Strahlungsbilanz der bewölkten Atmosphäre aus MSG1-SEVIRI Daten

Die Strahlungsbilanz der Erde treibt letztlich alle physikalischen Prozesse von Ozean und Atmosphäre an. Ein verbessertes Verständnis der Komponenten der Strahlungsbilanz liefert damit zwangsläufig auch ein besseres Verständnis des Klimasystems unserer Erde. Hierbei kommt den Wolken eine ganz besondere Rolle zu, immerhin bedecken sie ständig 66% unseres Planeten. Wolken wirken in zwei Richtungen auf die Strahlungsbilanz. Zum einen reflektieren sie mehr solare Strahlung als der Untergrund und verringern damit das dem System Erde-Atmosphäre zur Verfügung stehende Energieangebot. Auf der anderen Seite absorbieren Wolken Infrarotstrahlung und strahlen selbst mit ihrer kälteren Oberkantentemperatur ab. Somit verhindern sie, dass ein nicht unerheblicher Teil an Energie des Systems Erde-Atmosphäre an den Weltraum verloren geht. Diese beiden Wolkenstrahlungseffekte wirken in entgegegesetzte Richtung, wobei je nach Wolkentyp der eine oder der andere Effekt überwiegen kann. Diese Arbeit hat zum Thema, diese Effekte aus fernerkundeten Strahlungsbilanzkomponenten zu betrachten. Der erste Teil dieser Arbeit dient der Erarbeitung der Grundlagen und Algorithmen, mit deren Hilfe aus Messungen des SEVIRI Instruments auf den Meteosat-Satelliten der zweiten Generation (MSG) die Komponenten der Strahlungsbilanz am Oberrand der Atmosphäre gewonnen werden können. Darüberhinaus werden Strahlungsbilanzkomponenten aus zwei verschiedenen Instrumenten (SEVIRI und GERB) miteinander verglichen. Hierbei stellt sich heraus, dass die kurzwelligen Strahlungsflüsse eine signifikante Abhängigkeit von der betrachteten Szene zeigen, wohingegen die langwelligen Strahlungsflüsse eine gute Übereinstimmung zeigen. Ferner wird eine alternative Methode zur Gewinnung langwelliger Strahlungsflüsse aus SEVIRI-Daten entwickelt. Der zweite Teil befasst sich mit der zeitlichen Entwicklung der Strahlungsbilanzkomponenten während des Lebenszyklus konvektiver Bewölkungssysteme. Hierbei werden zunächst einzelne konvektive Wolkencluster dreier Klimazonen (mittlere Breiten Subtropen, Tropen) exemplarisch betrachtet und anschließend drei tropische Zyklonen der atlantischen Hurrikan-Saison 2006. Hierbei wird ein möglicher Zusammenhang zwischen der Rotations-Intensität eines tropischen Konvektionssystems und dessen Strahlungseffekten beschrieben. Im letzten Teil werden die Auswirkungen von Aerosole auf die Strahlungsbilanz am Beispiel von Sahara-Staub über dem Atlantik und die Wechselwirkungen von Rauch und Wolken am Beipiel einer Rauchwolke über den Wüstengebieten des Nahen Ostens sowie Rauchwolken über dem Kongobecken betrachtet. Die Strahlungseffekte der Aerosole werden im Falle der Rauchwolken lediglich exemplarisch beschrieben, im Falle des Staubausbruchs auch quantifiziert

Evaluation of seven European aerosol optical depth retrieval algorithms for climate analysis

Satellite data are increasingly used to provide observation-based estimates of the effects of aerosols on climate. The Aerosol-cci project, part of the European Space Agency's Climate Change Initiative (CCI), was designed to provide essential climate variables for aerosols from satellite data. Eight algorithms, developed for the retrieval of aerosol properties using data from AATSR (4), MERIS (3) and POLDER, were evaluated to determine their suitability for climate studies. The primary result from each of these algorithms is the aerosol optical depth (AOD) at several wavelengths, together with the Ångström exponent (AE) which describes the spectral variation of the AOD for a given wavelength pair. Other aerosol parameters which are possibly retrieved from satellite observations are not considered in this paper. The AOD and AE (AE only for Level 2) were evaluated against independent collocated observations from the ground-based AERONET sun photometer network and against “reference” satellite data provided by MODIS and MISR. Tools used for the evaluation were developed for daily products as produced by the retrieval with a spatial resolution of 10 × 10 km2 (Level 2) and daily or monthly aggregates (Level 3). These tools include statistics for L2 and L3 products compared with AERONET, as well as scoring based on spatial and temporal correlations. In this paper we describe their use in a round robin (RR) evaluation of four months of data, one month for each season in 2008. The amount of data was restricted to only four months because of the large effort made to improve the algorithms, and to evaluate the improvement and current status, before larger data sets will be processed. Evaluation criteria are discussed. Results presented show the current status of the European aerosol algorithms in comparison to both AERONET and MODIS and MISR data. The comparison leads to a preliminary conclusion that the scores are similar, including those for the references, but the coverage of AATSR needs to be enhanced and further improvements are possible for most algorithms. None of the algorithms, including the references, outperforms all others everywhere. AATSR data can be used for the retrieval of AOD and AE over land and ocean. PARASOL and one of the MERIS algorithms have been evaluated over ocean only and both algorithms provide good results

