Fernando Labbe

Uncertainty evaluation of spectral UV irradiance measurements

The reference instruments to measure the surface UV irradiance are based on a double monochromator system. The spectral irradiance yielded by these instruments is influenced by temporal instabilities and nonlinearities in the signal, the cosine error as well as uncertainties introduced in the needed prior calibrations. In this paper, we have carried out an uncertainty analysis of the spectral irradiances measured by using a mobile spectroradiometer that complies with the requirements of the Network for the Detection of Atmospheric Composition Change (NDACC). The spectral measurements were performed on 9th June 2005 (cloudless sky conditions) at the Izana Observatory (28.3° N, 16.5° E, 2367 m above sea level, Tenerife, Spain), during an international intercomparison campaign organized in the framework of the project Quality Assurance of Spectral Ultraviolet Measurements in Europe (QASUME). At solar zenith angles smaller than 30°, we found that despite the variations due to wavelength shifts induced by the high environmental temperature the relative expanded uncertainty was about 7% in the UV-A part of the spectrum; an increment was observed at wavelengths shorter than 315 nm such that the expanded uncertainty of the UV-B irradiance at 300 nm wavelength was about 9%. The measurements of the other five teams that participated in the campaign were within the bound specified by these expanded uncertainties. We also found that, regardless of the influence of the cosine error, the uncertainties involved in the absolute calibration procedure accounted for about 65% of the irradiance uncertainty. Although only a double monochromator was used in this work, the methodology applied to evaluate the uncertainty is general and it agrees with recommendations of the ISO Guide to the Expression of Uncertainty in Measurement.

Measuring displacement derivatives by electronic speckle pattern shearing interferometry (ESPSI)

Electronic speckle pattern shearing interferometry (ESPSI), also known as shearography, is a whole-field optical technique used to measure approximately the fields of displacement derivatives. The accurate measurements of these derivatives have two problems: first, although ESPSI results are approximately equal to the derivatives, they are equal to the derivatives only if the shear distance tends to zero, hence, if experimental data rendered by ESPSI are taken directly as equal to the derivatives, the measurements may carry an important shearing error; second, ESPSI yields values relative to a reference value at a specific location of the field that can be very difficult to determine accurately. In this paper, we propose a general procedure to compensate the shearing error and to introduce the reference by adding two quantities to the values rendered by ESPSI. As an example, we measured a displacement derivative field induced on a metallic sheet specimen by applying tensile load.

Uncertainty of experimental integrals: application to the UV index calculation

A procedure is presented to evaluate the uncertainty of integrals computed from an approximate function that interpolates a set of measured data. The procedure, a Monte Carlo-based uncertainty propagation technique, yields the integral uncertainty taking into account the uncertainties associated with the involved experimental data. As expected, we found that the uncertainty of the integral strongly depends on the error sources affecting the ordinates. As an example, we assessed the uncertainty of the so-called UV index, evaluated by integrating in the range 250 nm to 400 nm the biologically weighted spectral ultraviolet irradiance. The presented procedure agrees with international recommendations and is valid independently of the experimental technique from which data are obtained.

Monitoring the strain-rate progression of an aluminium sample undergoing tensile deformation by electronic speckle-pattern interferometry (ESPI)

We have used electronic speckle-pattern interferometry (ESPI), a whole-field optical technique, to follow the evolution of the strain-rate fields induced on an aluminium sample subjected to a uniaxial tensile test. At different load stages of the test, the strain-rate at each point of the field was obtained from fringe patterns generated by an interferometer with sensitivity along both in-plane coordinates. We observed that the strain-rate fields evolved from weakly to strongly heterogeneous as the test progressed. We found that, long before reaching the maximum of the tensile force, the strain localization began at a zone of the field where the local strain-rates presented the largest values. We conclude that the strain localization onset can be efficiently detected using ESPI.

Uncertainty evaluation of displacement gradients measured by electronic speckle pattern shearing interferometry (ESPSI)

Electronic speckle pattern shearing interferometry (ESPSI), also known as shearography, is a whole-field optical technique used to generate phase fringe patterns that allow us to evaluate displacement gradients. In this work, we report on a Monte Carlo computer simulation to calculate the standard uncertainties associated with the displacement gradients obtained from the experimental data yielded by ESPSI. The displacements were induced by applying tensile load to a metallic sheet sample. At each point of the illuminated area, the standard uncertainty of the gradient was taken as the standard deviation of the series of outcomes obtained by a large number of gradient evaluations. These evaluations were performed by using sets of values generated according to the probability density functions (PDFs) that we assigned to the experimental data. The reported method to evaluate the uncertainties of the displacement gradients agrees with international recommendations.

