Les nuages sont une incertitude majeure pour l'étude du climat. Parmi eux, les cirrus en sont des modulateurs essentiels, par leur effet de serre élevé et leur effet d'albedo. Cependant, leur haute altitude et leur composition atypique compliquent leur étude. Les nombreuses campagnes de mesure des vingt dernières années ont montré la complexité de leur composition et la forte variabilité de leurs propriétés microphysiques : taille, forme et orientation des cristaux qui les composent. Ce manque de connaissance entraîne de graves lacunes dans leur modélisation, et une incohérence certaine dans les résultats des simulations climatiques à grande échelle. Les avantages des instruments de télédétection pourraient conduire à de nouvelles méthodes d'étude des cristaux dans les cirrus. Cette thèse analyse l'apport potentiel de mesures spatiales pour caractériser les nuages de glace, en prévision de l'Aqua-Train, une formation de satellites pour l'étude de l'atmosphère. La diffusion lumineuse dans les cristaux a été modélisée en prenant en compte leurs multiples propriétés, menant à plusieurs techniques de restitution de la microphysique des cirrus par des mesures de l'état de polarisation de la lumière par télédétection active et passive. Ces techniques ont été appliquées à des données sol pour évaluer leur pertinence, et leur application potentielle à des mesures spatiales envisagée. Combinées dans un cadre théorique homogène, elles ont conduit à la restitution cohérente des propriétés des cristaux dans les cirrus : orientation spatiale et angle maximum de déviation à partir de mesures polarisées (lidar à balayage, radiomètre) ; facteur de forme par observations lidar polarisées ; taille par synergie multi-instruments (lidar, radiomètre IR). La restitution de ces paramètres à l échelle globale conduira à une meilleure compréhension des propriétés radiatives des cirrus, et à une meilleure interprétation de leur importance dans le contexte du bilan radiatif terrestre.

Les nuages jouent un rôle important dans l'évolution de l'atmosphère météorologique et dans le contrôle du climat. Or leur représentation paramétrique dans les modèles numériques de prévision météorologique et climatologique n'est toujours pas validée à ce jour. La préoccupation de la communauté scientifique internationale d'être à même de mieux contrôler à l'échelle globale l'effet radiatif des nuages a stimulé plusieurs programmes spatiaux mettant en œuvre une combinaison radar-lidar (CLOUDSAT/CALIPSO et EarthCare). De telles expériences spatiales ont besoin de démonstrateurs aéroportés tels que RALI (système synergique RAdar nuage - LIdar), qu'il s'agisse de la mise au point des algorithmes de traitement de données, ou de la validation des produits de la mission spatiale. Ainsi, le projet RALI vise le développement et l'exploitation d'une combinaison instrumentale radar-lidar aéroportée ou au sol, pour documenter les profils verticaux des propriétés microphysiques, dynamiques et radiatives des nuages glacés non précipitants. C'est dans ce cadre que s'inscrit cette thèse. Dans un premier temps, un modèle inverse reliant les paramètres de télédétection aux paramètres nuageux a été construit. Ce modèle permet de s'affranchir de la forme de la distribution dimensionnelle des particules; il sert de base à l'application de l'algorithme synergique radar-lidar. Celui-ci, prenant en compte la variation verticale de la concentration des particules nuageuses, a également été développé. Des tests de simulation ont été effectués afin de déterminer sa robustesse, sa précision et les conditions optimales d'application. Les résultas obtenus sur des cas d'expérimentations instrumentales (Clare'98, Carl'99 et Clare'2000), comparés à des mesures in-situ microphysiques, ont montré sa capacité à restituer le rayon effectif des particules nuageuses, le contenu en glace et l'épaisseur optique des nuages sondés. L'algorithme est également capable de restituer le contenu en eau des couches de phase mixtes et sa capacité a été testée sur un cas d'altocumulus de Clare'2000.

