Synoptic and mesoaerological study of radiation fog

Summary The present study was for the purpose of determining the conditions necessary to the formation of radiation fog. The temperature and relative humidity of the lowest stratum of the troposphere were sounded with a tethered balloon and radiosonde during a number of nights when the conditions were considered favourable for the formation of radiation fog: clear weather with effective net radiation, light wind, and high relative humidity at ground level. Advection fog was also studied on some occasions for comparison. The measurements were carried out in Uppsala. Meteorological phenomena of possible importance to the study were recorded concurrently with the measurements. The weather was analyzed synoptically and aerologically also on each occasion.The results of the investigation suggest that the conditions necessary to the formation of radiation fog are not only an effective net radiation, light wind, and high relative humidity at ground level, but also that the wind must be light and the relative humidity high at some distance above the ground. This permits the more or less marked surface inversion to leave ground level and to rise as a layer of rapid temperature transition. This is brought about by the radiation gradually emitting from the upper surface of the layer of mist or fog, formed in the lowest and chilled air strata, which are subsequently cooled from above until an unstable lapse rate develops. As a results, the layer of rapid temperature transition is maintained rather intact as it rises. If the radiation continues without obstruction and the wind is ligh aloft, this layer migrates upwards until it reaches a level at which the relative humidity is so low that the cooling necessary for condensation does not occur by radiation, limiting the extension of the underlying fog. If the relative humidity is high only in the undermost air strata, the surface inversion cannot leave ground level and no fog will be formed aloft. If the wind is strong aloft, the process is hindered by the increased turbulent exchange occurring between the high and low strata.

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Zusammenfassung Die vorliegende Untersuchung, die in Uppsala durchgeführt wurde, hat zum Zweck, die für die Bildung von Strahlungsnebel notwendigen Bedingungen festzulegen. Temperatur und relative Feuchtigkeit der untersten Troposphärenschicht wurden mit Fesselballon und Radiosonde während einer Anzahl Nächte ausgemessen, in denen die Bedingungen für die Bildung von Strahlungsnebel günstig erschienen, somit bei klarem Wetter mit effektiver Ausstrahlung, leichtem Wind und hoher relativer Feuchtigkeit am Boden. Zum Vergleich wurden auch einige Fälle von Mischungsnebel (Advektionsnebel) untersucht. Die Bestimmungen wurden in Uppsala durchgeführt. Gleichzeitig wurden auch weitere meteorologische Vorgänge, die für diese Untersuchung von Bedeutung sein konnten, aufgezeichnet. Zudem wurde das Wetter synoptisch und aerologisch bei jeder Gelegenheit un tersucht.Die Ergebnisse dieser Untersuchung legen die Annahme nahe, daß die für die Bildung von Strahlungsnebel not wendigen Bedingungen nicht nur in effektiver Ausstrahlung, leichtem Wind und hoher relativer Feuchtigkeit am Boden bestehen, sondern daß auch in gewisser Entfernung über dem Boden der Wind schwach und die Feuchtigkeit hoch sein muß. Dadurch wird es der mehr oder weniger ausgesprochenen Bodeninversion ermöglicht, das Bodenniveau zu verlassen und als Schicht eines schnellen Temperaturüberganges in die Höhe zu steigen. Dies wird bewirkt durch die Ausstrahlung, die allmählich von der Oberseite der Nebelschicht ausgeht, welche sich in den untersten und ausgekühlten Luftschichten bildet, die sich nach und nach von oben her abkühlen, bis sich ein instabiler Temperaturgradient entwickelt. Infolgedessen wird die Schicht des schnellen Temperaturübergangs beim Emporsteigen nahezu unverändert aufrechterhalten. Falls die Ausstrahlung ungehindert andauert und der Wind auch in der Höhe schwach ist, wandert diese Schicht aufwärts, bis sie ein Niveau erreicht, wo die relative Feuchtigkeit so niedrig ist, daß die für Kondensation notwendige Abkühlung nicht mehr durch die Ausstrahlung hervorgerufen wird, wodurch die Ausdehnung des darunter liegenden Nebels begrenzt wird. Wenn dagegen die relative Feuchtigkeit nur in den alleruntersten Luftschichten hoch ist, kann die Bodeninversion die Höhe des Erdbodens nicht verlassen und es bildet sich haher in der Höhe kein Nebel. Falls in der Höhe starker Wind weht, wird der Prozeß der Nebeldung durch die verstärkten Austauschvorgänge zwischen den hohen und den niederen Schichten verhindert.

