A weekly cycle in aerosol pollution and some meteorological quantities is observed over Europe. In the present study we exploit this effect to analyse aerosol-cloudradiation interactions. A weekly cycle is imposed on anthropogenic emissions in two general circulation models that include parameterizations of aerosol processes and cloud microphysics. It is found that the simulated weekly cycles in sulfur dioxide, sulfate, and aerosol optical depth in both models agree reasonably well with those observed indicating model skill in simulating the aerosol cycle. A distinct weekly cycle in cloud droplet number concentration is demonstrated in both observations and models. For other variables, such as cloud liquid water path, cloud cover, top-of-the-atmosphere radiation fluxes, precipitation, and surface temperature, large variability and contradictory results between observations, model simulations, and model control simulations without a weekly cycle in emissions prevent us from reaching any firm conclusions about the potential aerosol impact on meteorology or the realism of the modelled second aerosol indirect effects.

Modeled differences in predicted effects are increasingly used to help quantify the uncertainty of these effects. Here, we examine modeled differences in the aerosol indirect effect in a series of experiments that help to quantify how and why model-predicted aerosol indirect forcing varies between models. The experiments start with an experiment in which aerosol concentrations, the parameterization of droplet concentrations and the autoconversion scheme are all specified and end with an experiment that examines the predicted aerosol indirect forcing when only aerosol sources are specified. Although there are large differences in the predicted liquid water path among the models, the predicted aerosol first indirect effect for the first experiment is rather similar, about −0.6Wm−2 to −0.7Wm−2. Changes to the autoconversion scheme can lead to large changes in the liquid water path of the models and to the response of the liquid water path to changes in aerosols. Adding an autoconversion scheme that depends on the droplet concentration caused a larger (negative) change in net outgoing shortwave radiation compared to the 1st indirect effect, and the increase varied from only 22% to more than a factor of three. The change in net shortwave forcing in the models due to varying the autoconversion scheme depends on the liquid water content of the clouds as well as their predicted droplet concentrations, and both increases and decreases in the net shortwave forcing can occur when autoconversion schemes are changed. The parameterization of cloud fraction within models is not sensitive to the aerosol concentration, and, therefore, the response of the modeled cloud fraction within the present models appears to be smaller than that which would be associated with model “noise”. The prediction of aerosol concentrations, given a fixed set of sources, leads to some of the largest differences in the predicted aerosol indirect radiative forcing among the models, with values of cloud forcing ranging from −0.3Wm−2 to −1.4Wm−2. Thus, this aspect of modeling requires significant improvement in order to improve the prediction of aerosol indirect effects.

Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (tau a) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. cloud droplet number concentration (N d) compares relatively well to the satellite data at least over the ocean. The relationship between (tau a) and liquid water path is simulated much too strongly by the models. This suggests that the implementation of the second aerosol indirect effect mainly in terms of an autoconversion parameterisation has to be revisited in the GCMs. A positive relationship between total cloud fraction (fcld) and tau a as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong fcld–tau a relationship, our results indicate that none can be identified as a unique explanation. Relationships similar to the ones found in satellite data between tau a and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR - tau a relationship show a strong positive correlation between tau a and fcld. The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of tau a, and parameterisation assumptions such as a lower bound on Nd. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of −1.5±0.5Wm−2. In an alternative approach, the radiative flux perturbation due to anthropogenic aerosols can be broken down into a component over the cloud-free portion of the globe (approximately the aerosol direct effect) and a component over the cloudy portion of the globe (approximately the aerosol indirect effect). An estimate obtained by scaling these simulated clearand cloudy-sky forcings with estimates of anthropogenic tau a and satellite-retrieved Nd–tau a regression slopes, respectively, yields a global, annual-mean aerosol direct effect estimate of −0.4±0.2Wm−2 and a cloudy-sky (aerosol indirect effect) estimate of −0.7±0.5Wm−2, with a total estimate of −1.2±0.4Wm−2.

