Multidimensional method-of-lines transport for atmospheric flows over steep terrain using arbitrary meshes

Including terrain in atmospheric models gives rise to mesh distortions near the lower boundary that can degrade accuracy and challenge the stability of transport schemes. Multidimensional transport schemes avoid splitting errors on distorted, arbitrary meshes, and method-of-lines schemes have low computational cost because they perform reconstructions at fixed points. This paper presents a multidimensional method-of-lines finite volume transport scheme, “cubicFit”, which is designed to be numerically stable on arbitrary meshes. Stability conditions derived from a von Neumann analysis are imposed during model initialisation to obtain stability and improve accuracy in distorted regions of the mesh, and near steeply-sloping lower boundaries. Reconstruction calculations depend upon the mesh only, needing just one vector multiply per face per time-stage irrespective of the velocity field. The cubicFit scheme is evaluated using three, idealised numerical tests. The first is a variant of a standard horizontal transport test on severely distorted terrain-following meshes. The second is a new test case that assesses accuracy near the ground by transporting a tracer at the lower boundary over steep terrain on terrain-following meshes, cut-cell meshes, and new, slanted-cell meshes that do not suffer from severe time-step constraints associated with cut cells. The third, standard test deforms a tracer in a vortical flow on hexagonal-icosahedral meshes and cubed-sphere meshes. In all tests, cubicFit is stable and largely insensitive to mesh distortions, and cubicFit results are more accurate than those obtained using a multidimensional linear upwind transport scheme. The cubicFit scheme is second-order convergent regardless of mesh distortions

A mimetic, semi-implicit, forward-in-time, finite volume shallow water model: comparison of hexagonal–icosahedral and cubed-sphere grids

A new algorithm is presented for the solution of the shallow water equations on quasi-uniform spherical grids. It combines a mimetic finite volume spatial discretization with a Crank–Nicolson time discretization of fast waves and an accurate and conservative forward-in-time advection scheme for mass and potential vorticity (PV). The algorithm is implemented and tested on two families of grids: hexagonal–icosahedral Voronoi grids, and modified equiangular cubed-sphere grids. <br><br> Results of a variety of tests are presented, including convergence of the discrete scalar Laplacian and Coriolis operators, advection, solid body rotation, flow over an isolated mountain, and a barotropically unstable jet. The results confirm a number of desirable properties for which the scheme was designed: exact mass conservation, very good available energy and potential enstrophy conservation, consistent mass, PV and tracer transport, and good preservation of balance including vanishing &nabla; &times; &nabla;, steady geostrophic modes, and accurate PV advection. The scheme is stable for large wave Courant numbers and advective Courant numbers up to about 1. <br><br> In the most idealized tests the overall accuracy of the scheme appears to be limited by the accuracy of the Coriolis and other mimetic spatial operators, particularly on the cubed-sphere grid. On the hexagonal grid there is no evidence for damaging effects of computational Rossby modes, despite attempts to force them explicitly

A Standard Test Case Suite for Two-Dimensional Linear Transport on the Sphere: Results from a Collection of State-of-the-Art Schemes

Recently, a standard test case suite for 2-D linear transport on the sphere was proposed to assess important aspects of accuracy in geophysical fluid dynamics with a minimal set of idealized model configurations/runs/diagnostics. Here we present results from 19 state-of-the-art transport scheme formulations based on finite-difference/finite-volume methods as well as emerging (in the context of atmospheric/oceanographic sciences) Galerkin methods. Discretization grids range from traditional regular latitude–longitude grids to more isotropic domain discretizations such as icosahedral and cubed-sphere tessellations of the sphere. The schemes are evaluated using a wide range of diagnostics in idealized flow environments. Accuracy is assessed in single- and two-tracer configurations using conventional error norms as well as novel diagnostics designed for climate and climate–chemistry applications. In addition, algorithmic considerations that may be important for computational efficiency are reported on. The latter is inevitably computing platform dependent. The ensemble of results from a wide variety of schemes presented here helps shed light on the ability of the test case suite diagnostics and flow settings to discriminate between algorithms and provide insights into accuracy in the context of global atmospheric/ocean modeling. A library of benchmark results is provided to facilitate scheme intercomparison and model development. Simple software and data sets are made available to facilitate the process of model evaluation and scheme intercomparison

Convective precipitation simulated with ICON over heterogeneous surfaces in dependence on model and land-surface resolution

