Benoît Molineaux

Direct luminous efficacy and atmospheric turbidity—Improving model performance

Of all the atmospheric constituents, aerosol content is shown to be responsible for the greatest variations in direct luminous efficacy. Some clarity is brought to the comparison between Linkes and Angstroms turbidity coefficients, respectively TL and β. Greniers recent formulation of the optical thickness of a water and aerosol free atmosphere is presented here in a simplified expression. Based on these results and Dogniauxs illuminance turbidity factor, Til, two direct luminous efficacy models are derived, one of which is tuned to our experimental data. The input parameters are optical air mass, β, and water vapour content in the tuned version. These models perform significantly better than any of twelve other models found in the literature when compared to 1 yrs measurements from each of two sites in the U.S. and Switzerland. In both sites, β was derived from horizontal visibility estimated in a nearby airport.

Equivalence of pyrheliometric and monochromatic aerosol optical depths at a single key wavelength

The atmospheric aerosol optical depth (AOD) weighted over the solar spectrum is equal to the monochromatic AOD at a certain wavelength. This key wavelength is ~0.7 mum, which is only slightly influenced by air mass and aerosol content. On the basis of this result, simple relations are proposed to predict monochromatic AOD from pyrheliometric data and vice versa. The accuracy achieved is close to ?0.01 units of AOD at ~0.7 mum, estimated from simultaneous sunphotometer data. The precision required for the estimation of the precipitable water-vapor content is approximately ?0.5 cm.

Thermal analysis of five outdoor swimming pools heated by unglazed solar collectors

Abstract We have analysed measurements from five outdoor swimming pools located in Switzerland and heated by unglazed solar collectors. The main contributions to the daily energy balance of the swimming pools are evaluated. They include the active and passive solar gains, as well as the heat losses related to radiation, evaporation, convection, and water renewal (in order of importance). Coherent results are obtained using multilinear regressions in order to determine the best fitting values of the empirical parameters involved in the thermal equations.

Impact of Pinatubo aerosols on the seasonal trends of global, direct and diffuse irradiance in two northern mid-latitude sites

In June 1991, Mt Pinatubos eruption in the Philippines ejected a staggering 20 million metric tons of SO2 into the stratosphere which resulted in an aerosol cloud covering most of the Earth within a few months after the eruption. In this article we illustrate how the seasonal trends of global, direct and diffuse solar radiation were modified by the eruption in two mid-latitude sites in Europe and the U.S., totalling about 12 yr of hourly data. A spectacular increase in the diffuse fraction of solar radiation as opposed to a decrease in direct radiation which extended from late 1991 to mid 1993 was observed in both sites and for clear sky conditions. Global radiation was not altered significantly from these data sets. Sunphotometer measurements in one of the sites show the volcanic aerosols tripled the total atmospheric aerosol optical depth at 1 μm and approximately doubled the aerosol optical depth at 0.5 μm.

On the broad band transmittance of direct irradiance in a cloudless sky and its application to the parameterization of atmospheric turbidity

The total atmospheric attenuation of direct irradiance is often expressed as the product of independent broad band transmittances pertaining to the different depleting materials in the atmosphere. In theory, it is clear however that the broad band transmittances of the two most important attenuating factors, aerosols and water vapour, cannot be considered independent of each other. This is most relevant when trying to isolate the effect of atmospheric aerosols from the measurement of broad band direct irradiance. These questions are addressed in an analytical approach. Several broad band direct transmittance equations and corresponding turbidity parameters are discussed with respect to the analytical results. Simulations made with the SMARTS2 spectral radiative transfer code are used to illustrate the broad band effects.

Thermal analysis of five unglazed solar collector systems for the heating of outdoor swimming pools

The performance and behavior of five unglazed solar collector installations, devoted to the heating of five outdoor swimming pools located in Switzerland, is studied on the basis of experimental data collected in 1988. Due to the low temperatures involved, hourly efficiencies often exceed 80%. The mean daily collector efficiency is of the order of 60% in optimal conditions and is in good agreement with the G3 simulation program.

Retrieval of Pinatubo aerosol optical depth and surface bidirectional reflectance from six years of AVHRR global vegetation index data over boreal forests

Six years (1989–1994) of weekly composite satellite data are analyzed with the objective of retrieving the aerosol optical depth over boreal forests. The average surface bidirectional reflectance distribution (BRD) was estimated from a sample of the original data, yielding similar results in different regions around the world. Four analytical models are compared in their ability to reproduce the surface BRD. In channel 1 (0.58–0.68 μm) the surface reflectance averages 0.05 and exceeds 0.10 in the backscattering region. Thus the non-Lambertian ground contribution could not be neglected in the aerosol retrievals. A relevant radiative transfer code was run in iterative mode to retrieve the aerosol optical depth which best accounts for the difference between modeled surface BRD and measured satellite reflectance in all conditions. From comparisons with Sun photometer data we conclude that a large-scale, persistent aerosol enhancement such as that resulting from Mount Pinatubos eruption in 1991 is clearly discernible from weekly composite AVHRR data over boreal forests.