Analyse des précipitations annuelles à la station de Yaoundé de 1895 à 2006

Dans le cadre des changements climatiques nous nous proposons pour la station météorologique de Yaoundé, située dans la région du climat équatorial classique, d’étudier le comportement des pluies interannuelles. A ce jour, cette station est confrontée à diverses perturbations pluviométriques : Sur le plan de la pluviométrie mensuelle, les mois de décembre, de janvier et de février certes appartenant à la grande saison sèche n’enregistrent parfois aucune pluie. Or cette situation n’est pas normale entre 1926 et 2006 avec des moyennes de 21,4 mm pour le mois de janvier, 51 mm pour le mois de février et 135.7 mm pour le mois de mars. Certains de ces mois enregistrent des précipitations largement supérieures à la normale. C’est le cas de janvier 1975 avec 98 mm ; de février 1969 avec 125,9 mm et de décembre 1964 avec 97,5 mm. Sur le plan saisonnier, les répits pluviométriques des grandes saisons sèches s’érigent dans la plupart des cas à de véritables sécheresses. C’est le cas de 1987 qui a enregistré que 24,6 mm de précipitations durant les mois de décembre, janvier et février au lieu de 94,1mm en moyenne au total. Dans le même ordre, nous notons une modification dans la distribution des précipitations saisonnières. En outre, la petite saison sèche semble enregistrer de plus en plus des précipitations que jadis. Quant aux précipitations interannuelles, il ressort qu’il y a non seulement une diminution, mais aussi un prolongement des années sèches et, un déphasage entre l’arrivée des premières et la fin des dernières pluies.Mots-clés: Yaoundé, précipitations, saisons, sécheresse, El Niño. Analysis of the annual rainfalls in the Yaounde station from 1895 to 2006In the context of climate change, we propose for the Yaounde weather station, in the area of the classic equatorial climate, to study the behavior of interannual rainfalls. It follows a variety rainfall interference. In terms of monthly rainfall, the months of December, January and February certainly belonging to the long dry season, sometimes recorded no rain. But this situation is not normal between 1926 and 2006 with averages of 21.4 mm for the month of January, 51 mm for the month of February and 135.7 mm for the month of March. Some of these months are above normal precipitations. This is the case of January 1975 with 98 mm; February 1969 with 125.9 mm and December 1964 with 97.5 mm. At the seasonal level, rainfall respites large dry seasons are erected in most cases to real drought. This was the case from 1987 which recorded 24.6 mm rainfall during the months of December, January and February instead of average total 94,1mm. Similarly, we note a change in the distribution of seasonal rainfall. In addition, the short dry season record seems more rainfall than before. As for interannual rainfall, it is apparent that there has not only reduced but also an extension between the arrival of the first and the end of the last rains.Keywords: Yaounde, rainfalls, seasons, drought, El Niño

Deuterium-tritium plasmas in novel regimes in the Tokamak Fusion Test Reactor

Experiments in the Tokamak Fusion Test Reactor (TFTR) have explored several novel regimes of improved tokamak confinement in deuterium-tritium (D-T) plasmas, including plasmas with reduced or reversed magnetic shear in the core and high-current plasmas with increased shear in the outer region (high-l{sub i}). New techniques have also been developed to enhance the confinement in these regimes by modifying the plasma-limiter interaction through in-situ deposition of lithium. In reversed-shear plasmas, transitions to enhanced confinement have been observed at plasma currents up to 2.2 MA (q{sub a} {approx} 4.3), accompanied by the formation of internal transport barriers, where large radial gradients develop in the temperature and density profiles. Experiments have been performed to elucidate the mechanism of the barrier formation and its relationship with the magnetic configuration and with the heating characteristics. The increased stability of high-current, high-l{sub i} plasmas produced by rapid expansion of the minor cross-section, coupled with improvement in the confinement by lithium deposition has enabled the achievement of high fusion power, up to 8.7 MW, with D-T neutral beam heating. The physics of fusion alpha-particle confinement has been investigated in these regimes, including the interactions of the alphas with endogenous plasma instabilities and externally applied waves in the ion cyclotron range of frequencies. In D-T plasmas with q{sub 0} > 1 and weak magnetic shear in the central region, a toroidal Alfven eigenmode instability driven purely by the alpha particles has been observed for the first time. The interactions of energetic ions with ion Bernstein waves produced by mode-conversion from fast waves in mixed-species plasmas have been studied as a possible mechanism for transferring the energy of the alphas to fuel ions

