Impulsfluss an vertikalen Wänden – Messung und Parametrisierung

<p>Der globale Klimawandel hat einen großen Einfluss auf das städtische Klima, wobei sich durch die hohe Bebauungsdichte und Versiegelung viele Effekte wie Hitzewellen zusätzlich verstärken. Damit unsere Städte auch in Zukunft lebenswerte Orte bleiben, müssen diese an die veränderten klimatischen Bedingungen angepasst werden. Um diese Anforderungen umzusetzen, können dank der gestiegenen Rechenkapazitäten vermehrt wirbel- und gebäudeauflösende Large-Eddy-Simulations-(LES) Modelle wie das PALM-4U (Maronga et al., 2015) in der Praxis zur Stadtplanung eingesetzt werden. Die in diesen Modellen verwendeten Annahmen und Parametrisierungen zum Windprofil sowie Impulsfluss an vertikalen Wänden von Gebäuden basieren jedoch mangels geeigneter Daten zumeist auf Grenzschichtmessungen über nahezu homogenen Flächen (Businger, 1971). Daher stellt sich die Frage, wie gut diese Annahmen an vertikalen Wänden zutreffen. Wie sehen das Windprofil und der Impulsfluss an einer realen Fassade aus?</p> <p>Zur Untersuchung dieser Fragestellungen wurden im Rahmen des „Stadtklima im Wandel [UC]<sup>2</sup>“ Projektes zwei 6 m lange Ausleger in etwa 42 m (10. Stock) und 64 m (16. Stock) Höhe an der Fassade eines insgesamt 85 m hohen Gebäudes im Zentrum von Hamburg installiert. Um detaillierte Informationen zur Turbulenz zu erhalten, werden an beiden Auslegern in 2 m, 4 m und 6 m Entfernung zur Fassade die drei Windkomponenten mit 20 Hz erfasst. Die Messungen werden seit August 2021 durchgeführt, sodass unterschiedlichste Anströmungsrichtungen des Gebäudes als auch zahlreiche synoptische Situationen von schwachem bis stärkeren Wind gemessen wurden.</p> <p>Der einzigartige Messdatensatz an einer realen Hochhausfassade liefert detaillierte Einblicke in das Windprofil sowie den Impulsfluss an Gebäuden in Städten. Dies ermöglicht die Untersuchung der aktuell in vielen LES Modellen genutzten Annahmen wie zum Beispiel des logarithmischen Windprofils an Fassaden. Darüber hinaus wird die im PALM‑4U Modell verwendete Parametrisierung für den Impulsfluss mit den Messungen verglichen. Die Form des Windprofils an der Fassade ist unter anderem von der Anströmungsrichtung, der Geometrie sowie der Messposition am Gebäude abhängig. Die Turbulenzintensität nimmt unabhängig der Anströmung in allen drei Komponenten mit größerem Abstand zur Fassade hin ab. Die Ergebnisse werden in Hinblick auf verbesserte Parametrisierungen in Modellen diskutiert.</p> <p> </p> <p><strong>Literatur:</strong></p> <p>Businger, J. A., Wyngaard, J. C., Izumi, Y., and Bradley, E. F.: 1971, ‘Flux-Profile Relationships in the Atmospheric Surface Layer’, <em>J. Atmos. Sci.</em> 28, 181–189.</p> <p>Maronga, B., Gryschka, M., Heinze, R., Hoffmann, F., Kanani-Sühring, F., Keck, M., Ketelsen, K., Letzel, M. O., Sühring, M., and Raasch, S. (2015): The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives, Geosci. Model Dev., 8, 1539-1637, DOI:10.5194/gmd-8-2515-2015.</p>

Simulation of the Scalar Transport above and within the Amazon Forest Canopy

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms−1) and strong (6 ms−1) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere.

Influences of Entrainment-Mixing Parameterization on Numerical Simulations of Cumulus and Stratocumulus Clouds

Different entrainment-mixing processes can occur in clouds; however, a homogeneous mixing mechanism is often implicitly assumed in most commonly used microphysics schemes. Here, we first present a new entrainment-mixing parameterization that uses the grid-mean relative humidity without requiring the relative humidity of the entrained air. Second, the parameterization is implemented in a microphysics scheme in a large eddy simulation model. Third, sensitivity experiments are conducted to compare the new parameterization with the default homogeneous entrainment-mixing parameterization. The results indicate that the new entrainment-mixing parameterization has a larger impact on the number concentration, volume-mean radius, and cloud optical depth in the stratocumulus case than in the cumulus case. This is because inhomogeneous and homogeneous mixing mechanisms dominate in the stratocumulus and cumulus cases, respectively, which is mainly due to the larger turbulence dissipation rate in the cumulus case. Because stratocumulus clouds break up during the dissipation stage to form cumulus clouds, the effects of this new entrainment-mixing parameterization during the stratocumulus dissipation stage are between those during the stratocumulus mature stage and the cumulus case. A large aerosol concentration can enhance the effects of this new entrainment-mixing parameterization by decreasing the cloud droplet size and evaporation time scale. This study sheds new light on the improvement of entrainment-mixing parameterizations in models.

