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This chapter deals with an approach towards a physical description of the spreading of pollution in a turbulent atmosphere. We may witness this phenomenon quite commonly by, e.g., following the smoke of a chimney or the steam emerging from a power plant. It is remarkable that the equations to quantify atmospheric turbulence reveal close similarities with the formalism provided by the Fick’s diffusion laws. This is not only true for passive constituents (like smoke or moisture) but also for the turbulent diffusion of heat and momentum. Modelling of atmospheric turbulences, however, is still based on two conceptions which describe the same phenomenon from different perspectives. The development of an all-embracing view has thus remained a challenging task for the future.

Eddy covariance is an established technique for registering sensible heat fluxes. However, this method is less adequate for smaller surfaces that cannot match the associated footprint. As an alternative technique, a recently constructed acoustic anemometer (Ly-ATOM) is tested, which operates horizontally at an extension of circa 1 m and a data-acquisition frequency of 1 Hz. The Ly-ATOM device reproduces both the acoustic virtual temperature and the horizontal wind components registered by a three-dimensional sonic anemometer. As this Ly-ATOM device can be applied much closer to the ground compared with a sonic anemometer, the size of the related source area is significantly reduced (by a factor of 25). Two methods are used to retrieve the sensible heat flux from variance characteristics of temperature and the horizontal wind components recorded by the Ly-ATOM device: combining the flux-variance-similarity and alternative-flux-variance methods for use in unstable and stable stratification, respectively, yields good results for the sonic measurements. Therefore, these methods can be applied to the Ly-ATOM device as well. In investigating the sensitivity to detect modified surface characteristics, specifically increased evapotranspiration and decreased surface albedo, the Ly-ATOM device proves to be superior to the sonic anemometer which is more vertically removed from the surface of interest.

The Brazilian semiarid region (one million km²) is densely populated (25 million inhabitants), but its natural water availability is low. Despite the fact that evaporative processes are key to the regional water deficit, the actual evapotranspiration of natural environment has rarely been measured, especially in the native Caatinga dry forest. We hereby propose a simple method that demands the monitoring of five meteorological variables: relative humidity, global radiation, canopy and air temperature, as well as wind speed. These values are used to assess leaf energy balance, yielding net radiation (Rn) and actual evapotranspiration (LE). To estimate the actual Caatinga evapotranspiration under natural conditions and in different seasons, the proposed method was applied in situ during nine months. The application site was the Aiuaba Experimental Basin, situated in an environmental protection area in the North-eastern Brazil. The method provided consistent results when compared with independent measurements, such as atmospheric demand, leaf area, and soil water content variation. Results indicate that the daily average net radiation is 12 MJ m-². During the dry season, the actual evapotranspiration is very low, with negligible LE/Rn ratio. Contrastingly, in the rainy season, it raises to 6 mm per day, with average LE/Rn ratio equalling 0.89. The results show that the actual evapotranspiration in the Caatinga long-lasting dry season (up to nine months per year) is controlled by the water availability in the soil.

Die Eddy-Kovarianz-Methode ist ein bewährtes Verfahren zur Erfassung des fühlbaren Wärmeströms mit Hilfe dreidimensionaler Sonic Anemometer. Diese Methode eignet sich jedoch nicht für kleinere Flächen wie Gründächer, da diese nicht mit den räumlichen Dimensionen des entsprechenden Footprints übereinstimmen. Als eine alternative Methode wird ein kürzlich konstruiertes akustisches Anemometer (Ly-ATOM) getestet, das horizontal mit einer Ausdehnung von ca. 1 m und einer Datenerfassungsfrequenz von 1 Hz arbeitet. Das Ly-ATOM ist in der Lage sowohl die akustische virtuelle Temperatur als auch die horizontalen Windkomponenteneines dreidimensionalen Sonic Anemometers zuverlässig zu reproduzieren. Da das Ly-ATOM viel näher am Boden angebracht werden kann, kann die Größe des Footprint erheblich reduziert werden (um den Faktor 25). Zwei Methoden werden verwendet, um den fühlbaren Wärmestrom aus den Schwankungen der Temperatur und der horizontalen Windkomponenten, die vom Ly-ATOM aufgezeichnet wurden, zu ermitteln:Die Kombination der Fluss-Varianz-Ähnlichkeits-Methode und der alternativen Fluss-Varianz-Methode für die Anwendung bei labiler bzw. stabiler Schichtung führt zu guten Ergebnissen für die Sonic-Messungen. Daher können diese Methoden auch auf duie Messungen des Ly-ATOMs angewandt werden.Bei der Untersuchung der Senisitivität zur Detektion veränderter Oberflächeneigenschaften, insbesondere erhöhter Evapotranspiration und verringerter Oberflächenalbedo Albedo, erweist sich das Ly-ATOM-Gerät als geeigneter im Vergleich zum Sonic Anemometer, welches vertikal weiter von der zu untersuchenden Oberfläche entfernt ist.

