Lately a number of studies related with UV irradiance estimates from satellite data based on the Ozone Monitoring
Instrument (OMI) have shown a high correlation with ground-based measurements but a positive bias in many locations,
the satellite derived UV being higher. One of the key factors that this bias has been attributed to is the boundary layer
aerosol absorption not taken into account in the current OMI UV algorithm. In this work we have used a correction
procedure based on climatological global aerosol absorption data taken from AeroComm aerosol initiative. This dataset
includes aerosol optical depth and aerosol single scattering albedo assembled by combining, ground-based aerosol
measurements from AERONET and information from several global aerosol models. The results of this correction were
compared with synchronous ground-based measurements from 9 UV monitoring stations. The results generally showed a
significantly reduced bias of 7-20%, a lower variability, and an unchanged, high correlation coefficient.
In this study we have analysed the sensitivity of the erythemally effective radiation to uncertainties in measured total ozone content of the atmosphere (TOC).These uncertainties result from a restricted spatial resolution, a restricted temporal resolution or the restricted accuracy of measured TOC. Daily operational total ozone measurements from different instruments made over several years were applied. Measurements were gained space born by EPTOMS, ERS-2/GOME and TOVS and from the ground by Dobson and Brewer Spectrophotometers for the locations of Hradec Kralove (Czech Republic, 50°N), Nairobi (Kenya, 1°S) and Springbok (Rep. of South Africa, 30°S). The measurement uncertainties were analyzed by an inter-comparison of modeled erythemally effective UV radiation when using different sources of TOC. The evaluation of the uncertainties due to temporal delays was done in using TOC values with different temporal shifts. The influence of spatial gaps in TOC measurements was estimated separately in longitude and latitude up to distances of 1000 km around the measuring sites. From this analysis, requirements on the spatial resolution, temporal resolution and measuring uncertainties of total ozone measurements to calculate the erythemally effective UV radiation with a pre-selected accuracy can be derived in dependence of location and season.
The objective of this study, which is part of the project "crop drought stress monitoring by remote sensing" (DROSMON), is to assess the potential of hyperspectral imagery to determine drought stress of crops due to heterogeneous soil composition by estimating the leaf area index (LAI). LAI, which characterizes the actual status of the crops and therefore the potential yield, may be seen as the most important parameter indicating medium term drought stress. As a result of former river meanders, the soils in the Marchfeld region are interrupted by bands of lighter soil. The higher content of sand in the bands leads to a lower water storage capacity and consequently to a decrease in plant growth. An airborne HyMap image was acquired in June 2005 during anthesis stage of wheat. Inversion of a radiative transfer model by means of a look-up-table (LUT) approach was performed to retrieve LAI and other canopy parameters from wheat canopy reflectance. Additionally, the LAI was estimated by establishing empirical relationships between LAI and spectral indices (MSAVI, TVI and MTVI2). Both ways of LAI estimation showed a reasonable correlation to final yield measurements obtained one month after the image data acquisition. However, there was a slightly better agreement of model inversion results. The results suggest the applicability of hyperspectral imagery to map potential drought risk of (wheat) fields.
The ozone monitoring instrument (OMI) onboard the EOS/AURA satellite was launched on 15. July 2004. The OMI products include among other things total column ozone, total column NO2, cloud characteristics, aerosol optical thickness and single scattering albedo, SO2, UV-B, HCHO and BrO. A first validation of the products has been performed, a more thoroughly validation is now being performed. In this presentation we show comparisons of OMI retrieved ozone and ground UV with ground measured UV and ozone at Sonnblick (12.8 E, 47.05 N, 3106 m altitude).
The objectives of the COST action 726 are to establish long-term changes of UV-radiation in the past, which can only be derived by modelling with good and available proxy data. To find the best available models and input data, 16 models have been tested by modelling daily doses for two years of data measured at four stations distributed over Europe. The modelled data have been compared with the measured data, using different statistical methods. Models that use Cloud Modification Factors for the UV spectral range, derived from co-located measured global irradiance, give the best results.
Factors affecting changes of spectral UV irradiance at the Sonnblick Observatory are studied. Spectral UV measurements at wavelengths from 290 nm to 400 nm performed during the period from 1994 to 2003 are used in this investigation. These measurements have been performed with a Brewer ozone single spectrophotometer and with a Bentham DM 150 spectroradiometer (double monochromator). The radiative transfer model DISORT (based on the Discrete Ordinate Method) was used in the simulations. Measurements of UV-radiation at Sonnblick at 305 nm under clear sky conditions show, that the snowline may influence the irradiance by mean by 24%. Cloudiness enhances the influence of the albedo since multiple reflections between surface and lower bond of the clouds become more probable. In presence of 8/8 cloud cover, UV irradiance at 305 nm is enhanced by a factor of 1.7 when the snowline is 800 m instead of 3000 m. In addition to the snowline, clouds situated below the Sonnblick are responsible for increased albedo and consequently increased radiation. Model calculations show that average albedo is increased by 0.28 ± 0.15 due to 4/8 cloud cover or more below the top of the mountain. The influence of ozone, albedo and clouds on UV variability is evaluated separately using 10-year climatology. It was found that the effect of total ozone on short- term variability of UV irradiance at 305 nm can be more than 200% and on average 56%. Clouds can cause variability of 150% or more and on average 35%.