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Résumé Le but de la présente étude faite à Uppsala est de déterminer les conditions de formation du brouillard de rayonnement. On a mesuré à cet effet la température et l'humidité relative des couches basses de la troposphère à l'aide de ballons captifs et de radiosondes pendant un certain nombre de nuits apparement favorables à la formation de brouillard, done par temps clair, vent faible et forte humidité au sol. Quelques cas de brouillard d'advection ont été aussi retenus à titre de comparaison. En même temps on a relevé d'autres processus météorologiuqes pouvant être utiles; la situation météorologique fit l'objet d'une analyse synoptique et aérologique dans chaque cas.Les résultats obtenus font penser que ce ne sont pas seulement le rayonnement, la faible agitation de l'air et la forte humidité relative au sol qui sont déterminants dans la formation du brouillard; le vent doit être faible et l'humidité élevée également à une certaine hauteur au-dessus du sol: c'est alors que l'inversion plus ou moins prononcée peut s'élever. Cela résulte du refroidissement par rayonnement de la surface du brouillard bas; la couche froide au sol augmente alors d'épaisseur jusqu'à ce qu'un gradient vertical instable apparaisse. La couche de décroissance rapide de température se maintient tout en gagnant vers le haut, et le processus ne s'arrête que lorsqu'un niveau à faible humidité est atteint; la couche de brouillard a alors acquis son épaisseur maximum. Si l'humidité relative n'est forte que dans les couches les plus basses, l'inversion ne peut par contre quitter le sol et il ne se forme pas de brouillard plus haut. S'il y a du vent fort a une certaine hauteur, le processus de formation du brouillard est rendu impossible par le brassage des couches basses et moyennes.

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With 12 Figures

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Dedicated to Dr.Anders K. Ångström on the occasion of his 70th birthday.     

A description of how metal pollution occurs in the Tinto-Odiel rias (Huelva-Spain) through the application of cluster analysis

In the last few decades, the study of space-time distribution and variations of heavy metals in estuaries has been extensively studied as an environmental indicator. In the case described here, the combination of acid water from mines, industrial effluents and sea water plays a determining role in the evolutionary process of the chemical makeup of the water in the estuary of the Tinto and Odiel Rivers, located in the southwest of the Iberian Peninsula.Based on the statistical treatment of the data from the analysis of the water samples from this system, which has been affected by processes of industrial and mining pollution, the 16 variables analyzed can be grouped into two large families. Each family presents high, positive Pearson r values that suggest common origins (fluvial or sea) for the pollutants present in the water analyzed and allow their subsequent contrast through cluster analysis.     

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.     

Uncertainties in projected impacts of climate change on European agriculture and terrestrial ecosystems based on scenarios from regional climate models