TAE modes and MHD activity in TFTR DT plasmas

The high power deuterium and tritium experiments on TFTR have produced fusion a parameters similar to those expected on ITER. The achieved {beta}{sub {alpha}}/{beta} and the R{triangledown}{beta}{sub {alpha}} in TFRR D-T shots are 1/2 to 1/3 those predicted in the ITER EDA. Studies of the initial TFTR D-T plasmas find no evidence that the presence of the fast fusion {alpha} population has affected the stability of MHD, with the possible exception of Toroidal Alfven Eigenmodes (TAE`s). The initial TFTR DT plasmas had MHD activity similar to that commonly seen in deuterium plasmas. Operation of TFTR at plasma currents of 2.0--2.5 MA has greatly reduced the deleterious effects of MHD commonly observed at lower currents. Even at these higher currents, the performance of TFTR is limited by {beta}-limit disruptions. The effects of MHD on D-T fusion {alpha}`s was similar to effects observed on other fusion products in D only plasmas

Genetic engineering of apple (Malus domestica Borkh.) for resistance to fungal diseases using g2ps1 gene from Gerbera hybrida (Asteraceae)

In the present study, g2ps1 gene from Gerbera hybrida coding for 2-pyrone synthase which contribute for fungal and insect resistance was used. The aim was to work out an efficient approach of genetic transformation for apple cvs. ‘Golden Delicious’, ‘Royal Gala’ and ‘MM111’, ‘M26’ rootstocks for improving their fungal resistance using genetic engineering techniques. Adventitious shoot formation from leaf pieces of apples studied was achieved using middle leaf segments taken from the youngest leaves from in vitro-grown plants.Optimum conditions for ‚direct’ shoot organogenesis resulted in high regeneration efficiency of  0%, 95%, 92%, 94% in the studied apples respectively. Putative transgenic shoots could be obtained on MS media with B5 Vitamins, 5.0 mg l-1 BAP, or 2.0 mg l-1 TDZ with 0.2 mg l-1 NAA in the presence of the selection agent “PPT” at 3.0-5.0 mgl-1. Shoot multiplication of transgenic shoots was achieved on: MS + B5 vitamins + 1.0 mg l-1 BAP + 0.3 mg l-1 IBA, 0.2 mg l-1 GA3+1.0 g/l MES+ 30 g/l sucrose + 7.0 g/l Agar, with the selection agent PPT at 5.0 mg l-1 and were subcultured every 4 weeks in order to get sufficient material to confirm transformation of the putative shoots obtained. Six, seven, one and six transgenic clones of the apples studied respectively have been obtained and confirmed by selection on the media containing the selection agent “PPT” and by PCR analysis using the suitable primers in all clones obtained for the presence of the selection” bar gene (447 bp) and the gene-of- interest “g2PS1” (1244 bp), with transformation efficiency of 0.4%, 0.6%, 0.1% and 0.3% respectively. These transgenic clones were multiplied further in vitro in the presence of the selection agent ‘PPT’ and rooted in vitro. Rooted transgenic plantlets were successfully acclimatized and are being kept under-containment conditions according to the biosafety by-law in Syria to evaluate their performance for fungal resistance

Genetic engineering of apple (Malus domestica Borkh.) for resistance to fungal diseases using g2ps1 gene from Gerbera hybrida (Asteraceae)

