8 research outputs found
Entrainment temporal evolution across stably and unstably stratified vapor/clear air interfaces
No full textWarm clouds as stratocumuli swathe a significant part of earth’s surface and play a major role in the global dynamics of atmosphere by strongly reflecting incoming solar radiation so that an accurate representation of their dynamics is important in large-scale analyses of atmoshperic flows [Wood 2012].The mixing and entrainment processes at the cloud top have been identified as fundamental to determine the internal structure of warm clouds, so that a clear and complete understanding of their physics is required [Gerber et al 2013]. The aim of this work is to study some of the basic phenomena which occur at a stratified interface focusing on the smallest scales of the flow which influence. These scales are important to understand the global dynamic of clouds, as pointed out by Malinowski et al (2013). To achieve the results, a campaign of high-resolution simulation of the local transport through a dry/moist air were performed by the means of Direct Numerical Simulations (DNS) using our home produced computational code that implements a de-aliased pseudospectral Fourier-Galerkin spatial discretization and an explicit low storage fourth order Runge-Kutta time integration scheme [Iovieno et al 2001]. We consider the interface between clear air and moist air in a 6m × 6m × 12m parallelepipedic domain coupling two homogeneous and isotropic turbulent regions with different kinetic energy that interact through a mixing layer. The energy ratio is of the same order of the ones measured in warm clouds (see, e.g., [Malinowski et al 2013]) and, furthermore, it allows us to compare our results with experiments on shearless mixing (see [Veeravalli & Warhaft 1989, Tordella & Iovieno 2011]) in absence of any stratification. For each simulation two interfaces have been obtained, one in highly stably stratified condition, and one in unstable condition. The dynamics of interfaces is analyzed through an initial temperature perturbation located across one of the vapor/clear air interfaces thus generating a local stable layer; the water vapor is treated as a passive scalar. The level of stratification is quantified with the Froude number. For the stable cases, the Froude numbers considered ranges from 12.7 (weak stratification) to 0.6 (intense stratification), while for the unstable cases Fr^2 ranges from -250 to -16. In both stable and unstable cases the evolution of the system can be split in two different phases. In the first one, the buoyancy terms are negligible, and there are no significant differences with respect to a non-stratified case. As the system evolves, the effect of stratification becomes relevant (as soon as the stratification is intense). About the unstable case layer we observe a high intermittency and an intense growth rate of the layer, which becomes overdiffusive in the case Fr^2 = −16. In particular, the entrainment, after an initial decay, asimptotically always shows a positive growth rate. Here, for reason of space, we give details about the stably stratified layer which presents a more complex dynamics associated to the onset of a pocket very low turbulent kinetic energy. It can be observed the onset of a sub-layer characterized by the presence of low values of kinetic turbulent energy. At about 8 time scales, we observe the 8% of the energy in the wapor cloud and the 50% of the kinetic energy in the clear-air region. A similar trend was also observed in the LES cloud topped boundary layer simulations carried out by using Deardoff TKE model (NCAR group) and by using the ARAP TKE model (WVU group) [Moeng et al 1996]. The presence of such sublayer induces the formation of two local interfaces. Both of these interfaces present an intermittent behavior, and the entrainment (flux of dry air into the moist one) is blocked; the velocity of the moist air front can be considered a characteristic parameter, since the entrainment of clear air is responsible of the growth of the cloud [Mellado 2010, Moeng 2000]. As a consequence, the entrainment of clear air is confined to a thin interfacial layer. Also the dissipative terms inside the pit becomes relatively more important compared to the kinetic energy, making the pit deeper and deeper with respect to the external regions
Towards a Scholarship of School-Based Teaching and Learning That Embraces Hope, Change, and Social Justice in a South African University
No full textIn this commentary, the author presents an argument for embracing a critical southern paradigm and framework for a Scholarship of Teaching and Learning (SoTL) that advances decoloniality, social justice, and conscientisation. The kind of scholarship that is argued for in this paper proposes a SoTL that goes beyond the recognition of classrooms as sites of inquiry and teaching, but a SoTL that is generative, context responsive, carries moral and pedagogical imperatives, and can influence institutional and societal change. This commentary draws on the experiences of a Dean of Faculty through self-reflexive qualitative impressions. She frames her personal experience of implementing a School-Based Learning placement approach within a theoretical discussion of agency, conscientisation, and transformative learning
Turbulent dispersion in cloud-topped boundary layers
No full textCompared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion – a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.Infrastructures, Systems and ServicesTechnology, Policy and Managemen
A numerical study for turbulent flow and thermal influence over inhomogenous canopy of roughness elements
