Armin Raabe

Numerical Investigations on the Influence of Subgrid-Scale Surface Heterogeneity on Evapotranspiration and Cloud Processes

Abstract Numerical experiments were performed with a meso-β-scale meteorological model to investigate the influence of subgrid-scale surface heterogeneity on the prediction of evapotranspiration, cloud, and precipitation formation. The results of simulations using different horizontal grid resolutions and assuming the dominant land-use type within a grid box as the representative surface type for the entire grid element am compared with those obtained from model runs considering subgrid-scale heterogeneity by separately determining the fluxes of the respective subgrid-scale land-use types. The same surface parameterization scheme was applied in both cases. All of these numerical experiments show that the surface characteristics and, hence, the subgrid-scale surface processes strongly affect the predicted microclimate close to the ground. Furthermore, the model results also provide evidence that in the case of applying dominant land-use types the grid resolution may strongly affect the calculated water and...

Acoustic tomography on the basis of travel-time measurement

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.

Acoustic tomographic imaging of temperature and flow fields in air

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.

Estimates of the information provided by GPS slant data observed in Germany regarding tomographic applications

[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.

Acoustic tomography inside the atmospheric boundary layer

Abstract 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.

Acoustic Tomography as a Remote Sensing Method to Investigate the Near-Surface Atmospheric Boundary Layer in Comparison with In Situ Measurements

Abstract 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.

Observations of area averaged near-surface wind- and temperature-fields in real terrain using acoustic travel time tomography

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 earths 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.

Testing the effect of a two-way-coupling of a meteorological and a hydrologic model on the predicted local weather

Abstract 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.

STINHO - Structure of turbulent transport under inhomogeneous surface conditions. Part 1: The micro-α scale field experiment

A micrometeorological field experiment was performed within the frame of the STINHO-project (structure of turbulent transport under inhomogeneous conditions) at the boundary layer field site of the Meteorological Observatory Lindenberg of the German Meteorological Service (Deutscher Wetterdienst) in the summer of 2002 in order to investigate the interaction of thermally heterogeneous surfaces with the turbulent atmosphere. The intention was to compare conventional meteorological point and vertically integrated measurements with area-covering air flow observations and numerical simulations. To observe horizontally variable flow and temperature fields above a heterogeneous land surface, simultaneous acoustic methods (travel time tomography), optical observation methods (IR-camera and line-integrated scintillometer-measurements), as well as the airborne measurement system Helipod were used. The data set will be applied in future to validate large-eddy simulations adjusted to the area of investigation.

Flow field detection using acoustic travel time tomography

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.