The Brazilian semiarid region covers about 1 million km² and is characterised by irregular rainfall and high potential evaporation (above 2000 mm per year of water loss). To overcome the lack of water, the population depends on man-made reservoirs, which are the main source of water during the dry season (8 months of the year). In the federal state of Ceará there is on average one dam every 5 km, totalling more than 20,000. It is, therefore, noteworthy how the accurate estimation of evaporation would improve the management of scarce water resources. Moreover, climate change scenarios predict impacts on water availability in this region and better accuracy in evaporation assessments are fundamental to foster such simulations, which itself works at an intra-day time resolution. We suggest the most precise measurements possible, close to the lake and preferably on top of the water body. We used between October-December 2019 two direct measurement sensors in floating pans on the reservoir. A newly developed one based on the echo sounder principle and a standard differential pressure meter. Both show that at the reservoir more than 6 mm evaporate per day on average in the dry period. Uncertainty analyses for a single measurement show good performance of the acoustic sensor (± 0.11 mm) compared to the standard sensor (± 0.6 mm), sensitive enough to record the daily course of evaporation. The field measurements show 3.7% of uncertainty comparing the acoustic sensor with the standard pressuremeter. The minimum number of measurements with the acoustic sensor to obtain such uncertainty is 307. In this case, for 1h step, the uncertainty is 1.1%. The wave movement caused by wind influences the quality and frequency of the measurement. These results serve as a basis for future approaches to assess spatial variation of evaporation and wind on the lake. This might highlight the minimal number of instruments needed to solve the issue of spatial variability of evaporation.

Evaporation is a major factor controlling the hydrological dynamics of surface water reservoirs in dry environments, therefore quantification with minimal uncertainties is desired. The aim of this paper is to assess the spatial variability and impact of riparian vegetation on reservoir evaporation by remote sensing. Eight reservoirs located in subhumid and semi-arid climates in the Brazilian Drylands were studied. Scenes from Landsat 5 and Landsat 8 satellites (1985 and 2018) supplied the data for four evaporation models. For reference evaporation, the Class A Pan and Piche Evaporimeter closest to the reservoirs were considered. The occurrence/density of riparian vegetation was associated with the Normalized Difference Vegetation Index (NDVI) and its influence on evaporation was assessed. The Surface Energy Balance System for Water (AquaSEBS) model presented the best average performance (Nash-Sutcliffe Efficiency coefficient 0.40 ± 0.19). Evaporation was observed to be higher at the reservoirs' margins and near the dams, due to the contact of exposed soil and rock/concrete, respectively, which transfer heat to the water. Marginal areas near the riparian forest presented low evaporation rates with decreases between 18% and 31% in relation to the average. This interdependence was evidenced by the high negative correlation (R2 0.87–0.96) between NDVI and evaporation; vegetation reduces radiation because of the shading of the reservoir margin and changes local aerodynamics, reducing evaporation. Depending on the spatial variability of evaporation, it was found that the volumes transferred to the atmosphere may have variations of up to 30%. On average, the evaporated volume in all the studied reservoirs is 450,000 m3/day, a quantity enough to supply more than two million people. Overall, the results of this study contribute not only to a better understanding of the spatial variability of evaporation in surface reservoirs, but also of the interdependence between riparian vegetation and evaporation rates.

Wenn man davon ausgeht, dass man die Entfernungen zwischen den Schallsender-Empfanger-Kombinationen genau genug bestimmen kann und die Laufzeit von Schallsignalen auf der Strecke zwischen diesen Kombinationen genau genug messen kann, dann kann man auch Temperatur- und Stromungsverhaltnisse in Innenraumen ausmessen.

Die Geschwindigkeit, mit der die Schallwelle ausgehend von einer Schallquelle hin zu einem Schallempfanger unterwegs ist, hangt von der Temperatur und der Bewegung der Luft (Schalltransportmedium) ab. Im Umkehrschluss kann man Verfahren anwenden, die es ermoglichen, aus der auf einer bekannten Schallausbreitungsstecke gemessenen Schallgeschwindigkeit auf die Lufttemperatur bzw. Stromungseinfluss entlang dieser Strecke zu schliesen.

