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D. Guedalia, C. Allet, and J. Fontan

Abstract

Various methods of determining vertical eddy diffusion coefficients in the planetary boundary layer from two natural radioactive tracers, radon (Rn-222) and ThB (Pb-212), are presented and discussed. In all cases, it is more pertinent to measure vertical exchanges in the lower atmosphere using ThB (10.6 hr half-life) than radon (3.8 days half-life).

About 30 vertical profiles of Rn and ThB in the lower troposphere (100–3000 m) are determined over a 12-month period. Vertical eddy coefficient values are calculated according to the various methods discussed. The variation range, between 10−1 and several 100 m2 sec−1, is large and closely related to the vertical temperature gradient variation.

Low values (10−2 to 10−1 m2 sec−1) of the exchange coefficients determined in the inversion layer which frequently exists at the top of the planetary boundary layer show that vertical turbulent fluxes toward the free atmosphere are relatively small.

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A. Ben Mohamed, J-P. Frangi, J. Fontan, and A. Druilhet

Abstract

Spectral aerosol optical depths were measured in seven localities of Niger (West African Sahel) from February 1986 to June 1987. Results of these observations, together with related climatic parameters, are presented and discussed in this paper.

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A. Druilhet, J. P. Frangi, D. Guedalia, and J. Fontan

Abstract

We present a collection of experimental data concerning turbulent characteristics of the convective boundary layer. These data were obtained by means of instrumented aircraft during numerous experiments carried out above five flight areas from 1973 to 1978. Only dry convective conditions with weak dynamic instability are contained in this report. The computed quantities are vertical turbulent fluxes of sensible and latent heat, second- and third-order moments of vertical wind component, potential temperature and humidity, dissipative and spectral characteristics and length scales. All these quantities are normalized using the convective similarity hypothesis of Deardorff. A particular case is considered for humidity where it is necessary to choose between the two boundary conditions: evaporation and entrainment flux. The latter is chosen.

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M. L. Huertas, A. M. Marty, J. Fontan, and D. Blanc

Abstract

The mobilities and diffusion coefficients of the decay products of a natural radioactive gas, 86Tn220, and of an artificial radioactive gas, 86Kr88, have been measured in filtered air. The study of the mobility of these radioactive ions has demonstrated the presence of four groups of ions, ThB+ and Rb+.

Assuming that the different groups of ions correspond to an increase in the dimensions of these ions, it has been found that the agglomeration velocity of the most rapid ions increases with the irradiation of the air.

Under the same conditions, measurements have been made of diffusion coefficients of neutral particles.

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