This dissertation aims to study the broadband optical properties and radiative effects of dust aerosols and ice clouds. It covers three main topics: the uncertainty of dust optical properties and radiative effects from the dust particle shape and refractive index, the influence of ice particle surface roughening on the global cloud radiative effect, and the simulations of the global contrail radiative forcing. In the first part of this dissertation, the effects of dust non-spherical shape on radiative transfer simulations are investigated. We utilize a spectral database of the single-scattering properties of tri-axial ellipsoidal dust-like aerosols and determined a suitable dust shape model. The radiance and flux differences between the spherical and ellipsoidal models are quantified, and the non-spherical effect on the net flux and heating rate is obtained over the solar spectrum. The results indicate the particle shape effect is related to the dust optical depth and surface albedo. Under certain conditions, the dust particle shape effect contributes to 30% of the net flux at the top of the atmosphere. The second part discusses how the ice surface roughening can exert influence on the global cloud radiative effect. A new broadband parameterization for ice cloud bulk scattering properties is developed using severely roughened ice particles. The effect of ice particle surface roughness is derived through simulations with the Fu-Liou and RRTMG radiative transfer codes and the Community Atmospheric Model. The global averaged net cloud radiative effect due to surface roughness is around 1.46 Wm-2. Non-negligible increase in longwave cloud radiative effect is also found. The third part is about the simulation of global contrail radiative forcing and its sensitivity studies using both offline and online modeling frameworks. Global contrail distributions from the literature and Contrail Cirrus Prediction Tool are used. The 2006 global annual averaged contrail net radiative forcing from the offline model is estimated to be 11.3 mW m^(-2), with the regional contrail radiative forcing being more than ten times stronger. Sensitivity tests show that contrail effective size, contrail layer height, the model cloud overlap assumption, and contrail optical properties are among the most important factors.

This dissertation focuses on the global investigation of optically thin cirrus cloud optical thickness (tau) and microphysical properties, such as, effective particle size (D_(eff)) and ice crystal habits (shapes), based on the global satellite observations and fast radiative transfer models (RTMs). In the first part, we develop two computationally efficient RTMs simulating satellite observations under cloudy-sky conditions in the visible/shortwave infrared (VIS/SWIR) and thermal inferred (IR) spectral regions, respectively. To mitigate the computational burden associated with absorption, thermal emission and multiple scattering, we generate pre-computed lookup tables (LUTs) using two rigorous models, i.e., the line-by-line radiative transfer model (LBLRTM) and the discrete ordinates radiative transfer model (DISORT). The second part introduces two methods (i.e., VIS/SWIR- and IR-based methods) to retrieve tau and D_(eff) from satellite observations in corresponding spectral regions of the two RTMs. We discuss the advantages and weakness of the two methods by estimating the impacts from different error sources on the retrievals through sensitivity studies. Finally, we develop a new method to infer the scattering phase functions of optically thin cirrus clouds in a water vapor absorption channel (1.38-µm). We estimate the ice crystal habits and surface structures by comparing the inferred scattering phase functions and numerically simulated phase functions calculated using idealized habits.

Heidelberg, Univ., Diss., 2007.

The ultimate application of this work is constraining the optical properties of cirrus particles, which are poorly understood, by providing an automatic method, using all-sky cameras and an infrared radiometer, to identify the occurrence of the 22° halo formed by cirrus. This is done by interpreting all sky images in terms of a scattering phase function (SPF), from which the halo ratio (HR) is calculated, and by implementing a cirrus detection algorithm to associate HR measures to ice cloud occurrences. Cirrus reflectivity at solar wavelengths is inversely related to the HR which, being an indirect measure of the regularity of the shape of the ice crystals forming the cloud, relates in turn inversely to the asymmetry parameter g. Therefore, the method proposed here to derive statistics of HRs is expected to reduce the uncertainty over the optical and microphysical properties of cirrus. The light intensity measured by the all sky camera is transformed into a scattering phase function, from which the halo formation is identified. This is done by developing image transformations and corrections needed to interpret all sky images quantitatively in terms of scattering phase function, specifically by transforming the original image from the zenith-centred to the light-source-centred system of coordinates and correcting for the air mass and for vignetting. The SPF is then determined by averaging the image brightness over the azimuth angle and the HR by calculating the ratio of brightness at two scattering angles in the vicinity of the 22° halo peak. The instrument transformation and corrections are performed using a series of Matlab scripts. Given that the HR is an ice cloud characteristic and since the method needs additional temperature information if the halo observation is to be associated with cirrus, a cirrus detection algorithm is necessary to screen out non-ice clouds before deriving reliable HR statistics. Cloud detection is determined by quantifying the temporal fluctuations of sky radiance, expressed as brightness temperature (BT), through De-trended Fluctuation Analysis and setting a clear sky fluctuation threshold. Cloud phase discrimination instead is achieved through first constructing an analytic radiative transfer model to obtain an estimate for average molecular absorption cross-section of water vapour within the spectral window of the radiometer. This is done to model the down-welling clear sky radiance, which is in turn used to correct cirrus emissivity and ultimately determine a dynamic BT threshold for the transition from ice to liquid-containing clouds. In addition to the molecular cross section the screen level air temperature and integrated water vapour are used as input parameters to the model. The utilisation of the all sky camera for such quantitative measurement was the particularly novel aspect of this work; this has not been done previously to the best of my knowledge. The cirrus detection method proposed is also innovative in that with respect to previous works it does not rely on the use of additional techniques such as LIDAR or microwave radiometry for discriminating cloud phase. Furthermore, the cirrus threshold proposed is not fixed but accounts for the attenuating properties of the atmosphere below the cloud. Once the cirrus detection algorithm is validated and cirrus occurrences determinable, the HR could be extended to estimating the asymmetry parameter and crystal roughness. These are retrievable, for instance, from in-situ observations of single ice crystal 2D scattering patterns from cloud probes of the SID (Small Ice Detector) type. This would be significant for the constraining of the optical and microphysical properties of cirrus.

