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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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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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