In the present study, g2ps1 gene from Gerbera hybrida coding for 2-pyrone synthase which contribute for fungal and insectresistance was used. The aim was to work out an efficient approach of genetic transformation for apple cvs. ‘Golden Delicious’, ‘Royal Gala’and ‘MM111’, ‘M26’ rootstocks for improving their fungal resistance using genetic engineering techniques. Adventitious shoot formationfrom leaf pieces of apples studied was achieved using middle leaf segments taken from the youngest leaves from in vitro-grown plants.Optimum conditions for ‚direct’ shoot organogenesis resulted in high regeneration efficiency of 90%,95%,92%,94% in the studied applesrespectively. Putative transgenic shoots could be obtained on MS media with B5 Vitamins, 5.0 mg l-1 BAP, or 2.0 mg l-1 TDZ with 0.2 mg l-1NAA in the presence of the selection agent “PPT” at 3.0-5.0 mgl-1. Shoot multiplication of transgenic shoots was achieved on: MS + B5vitamins + 1.0 mg l-1 BAP + 0.3 mg l-1 IBA, 0.2 mg l-1 GA3+1.0 g/l MES+ 30 g/l sucrose + 7.0 g/l Agar, with the selection agent PPT at 5.0mg l-1 and were subcultured every 4 weeks in order to get sufficient material to confirm transformation of the putative shoots obtained. Six,seven, one and six transgenic clones of the apples studied respectively have been obtained and confirmed by selection on the media containingthe selection agent “PPT” and by PCR analysis using the suitable primers in all clones obtained for the presence of the selection” bar gene(447 bp) and the gene-of- interest “g2PS1” (1244 bp), with transformation efficiency of 0.4%, 0.6%, 0.1% and 0.3% respectively. Thesetransgenic clones were multiplied further in vitro in the presence of the selection agent ‘PPT’ and rooted in vitro. Rooted transgenic plantletswere successfully acclimatized and are being kept under-containment conditions according to the biosafety by-law in Syria to evaluate theirperformance for fungal resistance

The Roles of Electric Field Shear and Shafranov Shift in Sustaining High Confinement in Enhanced Reversed Shear Plasmas on the Tftr Tokamak

The relaxation of core transport barriers in TFTR Enhanced Reversed Shear plasmas has been studied by varying the radial electric field using different applied torques from neutral beam injection. Transport rates and fluctuations remain low over a wide range of radial electric field shear, but increase when the local E x B shearing rates are driven below a threshold comparable to the fastest linear growth rates of the dominant instabilities. Shafranov-shift-induced stabilization alone is not able to sustain enhanced confinement

Local Transport Barrier Formation and Relaxation in Reverse-Shear Plasmas on the TFTR Tokamak

The roles of turbulence stabilization by sheared E x B flow and Shafranov-shift gradients are examined for TFTR. Enhanced Reverse-Shear plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E x B and Shafranov shift effects was performed by varying the E x B shear through changes in the toroidal velocity with nearly-steady-state pressure profiles. Transport and fluctuation levels increase only when E x B shearing rates are driven below a critical value that is comparable to the fastest linear growth rates of the dominant instabilities. While a turbulence suppression criterion that involves the ratio of shearing to linear growth rates is in accord with many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was varied. The forward transition into the ERS regime has also been examined in strongly rotating plasmas. The power threshold is higher with unidirectional injection than with balanced injection

High performance deuterium-tritium plasmas in TFTR

Plasmas composed of nominally equal concentrations of deuterium and tritium (DT) have been created in TFTR with the goals of producing significant levels of fusion power and of examining the effects of DT fusion alpha particles. Conditioning of the limiter by the injection of lithium pellets has led to an approximate doubling of the energy confinement time, {tau}{sub E}, in supershot plasmas at high plasma current (I{sub p} {le} 2.5 MA) and high heating power (P{sub b} {le} 33 MW). Operation with DT typically results in an additional 20% increase in {tau}{sub E}. In the high poloidal beta, advanced tokamak regime in TFTR, confinement enhancement H {triple_bond} {tau}{sub E}/{tau}{sub E ITER-89P} > 4 has been obtained in a limiter H-mode configuration at moderate plasma current I{sub p} = 0.85 {minus} 1.5 MA. By peaking the plasma current profile, {beta}{sub N dia} {triple_bond} 10{sup 8} aB{sub 0}/I{sub p} = 3 has been obtained in these plasmas, exceeding the {beta}{sub N} limit for TFTR plasmas with lower internal inductance, l{sub i}. Confinement of alpha particles appears to be classical and losses due to collective effects have not been observed. While small fluctuations in fusion product loss were observed during ELMs, no large loss was detected in DT plasmas

Mode Structure of Disruption Precursors in TFTR Enhanced Reversed Shear Discharges

The mode structure of the disruption precursors in the TFTR enhanced reversed shear parameters has been studied by using T(subscript e) fluctuation profiles and q profiles obtained from Electron Cyclotron Emission (ECE) and Motional Stark Effect (MSE) measurements. The observed profiles of the radial displacement associated with the MHD modes were consistent with the displacement profiles expected from the ideal MHD external kink mode. The observed mode frequencies differ from the plasma toroidal rotation frequency measured with CVI charge-exchange recombination light. The independence of the mode frequency from the plasma rotation frequency supports the ideal MHD hypothesis. Possible causes of the frequency difference are discussed