On the nature of turbulent kinetic energy in a steep and narrow Alpine valley     

Andreas P. Weigel  Fotini K. Chow  Mathias W. Rotach 《Boundary-Layer Meteorology》2007,123(1):177-199

This contribution investigates the nature of turbulent kinetic energy (TKE) in a steep and narrow Alpine valley under fair-weather summertime conditions. The Riviera Valley in southern Switzerland was chosen for a detailed case study, in which the evaluation of aircraft data (obtained from the MAP-Riviera field campaign) is combined with the application of high-resolution (350-m horizontal grid spacing) large-eddy simulations using the numerical model ARPS. The simulations verify what has already been observed on the basis of measurements: TKE profiles scale surprisingly well if the convective velocity scale w _* is obtained from the sun-exposed eastern slope rather than from the surface directly beneath the profiles considered. ARPS is then used to evaluate the TKE-budget equation, showing that, despite sunny conditions, wind shear is the dominant production mechanism. Therefore, the surface heat flux (and thus w _*) on the eastern slope does not determine the TKE evolution directly but rather, as we believe, indirectly via the interaction of thermally-driven cross-valley and along-valley flows. Excellent correlation between w _* and the up-valley wind speed solidifies this hypothesis.  相似文献   

Predicting Effects of Temperature Increase on the Water Balance of Beech Forest – An Application of the ‘KAUSHA’ Model     

Mathias Herbst  Georg Hörmann 《Climatic change》1998,40(3-4):683-698

The water balance model KAUSHA (Halldin, 1989) was applied to a 100-year-old beech (Fagus sylvatica L.) forest in northern Germany. Overall, a satisfying agreement between modelled evapotranspiration values and independent micrometeorological measurements (Bowen ratio energy balance method) could be observed, although for rainy days KAUSHA showed a tendency to overestimate evapotranspiration. The model was used to predict the effects of a climate warming on the water budgets of the forest. It is shown that a temperature increase of 2°C due to a rising CO₂ content of the atmosphere will not change the yearly totals of evapotranspiration significantly, but could have serious effects on the soil water balance during the vegetation period. Because under climate change conditions a higher amount of the available soil water has already been evaporated in winter and spring, soil water content will limit the transpiration of the trees from July to September much more strongly. Therefore, the yield of beech forest might also suffer from drought effects. It can be concluded that a better knowledge of the seasonal distribution of rainfall under climate change conditions is indispensable for predicting effects of rising temperatures and CO₂ concentrations on ecosystems.  相似文献   

Genetic population structure of Austropotamobius pallipes (Lereboullet 1858) (Decapoda: Astacidae) in Switzerland, based on allozyme data     

Mathias Lörtscher  Martin Clalüna  Adolf Scholl 《Aquatic Sciences - Research Across Boundaries》1998,60(2):118-129

  相似文献   

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer     

Petra Kastner-Klein  Mathias W. Rotach 《Boundary-Layer Meteorology》2004,111(1):55-84

In this study, a detailed model of an urban landscape has been re-constructed inthe wind tunnel and the flow structure inside and above the urban canopy has beeninvestigated. Vertical profiles of all three velocity components have been measuredwith a Laser-Doppler velocimeter, and an extensive analysis of the measured meanflow and turbulence profiles carried out. With respect to the flow structure inside thecanopy, two types of velocity profiles can be distinguished. Within street canyons,the mean wind velocities are almost zero or negative below roof level, while closeto intersections or open squares, significantly higher mean velocities are observed.In the latter case, the turbulent velocities inside the canopy also tend to be higherthan at street-canyon locations. For both types, turbulence kinetic energy and shearstress profiles show pronounced maxima in the flow region immediately above rooflevel.Based on the experimental data, a shear-stress parameterization is proposed, inwhich the velocity scale, u_s, and length scale, z_s, are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, d_s, is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allowdetermination of appropriate scales for the mean wind profile. The roughnesslength, z₀, and displacement height, d₀, can both be described as fractions of the distance, z_s - d_s, between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth z_s - d_s as the roughness layer. With respect to the lower part of the canopy (z < d_s) the flow behaves as a skimming flow. Correlations between the length scales z_s and d_s and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic windlaw. A combination of morphometric estimation methods for d₀ and z₀ with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u_*, appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughnesssublayer, a local scaling approach results in good agreement between measuredand predicted mean wind profiles.  相似文献   

Flood Events of Selected European Rivers in the Sixteenth Century     

Rudolf Brázdil  Rüdiger Glaser  Christian Pfister  Petr Dobrovolný  Jean-Marc Antoine  Mariano Barriendos  Dario Camuffo  Mathias Deutsch  Silvia Enzi  Emanuela Guidoboni  Oldřich Kotyza  Fernando Sanchez Rodrigo 《Climatic change》1999,43(1):239-285

