Testing several models describing the change in surface solar irradiance with solar zenith angle in cloud-free skies

International audienceSeveral models aim at estimating surface downwelling solar irradiance (SSI) and are based on fast parameteri-zations of the radiative transfer in the atmosphere. This communication deals with the parameterization of the changes in SSI with solar zenith angle in cloudless conditions. It focuses on the total SSI and its direct and diffuse components. Several published and new models are tested against detailed calculations performed by the means of the radiative transfer model libRadtran in order to establish figures of merit. A methodology has been adopted whereby a large set of atmospheric properties (gas content, aerosols.. .) in cloud-free conditions is built by Monte-Carlo procedures and libRadtran is ran for each case of the set for solar zenith angle every 0.1 •. The selected models are tested against each case serving as reference and the discrepancies are computed and then summarized. The models comprise simple ones such as interpolation every 5 • of the irradiation itself or the clearness index, up to more complex ones such as Modified-Beer-Lambert model (MLB) or piecewise kernel-based functions. The accuracy of each model is assessed and compared to the accuracy currently achieved by the 5-piecewise MLB proposed by Qu et al. (2011). This 5-piecewise is made of five MLBs for five intervals (0 • , 60 •), (60 • , 75 •), (75 • , 80 •), (80 • , 85 •) and (85 • , 89.9 •). It exhibits satisfactory performances at any angle and outperforms standard linear interpolation techniques. 95% of errors in total SSI are less than 5 Wm-2. Conclusions on the respective performance of each model are drawn from this comparison. Finally, the strengths and weaknesses of each model are discussed having in mind their operational constraints. Reference: Qu, Z., Blanc, P., Lefevre, M., Wald, L., and Oumbe, A.: Study of the MLB parameterisation for change in surface solar irradiance with sun zenith angle in clear sky, Adv. Sci. Res., 6, 233–236, doi:10.5194/asr-6-233-2011, 2011

Study of the MLB parameterisation for change in surface solar irradiance with sun zenith angle in clear sky

International audienceThe MLB parameterisation (Modified Lambert-Beer, Mueller et al., 2004) describes the change in SSI with sun zenith angle (SZA) in clear-sky conditions. It applies to the direct and global SSI as well as their spectral distribution. We assess its performances by comparing its results to the outputs of the radiative transfer model libRadtran and standard interpolation procedures. The standard two-point fitting MLB function performs very well at SZA between 0° and 60° and fairly bad from 60° to 89.9°. A parameterisation made of four MLBs for four intervals (0°, 60°), (60°, 75°), (75°, 85°) and (85°, 89.9°) is also tested. This piecewise MLB parameterisation exhibits satisfactory performances at any SZA and outperforms standard linear interpolation techniques. 95 % of errors in global SSI are less than 1 W m−2 for each band and less than 5 W m−2 for total irradiance

Is the MLB parameterization accurate enough to describe change in solar radiation with solar zenith angle?

International audienceA new direct method, Heliosat-4, is currently being developed by the MINES ParisTech and the German Aerospace Center (DLR), aiming at estimating surface downwelling solar irradiance (SSI). This method is composed of two parts: a clear-sky module and a cloud-ground module (Oumbe et al. 2009). The clear-sky model is based on the radiative transfer model (RTM) libRadtran fed with advanced products on atmosphere optical parameters derived from recent Earth Observation missions. However, running a RTM in an operational mode is computer resources and time consuming. Therefore, it will be very beneficial to use parameterization to reduce the number of runs of RTM. Several parameterizations have been proposed in recent years, for example, the parameterization with water vapour density (Muller et al. 2009), with altitude (Gueymard, Thevenard 2009; Oumbe et al. 2009) and with sun zenith angle by using Modified Lambert-Beer function (MLB) (Muller et al. 2004, 2009). Qu et al. (2010) studied the MLB parameterization for beam and global irradiance. They proposed a 4-interval piecewise MLB which demonstrates better performances than the initial version based on a single interval of solar zenith angle. This piecewise MLB outperforms the current linear interpolation techniques. However, it is interesting to know whether the accuracy of this 4-piecewise MLB applied to global irradiance complies with standard performances expected for high quality pyranometers or for RTM. In order to assess it, we focused on two quality criteria: the systematic error (bias) and the percentile P95 of the absolute error. According to WMO standard for the measurement of radiation (WMO, 2008), the P95 should be less than 10 W/m2 (high quality). Myers et al. (2002) and Geuder et al. (2003) have compared different pyranometers. They wrote that as a whole a bias of 15 W/m2 is acceptable. Gueymard (2007) have compared results between different RTMs and estimated that a bias of 3 W/m2 is a proof of very high consistency between them. Therefore, we evaluated the performance of the 4-piecewise MLB by using two criteria: bias < 3 W/m2 and P95 < 10 W/m2. We observed that the 4-piecewise MLB does not satisfy the criteria of P95 when the ground albedo is greater than 0.1. Therefore, we propose a 5-piecewise MLB in adding another fitting angle at 40. We observe that this 5-piecewise MLB demonstrates satisfactory performance and could be used accurately in the Heliosat-4 method to reduce the number of runs of the RTM

