Günther Zängl

Intercomparison of Mesoscale Model Simulations of the Daytime Valley Wind System

AbstractThree-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameteriz...

Large eddy simulation using the general circulation model ICON

ICON (ICOsahedral Nonhydrostatic) is a unified modeling system for global numerical weather prediction (NWP) and climate studies. Validation of its dynamical core against a test suite for numerical weather forecasting has been recently published by Zangl et al. (2014). In the present work, an extension of ICON is presented that enables it to perform as a large eddy simulation (LES) model. The details of the implementation of the LES turbulence scheme in ICON are explained and test cases are performed to validate it against two standard LES models. Despite the limitations that ICON inherits from being a unified modeling system, it performs well in capturing the mean flow characteristics and the turbulent statistics of two simulated flow configurations—one being a dry convective boundary layer and the other a cumulus-topped planetary boundary layer.

Numerical errors above steep topography: A model intercomparison

This paper compares the behaviour of three mesoscale models (MMS, Penn State University/NCAR, LM, German Weather Service, and KAMM2, IMK Karlsruhe) in the vicinity of a steep isolated mountain under zero-wind conditions. The setup of the simulations is such that the model atmosphere would remain at rest for all times in the absence of numerical errors. Our results show that the errors occurring in the models under these idealized conditions differ strongly between the models. In the MM5 and the LM, the isentropes become heavily disturbed above the mountain after a few hours of integration, and peak vertical motions of several m s -1 are encountered. In KAMM2, however, isentropes remain virtually undisturbed and vertical motions do not exceed 5 cm s -1 except close to the upper boundary. Sensitivity tests indicate that the numerical errors in the MM5 disappear almost completely when the horizontal diffusion of temperature is computed truly horizontally rather than along the sloping coordinate surfaces. Experiments with moist physics show that the numerical errors over steep mountains can induce substantial amounts of spurious rain. The largest values are found for the LM, followed by the original MM5. The KAMM2 and the modified MM5 version produce only negligible amounts of rain.

Mesoscale Modeling over Complex Terrain: Numerical and Predictability Perspectives

This chapter presents an overview of numerical modeling techniques and methods that are relevant for the simulation and prediction of processes and phenomena over complex terrain. The formative years of numerical weather prediction provide an important reference perspective on the current state of the science for prediction of terrain-forced flows using modern complex modeling systems. New numerical methods and challenges for numerical weather prediction applications in complex terrain include basic issues that range from the formulation of model vertical coordinates for sloping terrain to the need to use consistent formulations to represent the key forcing contributions in the equations of motion. An overview of methods introduced to improve numerical simulations over steep terrain due to the pressure gradient representation and numerical filtering is provided. Advantages of higher-order accuracy methods for the prediction of mountain waves at high resolution are discussed as well.

Application of a hydrometeorological model chain to investigate the effect of global boundaries and downscaling on simulated river discharge

In the current study, two regional climate models (MM5 and REMO) driven by different global boundary conditions (the ERA40 reanalysis and the ECHAM5 model) are one-way coupled to the uncalibrated hydrological process model PROMET to analyze the impact of global boundary conditions, dynamical regionalization and subsequent statistical downscaling (bilinear interpolation, correction of subgrid-scale variability and combined correction of subgrid-scale variability and bias) on river discharge simulation. The results of 12 one-way coupled model runs, set up for the catchment of the Upper Danube (Central Europe) over the historical period 1971–2000, prove the expectation that the global boundaries applied to force the RCMs strongly influence the accuracy of simulated river discharge. It is, however, noteworthy that all efficiency criteria in case of bias corrected MM5 simulations indicate better performance under ERA40 boundaries, whereas REMO-driven hydrological simulations better correspond to measured discharge under ECHAM5 boundaries. Comparing the hydrological results achievable with MM5 and REMO, the application of bias-corrected MM5 simulations turned out to allow for a more accurate simulation of discharge, while the variance in simulated discharge in most cases was better reflected in case of REMO forcings. The correction of subgrid-scale variability within the downscaling of RCM simulations compared to a bilinear interpolation allows for a more accurate simulation of discharge for all model configurations and all discharge criteria considered (mean monthly discharge, mean monthly low-flow and peak-flow discharge). Further improvements in the hydrological simulations could be achieved by eliminating the biases (in terms of deviations from observed meteorological conditions) inherent in the driving RCM simulations, regardless of the global boundary conditions or the RCM applied. In spite of all downscaling and bias correction efforts described, the RCM-driven hydrological simulations remain less accurate than those achievable with spatially distributed meteorological observations.