The severity and frequency of sixteenth-century floods of the Rhine, the Main, the middle and upper Elbe with its tributaries, rivers of northern and central Italy, the Garonne and rivers in Catalonia and Andalusia are analyzed using documentary evidence. The basic topographical and hydrological characteristics of the rivers investigated as well as the synoptic causes of their flooding during the instrumental period are presented. Different examples of modifications of the run-off process due to anthropogenic activity are discussed. Prevalence in flood occurrence during the second half of the sixteenth century in comparison to the first half is typical for central European and Andalusian rivers (mainly in the 1560s and 1590s) and agrees with the evolution of precipitation patterns. On the other hand, Italian and Catalonian rivers, in part, had a higher occurrence of floods during the first half of the century. Changes in the flooding seasons in both halves of the century are not unambiguous. Results of an analysis on a broader European scale show floods to be a random natural phenomena with limited areal extent defined by the spatial influence of forcing meteorological factors (continuous heavy rains, sudden melting of thick snow cover, etc.). Despite some limitations of documentary evidence, series of reconstructed historical floods are valuable sources of proxy data which can be utilized for the study of the flooding fluctuations in the pre-instrumental period.  相似文献   

Inference for the truncated exponential distribution     

Mathias Raschke 《Stochastic Environmental Research and Risk Assessment (SERRA)》2012,26(1):127-138

The constructed estimator is introduced for the right truncation point of the truncated exponential distribution. The new estimator is most efficient in important ranges of truncation points for finite sample sizes. The introduced inverse mean squared error clearly indicates the good behaviour of the new estimator. The estimation of the scaling parameter is considered in all discussions and computations. The methods and models of the extreme value theory are not appropriate to estimate the truncation point because they work only in the case of very large sample sizes. Furthermore, a procedure for a first goodness-of-fit test is introduced. All this has been researched by extensive Monte Carlo simulations for different truncation points and sample sizes. Finally, the new inference methods are applied at the end for the random distribution of wildfire sizes and earthquake magnitudes.  相似文献   

Historical ecology of riverine fish in Europe     

Gertrud Haidvogl  Richard Hoffmann  Didier Pont  Mathias Jungwirth  Verena Winiwarter 《Aquatic Sciences - Research Across Boundaries》2015,77(3):315-324

  相似文献   

Channel and vegetation recovery from dredging of a large river in the Gulf coastal plain,USA     

Joann Mossa  Yin-Hsuen Chen  G. Mathias Kondolf  Scott Porter Walls 《地球表面变化过程与地形》2020,45(9):1926-1944

Anthropogenic impacts in large rivers are widely studied, but studies of recovery once a disturbance has stopped are uncommon. This study examines the biogeomorphic recovery of a 40-km river corridor on the mid-Apalachicola River, Florida following the cessation of dredging, disposal, and snag removal in 2002. This failed navigation project resulted in vegetation losses (~166 ha between 1941 and 2004), river widening, and increased point bar areas. We used paired sets of imagery for a 10-year period during the recovery process at two different flow levels to assess sand bar change, land cover change, and their spatial variations. Most large sand bars decreased significantly in area due to growth of pioneer species, typically from the bankside of the bar. Mean bar area shrank 0.17 and 0.20 ha for the 30th and 1st percentile flows, respectively. For the entire study area, both water-level comparisons showed gains in vegetation (23.36 and 15.83 ha), compensated by losses in the extent of water (16.83 and 8.55 ha) and sand bar losses (6.53 and 7.28 ha). Overall, these gains during the 10-year passive recovery period are equivalent to ~15% of the vegetation losses that resulted from the navigational dredging. As found in other studies, most of the pioneer vegetation grew approximately 2 m relative elevation above the low-water surface. The initial length of the tree line and the area of herbaceous growth both had a significant and positive relationship with the area of new vegetation growth over the study interval. As parts of the river are healing, reduced channel capacity from narrowing and tree growth will benefit the floodplain. As elsewhere, understanding of a river's biogeomorphology, hydrology, and disturbance history can help in selecting appropriate recovery metrics to further advance the understanding and management of disturbed floodplains. © 2020 John Wiley & Sons, Ltd.  相似文献   

Flux Footprints Over an Undulating Surface   总被引：2，自引：1，他引：1  

Weiguo Wang  Mathias W. Rotach 《Boundary-Layer Meteorology》2010,136(2):325-340

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.  相似文献   

The Budget of Turbulent Kinetic Energy in the Urban Roughness Sublayer   总被引：4，自引：4，他引：0  

Andreas Christen  Mathias W. Rotach  Roland Vogt 《Boundary-Layer Meteorology》2009,131(2):193-222

Full-scale observations from two urban sites in Basel, Switzerland were analysed to identify the magnitude of different processes that create, relocate, and dissipate turbulent kinetic energy (TKE) in the urban atmosphere. Two towers equipped with a profile of six ultrasonic anemometers each sampled the flow in the urban roughness sublayer, i.e. from street canyon base up to roughly 2.5 times the mean building height. This observational study suggests a conceptual division of the urban roughness sublayer into three layers: (1) the layer above the highest roofs, where local buoyancy production and local shear production of TKE are counterbalanced by local viscous dissipation rate and scaled turbulence statistics are close to to surface-layer values; (2) the layer around mean building height with a distinct inflexional mean wind profile, a strong shear and wake production of TKE, a more efficient turbulent exchange of momentum, and a notable export of TKE by transport processes; (3) the lower street canyon with imported TKE by transport processes and negligible local production. Averaged integral velocity variances vary significantly with height in the urban roughness sublayer and reflect the driving processes that create or relocate TKE at a particular height. The observed profiles of the terms of the TKE budget and the velocity variances show many similarities to observations within and above vegetation canopies.  相似文献