The uncertainties and sources of variation in projected impacts of climate change on agriculture and terrestrial ecosystems depend not only on the emission scenarios and climate models used for projecting future climates, but also on the impact models used, and the local soil and climatic conditions of the managed or unmanaged ecosystems under study. We addressed these uncertainties by applying different impact models at site, regional and continental scales, and by separating the variation in simulated relative changes in ecosystem performance into the different sources of uncertainty and variation using analyses of variance. The crop and ecosystem models used output from a range of global and regional climate models (GCMs and RCMs) projecting climate change over Europe between 1961–1990 and 2071–2100 under the IPCC SRES scenarios. The projected impacts on productivity of crops and ecosystems included the direct effects of increased CO₂ concentration on photosynthesis. The variation in simulated results attributed to differences between the climate models were, in all cases, smaller than the variation attributed to either emission scenarios or local conditions. The methods used for applying the climate model outputs played a larger role than the choice of the GCM or RCM. The thermal suitability for grain maize cultivation in Europe was estimated to expand by 30–50% across all SRES emissions scenarios. Strong increases in net primary productivity (NPP) (35–54%) were projected in northern European ecosystems as a result of a longer growing season and higher CO₂ concentrations. Changing water balance dominated the projected responses of southern European ecosystems, with NPP declining or increasing only slightly relative to present-day conditions. Both site and continental scale models showed large increases in yield of rain-fed winter wheat for northern Europe, with smaller increases or even decreases in southern Europe. Site-based, regional and continental scale models showed large spatial variations in the response of nitrate leaching from winter wheat cultivation to projected climate change due to strong interactions with soils and climate. The variation in simulated impacts was smaller between scenarios based on RCMs nested within the same GCM than between scenarios based on different GCMs or between emission scenarios.     

An investigation of the small ice cap instability in the Southern Hemisphere with a coupled atmosphere-sea ice-ocean-terrestrial ice model

 A simple climate model has been developed to investigate the existence of the small ice cap instability in the Southern Hemisphere. The model consists of four coupled components: an atmospheric energy balance model, a thermodynamic snow-sea ice model, an oceanic mixed layer model and a terrestrial ice model. Results from a series of experiments involving different degrees of coupling in the model show that the instability appears only in those cases when an explicit representation of the Antarctic ice sheet is not included in the model. In order to determine which physical processes in the ice sheet model lead to a stabilization of the system we have conducted several sensitivity experiments in each of which a given ice sheet process has been removed from the control formulation of the model. Results from these experiments suggest that the feedback between the elevation of the ice sheet and the snow accumulation-ice ablation balance is responsible for the disappearance of the small ice cap instability in our simulation. In the model, the mass balance of the ice sheet depends on the air temperature at sea level corrected for altitude and it is, therefore, a function of surface elevation. This altitude-mass balance feedback effectively decouples the location of the ice edge from any specific sea level isotherm, thus decreasing the model sensitivity to the albedo-temperature feedback, which is responsible for the appearance of the instability. It is also shown that the elevation-radiative cooling feedback tends to stabilize the ice sheet, although its effect does not seem to be strong enough to remove the instability. Another interesting result is that for those simulations which include the terrestrial ice model with elevation-dependent surface mass balance, hysteresis is exhibited, where for a given level of external forcing, two stable solutions with different, non-zero ice-sheet volume and area and different air and ocean temperature fields occur. However, no unstable transition between the two solutions is ever observed. Our results suggest that the small ice cap instability mechanism could be unsuitable for explaining the inception of glaciation in Antarctica. Received: 14 April 1997 / Accepted: 22 October 1997     

EURD observations of interstellar radiation

The hot interstellar medium (ISM) has far-reaching effect upon thestructure of galaxies. Although ISM heating processes are fairly wellunderstood, after decades of study, the processes that cool the hotinterstellar medium remain obscure. The EURD spectrograph was designed tomeasure the diffuse cosmic background from 350 to 1100 Å in order tostudy the hot ISM and the mechanisms by which it sheds its energy. Wepresent the first analysis of EURD observations of the cosmic background.These EURD observations have proven to be far more sensitive than previouswork; compared to previous results, we have improved the limits to theintensity of 450 to 900 Å line emission from the ISM by one to twoorders of magnitude. Our limit to OVI 1032 Å / 1038 Å doublet of 7900ph s^-1 cm^-2 str^-1 is the lowest yet reported. The EURDlimits to line emission are less intense than predicted by a varietytheoretical models of the local ISM.     

The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

Rocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO₂, Al₂O₃ etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.     

