Overview of the PALM model system 6.0

In this paper, we describe the PALM model system 6.0. PALM (formerly an abbreviation for Parallelized Large-eddy Simulation Model and now an independent name) is a Fortran-based code and has been applied for studying a variety of atmospheric and oceanic boundary layers for about 20 years. The model is optimized for use on massively parallel computer architectures. This is a follow-up paper to the PALM 4.0 model description in Maronga et al. (2015). During the last years, PALM has been significantly improved and now offers a variety of new components. In particular, much effort was made to enhance the model with components needed for applications in urban environments, like fully interactive land surface and radiation schemes, chemistry, and an indoor model. This paper serves as an overview paper of the PALM 6.0 model system and we describe its current model core. The individual components for urban applications, case studies, validation runs, and issues with suitable input data are presented and discussed in a series of companion papers in this special issue.Peer reviewe

Grid-Resolution Requirements for Large-Eddy Simulations of the Atmospheric Boundary Layer

Large-eddy simulations are widely used to study flows in the atmospheric boundary layer. As atmospheric boundary-layer flows of different atmospheric stratification have very different flow characteristics on different length scales, well-resolved simulations of these flows require very different meshes. The Parallelized Large-Eddy Simulation Model combined with a realizable dynamic subgrid model is used to identify the best method for evaluating the resolution requirements for boundary-layer flows simulated by large-eddy simulations. In particular, we consider three atmospheric boundary-layer set-ups with different stratifications (stable, neutral, convective) to investigate how the quality of the simulation changes with the grid resolution. By following the work of Davidson (Int J Heat Fluid Flow 30(5):1016–1025, 2009), the results are examined using criteria such as the convergence of mean profiles, the ratio of modelled and resolved turbulence kinetic energy, and the two-point correlation. We conclude that the two-point correlation is the best measure to evaluate whether the resolution demands for a specific flow are fulfilled

Low Level Jets over the Southern North Sea

An extensive analysis of Low Level Jets (LLJs) over the Southern North Sea is presented. The study is based on observational data from a wind LiDAR and a passive microwave radiometer, operated from May 2015 to October 2016 on the FINO1 platform, as well as on mesoscale simulations by WRF-ARW. Besides evaluations on LLJ occurrence, intensity, direction, height, wind shears and boundary layer stability based on 250 days of measurements, two case studies were investigated in detail. It indicates that LLJs are a very frequent phenomenon above the Southern North Sea as they occurred on the majority of the days during the measuring period. Our study suggests that highest probabilities for LLJs to occur are during winds within the sector East to South. Most detections were found for the period between the evening until morning while the lowest amount of detections was found at 1400 UTC. Considerable amounts of LLJs occurred at heights that are in the ranges of modern offshore wind turbine heights and rotor sizes. Moreover, the case studies showed strong wind shears and veering below the jet cores. Further findings suggest, that baroclinic effects in the coastal zone due to differential surface heating of land and sea as well as inertial oscillations may form and modify these jets

Low Level Jets over the Southern North Sea

An extensive analysis of Low Level Jets (LLJs) over the Southern North Sea is presented. The study is based on observational data from a wind LiDAR and a passive microwave radiometer, operated from May 2015 to October 2016 on the FINO1 platform, as well as on mesoscale simulations by WRF-ARW. Besides evaluations on LLJ occurrence, intensity, direction, height, wind shears and boundary layer stability based on 250 days of measurements, two case studies were investigated in detail. It indicates that LLJs are a very frequent phenomenon above the Southern North Sea as they occurred on the majority of the days during the measuring period. Our study suggests that highest probabilities for LLJs to occur are during winds within the sector East to South. Most detections were found for the period between the evening until morning while the lowest amount of detections was found at 1400 UTC. Considerable amounts of LLJs occurred at heights that are in the ranges of modern offshore wind turbine heights and rotor sizes. Moreover, the case studies showed strong wind shears and veering below the jet cores. Further findings suggest, that baroclinic effects in the coastal zone due to differential surface heating of land and sea as well as inertial oscillations may form and modify these jets