gain between detector elements is found to be significant, variable over the life of the detector, and different for different detectors. Fitting was performed by comparison of test scattering patterns (either modelled or experimental) to the reference database. Representation of the two dimensional scattering patterns by asymmetry factor, moment invariants, azimuthal intensity patterns (AIP) and the Fourier transform of the AIP are compared for fitting accuracy. Direct comparison of the AIP is found to be the most accurate method. Increased resolution of the AIP is shown to improve the fitting substantially. Case studies are presented for the fitting of two ice analogue crystals to the modelled database. Fitting accuracy is found to be negatively influenced by small amounts of surface roughness and detail not currently considered by the RTDF model. Fitting of in-situ data gathered by the SID-3 instrument during the HALO 02 campaign at the AIDA cloud chamber in Germany is presented and discussed. Saturation of detector pixels is shown to affect pattern fitting. In-flight operation of the instrument involves the variation of gain of the whole detector (as opposed to individual elements) in order to obtain unsaturated images of both large and small particles.

Identifying the dynamical processes that are active in tropical cirrus clouds is important for understanding the role of cirrus in the tropical atmosphere. This study focuses on analyzing turbulent motions inside tropical anvil cirrus with the use of a Cloud Resolving Model. Convection in the transition from shallow to deep convection has been simulated with Colorado State University Large Eddy Simulator/Cloud Resolving Model System for Atmospheric Model (SAM 6.3) in a high resolution three-dimensional simulation and anvil cirrus formed in the end of this simulation has been analyzed. For model set up, data gathered during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM LBA) field experiment in Amazonas, Brazil have been used as large scale forcing. 31 anvil clouds have been localized from a single time step of the simulation, “a snapshot”, of the entire simulated cloud field consisting of convective clouds of different scales and subsequently divided into three categories that represent different stages of the anvil lifetime; growing, mature and dissipating anvil stages. The classification is based on in-cloud properties such as cloud condensate content and vertical velocities. The simulated anvils have been analyzed both individually and as groups to examine the transition from isotropic three-dimensional turbulence in the convective core of the thunderstorm to stratified two-dimensional turbulence in the anvil outflow. A dimensionless number F is derived and used as a measure of the “isotropic” behavior of the turbulence inside the cloud. F is expressed as the ratio between the horizontal part of TKE and the total (horizontal + vertical) Experiments show that SAM 6.3 clearly can resolve turbulent structures and that the transition from isotropic three-dimensional turbulence to stratified two-dimensional turbulence occurs in the middle layers of the mature and dissipating anvil stages. / Sammanfattning av ”Studier av turbulenta rörelser i städmoln med hjälp av numeriska simuleringar av tropisk konvektion”  Städmoln i tropikerna har stor inverkan på strålningsballansen på grund av de är så vanligt förekommande och att de ligger på hög höjd i atmosfären. Att förstå de drivande krafterna som är aktiva i skapandet och underhållandet av städmoln är viktiga för att få en bra bild av rollen städmoln spelar i den tropiska atmosfären. Den här uppsatsen fokuserar på att studera turbulenta rörelser inuti tropiska städmoln med hjälp av en molnmodell. Tropisk konvektion har simulerats med Colorado State University’s molnmodell SAM 6.3 i en högupplöst tredimensionell simulering. Data från en ”ögonblicksbild” av det simulerade molnfältet har analyserats och 31 städmoln har valts ut och studerats vidare. De simulerade städmolnen indelades i tre olika kategorier baserat på utvecklingsstadier; växande städmoln, moget städmoln och skingrade städmoln. Stadieklassificeringen bestämdes beroende på isvatteninnehåll och vertikalhastigheter i molnet. Städmolnen har därefter analyserats både individuellt och som grupper för att lokalisera och analysera övergången från tredimensionell isotropisk turbulens i kärnan av Cb-molnet till tvådimensionell stratifierad turbulens i städmolnet. För att initiera simuleringen användes mätdata insamlade under fältexperimentet TRMM LBA (Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere) i Amazonas, Brasilien. För att beskriva turbulenta rörelser i molnen togs det dimensionslösa talet 𝐹 fram som ett mått på isotropin. 𝐹 uttrycks som kvoten mellan den horisontella delen av TKE och den totala (horisontell och vertikal). Den här studien visar att den undersökta molnmodellen SAM 6.3 klart kan simulera turbulenta i rörelser i övergången mellan isotropisk till horisontell turbulens i olika stadier av städmolnens livscykel. Mina analyser visar att övergången sker främst i de mellersta skikten av de mogna och skingrade stadierna av städmolnets utveckling.

In this study, Computational Fluid Dynamics (CFD) simulations have been carried out in order to investigate and improve the performance of the Standard Cirrus glider, using different Navier-Stokes methods and solving the equations for the steady flow. The work has been divided in two parts: First, a study is performed to test the quality of the transition model (Gamma-ReTheta). The two dimensional results of the glider´s airfoil are compared against the results from panel’s methods and the open-source CFD codes: SU2 and OpenFoam. In addition, three dimensional glider´s models are simulated using the transition model with the purpose of creating a validated reference model of the glider’s performance in steady level flight. The simulations are carried out in two dimensions for the outer wing airfoil for a 1.5 e+06 Reynolds number and in three dimensions for the Wing & Fuselage model and Tail & Fuselage model under a range of velocities. Both simulations are validated against experimental data. In the second part of the study, the validated model is used to developed possible improvements in the glider´s external geometry that could produce possible benefits in the performance and handling qualities of the glider.