Toward an improved representation of middle atmospheric dynamics thanks to the ARISE project

This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions

Standing-stock and potential of phytoplankton production in the Bay of Santos, Brazil

Phytoplankton primary production and the maximum photo synthetic index (Pb m) from the region of Bay of Santos were measured every two months during 1976 by simulated incubations using 14C method and incandescent lamps (737 µE.m-2 .s-140 klux) . The results obtained for production rates (maximum of 204.6 mgC.m-3.hr-1 in winter and 488.3 mgC m-3.hr-1 in summer) are among the highest recorded for tropical marine environments. A high capability of light adaptation under high temperatures was also verified. The photo synthetic indexes obtained were also very high and seems to be due to the high nutrient level of the region. The eutrophic state is supported by the high nutrient and chlorophyll-a concentrations and by the phytoplankton cells number

Denombrement d'hypotheses en association-fusion de donnees

En association-fusion de données sonar, on est confronté à un flot d'informations tel qu'un opérateur humain est incapable de traiter le problème. On introduit donc des algorithmes automatiques permettant de trouver parmi les partitions hypothèses celle qui est la plus vraisemblable. On montre comment, au moyen de techniques de dénombrement, une recherche exhaustive de la solution optimale est inenvisageable en pratique, du fait de l'explosion combinatoire. La structure en treillis de l'ensemble des partitions permet néanmoins de dégager un algorithme sous-optimal, dit de recherche hiérarchisé. Enfin une généralisation sur les mêmes principes est proposée au cas où l'on associe des sorties des sous-systèmes locaux de fusion

SIMULATION NUMÉRIQUE DE LA PROPAGATION D'UNE ONDE ACOUSTIQUE DANS UNE TURBULENCE BIDIMENSIONNELLE PAR UNE MÉTHODE DE RAYONS

Nous présentons des résultats de simulation numérique de la propagation d'une onde acoustique dans une turbulence cinématique bidimensionnelle. Le milieu turbulent est représenté par une superposition de modes de Fourier aléatoires. Pour chaque réalisation, on détermine les rayons issus d'une source ponctuelle, on calcule la variance du temps de parcours le long de ces trajectoires ainsi que la distance pour laquelle se forme la première caustique.A numerical technique for simulating the behavior of an acoustic wave propagating through a turbulent medium is presented. A bidimensional velocity field is generated using a superposition of random Fourier modes. The time variance along the rays and the probability density for the occurence of the first caustics are computed

FLUCTUATIONS D'INTENSITÉ ACOUSTIQUE D'UNE ONDE SPHÉRIQUE TRAVERSANT UNE TURBULENCE THERMIQUE

L'étude expérimentale présentée permet de caractériser les fluctuations d'intensité acoustique d'une onde sphérique traversant une turbulence thermique. Compte tenu des distances de propagation importantes nous mettons en évidence une saturation des fluctuations d'intensité. La loi de distribution de ces fluctuations est caractérisée depuis les faibles fluctuations jusqu'au fortes fluctuations. Nous comparons nos mesures avec les estimations déduites de différentes densités de probabilité (log-normale, exponentielle, I-K, gamma généralisée).The intensity fluctuations of a spherical wave that propagates through a turbulent thermal field are investigated. Results are given for the normalized variance and the probability-density function of intensity fluctuations under a variety of propagation conditions. Measured probability densities are compared with the log-normal, the exponential, the I-K and the generalized gamma distributions

Influence d'une turbulence thermique sur la propagation du son dans l'atmosphère en présence d'une zone d'ombre

A numerical technique for simulating the acoustic wave propagation through a turbulent atmosphere is introduced. We first generate a random homogeneous and isotropic medium by creating N independent realizations in terms of discrete Fourier modes. Then, for each realization, we solve a parabolic equation using a Split step Padé algorithm. Finally, we calculate the mean intensity of the acoustic field and its phase and amplitude fluctuations. Our numerical results are compared with outdoor measurements