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Maria G. Inclán, Jan Schween, and Ralph Dlugi

Abstract

The prognostic one-dimensional Forest–Land–Atmosphere Model (FLAME) has been further extended to simulate diurnal cycles of volatile organic compound (VOC) fluxes inside and above an idealized mixed forest mainly composed of oaks and pines. The tree height is 12 m and the leaf area index is 4. The canopy crown is divided into five layers of equal leaf area increment. The algorithms developed by Guenther et al. are applied to predict the monoterpene emission from leaves in each canopy layer. The value of photosynthetic active radiation (PAR) and foliage temperature (T leaf) required by these algorithms are provided by FLAME. The modeled PAR and T leaf are used for different leaf angle classes i from sun to shaded leaves within each layer j in the canopy.

In the present study the authors examine the VOC fluxes modeled at a reference level above the forest for two cases, A and B, for which the modeled canopy temperature T c reaches a maximum of approximately 21° and 31°C, respectively. It is supposed that VOC fluxes above the canopy are related to T c by a function of the form F(T c) = F s exp[β c(T cT S)]. The authors intend to study the temperature exponent at canopy level β c by deriving best-fit slopes.

The resulting mean values of β c = 0.125 ± 0.002 K−1 for the morning VOC fluxes, and β c = 0.267 ± 0.004 K−1 for the afternoon, are larger than those used to calculate the emission on leaf scale from cuvette data (β 1 = 0.124 K−1 and β 2 = 0.09 K−1 for oaks and pines, respectively). A sensitivity study was carried out using the modeled mean values of T leaf and PAR in a canopy layer instead of the angle dependence to simulate measurements by an infrared radiation thermometer. The authors also tested the importance of the basal emission rate E s on the total VOC fluxes and β c above the canopy.

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Thomas Spengler, Jan H. Schween, Markus Ablinger, Günther Zängl, and Joseph Egger

Abstract

The summertime thermal circulation in the region of an asymmetric valley exit is investigated by means of observations and high-resolution model simulations. The northeastward-oriented Alpine Lech Valley opening into the Bavarian Alpine foreland has an eastern slope exceeding the western slope by about 15 km. Northerly winds along the eastern slope are frequently observed, reaching substantial strength during fair weather conditions. A field experiment has been conducted to explore this phenomenon and to pinpoint the connection of the northeasterly flow to the Lech Valley wind circulation. Numerical simulations have also been carried out to support the interpretation of the observations. It is found that the northerlies owe their existence to the dominantly easterly flow along the foothills of the Alps, which is partly induced by the Alpine heat low but may be strengthened by favorable synoptic conditions. Examples for both situations will be discussed. The diurnal flow in the Lech Valley has little obvious impact on these northeasterlies. On days with moderate synoptic easterly flow, a wake is present on the lee of the eastern slope of the exit region, accompanied by a shear zone along the edge of the wake. This shear zone is forced southward during the daytime because of thermally initiated pressure gradients between the Alpine foreland and the Alps, leading to sudden wind changes in the exit area at the time of its passage.

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Tobias Marke, Susanne Crewell, Vera Schemann, Jan H. Schween, and Minttu Tuononen

Abstract

Low-level-jet (LLJ) periods are investigated by exploiting a long-term record of ground-based remote sensing Doppler wind lidar measurements supported by tower observations and surface flux measurements at the Jülich Observatory for Cloud Evolution (JOYCE), a midlatitude site in western Germany. LLJs were found 13% of the time during continuous observations over more than 4 yr. The climatological behavior of the LLJs shows a prevailing nighttime appearance of the jets, with a median height of 375 m and a median wind speed of 8.8 m s−1 at the jet nose. Significant turbulence below the jet nose only occurs for high bulk wind shear, which is an important parameter for describing the turbulent characteristics of the jets. The numerous LLJs (16% of all jets) in the range of wind-turbine rotor heights below 200 m demonstrate the importance of LLJs and the associated intermittent turbulence for wind-energy applications. Also, a decrease in surface fluxes and an accumulation of carbon dioxide are observed if LLJs are present. A comprehensive analysis of an LLJ case shows the influence of the surrounding topography, dominated by an open pit mine and a 200-m-high hill, on the wind observed at JOYCE. High-resolution large-eddy simulations that complement the observations show that the spatial distribution of the wind field exhibits variations connected with the orographic flow depending on the wind direction, causing high variability in the long-term measurements of the vertical velocity.

