143 research outputs found
THE WEST AFRICAN MONSOON: DYNAMICS, DECADAL VARIABILITY AND INTERACTION WITH THE EASTERN TROPICAL ATLANTIC SSTS
No full textThis is a PhD dissertation of Samson HagosThis dissertation documents regional modeling studies of various aspects of West African Monsoon dynamics and variability. The physical processes responsible for observed
characteristics seasonal evolution of the monsoon, its decadal variability and interaction
with the Eastern Atlantic Ocean are presented.
The primary mechanism for abrupt latitudinal shift of precipitation maximum from
the Guinea coast to the Sahel region, "the monsoon jump", in June is found to be the
deepening of the monsoon flow into the continent by inertial instability. This instability
is forced by the pressure gradient associated with low level condensational heating due
to the pre-existing boundary layer moisture supply onto the warm land surface.
The decadal variability of the West African precipitation, in particular, the causes of
the 1980s droughts over the Sahel and the partial recovery of the 1990s, are also studied
using the regional atmospheric model. The primary source of variability of precipitation
over the Sahel is found to be variability in the moisture transport across its western coast.
This moisture transport is influenced by circulation patterns associated with the regional
SST anomalies. The 1980s warming over the Indian Ocean, for example, introduces
divergence and anticyclonic easterly wind anomalies which drive moisture away from
the region and cause widespread drought. In a similar manner, warming over tropical
Atlantic Ocean also contributes to the drought. The modeled recovery of the 1990s is
found to be due to changes in the scale and distribution of the anomalous SSTs both over
the tropical Atlantic and Indian Oceans. These changes introduce cyclonic circulation
patterns that favor westerly transport of moisture towards the Sahel. The response of
the region to an SST forcing is related to the background moisture distribution and is
enhanced by circulation patterns associated with the changes in the local condensational
heating.
Finally, a coupled regional model is used to investigate the two way interaction between
theWest African Monsoon and seasonal SST variations over the Eastern Tropical
Atlantic Ocean. The model allows thermodynamic and dynamic interactions between
the atmospheric planetary boundary layer and the ocean mixed layer. The model results
show that the seasonal cooling over the Gulf of Guinea between spring and summer is
related to increased entrainment and evaporation due to acceleration of the southerly
winds driven by the African heat low. Over northeastern tropical Atlantic, the warming
due to solar heating is augmented by the suppression of evaporation. The northward
migration of the ITCZ leads to weakened northeasterly winds and reduced evaporation.
The warming of the northeastern Atlantic, on the other hand, weakens the anticyclonic
easterly winds associated with the Atlantic High and strengthen the cyclonic westerly
monsoon winds.NSF ATM-041548
Propriétés métrologiques du questionnaire « Copenhagen Hip and Groin Outcome Score » (HAGOS) chez les patients souffrant d’une pubalgie du sportif
No full textIntroduction: Groin pain in athlete is a complex and often misdiagnosed condition. The consensus on its nomenclature and pathophysiology is relatively recent (2015).Objective: To evaluate the test-retest reliability, internal consistency, measurement error, construct validity, responsiveness, interpretability) of the diagnostic tool HAGOS in groin pain in athlete.Method: Using the PICO acronym, several databases were explored (PubMed, DiTA, etc.), and three articles were included in this literature review. To analyze the results, the author of this review assessed the methodological quality of the included studies and interpreted several statistical parameters (ICC, Cronbach's alpha, SDC, Spearman’s correlation coefficient, floor et ceiling effects).Results: The overall psychometric properties of the HAGOS questionnaire are deemed satisfactory.Discussion: The low number of included articles and the population bias present in one of the included studies necessitate caution in interpreting the results obtained. It’s also important to note that an evaluation of the psychometric properties in the French language does not exist.Introduction : la pubalgie est une pathologie complexe et mal diagnostiquée. Le consensus autour de sa dénomination et sa physiopathologie est assez récente (2015).Objectif : évaluer la fiabilité test-retest, la cohérence interne, l’erreur de mesure, la validité par construit, réactivité et l’interprétabilité de l’outil diagnostique HAGOS chez la pubalgie du sportif. Méthode : à l’aide de l’acronyme PICO, plusieurs bases de données ont été explorées (PubMed, DiTA, etc.) et trois articles ont été inclus dans cette revue de littérature. Afin d’analyser les résultats, l’auteur de cette revue a procédé à une évaluation de la qualité méthodologiques des études incluses et a interprété plusieurs paramètres statistiques (ICC, α de Cronbach, SDC, coefficient de corrélation de Spearman, effets plancher et plafond).Résultats : l’ensemble des propriétés métrologiques du questionnaire HAGOS est jugé satisfaisant.Discussion : le faible nombre d’articles inclus ainsi que le biais de population présent dans une des études incluses nous oblige à modérer les résultats obtenus. Il est également important de souligner que l’évaluation des propriétés du questionnaire en langue français n’existe pas
