Toward Large-Eddy Simulations of Dust Devils of Observed Intensity: Effects of Grid Spacing, Background Wind, and Surface Heterogeneities

Dust devils are convective vortices with a vertical axis of rotation made visible by lifted soil particles. Currently, there is great uncertainty about the extent to which dust devils contribute to the atmospheric aerosol input and thereby influence Earth's radiation budget. Past efforts to quantify the aerosol transport and study their formation, maintenance, and statistics using large-eddy simulation (LES) have been of limited success. Therefore, some important features of dust devil-like vortices simulated with LES still do not compare well with those of observed ones. One major difference is the simulated value of the core pressure drop, which is almost 1 order of magnitude smaller compared to the observed range of 250 to 450 Pa. However, most of the existing numerical simulations are based on highly idealized setups and coarse grid spacings. In this study, we investigate the effects of various factors on the simulated vortex strength with high-resolution LES. For the fist time, we are able to reproduce observed core pressures by using a high spatial resolution of 2 m, a model setup with moderate background wind and a spatially heterogeneous surface heat flux. It is found that vortices mainly appear at the lines of horizontal flow convergence above the centers of the strongly heated patches, which is in contrast to some older observations in which vortices seemed to be created along the patch edges

Energy balance closure for the LITFASS-2003 experiment

In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft)

Turbulent heat exchange over polar leads revisited: A large eddy simulation study

Sea ice leads play an important role in energy exchange between the ocean and atmosphere in polar regions, and therefore must be considered in weather and climate models. As sea ice leads are not explicitly resolved in such models, lead-averaged surface heat flux is of considerable interest for the parameterization of energy exchange. Measurements and numerical studies have established that the lead-averaged surface heat flux depends not only on meteorological parameters, but also on lead width. Nonetheless, few studies to date have investigated the dependency of surface heat flux on lead width. Most findings on that dependency are based on observations with lead widths smaller than a few hundred meters, but leads can have widths from a few meters to several kilometers. In this parameter study, we present the results of three series of large-eddy simulations of turbulent exchange processes above leads. We varied the lead width and air temperature, as well as the roughness length. As this study focused on conditions without background wind, ice-breeze circulation occurred, and was the main driver of the adjustment of surface heat flux. A previous large-eddy simulation study with uncommonly large roughness length found that lead-averaged surface heat flux exhibited a distinct maximum at lead widths of about 3 km, while our results show the largest heat fluxes for the smallest leads simulated (lead width of 50 m). At more realistic roughness lengths, we observed monotonously increasing heat fluxes with increasing lead width. Further, new scaling laws for the ice-breeze circulation are proposed

Die technische Infrastruktur zur Teilnahme von Unternehmen an Gemeinschaften in Neuen Medien

Ein Internet-Auftritt eines Unternehmens setzt voraus, daß bezüglich der internen Informations-Infrastruktur bestimmte organisatorische und technische Voraussetzungen erfüllt sind. In diesem Beitrag soll dargelegt werden, daß eine komponentenbasierte Anwendungsarchitektur wegen ihrer leichten Erweiterbarkeit und wegen ihrer Übersichtlichkeit den Internet-Auftritt und damit die Beteiligung an Medien-Gemeinschaften erheblich erleichtern kann. Dies wird anhand einer Prototypentwicklung dargestellt und um Sicherheitsbetrachtungen, die für einen professionellen Einsatz der entwickelten Konzepte unabdingbar sind, ergänzt

Cell broadening revisited: Results from high-resolution large-eddy simulations of cold air outbreaks