Monte Carlo-based uncertainties of surface UV estimates from models and from spectroradiometers

Although some of the adverse effects of the ultraviolet (UV) radiation may be strictly proportional to cumulative UV doses, others may relate to the frequency of extreme UV events. Therefore, an improved understanding of the UV global climate, including variability and trends, has become of great interest. Variability and trend analyses require quality-ensured surface UV series. The quality of surface UV data depends on their uncertainty. Building upon our prior efforts, we have used a Monte Carlo-based method to compute, under different conditions, the uncertainties affecting UV data rendered by models (1D radiative transfer models) and by spectroradiometers (double monochromator-based and CCD array-based). We found that the uncertainty of spectral UV measurements is driven by the signal-to-noise ratio of the detector, while the uncertainty of spectral UV calculations strongly depends on the uncertainty of the ozone input. The presented uncertainty figures allow comparison of the performance of modern UV gathering techniques (models and instruments), and provide a frame to assess the significance of differences when intercomparing.

Exploitation of spectral direct UV irradiance measurements

Because of both scattering and absorption, ozone and aerosols lead to an attenuation in the surface UV irradiance. These factors can be efficiently retrieved from ground-based measurements of the spectral direct UV irradiance performed by using spectroradiometer systems. In this paper, we applied a Monte Carlo based exploitation technique of spectral direct UV irradiance measurements, which allowed us to obtain a bound within which the retrieved parameter (either the ozone column or an aerosol property) is expected to lie with a relatively high probability. The applied exploitation technique required sequentially comparing the ground-based measurements with a large number of spectra, each of them calculated by a radiative transfer model with randomly generated values of input parameters (i.e. the ozone column and aerosol properties). The dispersion of the generated values that led to acceptable matches between the measured and calculated spectra allowed us to compute both the estimates and the uncertainties of the retrieved parameters. The applied exploitation technique enabled us to describe the uncertainty propagation through the retrieval process and to assess the influence of the involved uncertainty sources. Methodological details are provided.

Satellite-derived UV irradiance for a region with complex morphology and meteorology: comparison against ground measurements in Santiago de Chile

Ground-based measurements of ultraviolet (UV) irradiance, carried out by a four-channel UV radiometer in Santiago de Chile from October 2004 to December 2011, have been used to estimate daily values of the UV index (UVI). These ground-based data have been compared with UVI estimates retrieved from the Ozone Measurement Instrument (OMI) on board the Aura spacecraft. Since the widely used OMI-gridded UVI data may not be suitable for the complex local morphology and meteorology, a careful screening of overpass OMI data was applied. Nevertheless, we found that OMI-derived UVI data overestimate ground-based values; depending on cloud-cover conditions, the mean bias (MB) and the root mean square error (RMSE) range from 34.53% to 30.29% and from 35.22% to 43.50%, respectively, with the lowest MB (and the highest RMSE) values occurring under overcast conditions. Moreover, the difference between satellite-derived and ground-based UVI data exhibits a limited seasonality with somewhat larger differences in the fall season. The detected overestimation seems to be linked with the boundary layer aerosol absorption that is not accounted for by the OMI algorithm. Indeed, we found that the difference in UVI increases with the aerosol concentration (which in Santiago shows seasonal variations). Ceilometer profiles of backscatter intensities, directly related to aerosol concentrations, and PM10 concentrations correlate with UVI differences (correlation coefficient r of approximately 0.6 and 0.4, respectively) under cloud-free conditions for time scales ranging from months to years. Additional comparisons were performed between UVI estimates retrieved from our ground-based measurements in Santiago and from the Tropospheric Emission Monitoring Internet Service (TEMIS) Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). Under cloudless conditions, also TEMIS-derived data overestimate ground-based UVI estimations (by about 31%) and exhibit a small seasonality.

Monitoring the plastic deformation progression of a specimen undergoing tensile deformation by moiré interferometry

By using moire interferometry we followed the deformation progression of an aluminium sheet metal sample subjected to a tensile test. At different load stages during the heterogeneous plastic deformation, we evaluated the strain fields from fringe patterns generated by using an interferometer with sensitivity vector perpendicular to the pulling direction. By analysing these strain fields, we determined the area where the process of strain localization began and the diffuse necking was formed. We observed that the localization started within the relatively large zone where the strains presented the fastest rate of change during the heterogeneous plastic deformation. In addition, by using a Monte Carlo-based technique, we carried out an uncertainty analysis of the performed strain measurements. We found that at each load stage, the relative standard uncertainties were constant and equal to 1.2%.

Satellite-derived UV climatology at Escudero Station, Antarctic Peninsula

Abstract We have used data from the Ozone Monitoring Instrument (OMI) aboard NASAs Earth Observing System (EOS) Aura satellite over the period 2004–11 to describe the characteristics of surface ultraviolet (UV) irradiance at Escudero Station (62°12′S, 58°57′W). The station is located on King George Island (northern Antarctic Peninsula). Temperatures in summer are frequently above 0°C, and the surrounding ocean is typically ice-free. We found that the UV irradiance at Escudero is driven by the Antarctic ozone hole (which annually in spring leads to significant variations in the ozone) and by clouds (which are more frequent and have a larger optical depth compared with other Antarctic sites). The combined effect of ozone and clouds led to significant variations in the surface UV. The variability (taken as the standard deviation of the UV estimates retrieved from OMI) is typically greater than 30% at Escudero, but may reach values greater than 50% in spring. The consistency of OMI-derived data was checked by using ground-based spectral measurements carried out under controlled conditions in January 2011.