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Daniel Guedalia
and
Thierry Bergot

Abstract

A 1D model adapted for forecasting the formation and development of fog, and forced with mesoscale parameters derived from a 3D limited-area model, was used to simulate three fog event observations made during the Lille 88 campaign. The model simulation correctly reproduced the time of fog formation and its vertical development when forcing terms derived from observations were used. It determined the influence of different physical processes and in particular that of dew deposition. The initial conditions deduced from the 3D model proved to be correct in two of the three events. On the other hand, the prediction of advection terms necessary for forecasting the vertical growth of fog was a more delicate matter.

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Thierry Bergot
and
Daniel Guedalia

Abstract

To improve the forecast of dense radiative fogs, a method has been developed using a one-dimensional model of the nocturnal boundary layer forced by the mesoscale fields provided by a 3D limited-area operational model. The 1D model involves a treatment of soil-atmosphere exchanges and a parameterization of turbulence in stable layers in order to correctly simulate the nocturnal atmospheric cooling.

Various sensitivity tests have been carried out to evaluate the influence of the main input parameters of the model (geostrophic wind, horizontal advections, cloud cover, soil moisture, etc.) on the predicted fog characteristics. The principal result concerns the difficulty of obtaining accurate forecasts in the case of fog appearing in the middle or at the end of the night, when the local atmospheric cooling is weak.

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Claude Estournel
and
Daniel Guedalia

Abstract

A dynamic radiative model was used to study the influence of the geostrophic wind on the nocturnal cooling processes. For weak wind conditions, an important difference appears between the top levels of the inversion and turbulent layers. The dimensionless vertical profile of turbulent heat flux presents an important curvature at the beginning of the night; afterwards this profile varies little during the night.

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Daniel Guedalia
,
Claude Estournel
, and
Raoul Vehil

Abstract

A study of the effect of a desert aerosol layer on the nocturnal cooling of the atmosphere is presented. The experimental data were obtained during the ECLATS experiment which was run in the Sahel region of the Niger in November 1980. This study uses measurements of thermodynamic and radiative parameters, aerosol size distribution, and a radiative model. The results show that the presence of a dust layer at night increases the downward infrared flux at the surface (and then modifies the energy budget) and increases the radiative cooling rate of the atmosphere, slightly in the layers near the ground (∼0.15 K h−1) and in a more important way at the top of the haze layer.

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Daniel Guedalia
,
Alain Lopez
,
Jacques Fontan
, and
André Birot

Abstract

Simultaneous measurements of the vertical distribution on Rn-222, Aitken nuclei and small ions have been carried out in southwest France, several hundred kilometers from the Atlantic Ocean, between ground level and 6000 m.

These measurements demonstrate the importance of radon as a tracer characterizing the continental nature of an air mass. Every one of the measurements shows an increase in the concentration of radon at altitudes in excess of 3000 m. The radon does not appear to have originated in the European continental land mass. The concentrations of Aitken nuclei, small ions and radon show a sudden variation at the upper limit of the planetary boundary layer which is due, at least in part, to the geographic location of the site at which the measurements were made.

With certain simplifying hypotheses, the mean size of atmospheric aerosols can be deduced from these measurements.

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Claude Estournel
,
Raoul Vehil
,
Daniel Guedalia
,
Jacques Fontan
, and
Aimé Druilhet

Abstract

Pollutants (gaseous and aerosol) contained in urban atmospheres alter radiative fluxes at the surface.Numerous radiative models have been developed, and while few experimental data are available, results areoften contradictory. We have taken measurements, over several weeks, of downward radiation (solar andinfrared) over the city of Toulouse and a rural reference site. The downward IR flux was larger at the urbansite by day and night (increase between 15 and 25 W m2). Attenuation of the incident solar radiation atthe urban site was observed (30 W m2 in the middle of the day). A radiative model enabled us to show thatthe IR flux increase is mainly due to higher air temperature associated with the urban "heat island." Theenhissivity increase due to the addition of absorbing constituents in the urban area was very weak. Theattenuation of solar radiation was due to absorption by urban aerosol. Total incoming radiation (solar+ infrared) was similar at the two sites by day and was slightly higher at the urban site during the night.

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Daniel Guedalia
,
André Ntsila
,
Aimé Druilhet
, and
Jacques Fontan

Abstract

This study presents a method of obtaining the quantitative intensity of vertical diffusion during periods of atmospheric stability. This method associates the continuous measurement of radon concentration at ground level and the use of a monostatic sodar. The value of “equivalent mixing height” he is calculated using the radon variation which can be related to the global exchange coefficient of the inversion layer. The sodar detects the thickness of the nocturnal inversion layer.

Both systems operated simultaneously over several months at two sites (urban and suburban). In most cases the nocturnal layer was indicated both by sodar echoes and by a large decrease of the he value. In other cases the use of sodar alone can lead to a lack of detection of stable periods which, nevertheless, are shown clearly by radon. The comparison of measurement at two sites shows a modification of nocturnal stability above the urban site. The equivalent exchange coefficients are ∼0.3 m2 s−1 (at the urban site) and 0.08 m2 s−1 (at the suburban site).

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