Die physikalischen Eigenschaften der Luft modifizieren die Schallgeschwindigkeit in Abhangigkeit von Temperatur und Stromungsverhaltnissen. Es wurde eine akustische Methode, die ursprunglich fur meteorologische Anwendungen zur Messung der Lufttemperatur bzw. der Windgeschwindigkeit entwickelt wurde, fur die Anwendung klimatischer Fragestellungen in Innenraumen beliebiger Grose modifiziert. Eine Kombination von mehreren Schallgeschwindigkeitsmesstrecken, die flachenhaft oder raumlich angeordnet werden, ermoglicht es, auf diese Weise die sich uber die Zeit hinweg andernden Schallgeschwindigkeitsfelder aufzuzeichnen.

The present paper analyses the influence of hydroclimatic forcing and water quality on the evaporation process of Lake Santo Anastacio, a tropical lake with capacity of about 0.4 hm3 and maximum depth of 5 m, located in the city of Fortaleza, Ceara, Brazil. The two-dimensional model CE-QUAL-W2 was used for modelling hydrodynamics and direct evaporation from the lake, and an evaporation estimation equation based on the mass transfer method was calibrated. Subsequently, direct evaporation was modelled while, at the same time, thermal characteristics of the water column were examined. Then, modelled evaporation was compared to measurements obtained over a period of 11 years (2009–2019) with a Class A pan. The results indicated a daily thermal stratification of up to 2 °C. When modelled evaporation was compared with the measurements obtained with the Class A pan, the mean Class A pan coefficients (K) were between 0.66 and 0.69, lower than the values reported in literature, possibly due to the high concentration of pollutants in the lake. Finally, the K coefficients were negatively correlated with wind speed (R2 of 0.51; p-value

The ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) model, with a higher order closure for tall vegetation, has already been successfully tested and validated for spruce forests. The aim of this paper is the further application for a clearing with a heterogeneous structure of the underlying surface. The comparison with flux data shows a good agreement with a footprint aggregated tile approach of the model. However, the results of a comparison with a tile approach on the basis of the mean land use classification of the clearing is not significantly different. It is assumed that the footprint model is not accurate enough to separate small scale heterogeneities. All measured fluxes are corrected for energy balance closure with a Bowen ratio and a buoyancy flux correction method. The comparison with the model – where the energy balance is closed – shows that the buoyancy correction for Bowen ratios > 1.5 better fits the measured data. For lower Bowen ratios, the correction probably lies between both methods, but the amount of available data was too small to make a conclusion. With an assumption of a similarity between water and carbon dioxide fluxes, no correction of the net ecosystem exchange is necessary for Bowen ratios > 1.5.

A remote sensing technique to detect area-averaged temperature and flow properties within an area under investigation, utilizing acoustic travel-time measurements, is introduced. This technique uses the dependency of the speed of acoustic signals on the meteorological parameters temperature and wind along the propagation path. The method itself is scalable: It is applicable for investigation areas with an extent of some hundred square metres as well as for small-scale areas in the range of one square metre. Moreover, an arrangement of the acoustic transducers at several height levels makes it possible to determine profiles and gradients of the meteorological quantities. With the help of two examples the potential of this remote sensing technique for simultaneously measuring averaged temperature and flow fields is demonstrated. A comparison of time histories of temperature and wind values derived from acoustic travel-time measurements with point measurements shows a qualitative agreement whereas calculated root-mean-square errors differ for the two example applications. They amount to 1.4 K and 0.3 m/s for transducer distances of 60 m and 0.4 K and 0.2 m/s for transducer distances in the range of one metre.

Im Beitrag werden einige Moglichkeiten vorgestellt, die akustische Verfahren fur energetische Fragestellungen im Bereich denkmalgeschutzter Gebaude, Raume, Wande usw. eroffnen. Im Mittelpunkt stehen akustische Verfahren, die fur Anwendungen im baulichen Bereich geeignet sind – typischerweise sind das Dimensionen mit Ausmasen wie bei Fenstern und Turen sowie von Raumen oder Hallen.

Within micrometeorology the term modeling is not uniquely defined. It refers to various methods covering a range of complexity extending from simple regressions up to complicated numerical models . In applied meteorology (agro meteorology and hydro meteorology) simple analytical models are very common. Modeling of evaporation is particularly important but sophisticated numerical methods are not yet widely used in this research area. The following chapter describes different types of models and their limitations beginning with simple analytical methods up to numerical models of near-surface energy and matter transport. The application of models in heterogeneous terrain receives special attention and related flux averaging approaches are addressed in a separate subchapter.