Effect of sidewalk vendors on pedestrian movement characteristics: A microscopic simulation study of Addis Ababa, Ethiopia
No full textStreet vending is an essential part of the urban informal economy in developing countries. Related to rapid urbanization and socio-economic challenges, studies have proposed comprehensive street design and management that accommodate sidewalk vendors efficiently. To this end, this paper evaluated the effect of three alternatives to integrate sidewalk vendors on average pedestrian density. A social-force based pedestrian micro-simulation model (PTV-Viswalk-11) was calibrated using a macroscopic approach. A calibrated model was then used to evaluate the effect of a kiosk, a sidewalk vendor in frontage, and a sidewalk vendor in furniture zones on pedestrian movement under different scenarios. Results indicated that the average pedestrian density varied with the location and width of the vending stall, width of the walkway, pedestrian flow and the presence of a customer interacting with a vendor. The paper concludes with recommendations for planners and local authorities.Hagos, KG (corresponding author), Hasselt Univ, Dept Transportat Sci, B-3590 Diepenbeek, Belgium ; Hasselt Univ, Transportat Res Inst IMOB, B-3590 Diepenbeek, Belgium
[email protected]; [email protected]; [email protected]
Cloud permitting modeling of shallow-to-deep convection transitions during the initiation and propagation of Madden-Julian Oscillation
No full textNon UBCUnreviewedAuthor affiliation: Pacific Northwest National LaboratoryOthe
The effect of aging on binder proporties of porous asphalt concrete
No full textIn this dissertation, the results of a research on the effects of aging on the binder properties of porous asphalt concrete (PAC) are described. The research has been done on laboratory and field aged specimens. In addition to the conventional aging methods for short and long term aging of the binder in the laboratory, a new mixture aging protocol in a 'weatherometer' was proposed that combines the main environmental factors influencing field aging, i.e. temperature, UV light, and humidity/moisture. The field binders were recovered from specimens taken from road sections immediately after construction and after 1, 3, 7, and 12 years service life. Rheological, mechanical, and chemical characterization of the binders was conducted on the laboratory and field binders to understand the effects of aging. To this end, the determination of fundamental rheological binder properties was performed in addition to conventional empirical properties such as penetration and softening point. Low temperature tensile and stiffness properties of the binder/mastic were determined using the Direct Tensile Test (DTT) and Bending Beam Rheometer (BBR). The Dynamic Shear Rheometer (DSR) was used to determine the viscoelastic properties (complex modulus and phase angle) of the binders in a wide range of temperatures. Furthermore, the DSR was used to determine the fatigue properties of the binders using strain controlled repeated loading at different strain levels. Chemical characterization using Infrared spectroscopy (IR) and Gel-Permeation Chromatography (GPC) were performed to determine, respectively, the development of the functional groups responsible for aging and the Molecular Weight Distribution (MWD). All the test results have provided insight into laboratory and field aging simulations, the impact of the filler, and at the same time has provided material properties for constitutive modelling. An important finding in this research signifies that aging greatly influences the low temperature performance of the binding material leading to a higher rate of damage accumulation (fatigue) and ultimately ravelling of PA. To illustrate the implications of the research results, a limited number of finite element calculations have been made to determine the stress conditions in the binder/mortar due to traffic loads. Because aging has a negative influence on the fatigue resistance and relaxation behaviour of the binder, aging negatively influences the performance of PAC.Civil Engineering and Geoscience
Diabatic heating, divergent circulation and moisture transport in the African monsoon system
No full textLarge-scale environmental variables and transition to deep convection in cloud resolving model simulations: A vector representation
No full textCloud resolving model simulations and vector analysis are used to develop a quantitative method of assessing regional variations in the relationships between various large-scale environmental variables and the transition to deep convection. Results of the CRM simulations from three tropical regions are used to cluster environmental conditions under which transition to deep convection does and does not take place. Projections of the large-scale environmental variables on the difference between these two clusters are used to quantify the roles of these variables in the transition to deep convection. While the transition to deep convection is most sensitive to moisture and vertical velocity perturbations, the details of the profiles of the anomalies vary from region to region. In comparison, the transition to deep convection is found to be much less sensitive to temperature anomalies over all three regions. The vector formulation presented in this study represents a simple general framework for quantifying various aspects of how the transition to deep convection is sensitive to environmental conditions