An investigation of the impact of the subgrid-scale variability of cloud liquid water on the autoconversion process as parameterized in a general circulation model is presented in this paper. For this purpose, a prognostic statistical probability density distribution (PDF) of the subgrid scale variability of cloud water is incorporated in a continuous autoconversion parameterization. Thus, the revised autoconversion rate is calculated by an integral of the autoconversion equation over the PDF of total water mixing ratio from the saturation vapor mixing ratio to the maximum of total water mixing ratio. An evaluation of the new autoconversion parameterization is carried out by means of one year simulations with the ECHAM5 climate model. The results indicate that the new autoconversion scheme causes an increase of the frequency of occurrence of high autoconversion rates and a decrease of low ones compared to the original scheme. This expected result is due to the emphasis on areas of high cloud liquid water in the new approach, and the non-linearity of the autoconversion with respect to liquid water mixing ratio. A similar trend as in the autoconversion is observed in the accretion process resulting from the coupling of both processes. As a consequence of the altered autoconversion, large-scale surface precipitation also shows a shift of occurrence from lower to higher rates. The vertically integrated cloud liquid water estimated by the model shows slight improvements compared to satellite data. Most importantly, the artificial tuning factor for autoconversion in the continuous parameterization could be reduced by almost an order of magnitude using the revised parameterization.

A climatology of thermodynamic phase of precipitating cloud is presented derived from global—land and ocean—, retrievals from Cloudsat, CALIPSO, and Moderate Resolution Imaging Spectroradiometer. Like precipitation rate, precipitation frequency is dominated by warm rain, defined as rain produced via the liquid phase only, over the tropical oceans outside the Intertropical Convergence Zone and by cold rain, produced via the ice phase, over the midlatitude oceans and continents. Warm rain is very infrequent over the continents, with significant warm rain found only in onshore flow in the tropics, and over India, China, and Indochina. Comparison of the properties of precipitating and nonprecipitating warm clouds shows that the scarcity of warm rain over land can be explained by smaller effective radii in continental clouds that delay the onset of precipitation. The results highlight the importance of ice-phase processes for the global hydrological cycle and may lead to an improved parameterization of precipitation in general circulation models.