On the Information Content of Hyperspectral Infrared Observations with Respect to Mineral Dust

Desert dust is characterized by strong silicate absorption bands located within the atmospheric window region in the terrestrial infrared (TIR) between 8 µm and 12 µm. These absorption bands and the corresponding optical properties (extinction efficiency, single scattering albedo, scattering phase function) have very specific spectral shapes for different silicate minerals, modulated by the particle size and shape. The asphericity of desert dust particles strongly affects the absorption band characteristics, for example due to surface wave modes for small particles. The use of the correct particle shape model significantly increases the spectral correlation between simulated dust optical properties for typical minerals and corresponding laboratory measurements for single minerals as well as for bulk dust from desert samples. The presence of absorption peaks and the spectral shape of the extinction signal carry dust information, which can be exploited for remote sensing purposes. With hyperspectral infrared methods it is thus possible to infer information beyond dust optical depth, that is to acquire information about dust particle size, composition and also vertical information. Examples of such information are shown for the Infrared Mineral Aerosol Retrieval Scheme (IMARS) which has been developed for the Infrared Atmospheric Sounding Interferometer (IASI) on board the European Metop satellite series. Another strong advantage of the hyperspectral signal from satellite instruments is the capability to minimize the influence of disturbing gas absorption lines within these bands. The probabilistic IMARS approach also directly provides the number of independent signals (variables) for each observation. For desert dust this number typically ranges from 2.5 to 4.0 depending on the characteristics of the observed dust plume. Consequently a lot more information beyond Aerosol Optical Depth (AOD) can be retrieved from these measurements

Large-scale analysis of relationships between mineral dust, ice cloud properties and precipitation from satellite observations using a Bayesian approach: Theoretical basis and first results for the tropical Atlantic Ocean

Mineral dust and ice cloud observations from the Infrared Atmospheric Sounding Interferometer (IASI) are used to assess the relationships between desert dust aerosols and ice clouds over the tropical Atlantic Ocean during the hurricane season 2008. Cloud property histograms are first adjusted for varying cloud top temperature or ice water path distributions with a Bayesian approach to account for meteorological constraints on the cloud variables. Then, histogram differences between dust load classes are used to describe the impact of dust load on cloud property statistics. The analysis of the histogram differences shows that ice crystal sizes are reduced with increasing aerosol load and ice cloud optical depth and ice water path are increased.The distributions of all three variables broaden and get less skewed in dusty environments. For ice crystal size the significant bimodality is reduced and the order of peaks is reversed.Moreover, it is shown that not only are distributions of ice cloud variables simply shifted linearly but also variance, skewness, and complexity of the cloud variable distributions are significantly affected.This implies that the whole cloud variable distributions have to be considered for indirect aerosol effects in any application for climate modelling