The German Research Centre for Geosciences (GFZ) operates a GNSS water vapour tomography system using about 350 German GNSS stations. The GNSS data processing at the GFZ works in near real-time and provides zenith total delays, integrated water vapour and slant delay data operationally. This large data set of more than 50,000 slant delays per hour is used to reconstruct spatially resolved humidity fields by means of tomographic techniques. It can be expected that additional observations from the future Galileo system provide more information with improved quality. A simulation study covering 12 h at 14 July 2009 was therefore started to estimate the impact of GPS, Galileo and GLONASS data on the GNSS tomography. It is shown that the spatial coverage of the atmosphere with slant paths is highly improved by combining observations from two or three satellite systems. Equally important for a reliable tomographic reconstruction is the distribution of slant path intersections as they are required to locate the integrated delay information. The number of intersection points can be increased by a factor of 4 or 8 if two or three systems are combined and their distribution will cover larger regions of the atmosphere. The combined data sets can be used to increase the spatiotemporal resolution of the reconstructed humidity fields up to 30 km horizontally, 300 m vertically and 15 min. The reconstruction quality could not be improved considerably using the currently available techniques.

Acoustic images of variable parameters of objects can be reconstructed by means of tomographic techniques which utilize the propagation of sound waves in the investigated medium. The technique described here utilizes sound frequencies in the audio range for acoustic imaging. The temperature-dependent sound speed as well as the flow field can be estimated by measuring the travel time of a defined acoustic signal between a sound source and a receiver when the distance between them is known exactly. The properties of the flow field are reconstructed using reciprocal sound rays to separate the direction-independent Laplace sound speed from the effective sound velocity. The temperatures in the flow field are then calculated by a combined inversion of all travel-time information resulting from the Laplace sound speed using an algebraic reconstruction technique. This reconstruction technique provides a cross section of the temperature distribution throughout the investigated area or volume. The tomographic system has been generalized to allow flow phenomena and temperature fields to be investigated with adapted sampling rates. The technique and procedures are exemplified by means of a scalable (from model-sized up to large-scale outdoor tomography) commercially oriented prototype of a tomograph which can utilize the whole audio and near-audio ultrasonic range. The software technology approach forms an inherent part of the realization.

An attempt is made to compare results oflarge-eddy simulation (LES) in a convective boundarylayer using the model PALM with experimental data obtained from acoustic travel time tomography.This method provides two-dimensional data arrays, which are considered as more suitable forLES-validation than classical local orline-integrated measurements, because the tomographic data are area- or volume-averaged.

Acoustic travel time measurements are used to investigate the spatial and temporal variability of air temperature and wind fields in the atmospheric surface layer. The observational system consists of several sound sources and receivers, which were distributed over an apparently horizontal homogeneous terrain to calculate a two-dimensional distribution of air temperature using a tomographic algorithm. Parallel to the tomographic measurements, all components of the energy balance of the investigated surface are observed using conventional micrometeorological methods. The sum of these components of the energy balance must theoretically be equal to zero. However, the sum of the observed components often add up to an unexplainable residuum. It can be shown here that the diurnal variability of the residuum of such energy balance measurements and the simultaneously observed diurnal variability of the horizontal gradients in the air temperature field seem to be closely related. This is a hint that the k...

The acoustic tomography method is applied in the atmospheric surface layer to observe near-surface temperature fields. Important advantages of this technique are the remote sensing capacity and the possibility of directly deriving area-average meteorological quantities. Combined observations of the air temperature using an acoustic tomography system and point measurements were carried out to validate the tomographic method. Results were used to compare representativeness for a designated area of direct measurements with the tomographic solution. The results demonstrate agreement between the two different measurement methods, except for some deviations of absolute values mainly caused by an imperfectly sheltered and ventilated thermocouple device.