While exact methods, such as DDA or T-matrix, can be applied to particles withsizes comparable to the wavelength, computational demands mean that they are size limited. For particles much larger than the wavelength, the Geometric Optics approximation can be employed, but in doing so wave effects, such as interference and diffraction, are ignored. In between these two size extremes there exists a need for computational techniques which are capable of handling the wide array of ice crystal shapes and sizes that are observed in cirrus clouds. The Beam Tracing model developed within this project meets these criteria. It combines aspects of geometric optics and physical optics. Beam propagation is handled by Snell's law and the law of reflection. A beam is divided into reflected and transmitted components each time a crystal facet is illuminated. If the incident beam illuminates multiple facets it is split, with a new beam being formed for each illuminated facet. The phase-dependent electric field amplitude of the beams is known from their ampli- tude (Jones) matrices. These are modified by transmission and reflection matrices, whose elements are Fresnel amplitude coefficients, each time a beam intersects a crystal facet. Phase tracing is carried out for each beam by considering the path that its 'centre ray' would have taken. The local near-field is then mapped, via a surface integral formulation of a vector Kirchhoff diffraction approximation, to the far-field. Once in the far-field the four elements of the amplitude matrix are trans- formed into the sixteen elements of the scattering matrix via known relations. The model is discussed in depth, with details given on its implementation. The physical basis of the model is given through a discussion of Ray Tracing and how this leads to the notion of Beam Tracing. The beam splitting algorithm is described for convex particles followed by the necessary adaptations for concave and/or ab- sorbing particles. Once geometric aspects have been established details are given as to how physical properties of beams are traced including: amplitude, phase and power. How diffraction is implemented in the model is given along with a review of existing diffraction implementations. Comparisons are given, first against a modified Ray Tracing code to validate the geometric optics aspects of the model. Then, specific examples are given for the cases of transparent, pristine, smooth hexagonal columns of four different sizes and orientations; a highly absorbing, pristine, smooth hexagonal column and a highly absorbing, indented, smooth hexagonal column. Analysis of two-dimensional and one-dimensional intensity distributions and degree of linear polarisation results are given for each case and compared with results acquired through use of the Amster- dam Discrete-Dipole Approximation (ADDA) code; with good agreement observed. To the author's best knowledge, the Beam Tracer developed here is unique in its ability to handle concave particles; particles with complex structures and the man- ner in which beams are divided into sub-beams of quasi-constant intensity when propagating in an absorbing medium. One of the model's potential applications is to create a database of known particle scattering patterns, for use in aiding particle classification from images taken by the Small Ice Detector (SID) in-situ probe. An example of creating such a database for hexagonal columns is given.

