In this study Lagrangian large‐eddy simulation of cloudy mixed layers in evolving warm air masses in the Arctic is constrained by in situ observations from the recent PASCAL field campaign. A key ...novelty is that time dependence is maintained in the large‐scale forcings. An iterative procedure featuring large‐eddy simulation on microgrids is explored to calibrate the case setup, inspired by and making use of the typically long memory of Arctic air masses for upstream conditions. The simulated mixed‐phase clouds are part of a turbulent mixed layer that is weakly coupled to the surface and is occasionally capped by a shallow humidity layer. All eight simulated mixed layers exhibit a strong time evolution across a range of time scales, including diurnal but also synoptic fingerprints. A few cases experience rapid cloud collapse, coinciding with a rapid decrease in mixed‐layer depth. To gain insight, composite budget analyses are performed. In the mixed‐layer interior the heat and moisture budgets are dominated by turbulent transport, radiative cooling, and precipitation. However, near the thermal inversion the large‐scale vertical advection also contributes significantly, showing a distinct difference between subsidence and upsidence conditions. A bulk mass budget analysis reveals that entrainment deepening behaves almost time‐constantly, as long as clouds are present. In contrast, large‐scale subsidence fluctuates much more strongly and can both counteract and boost boundary‐layer deepening resulting from entrainment. Strong and sudden subsidence events following prolonged deepening periods are found to cause the cloud collapses, associated with a substantial reduction in the surface downward longwave radiative flux.
Key Points
Lagrangian LES of Arctic cloudy mixed layers in evolving warm air masses is constrained by in situ observations from the PASCAL field campaign
A novel iterative method relying on LES on microgrids is applied to optimize the case configuration and adjust biases in GCM‐derived forcings
Budget studies give insight into local and remote controls on AML evolution, suggesting large‐scale subsidence events can cause low‐level cloud collapse over the sea ice
Springtime Arctic mixed-phase convection over open water in the Fram Strait as observed during the recent ACLOUD (Arctic CLoud Observations Using airborne measurements during polar Day) field ...campaign is simulated at turbulence-resolving resolutions. The first objective is to assess the skill of large-eddy simulation (LES) in reproducing the observed mixed-phase convection. The second goal is to then use the model to investigate how aerosol modulates the way in which turbulent mixing and clouds transform the low-level air mass. The focus lies on the low-level thermal structure and lapse rate, the heating efficiency of turbulent entrainment, and the low-level energy budget. A composite case is constructed based on data collected by two research aircraft on 18 June 2017. Simulations are evaluated against independent datasets, showing that the observed thermodynamic, cloudy, and turbulent states are well reproduced. Sensitivity tests on cloud condensation nuclei (CCN) concentration are then performed, covering a broad range between pristine polar and polluted continental values. We find a significant response in the resolved mixed-phase convection, which is in line with previous LES studies. An increased CCN substantially enhances the depth of convection and liquid cloud amount, accompanied by reduced surface precipitation. Initializing with the in situ CCN data yields the best agreement with the cloud and turbulence observations, a result that prioritizes its measurement during field campaigns for supporting high-resolution modeling efforts. A deeper analysis reveals that CCN significantly increases the efficiency of radiatively driven entrainment in warming the boundary layer. The marked strengthening of the thermal inversion plays a key role in this effect. The low-level heat budget shifts from surface driven to radiatively driven. This response is accompanied by a substantial reduction in the surface energy budget, featuring a weakened flow of solar radiation into the ocean. Results are interpreted in the context of air–sea interactions, air mass transformations, and climate feedbacks at high latitudes.
The Arctic has warmed more rapidly than the global mean during the past few decades. The lapse rate feedback (LRF) has been identified as being a large contributor to the Arctic amplification (AA) of ...climate change. This particular feedback arises from the vertically non-uniform warming of the troposphere, which in the Arctic emerges as strong near-surface and muted free-tropospheric warming. Stable stratification and meridional energy transport are two characteristic processes that are evoked as causes for this vertical warming structure. Our aim is to constrain these governing processes by making use of detailed observations in combination with the large climate model ensemble of the sixth Coupled Model Intercomparison Project (CMIP6). We build on the result that CMIP6 models show a large spread in AA and Arctic LRF, which are positively correlated for the historical period of 1951–2014. Thus, we present process-oriented constraints by linking characteristics of the current climate to historical climate simulations. In particular, we compare a large consortium of present-day observations to co-located model data from subsets that show a weak and strong simulated AA and Arctic LRF in the past. Our analyses suggest that the vertical temperature structure of the Arctic boundary layer is more realistically depicted in climate models with weak (w) AA and Arctic LRF (CMIP6/w) in the past. In particular, CMIP6/w models show stronger inversions in the present climate for boreal autumn and winter and over sea ice, which is more consistent with the observations. These results are based on observations from the year-long Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic, long-term measurements at the Utqiaġvik site in Alaska, USA, and dropsonde temperature profiling from aircraft campaigns in the Fram Strait. In addition, the atmospheric energy transport from lower latitudes that can further mediate the warming structure in the free troposphere is more realistically represented by CMIP6/w models. In particular, CMIP6/w models systemically simulate a weaker Arctic atmospheric energy transport convergence in the present climate for boreal autumn and winter, which is more consistent with fifth generation reanalysis of the European Centre for Medium-Range Weather Forecasts (ERA5). We further show a positive relationship between the magnitude of the present-day transport convergence and the strength of past AA. With respect to the Arctic LRF, we find links between the changes in transport pathways that drive vertical warming structures and local differences in the LRF. This highlights the mediating influence of advection on the Arctic LRF and motivates deeper studies to explicitly link spatial patterns of Arctic feedbacks to changes in the large-scale circulation.
