22 research outputs found
Large-eddy simulation of radiation fog with comprehensive two-moment bulk microphysics: impact of different aerosol activation and condensation parameterizations
In this paper we study the influence of the cloud microphysical
parameterization, namely the effect of different methods for calculating the
supersaturation and aerosol activation, on the structure and life cycle of
radiation fog in large-eddy simulations. For this purpose we investigate a
well-documented deep fog case as observed at Cabauw (the Netherlands) using
high-resolution large-eddy simulations with a comprehensive bulk cloud
microphysics scheme. By comparing saturation adjustment with a diagnostic and
a prognostic method for calculating supersaturation (while neglecting the
activation process), we find that, even though assumptions for saturation
adjustment are violated, the expected overestimation of the liquid water
mixing ratio is negligible. By additionally considering activation, however,
our results indicate that saturation adjustment, due to approximating the
underlying supersaturation, leads to a higher droplet concentration and hence
significantly higher liquid water content in the fog layer, while diagnostic
and prognostic methods yield comparable results. Furthermore, the effect of
different droplet number concentrations is investigated, induced by using
different common activation schemes. We find, in line with previous studies,
a positive feedback between the droplet number concentration (as a
consequence of the applied activation schemes) and strength of the fog layer
(defined by its vertical extent and amount of liquid water). Furthermore, we
perform an explicit analysis of the budgets of condensation, evaporation,
sedimentation and advection in order to assess the height-dependent
contribution of the individual processes on the development phases.</p
Improving collisional growth in Lagrangian cloud models: development and verification of a new splitting algorithm
Lagrangian cloud models (LCMs) are increasingly used in the cloud physics
community. They not only enable a very detailed representation of cloud
microphysics but also lack numerical errors typical for most other models.
However, insufficient statistics, caused by an inadequate number of
Lagrangian particles to represent cloud microphysical processes, can limit
the applicability and validity of this approach. This study presents the
first use of a splitting and merging algorithm designed to improve the warm
cloud precipitation process by deliberately increasing or decreasing the
number of Lagrangian particles under appropriate conditions. This new
approach and the details of how splitting is executed are evaluated in box
and single-cloud simulations, as well as a shallow cumulus test case. The
results indicate that splitting is essential for a proper representation of
the precipitation process. Moreover, the details of the splitting method
(i.e., identifying the appropriate conditions) become insignificant for
larger model domains as long as a sufficiently large number of Lagrangian
particles is produced by the algorithm. The accompanying merging algorithm is
essential to constrict the number of Lagrangian particles in order to
maintain the computational performance of the model. Overall, splitting and
merging do not affect the life cycle and domain-averaged macroscopic
properties of the simulated clouds. This new approach is a useful addition to
all LCMs since it is able to significantly increase the number of Lagrangian
particles in appropriate regions of the clouds, while maintaining a
computationally feasible total number of Lagrangian particles in the entire
model domain.</p
Vat photopolymerization of cemented carbide specimen
Numerous studies show that vat photopolymerization enables near-net-shape printing of ceramics and plastics with complex geometries. In this study, vat photopolymerization was investigated for cemented carbide specimens. Custom-developed photosensitive WC-12 Co (wt%) slurries were used for printing green bodies. The samples were examined for defects using quantitative microstructure analysis. A thermogravimetric analysis was performed to develop a debinding program for the green bodies. After sintering, the microstructure and surface roughness were evaluated. As mechanical parameters, Vickers hardness and Palmqvist fracture toughness were considered. A linear shrinkage of 26–27% was determined. The remaining porosity fraction was 9.0%. No free graphite formation, and almost no η-phase formation occurred. WC grain growth was observed. 76% of the WC grains measured were in the suitable size range for metal cutting tool applications. A hardness of 1157 HV10 and a Palmqvist fracture toughness of 12 MPa was achieved. The achieved microstructure exhibits a high porosity fraction and local cracks. As a result, vat photopolymerization can become an alternative forming method for cemented carbide components if the amount of residual porosity and defects can be reduced
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 Largeeddy 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