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Search Results: 1 - 10 of 27 matches for " Arona Diedhiou "
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Impacts of the Sahel-Sahara Interface Reforestation on West African Climate: Intra-Annual Variability and Extreme Temperature Events  [PDF]
Ibrahima Diba, Moctar Camara, Arona Diedhiou
Atmospheric and Climate Sciences (ACS) , 2019, DOI: 10.4236/acs.2019.91003
Abstract: The impacts of the reforestation of the Sahel-Sahara interface on the seasonal distribution of the surface temperature and thermal extremes are studied in the Sahel (West African region lying between 11°N and 18°N). We performed a simulation with the standard version of the RegCM4 model followed by another one using the altered version of the same model taking into account an incorporated forest. The impacts of the vegetation change are assessed by analyzing the difference between the two runs. The reforestation may influence strongly the frequency of warm days (TG90P) and very warm days (TX90P) by decreasing it over the reforested zone from March to May (MAM) and the entire Sahel during the June-August (JJA) period. These TG90P and TX90P indices decrease may be due to the strengthening of the atmospheric moisture content over the whole Sahel region and the weakening of the sensible heat flux over the reforested zone. The analysis also shows a decrease of the TN90P indice (warm nights) over the Sahel during the wet season (JJA) which could be partly associated with the strengthening of the evapotranspiration over the whole Sahel domain. The analysis of additional thermal indices shows an increase of the tropical nights over the entire Sahel from December to February (DJF) and during the warm season (MAM). The strengthening of the tropical night is partly associated with an increase of the surface net downward shortwave flux over the reforested zone. When considering the heat waves, an increase (a decrease) of these events is recorded over the southern Sahel during JJA and SON periods (over the whole Sahelian region during DJF), respectively. Changes in latent heat flux appear to be largely responsible for these extreme temperatures change. This work shows that the vegetation change may impact positively some regions like the reforested area by reducing the occurrence of thermal extremes; while other Sahel regions (eastern part of the central Sahel) could suffer from it because of the strengthening of thermal extremes.
African Easterly Waves and Cyclonic Activity over the Eastern Atlantic: Composite and Case Studies
Moctar Camara,Arona Diedhiou,Amadou Gaye
International Journal of Geophysics , 2011, DOI: 10.1155/2011/874292
Abstract: This study aims to understand the main differences over the African continent and the Eastern Atlantic Ocean between African Easterly Waves (AEWs) associated with Atlantic cyclones (developing AEWs) and non-developing AEWs. A statistical study showed that most of the named cyclones generated near the West African coast have a long lifecycle and all are associated with intense AEWs. Using NCEP/NCAR reanalyses, a composite study of the characteristics of developing AEWs is carried out and compared to those of non-developing AEWs. Developing AEWs exhibit the greatest baroclinic and barotropic conversions which are known to be the main processes involved in AEWs growth suggesting that these AEWs are stronger than the non-developing ones. Moreover, the developing AEWs are characterized by the existence of a relatively more unstable environment over West Africa and the Atlantic Ocean. A case study using rawinsonde data showed that the developing AEW is associated with dynamic and thermodynamic conditions conducive for deep convection and subsequent cyclogenesis compared to the non-developing AEW case.
Comparative influence of land and sea surfaces on the Sahelian drought: a numerical study
Arona Diedhiou,Jean-Franc?ois Mahfouf
Annales Geophysicae (ANGEO) , 2003,
Abstract: The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated anomalies in the atmospheric general circulation. This sensitivity study was done with the Météo-France climate model: ARPEGE. The sensitivity to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensitivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahel years observed since 1970. For each case, the spatiotemporal variability of the simulated rainfall anomaly and changes in the modelled tropical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant changes in the simulated TEJ and an enhancement of the AEJ are found; at the surface, the energy budget and the hydrological cycle are substantially modified. On the other hand, SST anomalies induce a negative rainfall anomaly over the Sahel and a positive rainfall anomaly to the south of this area. The rainfall deficit due to those anomalies is consistent with previous diagnostic and sensitivity studies. The TEJ is weaker and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated rainfall anomaly is similar to the observed mean African drought patterns. This work suggests that large-scale variations of surface conditions may have a substantial influence on Sahel rainfall and shows the importance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainfall.
