On a solution of the closure problem for dry convective boundary layer turbulence and beyond

We consider the closure problem of representing the higher order moments (HOMs) in terms of lower-order moments, a central feature in turbulence modelling based on the Reynolds-Averaged Navier-Stokes (RANS) approach. Our focus is on models suited for the description of asymmetric, non-local and semi-organized turbulence in the dry atmospheric convective boundary layer (CBL). We establish a multivariate probability density function (PDF) describing populations of plumes which are embedded in a sea of weaker randomly spaced eddies, and apply an assumed Delta-PDF approximation. The main content of this approach consists of capturing the bulk properties of the PDF. We solve the closure problem analytically for all relevant higher order moments (HOMs) involving velocity components and temperature and establish a hierarchy of new non-Gaussian turbulence closure models of different content and complexity ranging from analytical to semi-analytical. All HOMs in the hierarchy have a universal and simple functional form. They refine the widely used Millionshchikov closure hypothesis and generalize the famous quadratic skewness-kurtosis relationship to higher-order. We examine the performance of the new closures by comparison with measurement, LES and DNS data and derive empirical constants for semi-analytical models, which are best for practical applications. We show that the new models have a good skill in predicting the HOMs for atmospheric CBL. Our closures can be implemented in second-, third- and fourth-order RANS turbulence closure models of bi-, tri-and four-variate levels of complexity. Finally, several possible generalizations of our approach are discussed.

A Hybrid Visual-Based SLAM Architecture: Local Filter-Based SLAM with KeyFrame-Based Global Mapping

This work presents a hybrid visual-based SLAM architecture that aims to take advantage of the strengths of each of the two main methodologies currently available for implementing visual-based SLAM systems, while at the same time minimizing some of their drawbacks. The main idea is to implement a local SLAM process using a filter-based technique, and enable the tasks of building and maintaining a consistent global map of the environment, including the loop closure problem, to use the processes implemented using optimization-based techniques. Different variants of visual-based SLAM systems can be implemented using the proposed architecture. This work also presents the implementation case of a full monocular-based SLAM system for unmanned aerial vehicles that integrates additional sensory inputs. Experiments using real data obtained from the sensors of a quadrotor are presented to validate the feasibility of the proposed approach.

Ground-based lidar observations of vertical aersosol and water vapor profiles within the boundary layer over heterogeneous terrain

<p>Connecting the earth's surface with the free troposphere, the planetary boundary layer (PBL) comprises complex dynamics of turbulent behavior. This especially applies for areas with heterogeneous terrain. Relevant near-ground processes such as released energy fluxes and the emission of aerosols and trace gases directly interact with the atmosphere. Therefore, the PBL's physical state is determined both by the near-ground processes as well as entrainment of air parcels from higher layers. The mainly turbulence-driven transport of particles and properties throughout the PBL constrain a comprehensive understanding of the PBL's behavior. Hence, the energy balance closure problem as well as errors in precipitation forecast in long-term numerical weather predictions, amongst others, remain unresolved challenges. Here, ground-based lidar profiling is a well suitable method for observing the PBL, as data sampling allows for high temporal and vertical resolutions (Here: Sampling rate of 100\,Hz and 7.5\,m). During the CHEESEHEAD campaign, carried out in summer 2019, our newly developed ATMONSYS lidar performed measurements over complex terrain in northern Wisconsin. There, our lidar system was embedded in a dense network of multiple in-situ and remote sensing instruments. The central aim of this campaign was to further contribute to solve the energy balance closure problem. With the ATMONSYS lidar, vertical columns of aerosol backscatter coefficients, water vapor and temperature have been recorded. The presented work shows what the data is suitable for in terms of resolution and temporal extent in the first place. As a second point, focus is given on structure and variability of aerosol backscatter coefficient distributions and water vapor concentrations as well as their implications on the prevailing state of the PBL. Based on the presented findings, we discuss the potential and suitability of this experimental data for deriving transport processes within the PBL.</p>

On the way to realistic large eddy simulations – A comparison of virtual measurements with CHEESEHEAD19 field measurements

<p>Large-eddy simulations are useful tools to study transport processes by mesoscale structures in the atmospheric boundary layer, since in contrast to single-tower eddy covariance measurements, they provide not only temporally but also spatially highly resolved information. Therefore, they are well suited to study the energy balance closure problem, for which the mesoscale transport of latent and sensible heat, triggered by heterogeneous ecosystems, is suspected to be a major cause. However, this requires simulations that are as realistic as possible and thus allow a comparison of real measurements in the field and virtual measurements in the simulation.<br>During the Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors (CHEESEHEAD) experiment in the summer of 2019, a heterogeneous 10x10 square km domain was intensively sampled across scales. This data offers a unique possibility to set up large-eddy simulations with realistic surface heterogeneity. We use PALM to simulate two days and an area of 40 by 40 square kilometers incorporating the CHEESEHEAD site. The large scale atmospheric forcings to inform the boundary conditions are determined from the NCEP HRRR product. As the lower boundary condition, we use a soil and land-surface model coupled with a plant-canopy model, which we adapt to the CHEESEHEAD area based on ground-based and airborne measurements of plant physiological data.<br>In this study, we investigate how well the simulations match with real measurements by comparing simulated profiles and virtual tower measurements with field measurements from radiosonde ascents, lidar measurements of three-dimensional wind and water vapor, eddy-covariance measurements from the 400 meter tower in the center of the study domain, as well as from typical eddy-covariance stations distributed through the study area. This way, we investigate how realistic the simulations actually are and to what extent the knowledge gained from them concerning the energy balance closure problem can be transferred to field measurements.</p>