J. V. Dave

Effect of Aerosols on the Transfer of Solar Energy Through Realistic Model Atmospheres. Part I: Non-Absorbing Aerosols

Abstract Extensive calculations aimed at determining the effect of aerosols on the solar energy absorbed, reflected and transmitted by cloudless, nonhomogeneous, plane-parallel atmospheric models were recently carried out with the object of treating the radiation transfer in as comprehensive a manner as possible consistent with reasonable computing time. The concentration of aerosol (spherical particles with size distribution and refractive index independent of height), ozone and water vapor were specified for 160 layers of varying thickness from the surface to 45 km. The solar spectrum (0.285–2.5 μm) was divided into 83 intervals with appropriate functions representing the scattering and absorption of gases and aerosol assigned to each, the index of refraction of the aerosol taken to be wavelength-independent. Upward and downward fluxes for each spectral interval at each level were computed taking into account all orders of scattering. Results will be presented for four model atmospheres to show the abso...

Segmentation of a Thematic Mapper Image Using the Fuzzy c-Means Clusterng Algorthm

In this paper, a segmentation procedure that utilizes a clustering algorithm based upon fuzzy set theory is developed. The procedure operates in a nonparametric unsupervised mode. The feasibility of the methodology is demonstrated by segmenting a six-band Landsat-4 digital image with 324 scan lines and 392 pixels per scan line. For this image, 100-percent ground cover information is available for estimating the quality of segmentation. About 80 percent of the imaged area contains corn and soybean fields near the peak of their growing season. The remaining 20 percent of the image contains 12 different types of ground cover classes that appear in regions of diffferent sizes and shapes. The segmentation method uses the fuzzy c-means algorithm in two stages. The large number of clusters resulting from this segmentation process are then merged by use of a similarity measure on the cluster centers. Results are presented to show that this two-stage process leads to separation of corn and soybean, and of several minor classes that would otherwise be overwhelmed in any practical one-stage clustering.

Validity of the isotropic-distribution approximation in solar energy estimations

Abstract Results of numerical simulation are presented for the diffuse and direct energy passing through a Sun-facing flat surface located at the bottom of plane-parallel models of non-absorbing, homogeneous atmospheres. Computations of the diffuse component were carried out with a high degree of accuracy, and also were carried out using the well-known, isotropic-distribution approximation for the sky energy. It is shown that the results obtained with the isotropic-distribution approximation are consistently smaller than those obtained with the exact procedure. These two sets of results can differ by a factor 1–6 depending upon the optical characteristics of the model and the position of the Sun.

Effect of Cloudiness on the Transfer of Solar Energy Through Realistic Model Atmospheres

Abstract Extensive calculations of the effect of cloudiness on the solar energy absorbed, reflected and transmitted by nonhomogeneous plane-parallel atmospheric models have been carried out with the object of treating the radiation transfer in as comprehensive a manner as possible. The concentration of aerosol (spherical particles with size distribution and refractive index independent of height), ozone and water vapor were specified for 50 basic layers of equal geometric thickness from the surface to 50 km. A stratus cloud layer with a liquid water content of 0.0128 g m–3 was introduced between the 3 and 4 km levels of the models. The solar spectrum (0.285–2.5μm) was divided into 83 intervals with appropriate functions representing the scattering and absorption of gases, and the aerosol and liquid water drops assigned to each, the refractive indices of the aerosol and water drops taken to be wavelength-independent. For accurate computations of the upward and downward fluxes for a given model at a given w...

Sea-level solar radiation in the biologically active spectrum.

Calculations show a significant depletion of ultraviolet and visible radiation due to absorption and scattering by particulates and cloud drops for a fixed amount of ozone.

Effect of terrain orientation and solar position on satellite-level luminance observations

Abstract Changes in the luminance of a terrain along the local nadir direction are studied as a function of terrain slope, and the solar azimuth and elevation angles. The digital terrain data for the Santa Clara valley-Mt. Hamilton region of California, available from the U.S. National Cartographic Information Center, are used for this purpose. The elevation above mean sea-level of this selected region varies between 24 and 1,282 m. Luminance computations are performed by assuming the Lommel-Seeliger law of reflection, and the terrain reflectance to be a constant and independent of its geographic location. The atmospheric effects and contributions due to multiple reflections are neglected. It is shown that the ratio of the actual terrain luminance to that for the corresponding flat surface for 64 equal-size (∼ 4 km 2 ) finite subregions, changes by a very significant amount over the selected region, and also with changes in the solar position. Selected Landsat MSS band 7 data of the same region are presented for validation purposes.