Large-eddy simulations (LES) have been carried out in order to investigate the structure and development of organized mesoscale cellular convection (MCC) during cold air outbreaks (CAOB) and to reevaluate results by Müller and Chlond. Some limitations of this and other earlier LES studies of CAOBs have been removed by using a parallelized model with both a large horizontal domain and a fine grid resolution. These model simulations reveal a hitherto undiscovered insight into the development of MCC. It is shown that MCC with aspect ratios larger than 10 only develop in the presence of diabatic heat sources, that is, latent heat release within the clouds and cloud-top radiative cooling, which confirms results from previous studies. Simulated cells are of closed type. The wind field is seen to be correlated with the liquid water field, and thus, dynamic variables are organized on the mesoscale as well. Updrafts predominantly occur in regions with high liquid water content. Two-dimensional spectral analysis confirms clear peaks at the wavelength of the MCC. The dynamic variables are characterized by an organized conglomeration of randomly distributed up- and downdrafts, where each convective cell can hardly be detected by eye. Whereas scalar variables like temperature and the liquid water path field are characterized by large scales with aspect ratios of 9 to 11, in the vertical velocity field multiple scales are present simultaneously, with small scales having aspect ratios between 1 and 3 contributing most to the total energy, but also large scales correlated with scales seen in the thermodynamic variables. Copyright 2005 American Meteorological Societ

Formation of a diurnal thermocline in the ocean mixed layer simulated by LES

The formation of a diurnal thermocline in the ocean mixed layer under a stabilizing buoyancy flux was simulated successfully by large-eddy simulation, reproducing various features consistent with observation. The analysis of the simulation result revealed that the formation of a diurnal thermocline passes through two different phases: the formation of a thermocline (formation stage) and increasing thickness of the thermocline thereafter (growth stage). Turbulent kinetic energy (TKE) flux dominates TKE production within the mixed layer, but turbulence maintained by shear production at the thermocline causes stratification below the mixed layer. In addition, once the thermocline is formed, both the gradient and flux Richardson numbers maintain constant values at the thermocline. It was also found that a diurnal thermocline cannot be formed in the absence of both wave breaking and Langmuir circulation. Furthermore, the effects of stratification on turbulence were investigated based on the time series of various physical variables of turbulence at the diurnal thermocline and within the mixed layer, and the mechanism for diurnal thermocline formation is discussed. Copyright 2009 American Meteorological Societ

Beyond convergence rates: Exact recovery with Tikhonov regularization with sparsity constraints

The Tikhonov regularization of linear ill-posed problems with an $\ell^1$ penalty is considered. We recall results for linear convergence rates and results on exact recovery of the support. Moreover, we derive conditions for exact support recovery which are especially applicable in the case of ill-posed problems, where other conditions, e.g. based on the so-called coherence or the restricted isometry property are usually not applicable. The obtained results also show that the regularized solutions do not only converge in the $\ell^1$-norm but also in the vector space $\ell^0$ (when considered as the strict inductive limit of the spaces $\R^n$ as $n$ tends to infinity). Additionally, the relations between different conditions for exact support recovery and linear convergence rates are investigated. With an imaging example from digital holography the applicability of the obtained results is illustrated, i.e. that one may check a priori if the experimental setup guarantees exact recovery with Tikhonov regularization with sparsity constraints

Large eddy simulation of particle settling in the ocean mixed layer

The settling process of suspended particles in the turbulent flow was investigated by analyzing the Lagrangian motion of a large number of particles in the ocean mixed layer simulated by large eddy simulation (LES), focusing on the role of Langmuir circulation (LC). At the beginning, particles released near the surface are swept down rapidly following the downward jets in the presence of LC, contrary to the case of the horizontally uniform downward propagation in the absence of it. The strong vertical mixing by LC keeps the particle concentration almost uniform after the initial period in the presence of LC, while its mean concentration continues to decrease through particle settling. Under the influence of turbulence in the ocean mixed layer, the particle settling velocity W is always smaller than ws, and W∕ws decreases with decreasing ws∕u∗, where ws is the terminal velocity of a particle in the still fluid and u∗ is the frictional velocity. The presence of LC causes a further decrease of W ultimately, and the difference between the cases with and without LC is the largest at ws∕u∗∼1. The analysis of LES data reveals that particles spend more time in upward flows and that more particles tend to accumulate in the high vorticity region in the presence of LC. It suggests that particle trapping is more likely when the length scale of vortices is larger, which may lead to the decrease of W in the presence of LC. It was also found that the preferential concentration occurs away from the regions of high horizontal vorticity and divergence when ws∕u∗∼1