An adaptive method to derive direct irradiance from global irradiance

International audienceObservations and estimations of solar radiation at ground level deal most frequently with global horizontal irradiance (GHI) while direct irradiance is crucial notably for Concentrated Solar Technology (CST) such as solar energy conversion systems: parabolic through, solar towers, parabolic dish or concentrated photovoltaic. Several global-to-direct irradiance conversion schemes are proposed in the literature. They are obtained by regression of an empirical parametric function with global and direct irradiance measurements made at few stations over a time period. These schemes are well suited to the climatic region where they have been designed, but not likely to other climatic regions. A new method is proposed. A general shape of the relationship between the clearsky index (Kc) and the ratio of the diffuse to global irradiance (fD) is derived from literature and analysis of several data sets of ground measurements. This analytical function needs two parameters. One is defined by the case of the overcast skies; the other is changing depending on the clear-sky conditions for the location and time under concern. A clear-sky model provides the Kc and fD for the clear-sky conditions. The new method was validated against ground measurements made by BSRN ground stations located in Carpentras, Sede Boqer and Tamanrasset. It reveals itself accurate compared to other methods. Indeed, for the data sets used, the bias amounts to 0%, -9% and 10%, the root mean square deviation (RMSD) to 16%, 19% and 21% and the correlation coefficient to 0.98, 0.96 and 0.95. The proposed method is interesting because it is flexible, adaptive and does not rely on empirical parameter

Assessment of Heliosat-4 surface solar irradiance derived on the basis of SEVIRI-APOLLO cloud products

International audienceThe Heliosat-4 method developed by the MINES ParisTech and the German Aerospace Center (DLR), aims at estimating surface downwelling solar irradiance (SSI). It benefits from advanced products derived from recent Earth Observation missions, among which the cloud products are crucial for the assessment of SSI. The APOLLO cloud product provided by DLR includes abundant information about the cloud physical and optical properties. The performances of Heliosat-4 when using APOLLO product are evaluated for the period of 2004-2009. The estimated SSIs are compared to measurements made at six stations within the Baseline Surface Radiation Network. Extensive analysis of the discrepancies offers an in-depth view of the performance of Heliosat-4/APOLLO, an understanding of the advantages of this combination Heliosat-4/APOLLO when compared to existing methods and the identification of restrictions in both Heliosat-4 and the APOLLO product for future improvements

Use of OCA and APOLLO in Heliosat-4 method for the assessment of surface downwelling solar irradiance

International audienceWe test two cloud products: Optimal Cloud Analysis (OCA) of EUMETSAT, and APOLLO from the German Aerospace Center (DLR), for the assessment of surface downwelling solar irradiance (SSI). Each product is input to the Heliosat-4 method, and the SSI estimates are compared to accurate measurements performed in the Baseline Radiation Network (BSRN). The performances obtained by the two products are compared. The overall performance of Heliosat-4 method by using different cloud products is given and conclusions on the benefit of each product for an operational Heliosat-4 are drawn

Digital photoprogramming of liquid-crystal superstructures featuring intrinsic chiral photoswitches