Aerosol-Wolken Wechselwirkungen und ihre Auswirkungen auf den Wasserkreislauf

Aerosole bilden Kondensationskeime für Wolken. Eine Veränderung der atmosphärischen Aerosolbelastung sowie der Aerosoleigenschaften wirkt sich damit direkt auf die Wolkenbildung sowie deren Charakteristika aus. Ferner beeinflussen Aerosole durch Strahlungsabsorption die vertikale Temperaturstruktur der Atmosphäre und damit die durch Konvektion bedingte Niederschlagsbildung. Von Bedeutung ist daher die Kenntnis der aerosolphysikalischen Größen wie etwa Partikelform, Partikelgrößenverteilung und chemische Zusammensetzung. Eine Quantifizierung des Zusammenhanges zwischen Aerosolen und Wolken soll durch statistische Untersuchungen langer satellitenbasierter Zeitreihen vorgenommen werden. Wolkenparameter und Aerosolgehalte werden dazu aus Daten von AVHRR, MODIS, MSG, ENVISAT und Metop bestimmt, wie sie im DLR-DFD seit 1982 verfügbar sind. Der Einfluss von Aerosolgehalt auf die Größe der Wolkentröpfchen und damit auf das Reflexionsvermögen der Wolken („Twomey-Effekt“) wird bereits recht gut durch Fernerkundungsdaten wiedergegeben. Deutlich weniger dokumentiert ist jedoch der Effekt des Aerosolgehalts auf die makrophysikalischen Eigenschaften der Wolken (z.B. Wolkenbedeckungsgrad) sowie auf die thermodynamischen Effekte, welche die Niederschlagsbildung maßgeblich beeinflussen (Eisphasenanteil, Vertikalgeschwindigkeit, konvektiv erreichte Wolkenoberkantenhöhe). Für die typenabhängige Betrachtung von Wechselwirkungen zwischen Aerosolgehalt und Bewölkung bietet sich die westafrikanische Monsunregion zusammen mit dem tropisch-subtropischen Atlantik an. Hier liegen miteinander konkurrierende, aber deutlich voneinander abgrenzbare Wolkenregimes wie die flache Passat-Konvektion, die flache Seewind-Zirkula¬tion an den afrikanischen Küsten und die hoch reichende tropische Konvektion innerhalb der ITCZ vor. Der Aerosolgehalt in dieser Region wird maßgeblich durch Mineralstaub aus der Sahara (über Land und Ozean) und Rauchaerosol von der Verbrennung von Biomasse (hauptsächlich über Land) gebildet

Analysis of aerosol-cloud-interactions over semi-arid and arid subtropical land regions from three different satellite datasets (MODIS, AATSR/ENVISAT, IASI)

Indirect aerosol effects, i.e. the change of cloud physical properties by aerosol interactions, have been identified as one of the largest uncertainties in the current understanding of the climate system. Despite the uncertainties of the representations of aerosol-cloud interactions in current climate projections, they have large impact on the climate system itself – in terms of the radiation balance, but also in terms of precipitation, and thus vegetation cover, and re-distribution of water throughout the atmosphere. Nevertheless, so far only very few studies of large-scale statistics of aerosol-cloud interactions over land are available. Moreover most studies on the topic cover liquid water clouds only. Aerosol cloud interactions over arid and semi-arid land regions have been analysed from three different satellite datasets with respect to aerosol type and cloud phase. The regions of the analysis cover Southern Africa, the Sahel domain with the influence of the West African monsoon circulation, the North-Western African Maghreb region and the Arabian Peninsula. These regions have been chosen as they are dominated by one (Maghreb, Arabia) or two (Sahel, Southern Africa) aerosol types and as mineral dust is one of the dominating aerosol types in all of them. The second dominating aerosol type is biomass burning in the Sahel and Southern Africa. These aerosol types can be discriminated by separating the aerosol information into fine mode (biomass burning) and coarse mode (desert dust) aerosol. Thus they can generally also be discriminated from satellite, although these capabilities are limited over land. Over land the diurnal cycle of convection is much stronger and aerosol interactions with deep convective cloud systems over land have been identified to be of great importance not only for precipitation in regions under pressure of desertification, but also with respect to climate change. For liquid water clouds the well-known first indirect aerosol effect (“Twomey effect”), i.e. higher cloud albedo due to smaller droplet sizes, could be confirmed for all regions, if liquid water path is held constant. Nevertheless, liquid water path has been found to be affected by aerosol presence and the aerosol effect on liquid water path dominates the net effect of aerosols on cloud optical depth. For ice phase clouds the same effects are observed with ice water path controlling the net aerosol effect on optical depth. From thermal infrared retrievals of mineral dust and ice clouds an increase of ice particle size with respect to background conditions has been detected. Together with observations at solar wavelengths the differences can be interpreted as indications for an increase of optically thicker clouds at the cost of cirrus coverage. Although the Twomey effect has been identified to be active in all cases, cloud water path and cloud phase transitions could be identified to be of predominant importance for resulting cloud propertiy changes due to aerosol presence. The second indirect aerosol effect (“Albrecht effect”) could not be identified from the statistical analysis. Although cloud cover distributions as functions of aerosol optical depth (AOD) indicate an increase of cloud cover with AOD, these could not be related to any other cloud properties including cloud droplet size. Thus the satellite observations do not support the relatively simple formulation of the second indirect aerosol effect (longer cloud lifetime due to drizzle suppression as a consequence of smaller droplets). An aerosol effect on cloud phase has been identified with respect to cloud water path. It could not be confirmed in terms of cloud coverage. The statistical analysis of cloud macro- and microphysical properties has been performed after the observations have been projected all to the same cloud top temperature distribution. This method allows correcting for effects of the temperature 4 and moisture fields (meteorological conditions), which otherwise would dominate the statistical results. It has been shown that aerosol type is important for aerosol cloud interactions in subtropical land regions. Moreover the cloud water path (liquid and ice) has been identified to be a strong constraint on indirect aerosol effects, outweighing e.g. the optical depth increase by droplet size reduction (“Twomey-effect”). It could moreover be shown that aerosol-cloud interactions are also important for ice cloud properties in subtropical land regions, which have yet not fully been addressed in statistical analyses of indirect aerosol effects and consequently in climate projections. Nevertheless, by means of the large-scale statistical analysis, also some deficits of current satellite datasets have been identified, which have to be solved in order to furthermore reduce the uncertainties of indirect aerosol effects. It has been the first attempt to quantify aerosol-cloud interactions focussed on semi-arid and arid land regions, performing the same kind of analysis to liquid water and ice clouds at the same time with the same methods, comparing results from three different independent satellite datasets, using advanced statistical descriptions of the observed deviations from background in order to account for non-linearity and multimodal or non-Gaussian probability distributions of cloud properties, applying a newly developed method to account for variations in cloud top temperature affecting cloud property observations statistically and also introducing a newly developed dataset from IASI which is sensitive to desert dust and ice clouds only, adding information about aerosol type sensitivity of aerosol-cloud interactions