Acoustic travel time tomography is presented as a possibility for remote monitoring of near surface air temperature and wind fields. This technique provides line-averaged effective sound speeds changing with temporally and spatially variable air temperature and wind vector. The effective sound speed is derived from the travel times of sound signals which propagate at defined paths between different acoustic sources and receivers. Starting with the travel time data a tomographic algorithm (Simultaneous Iterative Reconstruction Technique, SIRT) is used to calculate area-averaged air temperature and wind speed. The accuracy of the experimental method and the tomographic inversion algorithm is exemplarily demonstrated for one day without remarkable differences in the horizontal temperature field, determined by independent in situ measurements at different points within the measuring field. The differences between the conventionally determined air temperature (point measurement) and the air temperature determined by tomography (area-averaged measurement representative for the area of the measuring field 200 m x 260 m) were below 0.5 K for an average time of 10 minutes. The differences obtained between the wind speed measured at a meteorological mast and calculated from acoustic measurements are not higher than 0.5 m s -1 for the same averaging time. The tomographically determined area-averaged distribution of air temperature (resolution 50 m x 50 m) can be used to estimate the horizontal gradient of air temperature as a pre-condition to detect horizontal turbulent fluxes of sensible heat.

Acoustic travel time tomography is demonstrated as a technique for remote monitoring of near surface air temperature and wind fields. An experimental procedure is presented which provides line-averaged values of the effective sound speed changing with air temperature and wind vector. In this study the travel times of sound signals at defined propagation paths between different acoustic sources and receivers were measured. The travel time data were inverted into effective sound speed values by using a special tomographic algorithm (Simultaneous Iterative Reconstruction Technique) to obtain area-averaged meteorological quantities (air temperature, wind speed). The method of acoustic travel time tomography will be applied to the atmosphere in order to directly provide spatially averaged data which are needed for evaluation of large-eddy simulation and microscale meteorological models as well as to complete conventional point measurements.

Acoustic tomography is presented as a technique for remote monitoring of meteorological quantities. This method and a special algorithm of analysis can directly produce area averaged values of meteorological parameters. As a result, consistent data will be delivered for validation of numerical atmospheric micro-scale models. Such a measuring system can complement conventional point measurements over different surfaces. The procedure of acoustic tomography uses the horizontal propagation of sound waves in the atmospheric surface layer. The state of the crossed atmosphere can be estimated from measurements of travel time of acoustic signals between sources and receivers on different points in a tomographic array. Derivation of area averaged values of the sound speed and furthermore of air temperature results from the inversion of travel time values for all possible acoustic paths. Thereby, the applied straight-ray two-dimensional tomography model is characterised as a method with small computational equirements and simple handling, especially, for online work.

A land-surface module to couple a meteorological and a hydrologic model was developed to simulate the water cycle in a closed manner. The module allows to consider the hydrologic processes of the river catchment (translation, retention, lateral discharge) in the meteorological model which itself drives the hydrologic model by predicted evapotranspiration and precipitation. Besides this two-way-coupling, the module allows to consider subgrid-scale surface processes and to heterogenize precipitation in the meteorological model. The results of 24-h simulations with and without a two-way-coupling of the models substantiate that even on a short time scale surface runoff and lateral water flows affect soil wetness, soil temperature, cloudiness and the thermal regime of the atmospheric boundary layer within the catchment. As expected, the coupling results in a slight trend towards moister valleys and drier hills.

The rapid growth of renewable energies is leading to increasing imbalances between electricity production and consumption. During high-wind periods with local overproduction of energy, the stability of the distribution grid can only be ensured if a fraction of (wind) farms is throttled or shut down. Precise infeed forecasts of wind and solar power production for individual sites assist in reducing these risks because controlling measures for renewables as well as the regulation of conventional power plants (coal, gas etc.) can be planned with longer lead times. A forecast system is presented which includes the whole chain from weather forecast to infeed forecast. The foundation is made up of weather model forecasts from the UK Met Office and from the high resolution regional model WRF. The conversion of forecast weather elements (e.g. wind profiles) into infeed energy from the turbines/farms is realized by artificial neural networks (ANNs). ANNs are able to recognize and eliminate the systematic errors produced by weather models. The quality of the infeed forecasts is validated with results for transmission nodes across the grid of the company E.ON edis, situated in the Northeast of Germany. (4 pages)

Measurements of land-surface emission rates of greenhouse and other gases at large spatial scales (10 000 m2) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micro-meteorological methods like flux-gradient methods which were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. Non-intrusive path-integrating measurements are utilized. Successful application of a flux-gradient method requires confidence in the gradients of trace gas concentration and wind, and in the applicability of boundary-layer turbulence theory; consequently the procedures to qualify measurements that can be used to determine the flux is critical. While there is relatively high confidence in flux measurements made under unstable atmospheres with mean winds greater than 1 m s−1, there is greater uncertainty in flux measurements made under free convective or stable conditions. The study of N2O emissions of flat grassland and NH3 emissions from a cattle lagoon involves quality-assured determinations of fluxes under low wind, stable or night-time atmospheric conditions when the continuous "steady-state" turbulence of the surface boundary layer breaks down and the layer has intermittent turbulence. Results indicate that following the Monin-Obukhov similarity theory (MOST) flux-gradient methods that assume a log-linear profile of the wind speed and concentration gradient incorrectly determine vertical profiles and thus flux in the stable boundary layer. An alternative approach is considered on the basis of turbulent diffusivity, i.e. the measured friction velocity as well as height gradients of horizontal wind speeds and concentrations without MOST correction for stability. It is shown that this is the most accurate of the flux-gradient methods under stable conditions.