The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, ...high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s−1crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.
This paper describes the use of granite filler produced during cutting, grinding and polishing of decorative stone. It mainly focuses on the consistency and workability of the fresh mixture in case ...of the cement substitution by granite filler. The comparative tests are carried out according to European standards dealing with fresh mortars and fresh concrete. The substitution of cement by granite filler is in multiples of 5% and 10% of the cement weight respectively. All results are also compared with a reference sample without granite filler. The results of flow table confirmed that workability and consistency decreased as the amount of granite filler increased. Altough this phenomena was also confirmed by spread flow test, specimens with 5 and 10% cement substitution had a better rheological properties. This phenomena was probably caused by improve of grain size curve of fresh mortar.
The article deals with the classification of fillers according to the type and method of rock processing and their use as a partial cement replacement in the concrete mixture. The aim is to better ...classify fillers by their origin, maximise their usability and design a concrete mixture with effective use of filler as a cement replacement. The article compares the fillers that originated from natural stone. The research is focused primarily on the granite filler, as it is not currently explored in detail. As the results of existing research differ, tests of the filler and its effect on the compressive strength of cement composites were performed, for better characterization of the effect of filler pozzolanic activity. The results of the tests are compared with each other, and the probable causes of differences in the individual research results are described.
The paper deals with the effect of various modifications of composition and homogenization procedure on micromechanical characteristics of high-performance concrete (HPC) containing supplementary ...cementitious materials (SCMs), namely silica fume, fly ash and metakaolin. The main motivation was to characterize the changes of microstructure induced by the type and amount of SCMs, by the time and order of mixing of components and by coarse aggregate washing. The effects of the changes of microstructure on macroscopic mechanical features of the material were also studied. Indentation moduli of particular phases of the material were measured by nanoindentation. Interfacial transition zone (ITZ) thickness was primarily measured by nanoindentation. An alternative method of ITZ thickness determination based on variations in chemical composition was tested for a selected sample with encouraging result. Compressive strength and bulk elastic modulus of concrete were determined by standard loading tests. The results showed that SCMs generally decrease the thickness of ITZ, but no direct relation to compressive strength of concrete was found for mixtures with variable SCMs content. In case of mixtures with optimized SCMs content prepared by different homogenization procedures or with the use of aggregate washing, qualitative dependence between ITZ thickness and compressive strength was found. Aggregate washing proved to be useful for improving both microscopic and macroscopic properties, having positive effect on ITZ thickness, compressive strength, bulk elastic modulus and indentation moduli of particular phases.
For the last several years, the attention has been devoted to the development of eco-friendly construction materials with the aim to reduce the environmental impact of construction. The integral part ...of this development is to find applications for which the recycled materials could be used efficiently. The presented study deals with the development of recycled aggregate fibre reinforced concrete which is going to be used for an innovative precast concrete pavement system. The system is being developed recently at Czech Technical University in Prague with the aim to provide a system for airfield pavement with rapid construction and recycled material utilization. The investigated material is going to be used for construction of a sub-base course which is underneath a surface course of precast concrete elements. Within the scope of the work, standard laboratory tests were conducted with the aim to observe mechanical properties of recycled aggregate fibre reinforced concrete. The obtained findings showed that the material exhibits ductile behaviour likely due to very long polypropylene fibres used for strengthening concrete matrix. On the contrary, the tested material has low modulus of elasticity in comparison with conventional concrete. Subsequently, the pilot construction of the sub-base course was carried out in order to examine both concrete mixture preparation and concrete course installation in practice. The obtained findings showed that a compaction ratio of the material is about 20%. Moreover, a flat surface of the sub-base course was hard to achieve considering the used manufacturing technology.
The article follows on previous research by authors dealing with the properties of concrete with the addition of recycled rubber. This paper describes the substitution of aeration additives with ...rubber recycled material and the effect of this substitution on strength and modulus of elasticity. For concrete mixes containing 40 kg/m3 of recycled rubber, the air content of the fresh concrete was measured. Concrete with the same amount of fresh air was made using an aeration additive. Compressive strength and modulus of elasticity were compared.
Supplementary cementitious materials (SCM) are used in concrete for two main reasons – to reduce the amount of cement used and to improve material properties. A material that is more sustainable, ...durable, environmental friendly and economical compared to the traditional Portland cement concrete can be obtained. This paper investigates the effect of two important factors on mechanical properties of highperformance concrete (HPC) containing SCM. The first factor is the content of selected SCM, the second one is the homogenization procedure used for preparation of concrete. In the first part of the research program, 10 different mixtures were compared: reference mixture with no SCM and mixtures where 10 %, 20 % or 30 % of cement weight were replaced by microsilica, fly ash or metakaolin. In the second part, three mixtures with selected replacement levels were prepared by four different homogenization procedures and studied. Tests of bulk density, compressive strength, splitting tensile strength, flexural tensile strength, dynamic and static elastic modulus and depth of penetration of water under pressure were carried out for the tested mixtures. The best results were reached when cement was partially replaced by fly ash. Resistance of concrete to penetration of water under pressure was significantly improved by all SCM. The homogenization procedure in which the SCM was added to the mixture after water led to slightly better properties than the standard mixing technique in case of mixtures containing microsilica and metakaolin. The paper provides an extensive database that can serve as a benchmark for the design of HPC containing SCM.
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