Performance of a tilted solar cell under various atmospheric conditions

Abstract Representative results of the numerical simulation of the response (viz. photocurrent, optimum power output, and optimum efficiency) of a Ga1−xAlxAsGaAs cell are presented as a function of its tilt angle for five different models of the terrestrial atmosphere, and for several elevations of the sun. The outward normal to the cell is in the suns meridian plane. These atmospheric models represent average, cloudfree, midlatitude summer conditions. The first model is free of aerosols and absorbing gases. Absorption by average amounts of carbon dioxide, oxygen, ozone, and water vapor is included in the remaining four models. The second model is also free of aerosols, but the last three models contain aerosols in the form of a spherical polydispersion made from a substance with a wavelength independent refractive index of 1.5-0.01i. Models 3 and 4 are expected to represent, respectively, the average, and strong turbid conditions encountered over large continental areas. Models 3 and 5 contain aerosols with different size distribution characteristics, but have the same amount of aerosol mass loading per unit horizontal area. Values of the direct solar radiation, and of the diffuse sky radiation incident upon the cell were calculated at 77 unequally spaced wavelengths in the spectral region 0.305-2.5 μm. This spectral resolution provides acceptable values of various wavelength integrals in the presence of absorption by the aforementioned gases. Furthermore, the field of the diffuse radiation was evaluated at sufficient number of points in the zenith and azimuth angles for obtaining reliable integrations over the respective parameters under various conditions. The diffuse radiation was evaluated after taking into account all orders of scattering. This study illustrates the manner in which the aforementioned solar-cell characteristics are affected by the atmospheric conditions, solar position, orientation of the cell, and reflectivity of the ground.

Importance of the diffuse sky radiation in evaluation of the performance of a solar cell

Abstract Representative results of the numerical simulation of responses (viz. photocurrent and optimum power output as well as efficiency) of the conventional and violet Si cells and of the Ga1−xAlxAsGaAs and GaAs cells are presented as a function of the solar zenith angle for seven different models of the terrestrial atmosphere. The atmospheric models used vary from an aerosol-free and cloud-free model with gaseous absorption to several models with moderately thick stratus cloud layer and high concentrations of aerosols. This study, restricted to horizontally situated solar cells, illustrates the manner in which characteristics are significantly affected by position of the sun, turbidity and cloudiness of the atmosphere, as well as reflectivity of the underlying surface.

Atmospheric Heating Rates Due to Solar Radiation for Several Aerosol-Laden Cloudy and Cloud-Free Models

Abstract From calculations of solar flux by direct numerical solution of the spherical harmonics approximation of the equation of transfer, heating rate profiles have been obtained for model atmospheres ranging from one with gaseous absorption only to one with additional heavy concentration of partly absorbing aersols with a tropospheric cloud of water drops. These calculations show that aersols can have a significant effect on the vertical distribution of absorbed solar energy, which is sensitive to the value of the ground reflectivity and solar zenith angle.

Isotropic distribution approximation in solar energy estimations

Abstract The isotropic distribution approximation commonly used for the estimation of the diffuse solar energy received by a tilted surface, is examined by studying the ratio of the energy computed using this approximation and that computed with an exact procedure. This examination is carried out for a tilted flat surface located at the bottom of five different models of the cloudfree, midlatitude, summer atmosphere. These models are assumed to rest on a ground absorbing all incident energy. The outward normal to the flat surface is confined to the suns meridian plane. The first model is free of aerosols and absorbing gases. Absorption by average amounts of carbon dioxide, oxygen, ozone, and water vapor is included in the remaining four models. The second model is also free of aerosols, but the last three models contain aerosols in the form of a spherical polydispersion made from a substance with a spectrally independent refractive index of 1.5-0.0li. Models 3 and 4 are expected to represent, respectively, the average and strongly turbid conditions encountered over large continental areas. Models 3 and 5 contain aerosols with different size distribution characteristics, but have the same amount of aerosol mass loading per unit horizontal area. Values of the direct solar radiation and of the diffuse sky radiation incident upon the tilted surface were calculated at 77 unequally spaced wavelengths in the spectral region 0.305-2.5 μm. This spectral resolution provides acceptable values of various wavelength integrals in the presence of absorption by the aforementioned gases. Furthermore, the field of the diffuse radiation was evaluated at a sufficient number of points in the zenith, and azimuth angles for obtaining reliable integrations over the respective parameters under various conditions. The diffuse radiation was calculated after taking into account all orders of scattering. It has been shown that the use of the isotropic distribution approximation results in a significant under- or over-estimation of the diffuse energy depending upon the tilt angle of the surface, position of the sun, and atmospheric conditions.