Die Eisbildung in Wolken wirkt sich auf die Niederschlagsbildung, die optischen Eigenschaften, und die Persistenz der Wolken aus und beeinflusst somit das Wetter und das Klima. Sogenannte eisnukleierende Partikel (ice nucleating particle; INP), katalysieren den Gefrierprozess von Wolkentröpfchen und tragen so zur primären Eisbildung in Wolken bei. In dieser Arbeit wurden die Häufigkeit und die Eigenschaften von INP in der Arktis untersucht. Hierzu wurde ein Vielzahl an Proben analysiert: Proben zweier Eisbohrkerne (aus Spitzbergen und Grönland); Filterproben von Aerosolpartikeln, die an Bord eines Flugzeuges über dem arktischen Ozean nordöstlich von Grönland gesammelt wurden; Filterproben von Aerosolpartikeln, die an Bord eines Schiffes in der Nähe von Spitzbergen gesammelt wurden. Zusätzlich wurden auch Meeresoberflächenfilm-, Meerwasser- und Nebelwasserproben gesammelt. Es wurde festgestellt, dass die INP-Konzentrationen in der Arktis im Allgemeinen niedriger sind als in den mittleren Breiten. Und obwohl die INP-Konzentrationen bei einer Temperatur von Probe zu Probe eine hohe Variabilität aufweisen, bewegen sie sich seit der Kleinen Eiszeit im 16. Jahrhundert auf einem ähnlichen Niveau und zeigen keinen langfristigen Trend. Außergewöhnlich eisaktive Proben zeichnen sich durch hohe INP-Konzentrationen bei wärmeren Temperaturen (ca. über -15°C) aus. Die in diesen Fällen aktiven INP können auf einen biogenen Ursprung zurückgeführt werden. Ferner wurden eindeutige Hinweise auf das Vorhandensein lokaler mariner INP-Quellen gefunden in der Arktis gefunden. Dies ist ein interessantes Ergebnis, da auch gezeigt wurde, dass ohne signifikante Anreicherung während des Transfers vom Ozean in die Aerosolphase, die vorhandenen INP im Meerwasser die INP-Konzentration in der Luft nicht erklären können. Die INP-Konzentrationen Temperaturbereich unterhalb von -26°C, hingegen scheinen eher durch Ferntransport von Staub aus den mittleren Breiten und/oder terrestrischer Quellen in der Arktis bestimmt zu sein.:1 Introduction 2 Experimental 2.1 Campaign Overviews 2.1.1 Arctic Ice Cores 2.1.2 PAMARCMiP 2.1.3 PASCAL 2.2 Instrumentation 2.2.1 Droplet Freezing Assays 2.2.2 HERA 2.2.3 Low Volume Filter Sampler 2.2.4 SPIN 2.2.5 Sea and fog water sampling 2.2.6 Other Aerosol Instrumentation 2.3 Data analysis 2.3.1 INP concentration 2.3.2 Back trajectories 2.3.3 Sea ice fraction and thickness 2.3.4 Transmission Electron Microscopy 3 Results 3.1 INP measurements on Arctic ice core samples 3.1.1 Results & Discussion 3.1.2 Summary 3.2 Airborne INP measurements during PAMARCMiP 3.2.1 Results & Discussion 3.2.2 Summary 3.3 Ship-borne INP measurements during PASCAL 3.3.1 Results & Discussion 3.3.2 Summary 4 Summary and Conclusion / Ice formation in clouds impacts precipitation initiation, cloud optical properties, and cloud persistence, and hence influences weather and climate. At the base of the primary ice formation in clouds stands the ice nucleating particle (INP), which catalyzes the freezing process of cloud droplets. In this thesis, the abundance and properties of Arctic INP were investigated in samples from two ice cores (Svalbard and Greenland), in samples of aerosol particles collected on an aircraft over the Arctic ocean northeast of Greenland, and in ship-borne aerosol filter samples, as well as sea surface microlayer, bulk sea water and fog water samples collected in the vicinity of Svalbard. It was found that INP concentrations in Arctic are generally lower than in mid-latitudes. And while they show a high inter-sample variability, INP concentrations have been on similar levels since the Little Ice Age in the 16th century and show no long-term trend. Exceptionally ice-active samples are characterized by high INP concentrations at warmer temperatures (approximately above -15°C). The INP active in these cases were attributed to a biogenic origin. Furthermore, clear evidence for the presence of local marine INP sources was found in the Arctic. This is an interesting finding as it was also shown that without significant enrichment during the transfer from the ocean to the aerosol phase, the INP in the sea water can not explain the INP concentration in the air. INP concentrations temperature range below -26°C, on the other hand, appear to be determined more by long-range transport of dust from mid-latitudes and/or terrestrial sources in the Arctic.:1 Introduction 2 Experimental 2.1 Campaign Overviews 2.1.1 Arctic Ice Cores 2.1.2 PAMARCMiP 2.1.3 PASCAL 2.2 Instrumentation 2.2.1 Droplet Freezing Assays 2.2.2 HERA 2.2.3 Low Volume Filter Sampler 2.2.4 SPIN 2.2.5 Sea and fog water sampling 2.2.6 Other Aerosol Instrumentation 2.3 Data analysis 2.3.1 INP concentration 2.3.2 Back trajectories 2.3.3 Sea ice fraction and thickness 2.3.4 Transmission Electron Microscopy 3 Results 3.1 INP measurements on Arctic ice core samples 3.1.1 Results & Discussion 3.1.2 Summary 3.2 Airborne INP measurements during PAMARCMiP 3.2.1 Results & Discussion 3.2.2 Summary 3.3 Ship-borne INP measurements during PASCAL 3.3.1 Results & Discussion 3.3.2 Summary 4 Summary and Conclusion

A climatology of thermodynamic phase of precipitating cloud is presented derived from global—land and ocean—, retrievals from Cloudsat, CALIPSO, and Moderate Resolution Imaging Spectroradiometer. Like precipitation rate, precipitation frequency is dominated by warm rain, defined as rain produced via the liquid phase only, over the tropical oceans outside the Intertropical Convergence Zone and by cold rain, produced via the ice phase, over the midlatitude oceans and continents. Warm rain is very infrequent over the continents, with significant warm rain found only in onshore flow in the tropics, and over India, China, and Indochina. Comparison of the properties of precipitating and nonprecipitating warm clouds shows that the scarcity of warm rain over land can be explained by smaller effective radii in continental clouds that delay the onset of precipitation. The results highlight the importance of ice-phase processes for the global hydrological cycle and may lead to an improved parameterization of precipitation in general circulation models.