Dynamic patterning of soft materials in a fully reversible and programmable manner with light enables applications in anti-counterfeiting, displays and labelling technology. However, this is a formidable challenge due to the lack of suitable chiral molecular photoswitches. Here, we report the development of a unique intrinsic chiral photoswitch with broad chirality modulation to achieve digitally controllable, selectable and extractable multiple stable reflection states. An anti-counterfeiting technique, embedded with diverse microstructures, featuring colour-tunability, erasability, reversibility, multi-stability and viewing-angle dependency of pre-recorded patterns, is established with these photoresponsive superstructures. This strategy allows dynamic helical transformation from the molecular and supramolecular to the macroscopic level using light-activated intrinsic chirality, demonstrating the practicality of photoprogramming photonics

cDNA-AFLP analysis reveals differential gene expression in compatible interaction of wheat challenged with Puccinia striiformis f. sp. tritici

<p>Abstract</p> <p>Background</p> <p><it>Puccinia striiformis </it>f. sp. <it>tritici </it>is a fungal pathogen causing stripe rust, one of the most important wheat diseases worldwide. The fungus is strictly biotrophic and thus, completely dependent on living host cells for its reproduction, which makes it difficult to study genes of the pathogen. In spite of its economic importance, little is known about the molecular basis of compatible interaction between the pathogen and wheat host. In this study, we identified wheat and <it>P. striiformis </it>genes associated with the infection process by conducting a large-scale transcriptomic analysis using cDNA-AFLP.</p> <p>Results</p> <p>Of the total 54,912 transcript derived fragments (TDFs) obtained using cDNA-AFLP with 64 primer pairs, 2,306 (4.2%) displayed altered expression patterns after inoculation, of which 966 showed up-regulated and 1,340 down-regulated. 186 TDFs produced reliable sequences after sequencing of 208 TDFs selected, of which 74 (40%) had known functions through BLAST searching the GenBank database. Majority of the latter group had predicted gene products involved in energy (13%), signal transduction (5.4%), disease/defence (5.9%) and metabolism (5% of the sequenced TDFs). BLAST searching of the wheat stem rust fungus genome database identified 18 TDFs possibly from the stripe rust pathogen, of which 9 were validated of the pathogen origin using PCR-based assays followed by sequencing confirmation. Of the 186 reliable TDFs, 29 homologous to genes known to play a role in disease/defense, signal transduction or uncharacterized genes were further selected for validation of cDNA-AFLP expression patterns using qRT-PCR analyses. Results confirmed the altered expression patterns of 28 (96.5%) genes revealed by the cDNA-AFLP technique.</p> <p>Conclusion</p> <p>The results show that cDNA-AFLP is a reliable technique for studying expression patterns of genes involved in the wheat-stripe rust interactions. Genes involved in compatible interactions between wheat and the stripe rust pathogen were identified and their expression patterns were determined. The present study should be helpful in elucidating the molecular basis of the infection process, and identifying genes that can be targeted for inhibiting the growth and reproduction of the pathogen. Moreover, this study can also be used to elucidate the defence responses of the genes that were of plant origin.</p

McClear: a new model estimating downwelling solar radiation at ground level in clear-sky conditions

International audienceA new fast clear-sky model called McClear was developed to estimate the downwelling shortwave direct and global irradiances received at ground level under clear skies. It is a fully physical model replacing empirical relations or simpler models used before. It exploits the recent results on aerosol properties, and total column content in water vapour and ozone produced by the MACC project (Monitoring Atmosphere Composition and Climate). It accurately reproduces the irradiance computed by the libRadtran reference radiative transfer model with a computational speed approximately 105 times greater by adopting the abaci, or look-up table, approach combined with interpolation functions. It is therefore suited for geostationary satellite retrievals or numerical weather prediction schemes with many pixels or grid points, respectively. McClear irradiances were compared to 1 min measurements made in clear-sky conditions at several stations within the Baseline Surface Radiation Network in various climates. The bias for global irradiance comprises between −6 and 25Wm−2. The RMSE ranges from 20Wm−2 (3% of the mean observed irradiance) to 36Wm−2 (5 %) and the correlation coefficient ranges between 0.95 and 0.99. The bias for the direct irradiance comprises between −48 and +33Wm−2. The root mean square error (RMSE) ranges from 33Wm−2 (5 %) to 64Wm−2 (10 %). The correlation coefficient ranges between 0.84 and 0.98. This work demonstrates the quality of the McClear model combined with MACC products, and indirectly the quality of the aerosol properties modelled by the MACC reanalysis