No full textA large-eddy simulation with transitional structure function(TSF) subgrid model we previously proposed was performed to investigate the turbulent flow with thermal influence over an inhomogeneous canopy, which was represented as alternative large and small roughness elements. The aerodynamic and thermodynamic effects of the presence of a layer of large roughness elements were modelled by adding a drag term to the three-dimensional Navier–Stokes equations and a heat source/sink term to the scalar equation, respectively. The layer of small roughness elements was simply treated using the method as described in paper (Moeng 1984, J. Atmos Sci. 41, 2052–2062) for homogeneous rough surface. The horizontally averaged statistics such as mean vertical profiles of wind velocity, air temperature, et al., are in reasonable agreement with Gao et al.(1989, Boundary layer meteorol. 47, 349–377) field observation (homogeneous canopy). Not surprisingly, the calculated instantaneous velocity and temperature fields show that the roughness elements considerably changed the turbulent structure within the canopy. The adjustment of the mean vertical profiles of velocity and temperature was studied, which was found qualitatively comparable with Belcher et al. (2003, J Fluid Mech. 488, 369–398)'s theoretical results. The urban heat island(UHI) was investigated imposing heat source in the region of large roughness elements. An elevated inversion layer, a phenomenon often observed in the urban area (Sang et al., J Wind Eng. Ind. Aesodyn. 87, 243–258)'s was successfully simulated above the canopy. The cool island(CI) was also investigated imposing heat sink to simply model the evaporation of plant canopy. An inversion layer was found very stable and robust within the canopy
Diversity and distribution of lepidopteran stemborer species and their host plants in Botswana
No full textDiversity and abundance of lepidopteran stem borer natural enemies in natural and cultivated habitats in Botswana
No full textLepidopteran stem-borers in Africa are attacked by diverse natural enemies in natural and cultivated environments. Field surveys of stem-borer natural enemies and associated host plants were conducted during the austral summers of 2014/15 and 2015/16 on natural and cultivated habitats across Botswana to determine their diversity and relative abundance. In cultivated habitats, the most common parasitoids of larvae were Cotesia flavipes Cameron, C. sesamiae (Cameron), and of pupae, Pediobius furvus Gahan and Gambroides nimbipennis Seyrig. In natural habitats, the larval parasitoids Chelonus curvimaculatus Cameron and Goniozus indicus Ashmead were recorded, along with the pupal parasitoid, Dentichasmias busseolae Heinrich. Furthermore, the predatory ants Linepithema humile Mayr, Crematogaster peringueyi Emery and Aenictus species were recorded in both cultivated and natural habitats. The major cultivated plants hosting stem-borers and related natural enemies were maize, sorghum, sweet sorghum, and the major wild plants were Echinochloa pyramidalis, Typha latifolia, Schoenopkctus corymbosus and Cyperus dives. Chilo partellus Swinhoe and Sesamia spp. were the major hosts for parasitoids, with C. partellus predominating in cultivated habitats and Sesamia jansei Tams & Bowden in natural habitats. Larval parasitism ranged from 2.1 to 34.7% and 3.3 to 14.3% in cultivated and natural habitats respectively, whereas pupal parasitism ranged from 6.1 to 10.6% and 6.7 to 9.1%, respectively. Parasitoid percentage abundance ranged from 1.1 to 41.6% and 4.8 to 38.1% in cultivated and natural habitats respectively, with C. flavipes dominating in cultivated and C. curvimaculatus in natural habitats. Our results show that cultivated and natural environments in Botswana harbor a diverse natural enemy community worthy of conserving for stem-borer biological control
The influence of physical processes on extratropical singular vectors
No full textAn investigation is made of the impact of a full linearized physical (moist) parameterization package on extratropical singular vectors (SVs) using the ECMWF integrated forecasting system (IFS). Comparison is made for one particular period with a dry physical package including only vertical diffusion and surface drag. The crucial extra ingredient in the full package is found to be the large-scale latent heat release. Consistent with basic theory, its inclusion results in a shift to smaller horizontal scales and enhanced growth for the SVs. Whereas, for the dry SVs, T42 resolution is sufficient, the moist SVs require T63 to resolve their structure and growth. A 24-h optimization time appears to be appropriate for the moist SVs because of the larger growth of moist SVs compared with dry SVs. Like dry SVs, moist SVs tend to occur in regions of high baroclinicity, but their location is also influenced by the availability of moisture. The most rapidly growing SVs appear to enhance or reduce large-scale rain in regions ahead of major cold fronts. The enhancement occurs in and ahead of a cyclonic perturbation and the reduction in and ahead of an anticyclonic perturbation. Most of the moist SVs for this situation are slightly modified versions of the dry SVs. However, some occur in new locations and have particularly confined structures. The most rapidly growing SV is shown to exhibit quite linear behavior in the nonlinear model as it grows from 0.5 to 12 hPa in 1 day. For 5 times this amplitude the structure is similar but the growth is about half as the perturbation damps a potential vorticity (PV) trough or produces a cutoff, depending on its sign
Regresion PLS y PCA como Solucion al Problema de Multicolinealidad en Regresion Multiple
No full textWe present and compare principal components regression and partial least squares regression, and their solution to the problem of multicollinearity. We illustrate the use of both techniques, and demonstrate the superiority of partial least squares