A Simulation Study on Risks to Wind Turbine Arrays from Thunderstorm Downbursts in Different Atmospheric Stability Conditions

Thunderstorm downbursts have been reported to cause damage or failure to wind turbine arrays. We extend a large-eddy simulation model used in previous work to generate downburst-related inflow fields with a view toward defining correlated wind fields that all turbines in an array would experience together during a downburst. We are also interested in establishing what role contrasting atmospheric stability conditions can play on the structural demands on the turbines. This interest is because the evening transition period, when thunderstorms are most common, is also when there is generally acknowledged time-varying stability in the atmospheric boundary layer. Our results reveal that the structure of a downburst’s ring vortices and dissipation of its outflow play important roles in the separate inflow fields for turbines located at different parts of the array; these effects vary with stability. Interacting with the ambient winds, the outflow of a downburst is found to have greater impacts in an “average” sense on structural loads for turbines farther from the touchdown center in the stable cases. Worst-case analyses show that the largest extreme loads, although somewhat dependent on the specific structural load variable considered, depend on the location of the turbine and on the prevailing atmospheric stability. The results of our calculations show the highest simulated foreaft tower bending moment to be 85.4 MN-m, which occurs at a unit sited in the array farther from touchdown center of the downburst initiated in a stable boundary layer.

A nested multi-scale system implemented in the large-eddy simulation model PALM model system 6.0

Large-eddy simulation (LES) provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES–LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains, which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by first simulating a purely convective boundary layer flow system and then three different neutrally stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs, while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

Simulating observed cloud transitions in the northeast Pacific during CSET

The goal of this study is to challenge a large eddy simulation model with a range of observations from a modern field campaign and to develop case studies useful to other modelers. The 2015 Cloud System Evolution in the Trades (CSET) field campaign provided a wealth of in situ and remote sensing observations of subtropical cloud transitions in the summertime Northeast Pacific. Two Lagrangian case studies based on these observations are used to validate the thermodynamic, radiative and microphysical properties of large eddy simulations (LES) of the stratocumulus to cumulus transition. The two cases contrast a relatively fast cloud transition in a clean, initially well-mixed boundary layer vs. a slower transition in an initially decoupled boundary layer with higher aerosol concentrations and stronger mean subsidence. For each case, simulations of two neighboring trajectories sample mesoscale variability and the coherence of the transition in adjacent air masses. In both cases, LES broadly reproduce satellite and aircraft observations of the transition. Simulations of the first case match observations more closely than for the second case, where simulations underestimate cloud cover early in the simulations and overestimate cloud top height later. For the first case, simulated cloud fraction and liquid water path increase if a larger cloud droplet number concentration is prescribed. In the second case, precipitation onset and inversion cloud breakup occurs earlier when the LES domain is chosen large enough to support strong mesoscale organization.

Comparison of urban climate measurements in Berlin and LES model output for a special observation period

<p>Surface (2 m) temperature and specific humidity data are measured at 5-minute intervals in a network comprising 33 stations distributed across the city of Berlin, Germany. These data are utilized in order to validate a LES (large eddy simulation) model designed to assess the local climate at a very high resolution of 10 m to 1 m. This model, was developed at the ​Institute of Meteorology and Climatology (IMUK) of the Leibniz Universität Hannover, Germany, and is developed into an application tool for city planners within the funding programme "[UC²] - Urban Climate under Change", of the German Federal Ministry of Education and Research (BMBF).</p><p>The evaluation distinguishes between the different Local climate zones (LCZ) in the city, which are defined following the concept of Stewart & Oke (2012). For Berlin, the following LCZ have been identified: 2 (compact midrise), 4 (open high-rise), 6 (open low-rise), 8 (large low-rise), A (dense trees), B (scattered trees), D (low Plants), G (water).</p><p>We analyzed one cold winter day during an intensive observation period from 06 UTC on 17<sup>th</sup> January to 06 UTC on 18<sup>th</sup> January, 2017. The minimum and maximum recorded temperatures were -8.1 °C and +2 °C, respectively, the sun shine duration was 6.5 hours. Daily and hourly mean absolute error, mean square error and root mean square error confirm that the deviation between measurements and the PALM-4U model differs between the LCZ for Berlin, with particularly large negative deviations of up to 5 K in forest areas, as they are not yet well represented in the model. Smallest deviations are found for the industrial zone. In all cases, the observed amplitude of the diurnal cycle is underestimated. The role of the driving model for the deviations found is addressed.</p><p>Stewart, I.D., Oke, T.R. (2012) Local climate zones for urban temperature studies. Bull. Amer. Meteor. Soc. 93 1879-1900. DOI: 10.1175/BAMS-D-11-00019.1.</p><p> </p>

WRF4PALM v1.0: A Mesoscale Dynamic Driver for the Microscale PALM Model System 6.0

<p>A set of Python-based tools, WRF4PALM, has been developed for offline-nesting of the PALM model system 6.0 into the Weather Research and Forecasting (WRF) modelling system. Time-dependent boundary conditions of the atmosphere are critical for accurate representation of microscale meteorological dynamics in high resolution real-data simulations. WRF4PALM generates initial and boundary conditions from WRF outputs to provide time-varying meteorological forcing for PALM. The WRF model has been used across the atmospheric science community for a broad range of multidisciplinary applications. The PALM model system 6.0 is a turbulence-resolving large-eddy simulation model with an additional Reynolds averaged Navier–Stokes (RANS) mode for atmospheric and oceanic boundary layer studies at microscale (Maronga et al., 2020). Currently PALM has the capability to ingest output from the regional scale Consortium for Small-scale Modelling (COSMO) atmospheric prediction model. However, COSMO is not an open source model which requires a licence agreement for operational use or academic research (). This paper describes and validates the new free and open-source WRF4PALM tools (available on ). Two case studies using WRF4PALM are presented for Christchurch, New Zealand, which demonstrate successful PALM simulations driven by meteorological forcing from WRF outputs. The WRF4PALM tools presented here can potentially be used for micro- and mesoscale studies worldwide, for example in boundary layer studies, air pollution dispersion modelling, wildfire emissions and spread, urban weather forecasting, and agricultural meteorology.</p>