To date, no observation-based proxy for climate change has been successful in quantifying the feedbacks between clouds and climate. The most promising, yet demanding, avenue to gain confi dence in cloud–climate feedback estimates is to utilize observations and large-eddy simulations (LES) or cloud-resolving modeling (CRM) to improve cloud process parameterizations in large-scale models. Sustained and improved satellite observations are essential to evaluate large-scale models. A reanalysis of numerical prediction models with assimilation of cloud, aerosol, and precipitation observations would provide a valuable dataset for examining cloud interactions. The link between climate modeling and numerical weather prediction (NWP) may be exploited by evaluating how accurate cloud characteristics are represented by the parameterization schemes in NWP models. A systematic simplifi cation of large-scale models is an important avenue to isolate key processes linked to cloud–climate feedbacks and would guide the formulation of testable hypotheses for fi eld studies. Analyses of observation-derived correlations between cloud and aerosol properties in combination with modeling studies may allow aerosol–cloud interactions to be detected and quantifi ed. Reliable representations of cloud dynamic and physical processes in large-scale models are a prerequisite to assess aerosol indirect effects on a large scale with confi dence. To include aerosol indirect effects in a consistent manner, we recommend that a “radiative fl ux perturbation” approach be considered as a complement to radiative forcing.

Aerosole dienen als Kondensationskerne (CCN) und verändern die Wolkentropfenkonzentration (Nd) und weitere Wolkeneigenschaften. Nd ist daher ein Schlüsselparameter von Aerosol-Wolken-Wechselwirkungen (ACI). ACI sind bezüglich ihrer Klimarelevanz nur unzureichend bestimmt. ACI wurden auf verschiedenen Skalen und mit unterschiedlichen Methoden von unterschiedlichen Perspektiven aus untersucht, was zu einer großen Variabilität von ACI Metriken in der Literatur führt. Für eine genauere Quantifizierung von ACI, wurden in dieser Arbeit damit verbundene, entscheidende Unsicherheiten diskutiert, u.a. das Einmischen trockener Luft, Unsicherheiten von Retrieval-Methoden und die Konsistenz von Schlüsselparametern auf verschiedenen Skalen. Um Nd und weitere Wolkeneigenschaften aus passiven Satellitenbeobachtungen für Flüssigwasserwolken abzuleiten, wird oft das adiabatische Modell herangezogen. Es wurde untersucht, inwiefern dieses Modell reale Bedingungen wiedergibt. Es wurde gezeigt, dass Wolken typischerweise subadiabatisch sind, mit stark reduziertem Flüssigwassergehalt nahe der Wolkenoberkante. Der Einmischprozess variiert zeitlich stark und erschwert die Quantizierung von ACI. Für unterschiedliche Cloudnet-Stationen variieren die Medianwerte des subadiabatischen Faktors von 0.35+/-0.12 bis 0.48+/-0.22. Dieser hängt stark von der geometrischen Wolkendicke ab, wobei dickere Wolken subadiabatischer sind. Die Ableitung von Nd aus bodenbasierten Radar-Radiometer-Beobachtungen ist sehr sensitiv gegenüber a-priori Annahmen. Um zusätzliche Beobachtungen und Unsicherheiten berücksichtigen zu können, wurde ein neuartiges Optimal Estimation (OE) Retrieval entwickelt und mittels synthetischer Wolkenprofile evaluiert. Mittels einer Sensitivitätsstudie konnte die Verbesserung der Retrieval-Genauigkeit für die OE Methode gezeigt werden, wenn die Beobachtungen der optischen Dicke mit berücksichtigt werden. Die Annahme einer typischen Breite der Tropfengrößenverteilung ist dabei eine der größten Unsicherheiten von Nd (Unsicherheit für die OE Methode etwa 150%, für die Radar-Radiometer-Methode etwa 200%). Desweiteren wurden aus unterschiedlichen Perspektiven abgeleitete wolkenmikrophysikalische Größen hinsichtlich ihrer Konsistenz untersucht. Beim Vergleich von Nd und des Effektivradius (re) vom bodengebundenen Retrieval mit in-situ Beobachtungen für einen Falltag wurde eine gute Übereinstimmung gefunden. Beim Vergleich der von SEVIRI und Bodenstationen abgeleiteten Wolkeneigenschaften haben sich mittlere quadratische Abweichungen des Flüssigwasserpfads und der optischen Dicke von jeweils 65 gm2 und 14 ohne signifikanten Bias gezeigt. Damit zeigt sich SEVIRI für großskalige, statistische ACI-Untersuchungen repräsentativ zur Bodenperspektive. Für individuelle Falltage traten jedoch teils größere Unterschiede auf, welche durch Inhomogenitäten und Auflösungseffekte erklärt werden können. Mit SEVIRI Beobachtungen und aus der MACC Reanalyse abgeleiteten CCN Konzentrationen wurde eine Quantifizierung von ACI für das Jahr 2012 durchgeführt. Dabei wurde für Europa ein deutlicher, mikrophysikalischer Effekt gefunden, d.h. eine Abnahme von re und eine Zunahme von Nd mit zunehmender CCN Konzentration. Im Gegensatz dazu wurde für die Wolkenalbedo kein eindeutiger Anstieg mit der CCN-Konzentration gefunden. Verwendet man die Aerosol-optische Dicke (AOD) anstelle der CCN-Konzentration, sind die Werte der ACI Metrik nur halb so groß. Dieses Ergebnis deutet darauf hin, dass die AOD kein optimaler CCN-Proxy ist.