A GNSS water vapour tomography system developed to reconstruct spatially resolved humidity fields in the troposphere is described. The tomography system was designed to process the slant path delays of about 270 German GNSS stations in near real-time with a temporal resolution of 30 min, a horizontal resolution of 40 km and a vertical resolution of 500 m or better. After a short introduction to the GPS slant delay processing the framework of the GNSS tomography is described in detail. Different implementations of the iterative algebraic reconstruction techniques (ART) used to invert the linear inverse problem are discussed. It was found that the multiplicative techniques (MART) provide the best results with least processing time, i.e., a tomographic reconstruction of about 26,000 slant delays on a 8280 cell grid can be obtained in less than 10 min. Different iterative reconstruction techniques are compared with respect to their convergence behaviour and some numerical parameters. The inversion can be considerably stabilized by using additional non-GNSS observations and implementing various constraints. Different strategies for initialising the tomography and utilizing extra information are discussed. At last an example of a reconstructed field of the wet refractivity is presented and compared to the corresponding distribution of the integrated water vapour, an analysis of a numerical weather model (COSMO-DE) and some radiosonde profiles.

The path-averaging, multi-component Fourier Transform Infrared (FTIR) absorption spectrometry at an open path of 100 m length is applied for the up-scaling of greenhouse gas (GHG) flux measurements from soil surfaces. For the detection of the emissions of N 2 O and further GHG from arable field soils a measuring tunnel for controlled enrichment of released gases was installed at the soil surface covering an area of 495 or 306 m 2 . The concentrations of GHG were measured by FTIR across the whole measuring tunnel. The precision of the FTIR system is discussed to detect the concentration increases during a time period of up to two hours. During a 2-years-time frame the N 2 O fluxes between the soil and the atmosphere at the agricultural field varied between 1.0 and 21 μg N 2 O-N m -2 h -1 . A non-intrusive emission and flux measurement method at a scale from 100 m up to 27.000 m 2 on the basis of the fluxgradient method (0.50 and 2.70 m height above surface) was developed and tested by means of FTIR (N 2 O and further GHG concentrations) and area averaging meteorological measurements (determination of horizontal winds and friction velocity using acoustic tomography). To detect the concentration gradient between the two heights the precision of the FTIR system is discussed. Two campaigns in October 2007 and June 2008 were performed with this new methodology when wind speeds were low. The measurement errors are discussed and the results compared with the measurement tunnel results that were higher by up to 25 %.

[1] The observation of GPS slant delays from ground GPS networks can be used to reconstruct spatially resolved humidity fields in the troposphere by means of tomographic techniques. Tomography is always related to the solution of inverse problems which are very sensitive to the quality of the input data. Prior to a tomographic reconstruction, it is therefore necessary to quantify the information provided by a given set of GPS slant delay data. This work describes the properties and the information content of more than two million GPS slant delays taken in March 2006 by a continuously operating German GPS network. The temporal and spatial distribution of the slant paths in the atmosphere and their angular distribution in the local system of the GPS station is given. These distributions depend on the satellite orbits and show some characteristic pattern. The available information is estimated by investigating the distribution of intersection points between the slant paths. From these data it is possible to identify regions that are well covered by GPS slant paths and to evaluate the applicability of the existing German GPS stations for continuous atmosphere sounding.

Temperature can be estimated by acoustic travel time measurements along known sound paths. By using a multitude of known sound paths in combination with a tomographic reconstruction technique a spatial and temporal resolution of the temperature field can be achieved. Based on it, this article focuses on an experimental method in order to determine the spatially differentiated development of room temperature with only one loudspeaker and one microphone. The theory of geometrical room acoustics is being used to identify sound paths under consideration of reflections. The travel time along a specific sound path is derived from the room impulse response. Temporal variances in room impulse response can be attributed primarily to a change in air temperature and airflow. It is shown that in the absence of airflow a 3D acoustic monitoring of the room temperature can be realized with a fairly limited use of hardware.