Union américaine de géophysique

Résumé

L'Union américaine de géophysique (en anglais American Geophysical Union, AGU) est une organisation à but non lucratif de géophysiciens qui compte (chiffre de l'année 2004) membres provenant de 130 pays. Les activités de l'AGU sont concentrées sur l'organisation et la dissémination de l'information scientifique dans le domaine de la géophysique, domaine interdisciplinaire et international. L'AGU divise la géophysique en quatre branches fondamentales : l'atmosphère et l'océan, la Terre solide, l'hydrologie et l'espace. Mission La mission de l'AGU est

la promotion de l'étude scientifique de la Terre et de son environnement dans l'espace, ainsi que la dissémination des résultats au public,
la promotion de la coopération entre les organisations scientifiques impliquées dans la géophysique ou dans des disciplines connexes,
l'initiation et la participation à des programmes de recherches géophysiques,
l'avancée des divers disciplines de la géophysique à travers la discus

Source officielle

https://fr.wikipedia.org/wiki/Union_am%C3%A9ricaine_de_g%C3%A9ophysique

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Publications associées (74)

Measurements of cloud condensation nuclei (CCN), aerosol size distribution and chemical composition were obtained at the UNH-AIRMAP Thompson Farms site, during the ICARTT 2004 campaign. This work focuses on the analysis of a week of measurements, during which semiurban and continental air were sampled. Predictions of CCN concentrations were carried out using "simple" Köhler theory; the predictions are subsequently compared with CCN measurements at 0.2%, 0.3%, 0.37%, 0.5% and 0.6% supersaturation. Using size-averaged chemical composition, CCN are substantially overpredicted (by 35.8 ± 28.5%). Introducing size-dependent chemical composition substantially improved closure (average error 17.4 ± 27.0%). CCN closure is worse during periods of changing wind direction, suggesting that the introduction of aerosol mixing state into CCN predictions may sometimes be required. Finally, knowledge of the soluble salt fraction is sufficient for description of CCN activity. Copyright 2007 by the American Geophysical Union.

Blackwell Publishing Ltd2007

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Characteristic updrafts for computing distribution-averaged cloud droplet number and stratocumulus cloud properties

Athanasios Nenes

A computationally effective framework is presented that addresses the contribution of subgrid-scale vertical velocity variations in predictions of cloud droplet number concentration (CDNC) in large-scale models. Central to the framework is the concept of a "characteristic updraft velocity" w*, which yields CDNC value representative of integration over a probability density function (PDF) of updraft (i.e., positive vertical) velocity. Analytical formulations for w* are developed for computation of average CDNC over a Gaussian PDF using the Twomey droplet parameterization. The analytical relationship also agrees with numerical integrations using a state-of-the-art droplet activation parameterization. For situations where the variabilities of vertical velocity and liquid water content can be decoupled, the concept of w* is extended to the calculation of cloud properties and process rates that complements existing treatments for subgrid variability of liquid water content. It is shown that using the average updraft velocity w* (instead of w*) for calculations of Nd, re, and A (a common practice in atmospheric models) can overestimate PDF-averaged Nd by 10%, underestimate re by 10%-15%, and significantly underpredict autoconversion rate between a factor of 2-10. The simple expressions of w* presented here can account for an important source of parameterization "tuning" in a physically based manner. Copyright 2010 by the American Geophysical Union.

Blackwell Publishing Ltd2010

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Sensitivity of the global distribution of cirrus ice crystal concentration to heterogeneous freezing

Athanasios Nenes

This study presents the sensitivity of global ice crystal number concentration, Nc, to the parameterization of heterogeneous ice nuclei (IN). Simulations are carried out with the NASA Global Modeling Initiative chemical and transport model coupled to an analytical ice microphysics parameterization. Heterogeneous freezing is described using nucleation spectra derived from theoretical considerations and empirical data for dust, black carbon, ammonium sulfate, and glassy aerosol as IN precursors. When competition between homogeneous and heterogeneous freezing is considered, global mean Nc vary by up to a factor of twenty depending on the heterogeneous freezing spectrum used. IN effects on Nc strongly depend on dust and black carbon concentrations and are strongest under conditions of weak updraft and high temperature. Regardless of the heterogeneous spectrum used, dust is an important contributor of IN over large regions of the Northern Hemisphere. Black carbon however exhibits appreciable effects on Nc when the freezing fraction is greater than 1%. Compared to in situ observations, Nc is overpredicted at temperatures below 205 K, even if a fraction of liquid aerosol is allowed to act as glassy IN. Assuming that cirrus formation is forced by weak updraft addressed this overprediction but promoted heterogeneous freezing effects to the point where homogeneous freezing is inhibited for IN concentrations as low as 1 L-1. Chemistry and dynamics must be considered to explain cirrus characteristics at low temperature. Only cloud formation scenarios where competition between homogeneous and heterogeneous freezing is the dominant feature would result in maximum supersaturation levels consistent with observations. Copyright 2010 by the American Geophysical Union.

Blackwell Publishing Ltd2010

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