The impact of land-surface properties like vegetation, soil type and soil moisture, and the orography on the atmosphere is manifold. On the one hand, these features determine the evolution of the atmospheric boundary layer, in particular, the convective conditions and further the exchange of mass, momentum, heat, and humidity with the free troposphere. Subsequently, the land surface also influences the pre-convective environment. On the other hand, land-surface heterogeneity results in the spatial variability of the land-surface parameters, which often leads to thermally-induced circulations and associated convergence zones in the convective boundary layer. These, in turn, act as trigger mechanisms for convective clouds and precipitation. That means in simulations, the distribution and amount of clouds and subsequent precipitation depend on the resolution of land-surface and model grid spacing ($\Delta_h$) alike. Therefore, the focus of the study is to (i) compare areal mean precipitation for different model grid spacings and land-surface resolutions, (ii) analyse reasons for their differences, (iii) investigate spatial precipitation patterns, and describe relevant trigger mechanisms of convection. The impact of model grid spacing and land-surface resolution on convective precipitation is investigated within the framework of the \hdcp project (High Definition Clouds and Precipitation for advancing Climate Prediction). For that purpose, geographical areas with different types of complexity in the orography and considerable number density of lightning strikes (deep convection) are selected. The areas are: the flat terrain near Berlin (A1), the isolated Harz mountain range in central Germany (A2), and the complex terrain, the Black Forest mountains (A3). Six suitable days with weak large-scale forcing but a considerable number of lightning strikes are chosen. \ac{ICON} simulations in large eddy model setup have been performed using six model grid spacings: \ac{NWP} mode ($\Delta_{5000\,m}$, $\Delta_{2500\,m}$), LES mode ($\Delta_{1250\,m}$, $\Delta_{625\,m}$, $\Delta_{312\,m}$ and $\Delta_{156\,m}$) in a nested domain setup (control runs). The $\Delta_{156\,m}$ control run is the reference run. The impact of land-surface resolution on areal mean precipitation and precipitation patterns has been deduced by reducing the resolution of land-surface properties, e.g. vegetation, soil type, and the orography. The differences in simulated areal mean precipitation are explained through heat and moisture budget calculations. Variations in the precipitation patterns are analysed by investigating relevant triggering mechanisms. The source regions of the convective precipitation are identified by applying a backward trajectory model. To diagnose the turbulent sensible and latent heat fluxes at the Earth’s surface in the source regions of convection, their dependence on parameters like orography, soil moisture index, transpiration area index, and net radiation is determined using the standardised multiple regression techniques. The results show that the areal mean accumulated precipitation amount for most of the cases decreases systematically across the LES grid spacings from $\Delta_{1250\,m}$ to $\Delta_{156\,m}$. The relative precipitation difference normalised by the precipitation in the reference run is in the range of -26 to 400 \% with the 75th percentile of 155 \%. In four out of the six days, $\Delta_{1250\,m}$ results in intenser precipitation patterns and an earlier onset of precipitation by 1 to 2 hours in comparison to the reference run. The modification of land-surface resolution from 156 m to 1250 m leads to variability in the mean precipitation in the range of 17 to 37 \% with the 75th percentile of 7 \% which increases to a range of -17 to 49 \% and the 75th percentile of 22 \% with the land-surface resolution of 5000 m. The land-surface sensitivity experiments show a negligible impact on the onset time of precipitation and the precipitation patterns. Thus, the modification in land-surface resolution results in much smaller variability in the areal mean precipitation amount in comparison to the model grid spacing. To understand the differences of areal mean accumulated precipitation and onset of precipitation between the control runs, the heat and moisture budgets are analysed in detail for one day, for which the relative difference in the mean precipitation by $\Delta_{1250\,m}$ and $\Delta_{156\,m}$ is $\simeq$175 \%. Unlike $\Delta_{1250\,m}$, $\Delta_{156\,m}$ first shows intensive evaporative cooling due to the formation of numerous small clouds. Evaporative cooling is generated at the edge and shell regions of the small clouds. As a result, the clouds often dissolve before they could grow deep enough to precipitate. In the subsequent hours cloud aggregation is a crucial step causing precipitation generation in $\Delta_{156\,m}$. Concerning initiation of convection, overall the LES grid spacings show the similar thermally- and orographically-induced circulations in all areas (A1, A2, and A3). However, as demonstrated for A1 considerable differences in triggering could occur when the land-surface resolution is reduced down from 156 m to 5000 m. This finding holds when the resolution of 5000 m smoothed out those land-surface heterogeneities (e.g. lake breezes and urban heat island) which are responsible for convection initiation at 156 m land-surface resolution

Un modèle de transport et de chimie atmosphérique à grande échelle adapté aux calculateurs massivement parallèles