APOLLO_NG – a probabilistic interpretation of the APOLLO legacy for AVHRR heritage channels

The cloud processing scheme APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) has been in use for cloud detection and cloud property retrieval since the late 1980s. The physics of the APOLLO scheme still build the backbone of a range of cloud detection algorithms for AVHRR (Advanced Very High Resolution Radiometer) heritage instruments. The APOLLO_NG (APOLLO_NextGeneration) cloud processing scheme is a probabilistic interpretation of the original APOLLO method. It builds upon the physical principles that have served well in the original APOLLO scheme. Nevertheless, a couple of additional variables have been introduced in APOLLO_NG. Cloud detection is no longer performed as a binary yes/no decision based on these physical principles. It is rather expressed as cloud probability for each satellite pixel. Consequently, the outcome of the algorithm can be tuned from being sure to reliably identify clear pixels to conditions of reliably identifying definitely cloudy pixels, depending on the purpose. The probabilistic approach allows retrieving not only the cloud properties (optical depth, effective radius, cloud top temperature and cloud water path) but also their uncertainties. APOLLO_NG is designed as a standalone cloud retrieval method robust enough for operational near-realtime use and for application to large amounts of historical satellite data. The radiative transfer solution is approximated by the same two-stream approach which also had been used for the original APOLLO. This allows the algorithm to be applied to a wide range of sensors without the necessity of sensorspecific tuning. Moreover it allows for online calculation of the radiative transfer (i.e., within the retrieval algorithm) giving rise to a detailed probabilistic treatment of cloud variables. This study presents the algorithm for cloud detection and cloud property retrieval together with the physical principles from the APOLLO legacy it is based on. Furthermore a couple of example results from NOAA-18 are presented

Observation of volcanic ash from Puyehue-Cordon Caulle with IASI

On 4 June 2011 an eruption of the Chilean volcano complex Puyehue-Cordon Caulle injected large amounts of volcanic ash into the atmosphere and affected local life as well as hemisphere-wide air traffic. Observations of the Infrared Atmospheric Sounding Interferometer (IASI) flown on board of the MetOp satellite have been exploited to analyze the evolution of the ash plume around the Southern Hemisphere. A novel singular vector-based retrieval methodology, originally developed for observation of desert dust over land and ocean, has been adapted to enable remote sensing of volcanic ash. Since IASI observations in the 8–12 μm window are applied in the retrieval, the method is insensitive to solar illumination and therefore yields twice the observation rate of the ash plume evolution compared to solar backscatter methods from polar orbiting satellites. The retrieval scheme, the emission characteristics and the circumpolar transport of the ash are examined by means of a source-receptor analysis