Simulations assuming open-pit mines and different cessation landscapes of open-pit mines with urbanization were performed with a non-hydrostatic meteorological model. The possible enhancement or counteracting of the atmospheric responses to these simultaneously occurring (= accumulated) land-use changes are examined applying a formula drived from the principle of superposition. Although accumulated land-use changes substantially affect the local water and energy fluxes and significantly influence cloud and precipitation microphysics, they do not necessarily provide more significant changes in these quantities than the change of different land-use types to only one land-use type (=single land-use changes). Where the atmospheric responses to accumulated land-use changes are enhanced or inhibited, depends on the thermal, dynamical and hydrologic characteristics of the undergoing accumulated land-use changes as well as on the land-use adjacent to the land-use conversion. In regions dominated by drier surfaces (e.g., agriculture), no enhancement or counteraction according to the principle of superposition could be detected in this study. / Simulationen mit einer Tagebaulandschaft sowie verschiedenen Tagebaufolgelandschaften inklusive Urbanisierung wurden mit einem nicht-hydrostatischen meteorologischen Modell durchgeführt. Eine mögliche Verstärkung oder Abschwächung der atmosphärischen Reaktion auf gleichzeitig auftretende (akkumulierte) Landnutzungsänderungen wird mittels einer auf dem Prinzip der Superposition erstellten Formel untersucht. Obgleich die akkumulierten Landnutzungsänderungen die lokalen Energie- und Wasserflüsse merklich sowie die Wolken und Niederschlagsmikrophysik signifikant beeinflussen, führen sie nicht notwendigerweise zu stärkeren Änderungen dieser Größen als einfache Landnutzungsänderungen. Wo sich akkumulierte Landnutzungsänderungen in ihrer Wirkung auf die Atmosphäre verstärken oder abschwächen, hängt davon ab, wie stark sich die dynamischen, hydrologischen und energetischen Eigenschaften der veränderten Landnutzung von der vorherigen und denen der Umgebung unterscheiden. In Regionen, die durch trockene Flächen charakterisiert sind (z.B. Ackerland), konnte in dieser Studie kein Verstärken oder Abschwächen im Sinne einer Abweichung vom Prinzip der Superposition festgestellt werden.