KEYWORDS: Satellites, Clouds, Solar radiation, Error analysis, Solar radiation models, Fractal analysis, Linear filtering, Radiometry, Statistical analysis, Data modeling
There are large errors in satellite estimation of downward surface solar radiation (DSSR) at hourly time scales. This is due to several factors including mismatch in the spatial scale of the satellite vs. point measurements from surface pyranometers; and most importantly, structural variability in cloud properties. The authors examined the temporal and spatial variability of UV-B erythemal irradiance under cloudy stratocumulus conditions in Hobart Australia. Three radiometers were deployed at distances under 5 km. Short-term statistics were analysed and related to estimates from a three-dimensional radiation/cloud model with fractal properties in the horizontal. Results indicate that accuracy in satellite-derived hourly solar radiation may be improved with several satellite scans per hour, ideally every 10 minutes. However ground validation is a problem because an hourly measurement of irradiance in cloudy conditions is not likely to represent well the regional average as estimated from satellite.
KEYWORDS: Clouds, Liquids, Fractal analysis, Solar radiation models, 3D modeling, Ultraviolet radiation, Statistical modeling, Monte Carlo methods, Systems modeling, Modeling
The authors examined the temporal and spatial variation of UV-B erythemal irradiance under cloudy stratocumulus conditions. A three-dimensional cloud model was constructed which had fractal properties in the horizontal. Fractal parameters were derived from 98 aircraft measurements in northwest Tasmania. Modelled radiation was compared with spatial sky radiance measurements using a Variable Sky Platform. Temporal measurements of irradiance were also compared with model predictions. In both areas both model and measurements agree well.
Previous studies have shown that the albedo of clouds with inhomogeneous liquid water fields is lower than that of homogeneous clouds with the same average liquid water content. This can lead to biases in the retrieval of cloud properties from satellite images with pixel sizes significantly greater than the photon mean free path length. In this work we present a three-dimensional multifractal cloud model for use in radiative transfer calculations. The model is based on aircraft measurements of liquid water
content taken during 98 flights over Tasmania, Australia. Monte Carlo
radiative transfer is used to calculate the optical properties of clouds that were constructed according to this model. The reflectance of the cloud not only varies with the fractal parameters and mean liquid water content, but also with the area size over which it is averaged, i.e. the pixel size used. An "effective optical depth" is defined as the optical depth of a homogeneous cloud with the same reflectance as the 3D-multifractal cloud, and is parameterized as a function of the mean optical depth and the pixel size. This parameterization allows for fast radiation calculations in the remote sensing of cloud properties, by the replacement of an inhomogeneous cloud with a plane-parallel homogeneous one.
In the polishing process, the wear tends to be greater when the tool extends beyond the edge of the workpiece. A linear pressure distribution (between the tool and the workpiece) has been used to explain this effect, however, this model also can predict negative pressures. This could mean that material is deposited instead of being removed. We present a new pressure distribution proposal, which presents like a skin effect. This means that the pressure is significantly higher at the border points than at internal points of the glass. With this model the material removal at the border points is increased considerably since, according to Preston, the wear is proportional to the pressure. This pressure distribution model is applied to calculate the wear produced by a square tool on a glass border moving along straight lines.
We report experimental results and analysis about a new hydrodynamic radial tool (HyDra, patent pending), which expels a suspension of water and polishing particles radially on glass. With this method it is possible to locally shape optical surfaces. The depth of material removed by HyDra grows linearly with the time. The removal rate is independent of the velocity between the tool and the glass element. The HyDra has been used to fabricate successfully an optical flat and Schmidt surface.
Since October 1995 a global daily forecast of the UV index and the daily dose, as the irradiance of the biologically effective ultraviolet radiation, for clear sky is calculated. The Austrian model as well as the input parameters are described. By connecting the daily dose with the sensitivity of the photobiological skin types, a recommendation is given to select an appropriate sun protection factor of a sunscreen to avoid overexposure of the skin. The validation of the Austrian forecast model is done by long-term measurements of the biologically effective ultraviolet radiation. Measurements were taken from 6 different sites at 4 continents (Antarctica, Australia, America and Europe) covering the latitudinal range from 67 degree(s)N to 60 degree(s)S. By using the underestimation as criteria in the sense of radiation protection, the Austrian model shows less than 12% underestimation over the whole period for the UV index and less than 10% for the daily dose. The evaluation shows further that the forecast of the daily dose is much more influenced by the attenuation due to clouds than the UV index.
Presented is a ground-based measurement technique for quantifying the directional properties of the IR radiation regime within a forest canopy with regard to mapping the canopy transmission over the upwards facing hemisphere. Implications are discussed for remotely sensed canopy temperature measurements and models of canopy transmission. The technique involves taking narrowband measurement scans throughout the hemisphere using a narrow view angle, narrow spectral band radiometer that measures predominantly within the water vapor transmission window. Measurements made under both clear skies and within a dry sclerophyll Eucalyptus canopy are presented and discussed.
In low spatial resolution remote sensing the plane parallel albedo bias caused by sub-pixel cloud inhomogeneities leads to underestimation of cloud properties. RF Cahalan et al. Have suggested the effective thickness approximation as a method of correcting this bias, assuming a single parameter fractal cloud model.. The magnitude of the reduction factor applied to the optical depth in this method is dependent on the cloud fractal parameter, determined from spatial liquid water distribution. We present here a study using in situ aircraft liquid water measurements in northern Tasmania, Australia, to first locally determine the cloud fractal parameter in local conditions, and then to test the satellite retrieval of cloud properties using these results. Four categories of cloud with different fractal parameters are identified and the retrieval method showed encouraging results, with further testing and refinement required.
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