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Wang W.,Plurality | Rotach M.W.,Federal Office for Meteorology and Climatology | Rotach M.W.,University of Innsbruck
Boundary-Layer Meteorology | Year: 2010

The flux footprint probability distribution (FPD) functions for near-surface receptors over an idealised undulating surface are evaluated using a backward Lagrangian stochastic model. The wind and turbulence fields employed to drive the stochastic model are derived from large-eddy simulations, in which the horizontal wind aligns with the surface-elevation-varying direction. The flux FPD for a receptor is affected by flow divergence or convergence, and varies with the receptor's location. The widest crosswind-integrated FPD (CIFPD) curve with the smallest peak value appears when the receptor is located in the crest area, while the narrowest CIFPD curve with the largest peak value appears when the receptor is located in the windward area. Experiments are designed to highlight the impact of the horizontal homogeneity assumption on the estimation of the FPD. When the receptor is located in the area with surface-wind convergence, the peak value of the CIFPD is larger than its counterpart under assumed horizontally homogeneous flow conditions, with the peak position being closer to the receptor. The case is reversed when the receptor is located in the area with surface-wind divergence. Similar results are obtained when the CIFPD derived from an analytical footprint model (developed under the assumption of horizontally homogeneous flow conditions) is compared with that from the stochastic model over the undulating surface. The analytical model fails to simulate the CIFPD in the local downwind area under weak wind conditions due to the longitudinal wind fluctuation not being considered. © 2010 Springer Science+Business Media B.V.

Wunderli J.M.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Rotach M.W.,Federal Office for Meteorology and Climatology
Acta Acustica united with Acustica | Year: 2011

sonRAIL is a new Swiss simulation model for railway noise. Its propagation model is applying a ray tracing approach to take meteorological influences on sound propagation into account. The ray tracing algorithm is designed to allow for arbitrary sound speed profiles, which additionally can vary along the propagation path, for example as a consequence of changing surface properties. In order to do noise mapping for long-term averages, weather statistics for entire Switzerland were derived on the basis of model data of COSMO-2, the most recent version of the operational numerical weather prediction model of the Swiss Federal Office of Meteorology and Climatology, MeteoSwiss, with a grid resolution of 2.2 x 2.2 km2. Especially in regions with complex topography, as it is the case in Switzerland, this high spatial resolution is a major advantage. Additionally, a profile generator was devised, which allows to calculate vertical profiles of wind speed, temperature and humidity based on input data describing the general meteorological conditions and the local surface properties. Based on this profile generator a classification scheme for meteorological conditions was developed and a set of standard profile functions for wind speed and temperature is given for each class. The validation of the profile generator in comparison with profiles directly taken from COSMO-2 showed good agreement in general, but with a tendency to overestimate temperature gradients under highly stable conditions. A comparison of the probability of occurrence of the meteorological classes derived from COSMO-2 with observation data of conventional stations also yielded a high compliance. However nighttime situations with hardly any wind were underpredicted by COSMO-2. These findings confirm the fact that meteorological modeling is generally most difficult for low wind stable conditions. Yet the advantage of having meteorological statistics available in a small spatial resolution clearly outweighs this restraint. © S. Hirzel Verlag · EAA.

Arteaga A.,ETH Zurich | Fuhrer O.,Federal Office for Meteorology and Climatology | Hoefler T.,ETH Zurich
Proceedings of the International Parallel and Distributed Processing Symposium, IPDPS | Year: 2014

Bit-reproducibility has many advantages in the context of high-performance computing. Besides simplifying and making more accurate the process of debugging and testing the code, it can allow the deployment of applications on heterogeneous systems, maintaining the consistency of the computations. In this work we analyze the basic operations performed by scientific applications and identify the possible sources of non-reproducibility. In particular, we consider the tasks of evaluating transcendental functions and performing reductions using non-associative operators. We present a set of techniques to achieve reproducibility and we propose improvements over existing algorithms to perform reproducible computations in a portable way, at the same time obtaining good performance and accuracy. By applying these techniques to more complex tasks we show that bit-reproducibility can be achieved on a broad range of scientific applications. © 2014 IEEE.

Liechti K.,Swiss Federal Institute of forest | Zappa M.,Swiss Federal Institute of forest | Fundel F.,Swiss Federal Institute of forest | Germann U.,Federal Office for Meteorology and Climatology
Hydrological Processes | Year: 2013

The quality of hydrological discharge simulations depends to a great extent on the uncertainties in the meteorological input and the model parameterization. To quantify these uncertainties, we adopt ensemble techniques in a 4-year nowcast experiment for two nested flash-flood-prone basins in the southern Swiss Alps. The spatiotemporal uncertainties in the weather radar quantitative precipitation estimates (QPE) were accounted for by applying an ensemble of 25 radar fields. To account for uncertainties in model parameterization, a Monte Carlo experiment was run to find 26 equifinal model realizations. The resulting parameter ensemble, consisting of 26 members, was run with precipitation input obtained from interpolated pluviometer data and with the deterministic operational weather radar QPE. To produce the discharge nowcast, the PREcipitation-Runoff-EVApotranspiration HRU Model (PREVAH) was used. PREVAH was calibrated for the main catchment Verzasca. The results for the sub-catchment Pincascia are an independent internal verification of the nowcasting system. The three ensemble nowcasts and the two deterministic nowcasts are evaluated for a 4-year time series and for two events included in that period. The event analysis shows no clear superiority for either pluviometer-based or radar-based nowcasts. The performance for single events depends heavily on the storm characteristics. However, the evaluation of the 4-year nowcast shows that pluviometer-based nowcasts outperform radar-based nowcasts in the gauged and calibrated catchment and that there is added value in the application of parameter ensembles. For the small, ungauged catchment, the results achieved by the radar-based nowcasts are superior to the pluviometer-based nowcasts. Especially the radar ensemble proves to be of significant advantage for flash flood nowcasts in such catchments. © 2012 John Wiley & Sons, Ltd.

Baklanov A.A.,Danish Meteorological Institute | Grisogono B.,University of Zagreb | Bornstein R.,San Jose State University | Mahrt L.,Oregon State University | And 6 more authors.
Bulletin of the American Meteorological Society | Year: 2011

The scientific problem areas, research needs, and new efforts necessary for development of more accurate atmospheric planetary boundary layers (PBL) schemes, are reviewed. Between-site comparisons are critical, as stable PBLs are sensitive to weak surface heterogeneity and to the occurrence of low-level jets. Creation of a catalog of data from complex PBL field studies is useful to develop a unifying theory of mesoscale PBL flows over complex terrain and to develop PBL schemes necessary for accurate high-resolution operational modeling. A deeper understanding of urban PBL dynamics requires development of long-term urban test beds in a variety of geographic regions in various climates, and with a variety of urban core types. The success of PBL descriptions in operational models is dependent upon improved assimilation techniques for PBL data, accounting for not only statistical but also physical flow properties.

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