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Bi-modal Structure and Variability of Large-Scale Diabatic Heating in the Tropics
No full textAbstract Tropical diabatic heating profiles estimated using sounding data from eight field campaigns were diagnosed to document their common and prevailing structure and variability that are relevant to the large-scale circulation. The first two modes of a rotated empirical orthogonal function analysis—one deep, one shallow—explain 85% of the total variance of all data combined. These two modes were used to describe the heating evolution, which led to three composited heating profiles that are considered as prevailing large-scale heating structures. They are, respectively, shallow, bottom heavy (peak near 700 hPa); deep, middle heavy (peak near 400 hPa); and stratiform-like, top heavy (heating peak near 400 hPa and cooling peak near 700 hPa). The amplitudes and occurrence frequencies of the shallow, bottom-heavy heating profiles are comparable to those of the stratiform-like, top-heavy ones. The sequence of the most probable heating evolution is deep tropospheric cooling to bottom-heavy heating, to middle heavy heating, to stratiform-like heating, then back to deep tropospheric cooling. This heating transition appears to occur on different time scales. Each of the prevailing heating structures is interpreted as being composed of particular fractional populations of various types of precipitating cloud systems, which are viewed as the building blocks for the mean. A linear balanced model forced by the three prevailing heating profiles produces rich vertical structures in the circulation with multiple overturning cells, whose corresponding moisture convergence and surface wind fields are very sensitive to the heating structures
Moist Thermodynamics of the Madden–Julian Oscillation in a Cloud-Resolving Simulation
No full textAbstract
The moist thermodynamic processes that determine the time scale and energy of the Madden–Julian oscillation (MJO) are investigated using moisture and eddy available potential energy budget analyses on a cloud-resolving simulation. Two MJO episodes observed during the winter of 2007/08 are realistically simulated. During the inactive phase, moisture supplied by meridional moisture convergence and boundary layer diffusion generates shallow and congestus clouds that moisten the lower troposphere while horizontal mixing tends to dry it. As the lower troposphere is moistened, it becomes a source of moisture for the subsequent deep convection during the MJO active phase. As the active phase ends, the lower troposphere dries out primarily by condensation and horizontal divergence that dominates over the moisture supply by vertical transport. In the simulation, the characteristic time scales of convective vertical transport, mixing, and condensation of moisture in the midtroposphere are estimated to be about 2 days, 4 days, and 20 h respectively. The small differences among these time scales result in an effective time scale of MJO moistening of about 25 days, half the period of the simulated MJO. Furthermore, various cloud types have a destabilizing or damping effect on the amplitude of MJO temperature signals, depending on their characteristic latent heating profile and its temporal covariance with the temperature. The results are used to identify possible sources of the difficulties in simulating MJO in low-resolution models that rely on cumulus parameterizations.</jats:p
On the Relationship between Uncertainties in Tropical Divergence and the Hydrological Cycle in Global Models
No full textAbstract
A survey of tropical divergence from three GCMs, three global reanalyses, and four in situ soundings from field campaigns shows the existence of large uncertainties in the ubiquity of shallow divergent circulation as well as the depth and strength of the deep divergent circulation. More specifically, only two out of the three GCMs and three global reanalyses show significant shallow divergent circulation, which is present in all in situ soundings, and of the three GCMs and three global reanalyses, only two global reanalyses have deep divergence profiles that lie within the range of uncertainty of the soundings. The relationships of uncertainties in the shallow and deep divergent circulation to uncertainties in present-day and projected strength of the hydrological cycle from the GCMs are assessed. In the tropics and subtropics, deep divergent circulation is the largest contributor to moisture convergence that balances the net precipitation (precipitation minus evaporation), and intermodel differences in the present-day simulations carry over onto the future projections. In comparison to the soundings and reanalyses, the GCMs are found to have deeper and stronger divergent circulation. While these two characteristics of GCM divergence affect the strength of the hydrological cycle, they tend to compensate for each other so that their combined effect is relatively modest.</jats:p
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