Scheduling the EChO survey with known exoplanets

The Exoplanet Characterization Observatory (EChO) is a concept of a dedicated space telescope optimized for low-resolution transit and occultation spectroscopy to study the exoplanet diversity through the composition of their atmospheres. The scope of this paper is to answer the following question: Can we schedule a nominal EChO mission, with targets known today (in mid 2013), given the science requirements, realistic performances and operational constraints? We examine this issue from the point of view of duration of the mission and the scheduling restrictions with a sample of exoplanet systems known nowadays. We choose different scheduling algorithms taking into account the science and operational constraints and we verified that it is fairly straightforward to schedule a mission scenario over the lifetime of EChO compliant with the science requirements. We identified agility as a critical constraint that reduces significantly the efficiency of the survey. We conclude that even with known targets today the EChO science objectives can be reached in the 4.5 years duration of the mission. We also show that it is possible to use gaps between exoplanet observations, to fit the required calibration observations, data downlinks and station keeping operations or even to observe more exoplanet targets to be discovered in the coming years.     

Geostrophic Adjustment Problems in a Polar Basin

The geostrophic adjustment of a homogeneous fluid in a circular basin with idealized topography is addressed using a numerical ocean circulation model and analytical process models. When the basin is rotating uniformly, the adjustment takes place via excitation of boundary propagating waves and when topography is present, via topographic Rossby waves. In the numerically derived solution, the waves are damped because of bottom friction, and a quasi-steady geostrophically balanced state emerges that subsequently spins-down on a long time scale. On the f-plane, numerical quasi-steady state solutions are attained well before the system's mechanical energy is entirely dissipated by friction. It is demonstrated that the adjusted states emerging in a circular basin with a step escarpment or a top hat ridge, centred on a line of symmetry, are equivalent to that in a uniform depth semicircular basin, for a given initial condition. These quasi-steady solutions agree well with linear analytical solutions for the latter case in the inviscid limit.

On the polar plane, the high latitude equivalent to the β-plane, no quasi-steady adjusted state emerges from the adjustment process. At intermediate time scales, after the fast Poincaré and Kelvin waves are damped by friction, the solutions take the form of steady-state adjusted solutions on the f-plane. At longer time scales, planetary waves control the flow evolution. An interesting property of planetary waves on a polar plane is a nearly zero eastward group velocity for the waves with a radial mode higher than two and the resulting formation of eddy-like small-scale barotropic structures that remain trapped near the western side of topographic features.

RÉSUMÉ?Traduit par la rédaction] Nous étudions l'ajustement géostrophique d'un fluide homogène dans un bassin circulaire ayant une topographie idéalisée à l'aide d'un modèle numérique de circulation océanique et de modèles analytiques de processus. Quand le bassin est en rotation uniforme, l'ajustement se fait par l'excitation d'ondes de propagation aux limites, et en présence de topographie, par des ondes de Rossby topographiques. Dans la solution numériquement dérivée, les ondes sont amorties à cause frottement contre le fond, et un état quasi-stable géostrophiquement équilibré s’établit pour ensuite décélérer sur une longue période de temps. Sur le plan f, les solutions numériques d’états quasi-stables sont atteintes bien avant que l’énergie du système mécanique soit entièrement dissipée par le frottement. Nous démontrons que les états ajustés apparaissant dans un bassin circulaire avec un accore en forme de marche ou une crête en forme de merlon, centrés sur une ligne de symétrie, sont équivalents à ceux d'un bassin semi-circulaire de profondeur uniforme pour une condition initiale donnée. Les solutions quasi-stables s'accordent bien avec les solutions analytiques linéaires pour le dernier cas dans la limite de la non-viscosité.

Sur le plan polaire, la haute latitude équivalente au plan β, le processus d'ajustement n'aboutit à aucun état ajusté quasi-stable. Aux échelles de temps intermédiaires, après l'amortissement des ondes rapides de Poincaré et Kelvin par le frottement, les solutions prennent la forme de solutions ajustées d’états stables sur le plan f. Aux échelles de temps plus longues, les ondes planétaires déterminent l’évolution de l’écoulement. Une propriété intéressante des ondes planétaires sur un plan polaire est une vitesse de groupe presque nulle vers l'est pour les ondes ayant un mode radial plus grand que deux et la formation résultante de structures barotropiques de petite échelle de type remous qui demeurent emprisonnées près du bord ouest des caractéristiques topographiques.