Remote sensing at optical wavelengths provides information on agricultural crop status, therefore being a useful tool for the detection and monitoring of drought stress in crop production. In the project "crop drought stress monitoring by remote sensing" (DROSMON) led by the University of Natural Resources and Applied Life Sciences in Vienna, which started in January 2005, remote sensing methods for drought stress classification were based on physical models of canopy reflectance using a combination of SAILH and PROSPECT.
Spectral reflectance of maize and wheat were measured in situ using a field spectroradiometer FieldSpec Pro FR for different crop development stages and drought stress levels at a test site in Vienna, Austria.
An extensive validation program was carried out measuring various physiological properties of the crops. A significant difference in reflectance was observed between the canopies experiencing distinct drought stress levels. The observed differences could be confirmed by model simulations based on the measured biophysical variables. These suggest that there will be a change in spectral reflectance in drought stressed crops, varying according to the different growth stages. This is most marked in the near (NIR) and mid (MIR) infrared wavelength region, probably due to modifications of leaf internal structure, variations in leaf inclination (e.g. due to wilting) and leaf area index.
We present initial results from this research, which partly support these ideas. Further investigations are necessary.
The determination of UV- and light doses received by people as a function of their activities and their environment, for present and future conditions, is the aim of the presented study. In this paper we present first preliminary results. Measurements of the total daily UV dose received by horizontal and vertical parts of the human body were performed on three chosen days in the region of Vienna, Austria. The measurements were performed in the UV and in the visible spectral range using ultraviolet selective sensors and sensors adapted to human eye sensitivity. Data acquisition was performed by using dataloggers. In this way it was also possible to determine the UV intensity and dose as a function of time and location. The UV intensity was determined for typical outdoor and indoor activities such as walking in a street, in a forest or in flat unobstructed areas. Indoors the determination of UV doses is more straightforward, the determination of the visible dose is however much more complex. A software was developed to determine the total daily dose received by the human body as a function of day and occupation.
Beside the sun elevation angle, the total ozone content (TOC) of the atmosphere is a main input parameter of models to predict the biologically-effective UV radiation. Therefor TOC was analysed for the domain of Central Europe. The data domain contains the area from 7 degree(s)E to 17 degree(s)E in longitude and from 45 degree(s) to 50 degree(s) in latitude. Ground based as well as satellite based measurements from EPTOMS were involved. The temporal and spatial variability of TOC was analysed using auto-correlation and fitting auto- correlation functions. It becomes evident that temporal correlation decrease rapidly within the first 9 days down to the level of 1/e. From spatial correlation functions de- correlation distances (1/e) are estimated with 180 km E-W and 145 km for S-N direction. The analysis of TOC variability per 100 km for the domain of Austria was also examined. The results show that the highest values can reach up to 18%/100 km. Day-to-day variability for a certain location may exceed this level by a few percent (25%/day). These results show that for the calculation of the biologically effective UV radiation a high spatial resolution of TOC data is necessary to ensure an appropriate quality.
One aim of the project CUVRA was to improve the modeling of effects of inhomogeneous albedo fields on ground UV irradiance. For the determination of the effect of heterogeneous illumination (shading effects, inclination and orientation of the facet) on the reflectivity of a facet in a mountainous region more complex approaches (than for the calculation of the reflectivity of flat surfaces) have to be chosen. We determined the 'effective' albedo for the surroundings of the Observatory Sonnblick (3106 m) by using three different methods. The first method consisted of a combination of a 3D albedo model calculation and 1D radiative transfer calculation. By using this method and a digital elevation map, the reflectivity of the region surrounding Sonnblick Observatory was calculated. The second method was an inversion method using a 1D radiative transfer model. The routine spectral UV measurements performed at Sonnblick were used to calculate the average effective albedo. The third method was entirely experimental. An albedo measuring system was used to perform reflectivity measurements of the surrounding of Sonnblick. Overall, the results showed that the average albedo of a topographically structured surface is lower than the average albedo of a corresponding (surface with same ground characteristics) flat surface.
Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.