Upstream effects of an Alpine-scale mountain ridge under various flow angles

Numerical simulations are presented to investigate the upstream effects induced by an east-west-oriented, Alpine-scale mountain ridge under various flow angles. Due to the influence of the Coriolis force, the flow around the mountain is strongly asymmetric in the case of northerly flow, most of the air being deflected to the east. When the ambient flow turns eastward, the flow-splitting point also moves eastward, but this movement does not depend linearly on the large-scale flow direction. Surface friction induces a westward shift of the split point, which is consistent with the friction-induced wind turning in the boundary layer. The force balance in the vicinity of the split point shows that the low-level air flow is in approximate geostrophic balance when surface friction is neglected. The mass field adjusts such that the pressure increases (decreases) towards the mountain to the east (west) of the flow-splitting point, implying westerly (easterly) geostrophic wind.

Large-scale-flow interactions with the Alps and their impact on the low-level temperature field in the northern foreland

Numerical simulations are presented to investigate the impact of large-scale-flow interactions with the Alps on the temperature field in the northern Alpine foreland. The simulations use realistic topography but idealized large-scale conditions that allow for changing the wind direction without creating systematic temperature advection effects. The results show that the dependence of the surface temperatures on the wind direction differs substantially between the morning and the afternoon. In the afternoon, temperatures tend to be higher for southerly flow directions than for northerly ones, as might be expected from the fact that subsidence in the lee of the Alps causes warming while upslope flow is related to cooling. In the morning, however, the lowest temperatures are found for easterly directions while northwesterly flow shows the highest temperatures, followed by westerly flow. Part of this behaviour is related to the fact that in the northern Alpine foreland, westerly wind directions tend to be associated with higher wind speeds than easterly directions, which in turn is caused by the fact that the flow around the Alps is asymmetric. As a consequence, turbulent mixing tends to be stronger for westerly directions. For northwesterly flow, turbulent mixing is particularly strong because a barrier jet forms along the Alps. Another important contribution arises from the friction-induced wind turning in the surface layer, which causes a northerly (southerly) component for easterly (westerly) flow. Thus, the radiatively cooled near-surface air is piled up along the northern rim of the Alps for easterly flow but is advected away from the Alps for westerly flow.

The Mini-Foehn of Mittenwald: A comparison of measurements and modelling

Strong nocturnal downvalley winds can often be observed in the Isar Valley near Mittenwald (Bavarian Alps) under the influence of high pressure systems over Central Europe or in the presence of ambient southerly winds. The properties of this downvalley wind are similar to the Alpine foehn, bringing about a pronounced increase in temperature and a concomitant decrease in relative humidity. We will therefore refer to this wind as mini-foehn. Measurements with automatic surface stations as well as numerical simulations with the MM5 model show that the mini-foehn is a part of the strong nocturnal drainage flow in the adjacent Leutasch Valley in Tyrol. This flow is driven by the temperature difference between the nearby plateau of Seefeld and the free atmosphere above Mittenwald, lying some 250 m below Seefeld. The foehn effect arises from the fact that the cold low-level air leaves the Leutasch Valley through a narrow gorge while potentially warmer air from higher levels reaches Mittenwald via a small mountain ridge. A second jet-like downvalley wind is found in the Isar Valley between Scharnitz and Mittenwald. The temperature of this air mass is usually lower than that of the Leutasch Valley jet, leading to pronounced temperature contrasts around Mittenwald.

A Compact Parallel Algorithm for Spherical Delaunay Triangulations

We present a data-parallel algorithm for the construction of Delaunay triangulations on the sphere. Our method combines a variant of the classical Bowyer-Watson point insertion algorithm [2, 14] with the recently published parallelization technique by Jacobsen et al. [7]. It resolves a breakdown situation of the latter approach and is suitable for practical implementation due to its compact formulation. Some complementary aspects are discussed such as the parallel workload, floating-point arithmetics and an application to interpolation of scattered data.

A compact parallel algorithm for spherical Delaunay triangulations: A COMPACT PARALLEL ALGORITHM FOR SPHERICAL DELAUNAY TRIANGULATIONS

We present a data‐parallel algorithm for the construction of Delaunay triangulations on the sphere. Our method combines a variant of the classical Bowyer–Watson point insertion algorithm with the recently published parallelization technique by Jacobsen et al. It resolves a breakdown situation of the latter approach and is suitable for practical implementation because of its compact formulation. Some complementary aspects are discussed such as the parallel workload and floating‐point arithmetics. In a second step, the generated triangulations are reordered by a stripification algorithm. This improves cache performance and significantly reduces data‐read operations and indirect addressing in multi‐threaded stencil loops. This paper is an extended version of our Parallel Processing and Applied Mathematics conference contribution. Copyright