African Easterly Waves and Cyclonic Activity over the Eastern Atlantic: Composite and Case Studies
Moctar Camara,Arona Diedhiou,Amadou Gaye
International Journal of Geophysics , 2011, DOI: 10.1155/2011/874292
Abstract: This study aims to understand the main differences over the African continent and the Eastern Atlantic Ocean between African Easterly Waves (AEWs) associated with Atlantic cyclones (developing AEWs) and non-developing AEWs. A statistical study showed that most of the named cyclones generated near the West African coast have a long lifecycle and all are associated with intense AEWs. Using NCEP/NCAR reanalyses, a composite study of the characteristics of developing AEWs is carried out and compared to those of non-developing AEWs. Developing AEWs exhibit the greatest baroclinic and barotropic conversions which are known to be the main processes involved in AEWs growth suggesting that these AEWs are stronger than the non-developing ones. Moreover, the developing AEWs are characterized by the existence of a relatively more unstable environment over West Africa and the Atlantic Ocean. A case study using rawinsonde data showed that the developing AEW is associated with dynamic and thermodynamic conditions conducive for deep convection and subsequent cyclogenesis compared to the non-developing AEW case. 1. Introduction Gray et al. [1, 2] suggested some necessary but not sufficient conditions for the initiation of North Atlantic tropical cyclones (TCs): warm waters (at least 26°C), an atmosphere which cools fast enough with height, a high mid-levels humidity because dry air layer at middle levels is not conducive for promoting the development of deep convection, and a low vertical wind shear between the low and upper troposphere. Moreover, a minimum distance of at least 500?km from the equator (nonnegligible amounts of Coriolis force) is required to maintain the circulation of the disturbance. To develop, a tropical cyclone needs a precursor. African Easterly Waves (AEWs, hereinafter) are the main precursors of cyclonic activity over the North Atlantic [3–7]. AEWs are important features of the West African region and tropical Atlantic Ocean. They propagate westward with a period of 3–5 days and are generated by a mixed baroclinic—barotropic instability of the African Easterly Jet [8]. Thorncroft et al. [9] suggested that these synoptic disturbances tend to be triggered by convection over orographic regions of East Africa. Their associated cyclonic vortices usually propagate along two tracks over West Africa (North and South of the AEJ) and merge into one over the North Atlantic Ocean [10, 11]. AEWs are also known to modulate Mesoscal Convective Systems activity and the daily rainfall over West Africa [12–15]. Many studies have focused on the role of large-scale
Assessment of Regional Climate Models over C?te D'Ivoire and Analysis of Future Projections over West Africa  [PDF]
Kouakou Kouadio, Abdourahamane Konare, Adama Diawara, Bernard Kouakou Dje, Vincent Olanrewaju Ajayi, Arona Diedhiou
Atmospheric and Climate Sciences (ACS) , 2015, DOI: 10.4236/acs.2015.52005
Abstract: The ability of six Regional Climate Models (RCMs) used in AMMA-ENSEMBLES project is assessed over six meteorological stations in Côte d’Ivoire. The ensemble mean of the models is also used for the prediction of climate change over West Africa. The study focused on two periods: the period 1995-2005, the present-day simulations, is used to evaluate the skills of the models over the country and the years 2010-2013, for assessment of the future climate change scenario used. The results show that the skills of the models vary from one station to another and from one season to another. None of the models considered, presents an excellent performance over the entire country and in all the seasons. Generally, the ensemble mean of all the models presents better results when compared with the observation. These results suggest that the choice of any model for study over the country may depend on the focus of interest: intensity or variability of the rain and also on area of interest. The projection for 2020-2040, future climate change over West Africa shows that the Sahel exhibits a tendency to be drier while wetter Guinean coast is observed.
Mean Kinematic Characteristics of Synoptic Easterly Disturbances over the Atlantic
Arona Diedhiou,Luiz A T Machado,Henri Laurent,
Arona DIEDHIOU
,Luiz A. T. MACHADO,Henri LAURENT

大气科学进展 , 2010,
Abstract: This study investigates the mean kinematic characteristics of the tropical Atlantic easterly disturbances in January--March (JFM), April--June (AMJ), July--September (JAS) and October--December (OND) from 1968--1998. For each season, the preferential tracks of these disturbances in the 3--10-day band periods were computed and spatialized, as well as their associated wavelength, velocity and main period, which lies between 3--5 days and between 6--9 days depending on the track and the season. Two main tracks are highlighted over the Atlantic Ocean. During OND and JFM these two tracks are well separated and located in each hemisphere around 15oS and 12.5oN. From AMJ to JAS these tracks migrate northward; in JAS, they merge into one over the northern tropical Atlantic along 17.5oN.The associated wavelength fields exhibit a meridional gradient, with large wavelengths (greater than 4000 km) around the equator, between 5oN and 5oS, and smaller wavelengths outside of this latitude band (between 2500--3500 km). The phase speed is also found to exhibit poleward decreasing values from 12--6 m s-1. Over the north Atlantic track, 6--9-day disturbances were found to occur from January to May and approximately from October to December. From June to September, the 3--5-day waves dominate the synoptic activity. Over the south Atlantic track, between May and August the synoptic variability is mainly explained by the 3--5-day disturbances but from January to April and from September to December both 3--5-day waves and 6--9-day waves can occur.
