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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.

Acoustic travel time tomography uses the dependence of sound speed on temperature and flow properties along the propagation path to measure these parameters. An algorithm is introduced that is capable of resolving the two-dimensional flow field within a specified measuring area. Different flow fields have been simulated to explore the reconstruction properties of the algorithm. Furthermore, an experiment within the Barrel of Ilmenau has been carried out, which demonstrates that the acoustic tomographic method is able to detect flow fields in a closed convection chamber. The simulations reveal that the vector tomographic algorithm can resolve the flow field characteristics speed and direction. Furthermore it is demonstrated that the distribution of sound sources and receivers has to be homogeneous around the measuring site. For the reconstruction of flow distributions, this requirement is even more important compared to temperature reconstructions, since information along inclined sound ray paths within each grid cell are needed to reconstruct the flow vector properly.

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.

Any physical description setting for the distribution of momentum, energy and matter within the turbulent boundary layer of the atmosphere is usually made according to models based on the assumption of horizontal homogeneity. However, the earth's surface not being homogeneous, the consequence of this approach is that these physical models cannot be transferred nor applied under non-homogeneous conditions. The conventional methods used for observing micro-meteorology data do not appear to be adequate in interpreting data observed on heterogeneous surfaces. In this connection, the method of acoustic travel-time tomography is here introduced. This method uses the variability of the speed of sound according to meteorological quantities such as wind and temperature in order to observe synchronous data of wind- and air-temperature fields under natural conditions in a given area. This allows to shun a main requirement of conventional micro-meteorological experiments - the horizontally homogeneous conditions.

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 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.

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.

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.

Acoustic travel-time tomography is a remote sensing technique that uses the dependence of sound speed in air on temperature and wind speed along the sound propagation path. Travel-time measurements of acoustic signals between several sound sources and receivers travelling along different paths through a measuring area give information on the spatial distribution of temperature and flow fields within the area. After a separation of the two influences, distributions of temperature and flow can be reconstructed using inverse algorithms. As a remote sensing method, one advantage of acoustic travel-time tomography is its ability to measure temperature and flow field quantities without disturbing the area under investigation due to insertion of sensors. Furthermore, the two quantities—temperature and flow velocity—can be recorded simultaneously with this measurement method. In this paper, an acoustic tomographic measurement system is introduced which is capable of resolving three-dimensional distributions of temperature and flow fields in air within a certain volume (1.3 m × 1.0 m × 1.2 m) using 16 acoustic transmitter–receiver pairs. First, algorithms for the 3D reconstruction of distributions from line-integrated measurements are presented. Moreover, a measuring apparatus is introduced which is suited for educational purposes, for demonstration of the method as well as for indoor investigations. Example measurements within a low-speed wind tunnel with different incident flow situations (e.g. behind bluff bodies) using this system are shown. Visualizations of the flow illustrate the plausibility of the tomographically reconstructed flow structures. Furthermore, alternative individual measurement methods for temperature and flow speed provide comparable results.

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 ...