Séminaire du CEREA - 9 mai 2005

Turbulent reacting flows in the atmospheric convective boundary layer

The reactivity of chemical species in the atmosphere occurred in a wide range of time and spatial scales. In the convective boundary layer (CBL), different behaviors can be distinguished according to the lifetime of the reactants. For chemical species with a lifetime higher than the turnover time of the CBL (typically 10-20 minutes), the turbulence mixes homogeneously the species and the vertical profile of their fluxes follow a linear shape. For the others, with lifetimes similar or lower than the turbulent timescale, the ability of turbulence to bring the reactants together controls their chemical transformations. This process influences the distribution of reacting scalars and the chemical composition of the CBL, and in particular the second-order moments of the concentration distributions, i.e. fluxes and (co-)variances. By including the chemical terms in the governing equations for reactants, one can study the relevance of accounting for these terms in fluxes and (co-)variances. When turbulence and chemistry have similar timescales, one would expect the chemical terms to make a contribution similar to that made by dynamical terms and, as a result, fluxes and (co-)variances will deviate from the inert linear profiles. Since these second-order moments, which describe the transport, the variability and the mixing of reacting scalars, are relevant for atmospheric chemistry, we intend to analyse the magnitude of these deviations. To do this, we use large-eddy simulation (LES) to calculate explicitly the different terms of the flux and (co-)variance budget equations. The CBL is characterised by vigorous thermals (updraft motions) surrounded by subsidence motions. In such a flow, reactants are normally segregated and as a consequence the reaction rate is slowed down. In large-scale atmospheric models, this process occurs in scales smaller than the grid length (sub-grid process), and therefore it requires a description in term of a parameterisation. Based on our LES results, we develop such a parameterisation that represents the segregation of reacting species in large-scale models under convective conditions. In its derivation, we use the LES results to account explicitly for the chemical contribution to the covariance of the concentration distributions. The parameterisation is applied to an atmospheric chemical mechanism that accounts for ozone formation and depletion in the CBL.

Le séminaire aura lieu dans la salle de réunion du CEREA B220 à 11h00.

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