Comparative Study of the West African Continental, Coastal, and Marine Atmospheric Profiles during the Summer of 2006
Ibrahima Kalil Kante,Daouda Badiane,Sa dou Moustapha Sall,Abdoulaye Deme,Arona Diedhiou
International Journal of Geophysics , 2012, DOI: 10.1155/2012/603949
Abstract: We used sounding data of the Multidisciplinary Analysis of the African Monsoon experience in summer 2006 at continental and coastal sites of West Africa, respectively, to analyze the vertical profiles of relative humidity, temperature, dew point, and speed and wind direction for the JJAS rainy period. The vertical gradient method is applied to the profiles of some thermodynamic parameters estimated from sounding data to do a comparative study of the structure and thermal properties, moisture, and static stability of the atmospheric boundary layer of inland, coastal, and marine sites to show consistent differences related to geographic factors. In vertical profiles of relative humidity, the intensity is higher in Dakar than in Niamey particularly in the core of the season. There are dry intrusions in the low levels at the beginning and end of the season in Dakar, which do not exist in Niamey. The mixing layer on the continent during the day can reach a height greater than 1100 m, and the inversion layer height can exceed 1700 m. Therefore, the maximum thickness of the boundary layer is observed on the continent during the day, while at night the marine boundary layer is the thickest. The diurnal evolution shows that the mixing layer thickness decreases during the night over the continent but increases at the coast and at sea. In the night at the continental site there is a division of the mixing layer with a consistent residual mixing layer. Continental boundary layer is more unstable during the day, while at night it is the marine boundary layer that is more unstable than the coastal and inland ones.
Comparative Study of the West African Continental, Coastal, and Marine Atmospheric Profiles during the Summer of 2006
Ibrahima Kalil Kante,Daouda Badiane,Sa?dou Moustapha Sall,Abdoulaye Deme,Arona Diedhiou
International Journal of Geophysics , 2012, DOI: 10.1155/2012/603949
Abstract: We used sounding data of the Multidisciplinary Analysis of the African Monsoon experience in summer 2006 at continental and coastal sites of West Africa, respectively, to analyze the vertical profiles of relative humidity, temperature, dew point, and speed and wind direction for the JJAS rainy period. The vertical gradient method is applied to the profiles of some thermodynamic parameters estimated from sounding data to do a comparative study of the structure and thermal properties, moisture, and static stability of the atmospheric boundary layer of inland, coastal, and marine sites to show consistent differences related to geographic factors. In vertical profiles of relative humidity, the intensity is higher in Dakar than in Niamey particularly in the core of the season. There are dry intrusions in the low levels at the beginning and end of the season in Dakar, which do not exist in Niamey. The mixing layer on the continent during the day can reach a height greater than 1100?m, and the inversion layer height can exceed 1700?m. Therefore, the maximum thickness of the boundary layer is observed on the continent during the day, while at night the marine boundary layer is the thickest. The diurnal evolution shows that the mixing layer thickness decreases during the night over the continent but increases at the coast and at sea. In the night at the continental site there is a division of the mixing layer with a consistent residual mixing layer. Continental boundary layer is more unstable during the day, while at night it is the marine boundary layer that is more unstable than the coastal and inland ones. 1. Introduction The term boundary layer was first introduced in the literature by Prandtl and Lustig [1]. Since then, many authors such as Turner [2], Monin [3], Zeman [4] and more, recently, Cushman-Roisin and Beckers [5] have studied the detailed description. The atmospheric boundary layer (ABL) has been variously defined, but it is commonly recognized as the lower part of the atmosphere, which is strongly influenced by the presence of the earth’s surface and responds to surface forcing with a timescale of about one hour or less. The ABL is the place where many processes, such as turbulence, friction, dispersion, energy dissipation, and wind shear occur. These processes are poorly parameterized in atmospheric models. Several ABL studies have focused on the turbulence but also on the mixing process triggered by significant warming or cooling [6]. Within the ABL, the transport of various quantities (heat, pollution, moisture, momentum, etc.) is mainly
Sensitivity of Solar Photovoltaic Panel Efficiency to Weather and Dust over West Africa: Comparative Experimental Study between Niamey (Niger) and Abidjan (C?te d’Ivoire)  [PDF]
Alima Dajuma, Saleye Yahaya, Siaka Touré, Arona Diedhiou, Rabani Adamou, Abdourahamane Konaré, Mariama Sido, Michel Golba
Computational Water, Energy, and Environmental Engineering (CWEEE) , 2016, DOI: 10.4236/cweee.2016.54012
Abstract: Energy demand is increasing while we are facing a depletion of fossils fuels, the main source of energy production in the world. These last years, photovoltaic (PV) system technologies are growing rapidly among alternative sources of energy to contribute to mitigation of climate change. However, PV system efficiency researches operating under West African weather conditions are nascent. The first objective of this study is to investigate the sensitivity of common monocrystalline PV efficiency to local meteorological parameters (temperature, humidity, solar radiation) in two contrasted cities over West Africa: Niamey (Niger) in a Sahelian arid area and Abidjan (Cote d’Ivoire) in atropical humid area. The second objective is to quantify the effect of dust accumulation on PV efficiency in Niamey (Niger). The preliminary results show that PV efficiency is more sensitive to high temperature change especially under Niamey climate conditions (warmer than Abidjan) where high ambient temperatures above 33°C lead to an important decrease of PV efficiency. Increase of relative humidity induces a decrease of PV efficiency in both areas (Niamey and Abidjan). A power loss up to 12.46% is observed in Niamey after 21 days of dust accumulation.
Limit of the Solution of a PDE in the Degenerate Case  [PDF]
Alassane Diedhiou
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.42051
Abstract:

In this paper we show that we can have the same conclusion for the limit of the solution if we suppose the case of hypoellipticity.

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