We present in this thesis the development of a large-scale bidimensional atmospheric transport scheme designed for parallel architectures with scalability in mind. The current version, named Pangolin, contains a bi-dimensional advection and a simple linear chemistry scheme for stratospheric ozone and will serve as a basis for a future CTM. For mass-preservation, a van Leer finite-volume scheme was chosen for advection and extended to 2D with operator splitting. To ensure mass preservation, winds are corrected in a preprocessing step. We aim at addressing the "pole issue" of the traditional regular latitude-longitude by presenting a new quasi-area-preserving grid mapping the sphere uniformly. The parallelization of the model is based on the advection operator and a custom domain-decomposition algorithm is presented here to attain load-balancing in a message-passing context. To run efficiently on current and future parallel architectures, algebraic features of the grid are exploited in the advection scheme and parallelization algorithm to favor the cheaper costs of flops versus data movement. The model is validated on algebraic test cases and compared to other state-of-the-art schemes using a recent benchmark. Pangolin is also compared to the CTM of Météo-France, MOCAGE, using a linear ozone scheme and isentropic coordinates.Cette thèse présente un modèle bi-dimensionnel pour le transport atmosphérique à grande échelle, nommé Pangolin, conçu pour passer à l'échelle sur les achitectures parallèles. La version actuelle comporte une advection 2D ainsi qu'un schéma linéaire de chimie et servira de base pour un modèle de chimie-transport (MCT). Pour obtenir la conservation de la masse, un schéma en volume-finis de type van Leer a été retenu pour l'advection et étendu au cas 2D en utilisant des opérateurs alternés. La conservation de la masse est assurée en corrigeant les vents en amont. Nous proposons une solution au problème "des pôles" de la grille régulière latitude-longitude grâce à une nouvelle grille préservant approximativement les aires des cellules et couvrant la sphère uniformément. La parallélisation du modèle se base sur l'advection et utilise un algorithme de décomposition de domaines spécialement adapté à la grille. Cela permet d'obtenir l'équilibrage de la charge de calcul avec MPI, une librairie d'échanges de messages. Pour que les performances soient à la hauteur sur les architectures parallèles actuelles et futures, les propriétés analytiques de la grille sont exploitées pour le schéma d'advection et la parallélisation en privilégiant le moindre coût des flops par rapport aux mouvement de données. Le modèle est validé sur des cas tests analytiques et comparé à des schémas de transport à l'aide d'un comparatif récemment publié. Pangolin est aussi comparé au MCT de Météo-France via un schéma linéaire d'ozone et l'utilisation de coordonnées isentropes

The WWRP Polar Prediction Project (PPP)

Mission statement: “Promote cooperative international research enabling development of improved weather and environmental prediction services for the polar regions, on time scales from hours to seasonal”. Increased economic, transportation and research activities in polar regions are leading to more demands for sustained and improved availability of predictive weather and climate information to support decision-making. However, partly as a result of a strong emphasis of previous international efforts on lower and middle latitudes, many gaps in weather, sub-seasonal and seasonal forecasting in polar regions hamper reliable decision making in the Arctic, Antarctic and possibly the middle latitudes as well. In order to advance polar prediction capabilities, the WWRP Polar Prediction Project (PPP) has been established as one of three THORPEX (THe Observing System Research and Predictability EXperiment) legacy activities. The aim of PPP, a ten year endeavour (2013-2022), is to promote cooperative international research enabling development of improved weather and environmental prediction services for the polar regions, on hourly to seasonal time scales. In order to achieve its goals, PPP will enhance international and interdisciplinary collaboration through the development of strong linkages with related initiatives; strengthen linkages between academia, research institutions and operational forecasting centres; promote interactions and communication between research and stakeholders; and foster education and outreach. Flagship research activities of PPP include sea ice prediction, polar-lower latitude linkages and the Year of Polar Prediction (YOPP) - an intensive observational, coupled modelling, service-oriented research and educational effort in the period mid-2017 to mid-2019

Investigation of swirl pipe for improving cleaning efficiency in closed processing system

This thesis provides unique insights into the fundamentals of improving the efficiency of ‘Clean-In-Place’ procedures in closed processing systems by locally introducing intensified hydrodynamic force from swirl flows induced by an optimised four-lobed swirl pipe without increasing the overall flow velocities. The studies, carried out employing Computational Fluid Dynamics (CFD) techniques, pressure transmitters and a fast response Constant Temperature Anemometer (CTA) system, covered further optimisation of the four-lobed swirl pipe, RANS-based modelling and Large Eddy Simulation of the swirl flows, and experimental validation of the CFD models through the measurements of pressure drop and wall shear stress in swirl flows with various Reynolds Number. The computational and experimental work showed that the swirl pipe gives rise to a clear increase of mean wall shear stress to the downstream with its value and variation trend being dependent on swirl intensity. Moreover, it promotes a stronger fluctuation rate of wall shear stress to the downstream especially further downstream where swirl effect is less dominant. As the increase of either the mean or the fluctuation rates of wall shear stress contributes to the improvement of CIP procedures in the closed processing systems. This thesis demonstrates that, with the ability to exert strengthened hydrodynamic force to the internal surface of the pipe downstream of it without increasing the overall flow velocity, the introduction of swirl pipe to the CIP procedures should improve the cleaning efficiency in the closed processing systems, consequently shortening the downtime for cleaning, and reducing the costs for chemicals and power energy