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Joseph Egger, Sapta Bajrachaya, Richard Heinrich, Philip Kolb, Stephan Lämmlein, Mario Mech, Joachim Reuder, Wolfgang Schäper, Pancha Shakya, Jan Schween, and Hilbert Wendt

Abstract

In 1998 a field campaign has been conducted in the north–south-oriented Kali Gandaki valley in Nepal to explore the structure of its extreme valley wind system. Piloted ballon (pibal) observations were made to map the strong upvalley winds as well as the weak nocturnal flows (Part I). The stratification of the valley atmosphere was not explored. In Part II of this multipart paper, numerical simulations are presented that successfully simulate most of the wind observations. Moreover, the model results suggest that the vigorous upvalley winds can be seen as supercritical flow induced by contractions of the valley. Here, the results of a further campaign are reported where remotely piloted airplanes were used to obtain vertical profiles of temperature and humidity up to heights of ∼2000 m above the ground. Such profiles are needed for an understanding of the flow dynamics in the valley and for a validation of the model results. This technique is novel in some respects and turned out to be highly reliable even under extreme conditions. In addition four automatic stations were installed along the valley's axis. Winds were observed via pibal ascents. These data complement the wind data of 1998 so that the diurnal wind system of the Kali Gandaki valley is now documented reasonably well.

It is found that the fully developed upvalley flow is confined to a turbulent layer that tends to be neutrally stratified throughout the domain of observations. The stratification above this layer is stable. A capping inversion is encountered occasionally. This finding excludes explanations of the strong winds in terms of hydraulic theories that rely on the presence of strong inversions. Pairs of simultaneous ascents separated by 5–10 km along the valley axis reveal a remarkable variability induced by the topography and, perhaps, by an instability of the flow. The analysis of the surface data as well as that of the soundings shows that the flow above the neutral layer affects the surface pressure distribution and, therefore, the acceleration of the extreme upvalley winds.

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Joseph Egger, Luis Blacutt, Flavio Ghezzi, Richard Heinrich, Philip Kolb, Stephan Lämmlein, Martin Leeb, Stephanie Mayer, Eduardo Palenque, Joachim Reuder, Wolfgang Schäper, Jan Schween, Rene Torrez, and Francesco Zaratti

Abstract

In July and August 2003 a field campaign was conducted to explore the diurnal circulation of the Bolivian Altiplano. Vertical soundings by remote-controlled aircraft yielded profiles of temperature, pressure, and humidity at six passes and in a valley. Pilot balloon observations provided wind profiles. Two permanent stations collected additional data. Typically, inflow toward the Altiplano commences a few hours after sunrise at about the time when the stable nocturnal layer near the ground is transformed by the solar heating into an almost neutrally stratified convective boundary layer. The depth of the inflow layer is comparable to but normally less than that of this boundary layer. There are indications of return flow aloft. The inflow continues at least until sunset. Moisture is imported at the passes leading to the Yungas in the east. Strong upvalley flows were found in the valley of the Rio de La Paz, which connects the wide canyon of La Paz with the tropical lowlands to the east. Inflow was absent at one of the passes despite favorable synoptic conditions. Cases of synoptically forced flows are presented as well where the diurnal signal is difficult to separate. A simple flow scheme is presented that fits the observations reasonably well.

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Clemens Simmer, Insa Thiele-Eich, Matthieu Masbou, Wulf Amelung, Heye Bogena, Susanne Crewell, Bernd Diekkrüger, Frank Ewert, Harrie-Jan Hendricks Franssen, Johan Alexander Huisman, Andreas Kemna, Norbert Klitzsch, Stefan Kollet, Matthias Langensiepen, Ulrich Löhnert, A. S. M. Mostaquimur Rahman, Uwe Rascher, Karl Schneider, Jan Schween, Yaping Shao, Prabhakar Shrestha, Maik Stiebler, Mauro Sulis, Jan Vanderborght, Harry Vereecken, Jan van der Kruk, Guido Waldhoff, and Tanja Zerenner

Abstract

Most activities of humankind take place in the transition zone between four compartments of the terrestrial system: the unconfined aquifer, including the unsaturated zone; surface water; vegetation; and atmosphere. The mass, momentum, and heat energy fluxes between these compartments drive their mutual state evolution. Improved understanding of the processes that drive these fluxes is important for climate projections, weather prediction, flood forecasting, water and soil resources management, agriculture, and water quality control. The different transport mechanisms and flow rates within the compartments result in complex patterns on different temporal and spatial scales that make predictions of the terrestrial system challenging for scientists and policy makers. The Transregional Collaborative Research Centre 32 (TR32) was formed in 2007 to integrate monitoring with modeling and data assimilation in order to develop a holistic view of the terrestrial system. TR32 is a long-term research program funded by the German national science foundation Deutsche Forschungsgemeinschaft (DFG), in order to focus and integrate research activities of several universities on an emerging scientific topic of high societal relevance. Aiming to bridge the gap between microscale soil pores and catchment-scale atmospheric variables, TR32 unites research groups from the German universities of Aachen, Bonn, and Cologne, and from the environmental and geoscience departments of Forschungszentrum Jülich GmbH. Here, we report about recent achievements in monitoring and modeling of the terrestrial system, including the development of new observation techniques for the subsurface, the establishment of cross-scale, multicompartment modeling platforms from the pore to the catchment scale, and their use to investigate the propagation of patterns in the state and structure of the subsurface to the atmospheric boundary layer.

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