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The interplay between wind and clouds in the trades
No full textCumulus clouds ('fair-weather clouds') form as a result of atmospheric convection and have vertical extents between a few hundred metres (humilis species) and several kilometres (congestus species). They are a major source of uncertainty in the estimation of climate sensitivity by climate models. In order to reach more agreement in cloud changes due to global warming as predicted by different climate models, a better understanding of the physics of these clouds is needed. Shallow cumulus clouds are particularly common over the oceans of Earth's trade-wind regions, which are situated roughly between the 10° and 30° parallels on both hemispheres and are characterised by steady easterly surface winds. These winds are part of the Hadley cell, a large-scale circulation system in which air flows away from the equator at high altitudes and towards the equator near the surface. As a consequence, vertical shear (i.e. vertical differences in wind speed and direction) is common in this region. While recent studies have shown that (surface) wind speed is an important predictor of cloudiness in this region, little work has been done to elucidate how shear affects clouds. Vice versa, clouds also affect the wind by vertically transporting it. While this convective momentum transport (CMT) undoubtedly plays an important role in the force balance that sets the trade winds, only little is known about the details of how CMT sets the vertical structure of the wind and of the spatial scales of the momentum-transporting eddies. In this thesis, more light is shed on the bidirectional interaction between shallow cumulus convection and the wind. Particular focus is put on the effect of wind shear on convection and on the different spatial scales (convective and turbulent) at which convection affects the wind at different heights. To this end, results from numerical large-eddy-simulation (LES) experiments are utilised in this thesis. Due to their fine horizontal resolution (of hundreds of metres and less), LES is able to numerically resolve clouds and the largest turbulent eddies explicitly. This leads to a high degree of realism in the simulation. Together with the possibility to artificially simplify the experimental set-up as well as the completeness of the output (in terms of time, space and quantities), this makes LES the ideal tool to understand physical mechanisms in the atmosphere. To identify and understand the effect of wind shear on cumulus convection, LES experiments were carried out in which typical conditions of the trades were simulated, while the amount of wind shear was systematically varied. In these idealised LES, vertical wind shear effectively limits the deepening of trade-wind convection. Several mechanisms are responsible for this, which depend on the direction of the shear vector (vertically decreasing or increasing wind speed) as well as the altitude at which shear is present. A situation with easterly surface winds that weaken with height and eventually turn westerly is referred to as backward shear, and the opposite situation with easterlies that strengthen with height is called forward shear. Different directions of wind shear cause different surface winds due to CMT, which in turn affect the surface evaporation: Faster surface winds occur in the presence of forward shear and lead to stronger evaporation of sea water, resulting in deeper convection. Forward shear in the subcloud layer also leads to a spatial separation of precipitative downdrafts and emerging updrafts, as clouds move faster than their subcloud-layer roots; this is favourable for convective development. Conversely, under backward shear, the surface evaporation is weakened and precipitative downdrafts interfere with updrafts, hindering convective deepening. However, once clouds grow to sufficient depths, they may produce precipitation so strong that the associated downdrafts spread out laterally near the surface, forming a distinct circular region of cold air, a cold pool. The spreading of this cold pool can cause uplift at its edges, triggering new convection. Backward shear limits the triggering of such secondary convection at cold-pool fronts, while forward shear facilitates it. Finally, shear of any direction in the cloud layer weakens cloud updrafts through an enhanced downward-oriented pressure perturbation force. The limiting effect of wind shear on cloud depth also affects the thermodynamic properties of the cloud layer: The relative humidity is larger and its decrease near the trade-wind inversion is more distinct if clouds are shallower. Large-domain LES hindcasts of specific days during the NARVAL measurement campaigns (which took place in December 2013 and August 2016 in the North Atlantic trades) give a uniquely realistic and complete view on the momentum balance of the trade winds. The combined effect of advection resolved by the model — which here is interpreted as CMT — and unresolved small-scale turbulence is to decelerate the wind in a layer that extends from the surface up to a height of about 2 km in winter and 1 km in summer. However, the role of each term in the balance depends on the altitude. CMT itself acts to accelerate near-surface winds, and only due to strong small-scale turbulence, there is still an overall frictious force at this height. Halfway into the subcloud layer, CMT starts to act as a frictious force. This friction is strengthened by small-scale turbulence from cloud base upwards and quickly diminishes with height. Thus, the cumulus clouds themselves do not introduce significant friction at the altitude where the zonal trade-wind jet resides, which coincides with cloud base. In fact, combined with momentum transport against the wind gradient (counter-gradient momentum transport), they may help to sustain this jet. Overall, wind shear appears to be an important player in setting the typical structure of the trade-wind atmosphere by modulating the depth of convection and may thus even affect cloud-radiative effects. Conversely, convection and turbulence give rise to an overall frictious force on the trade winds. CMT alone acts to accelerate the winds near the surface, which may weaken the Hadley circulation, while in the cloud layer, CMT hardly affects the wind
The Morphology of Simulated Trade-Wind Convection and Cold Pools Under Wind Shear
No full textA growing body of literature investigates convective organization, but few studies to date have sought to investigate how wind shear plays a role in the spatial organization of shallow (trade-wind) convection. The present study hence investigates the morphology of precipitating marine cumulus convection using large-eddy-simulation experiments with zonal forward and backward shear and without shear. One set of simulations includes evaporation of precipitation, promoting cold-pool development, and another set inhibits the evaporation of precipitation and thus cold-pool formation. Without (or with only weak) subcloud-layer shear, conditions are unfavorable for convective deepening, as clouds remain stationary relative to their subcloud-layer roots so that precipitative downdrafts interfere with emerging updrafts. Under subcloud-layer forward shear (FS), where the wind strengthens with height (a condition that is commonly found in the trades), clouds move at greater speed than their roots and precipitation falls downwind away from emerging updrafts. FS in the subcloud layer appears to promote the development of stronger subcloud circulations, with greater divergence in the cold-pool area downwind of the original cell and larger convergence and stronger uplift at the gust front boundary. As clouds shear forward, a larger fraction of precipitation falls outside of clouds, leading to more moistening within the cold pool (gust front).Atmospheric Remote Sensin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
On the Value of LES Models for Evaluating Spatio-Temporal Tropospheric Variability in Multitemporal SAR Interferograms
No full textAtmospheric delay has a profound impact on synthetic aperture radar (SAR) interferometry, inducing a spatial signal that significantly devaluates interferometric products. While the wide-scale variability of the atmosphere can be adequately modeled with global or regional weather models, it is especially the turbulent and convective part of the atmosphere at smaller spatial and temporal scales that is typically poorly captured. Due to the high resolution and precision of InSAR, there is a need for a realistic modeling of the 3-D distribution of turbulent refractivity in the boundary layer. This would enable assessment of the impact of a temporal or spatial model misalignment on the interferometric products, and contribute to studying the impact for future SAR missions. Here we test the feasibility of an advanced large Eddy simulation (LES) model to simulate a time-series refractivity distribution with a high spatio-temporal resolution to show the spatio-temporal variability of the troposphere on short time scales. We found for a fair-weather situation that the LES model produces realistic atmospheric simulations that match stochastically with results found in interferometric studies and that tropospheric delay variation leads to significant phase gradients within several minutes. This implies that even when using an (unrealistic) perfect weather model with resolutions similar to the SAR image, realizations that are several minutes apart from the time of the SAR acquisition will lead to significant phase errors. We propose the use of LES models as a realistic instrument to perform InSAR quality-assessments and for the development and simulation for future missions.Mathematical Geodesy and PositioningAtmospheric Remote Sensin
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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