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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.

A hot-spot region of climate change is the Middle East where temperatures actually have a rising tendency and this will increase in future. To mitigate the progressing thermal burden urban planning has to develop adaptation measures. On the basis of micrometeorological simulations for a quarter in Cairo we suggest a decomposition of air temperatures into two temporal and two spatial patterns, respectively explaining 97% and 94% of the temperature variability. We find that land-use has a significant impact on the spatial temperature distribution and should be modified for the purpose of heat adaptation. However, just 13% of the spatial temperature variability are explained by land-use, which is a quite limited impact. Regional weather conditions are the dominant factor for the spatial as well as the temporal development of urban heat.

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.

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.

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.

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.

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.

[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.

The GPS water vapour tomography is a new technique which provides spatially resolved water vapour distributions in the atmosphere under all weather conditions. This work investigates the information contained in a given set of GPS signals as a precondition to an optimal tomographic reconstruction. The spatial distribution of the geometric intersection points between different ray paths is used to estimate the information density. Different distributions of intersection points obtained from hypothetical GPS networks with varying densities of GPS stations are compared with respect to the horizontal and vertical resolution of a subsequent tomographic reconstruction. As a result some minimum requirements for continuously operating extensive GPS networks for meteorological applications are given.

A Multiplicative Algebraic Reconstruction Technique (MART) assimilation process of calibrated ground-based and occultation GPS TEC data into combined IRI/GCPM model data is described. The resulting electron density fields are assessed with in situ records. An improvement of the electron density estimations by IRI/GCPM of 27.25% at northern high latitudes is obtained through the assimilation of GPS data. A specification of NmF2 and hmF2 within a mean bias of only 8.5 × 10 10 m −3 and −6.1 km, respectively, is achieved in comparison with soundings of seven European ionosondes.

Integrated electron density measurements along GPS signal ray paths provide a useful data source for 3-dimensional ionospheric reconstruction. Since a substantial data amount is required for the application of the inverse problem, ground-based inversions are restricted to regions with high ground receiver density. The additional incorporation of space-based GPS data from LEO satellites allows the monitoring in areas less densely covered by ground receivers. We present a case study of 3-dimensional reconstruction for Nov. 2001 in the North polar region. Input data are calibrated TEC derived from IGS and CHAMP measurements. The results show increased electron densities near the geomagnetic pole. The comparison of the reconstruction results with measurements of the Langmuir Probe onboard CHAMP shows a good agreement along the CHAMP orbit. The large-scale structure of the ionospheric observations are closely related to the present geomagnetic conditions.

An acoustic tomographic system has been used to estimate area averaged wind and temperature data within an area of 97 m × 12 m considering the dependence of sound speed on meteorological conditions To obtain information about vertical structure of meteorological data, eight sound sources and receivers were placed in two different heights above the ground (0.5 m and 2.7 m). Spatially, the acoustic measurements correspond to line integrated N2O concentration measurements (98 m) using FTIR-spectrometers Taking stability of atmospheric layering into account, acoustic tomographic measurements serve as basis for estimating vertical fluxes of momentum and sensible heat.

Acoustic travel time tomography will be applied on the atmosphere to directly provide spatially averaged values of meteorological quantities (temperature and wind field) which are needed for the evaluation of numerical models as well as to complete in situ point measurements and conventional studies of the atmospheric surface layer over natural surfaces. Sound waves propagate through the atmosphere with different sound velocities according to the distribution of air temperature and wind vector and become, therefore, an information carrier about the medium properties. In the presented study acoustic travel time data were measured between sources and receivers placed around an array of 200×260 m2 as initial values for the tomographic reconstruction. Thereby, a frequently applied procedure of ray tomography, the simultaneous iterative reconstruction technique, characterized by simple handling and small computational requirements, was used. Additionally, a special system of equations for the isolation of the ...

Results of case studies with a mesoscale β meteorological model applying two different strategies to treat subgrid-scale surface heterogeneity are compared with each other to evaluate the effects of these strategies on the predicted hydrologically relevant quantities. In the first strategy, the mosaic approach, different land-use types are considered as separate patches within a grid cell independently interacting with mean atmospheric field quantities of that grid cell. Feedback to the grid scale is accomplished by forming area-weighted quantities from the fluxes provided by the individual patches for the soil–biosphere–atmosphere interaction. In the second strategy, a higher resolution subgrid is established within each model grid cell and the soil–biosphere–atmosphere interaction is determined for each subgrid cell with its individual soil and biosphere conditions and near-surface meteorological forcing. Probability density functions are used to evaluate the statistical behavior of both the strategies. It is substantiated that the partitioning of the atmospheric radiative and moisture forcing at the surface as well as cloud and precipitation formation can significantly be affected by the type of strategy. Using the explicit subgrid strategy results in a shift in the partitioning of energy towards decreasing Bowen ratios as compared to the mosaic approach. For very heterogeneous surfaces with strongly varying soil types and plant species an area-weighted meteorological near-surface forcing as used in the mosaic approach may artificially reduce evapotranspiration. An explicit subgrid strategy or individual near-surface meteorological forcing within the mosaic approach seems to be more adequate under such surface conditions. DOI: 10.1034/j.1600-0870.1996.00012.x