An imaging spectrometer (AisaEAGLE) is applied for ground-based measurements of downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7° field of view), spectral (488 spectral pixels, 400-970 nm, 1.25 nm full width at half maximum) and temporal (4-30 Hz) resolution. The calibration, measurement, and data evaluation procedures are introduced. A method is presented that retrieves the cirrus optical thickness tci using the spectral radiance data collected by AisaEAGLE. On the basis of four measurement cases during the second campaign of the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011 the spatial inhomogeneity of the investigated cirrus is characterized by the standard deviation of the retrieved tci, as well as the width of the frequency distribution of the retrieved tci. By comparing measured and simulated downward solar radiances as a function of scattering angle, a first estimation of the detected cirrus ice crystal shape is given and used in the retrieval of tci. / Ein abbildendes Spektrometer (AisaEAGLE) wurde bodengebunden zur Messung von Feldern abwärts gerichteter spektraler Strahldichten mit hoher räumlicher (1024 Raumpixel auf 36.7° FOV), spektraler (488 spektrale Pixel, 400-970 nm, 1.25 nm FWHM) und zeitlicher (4-30 Hz) Auflösung verwendet. Die Kalibrierungsprozedur, das Messverfahren sowie die Datenauswertung werden hier vorgestellt. Weiter wird eine Methode zur Ableitung der Zirrus optischen Dicke tci unter Verwendung dieser bodengebundenen spektralen Strahldichtedaten vorgestellt. Auf der Grundlage von vier Messzeiträumen während der zweiten Kampagne des Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) Projektes in 2011 wird die räumliche Inhomogenität der untersuchten Zirren durch die Standardabweichung der abgeleiteten tci, wie auch der Breite ihrer Häufigkeitsverteilungen charakterisiert. Vergleiche der gemessenen Strahldichten mit Simulationen abwärts gerichteter solarer Strahldichten als Funktion der Streuwinkel ermöglichen eine erste Abschätzung der Eiskristallform im detektierten Zirrus und gehen in die Ableitung der tci ein.

From June to July 2021, the Leipzig Institute for Meteorology (LIM) participated in the Cirrus in High Latitudes (CIRRUS-HL) campaign. Utilizing the German High Altitude Long Range Research Aircraft (HALO), 24 research flights were conducted out of Oberpfaffenhofen, Germany. The initial goal of the campaign was to sample high-latitude cirrus clouds with a combination of in-situ and remote sensing instrumentation. However, due to the global coronavirus pandemic, the flights had to be carried out from southern Germany instead of northern Sweden. Thus, the flight time in Arctic latitudes was limited. Therefore, more objectives concerning midlatitude cirrus were included in the campaign goals. LIM contributed to CIRRUS-HL with measurements by the Broadband AirCrAft RaDiometer Instrumentation (BACARDI) and the Spectral Modular Airborne Radiation measurement sysTem (SMART). While BACARDI measured broadband solar and terrestrial upward and downward irradiance, SMART measured spectrally resolved solar upward radiance as well as upward and downward irradiance. / Von Juni bis Juli 2021 nahmen einige Mitarbeitende des LIM an der CIRRUS-HL Kampagne teil. Mit dem deutschen Forschungsflugzeug HALO (High Altitude Long Range Research Aircraft) wurden 24 Forschungsflüge von Oberpfaffenhofen, Deutschland, aus durchgeführt. Ursprüngliches Ziel der Kampagne war es, Zirruswolken in hohen Breitengraden mit einer Kombination aus In-situ- und Fernerkundungsinstrumenten zu untersuchen. Aufgrund der weltweiten Corona-Pandemie mussten die Flüge jedoch von Süddeutschland statt von Nordschweden aus durchgeführt werden. Daher wurden weitere Ziele in Bezug auf Zirruswolken in mittleren Breiten in die Ziele der Kampagne aufgenommen. Das LIM-Team betrieb die breitbandigen und spektralen Strahlungssensoren BACARDI (Broadband AirCrAft RaDiometer Instrumentation) und SMART (Spectral Modular Airborne Radiation measurement sysTem), wobeiBACARDI die breitbandige solare und terrestrische Auf- und Abwärtsstrahlung und SMART die spektral aufgelöste solareAuf- undAbwärtsstrahlung sowie dieAufwärtsstrahlungsdichte maß.

Aircraft produce condensation trails, which are thought to increase high-level cloudiness under certain conditions. However the magnitude of such an effect and whether this contributes substantially to the radiative forcing due to the aviation sector remain uncertain. The very substantial, near-global reduction in air traffic in response to the COVID-19 outbreak offers an unprecedented opportunity to identify the anthropogenic contribution to the observed cirrus coverage and thickness. Here we show, using an analysis of satellite observations for the period March–May 2020, that in the 20% of the Northern Hemisphere mid-latitudes with the largest air traffic reduction, cirrus fraction was reduced by ∼9 ± 1.5% on average, and cirrus emissivity was reduced by ∼2 ± 5% relative to what they should have been with normal air traffic. The changes are corroborated by a consistent estimate based on linear trends over the period 2011–2019. The change in cirrus translates to a global radiative forcing of 61 ± 39 mW m−2 . This estimate is somewhat smaller than previous assessments.