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Multiple satellite products are available to monitor the spatiotemporal dynamics of surface albedo. They are extensively assessed over snow-free surfaces but less over snow. However, snow albedo is critical for climate monitoring applications, so a better understating of the accuracy of these products over snow is needed. This work analyzes long-term (+20 years) products (MCD43C3 v6/v6.1, GLASS-AVHRR, C3S v1/v2) by comparing them against the 11 most spatially representative stations from FLUXNET and BSRN during the snow-free and snow-covered season. Our goal is to understand how the performance of these products is affected by different snow cover conditions to use this information in an upcoming product inter-comparison that extends the analysis spatially and temporally. MCD43C3 has the smallest bias during the snow season (−0.017), and more importantly, the most stable bias with different snow cover conditions. Both v6 and v6.1 have similar performance, with v6.1 just increasing slightly the coverage at high latitudes. On the contrary, the quality of both GLASS-AVHRR and C3S-v1/v2 albedo decreases over snow. Particularly, the bias of both products varies strongly with the snow cover conditions, underestimating albedo over snow and overestimating snow-free albedo. GLASS bias strongly increases during the melting season, which is most likely due to an artificially extended snow season. C3S-v2 has the largest negative bias overall over snow during both the AVHRR (−0.141) and SPOT/VGT (−0.134) period. In addition, despite the improvements from v1 to v2, C3S-v2 still is not consistent enough during the transition from AVHRR to SPOT/VGT. [ABSTRACT FROM AUTHOR]

This paper aims to assess the relationship between the surface reflectance derived from ground based and aircraft measurements. The parameters of the Rahman–Pinty–Verstraete (RPV) and Ross Thick-LiSparse (RTLS) kernel based bi-directional reflectance distribution functions (BRDF), have been derived using actual measurements of the hemispherical-directional reflectance factor (HDRF), collected during different campaigns over the Railroad Valley Playa. The effect of the atmosphere, including that of the diffuse radiation on bi-directional reflectance factor (BRF) parameter retrievals, assessed using 6S model simulations, was negligible for the low turbidity conditions of the site under investigation ( τ 550 ≤ 0.05 ). It was also shown that the effects of the diffuse radiation on RPV spectral parameters retrieval is linear for the isotropic parameter ρ 0 and the scattering parameter Θ , and can be described with a second order polynomial for the k-Minnaert parameter. In order to overcome the lack of temporal collocations between aircraft and in-situ measurements, Monte Carlo 3-D radiative transfer simulations mimicking in-situ and remote sensing techniques were performed on a synthetic parametric meshed scene defined by merging Landsat and Multianglhe Imaging Spectroradiometer (MISR) remote sensing reflectance data. We simulated directional reflectance measurements made at different heights for PARABOLA and CAR, and analyzed them according to practices adopted for real measurements, consisting of the inversion of BRF functions and the calculation of the bi-hemispherical reflectance (BHR). The difference of retrievals against the known benchmarks of kernel parameters and BHR is presented. We associated an uncertainty of up to 2% with the retrieval of area averaged BHR, independently of flight altitudes and the BRF model used for the inversion. As expected, the local nature of PARABOLA data is revealed by the difference of the anisotropic kernel parameters with the corresponding parameters retrieved from aircraft loops. The uncertainty of the resultant BHR fell within ± 3 % . [ABSTRACT FROM AUTHOR]

The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral observations via the Environmental Mapping and Analysis Program (EnMAP). The location close to the Concordia station at the Dome C Plateau (Antarctica) was selected. At this location, the atmospheric effects (except molecular light scattering and absorption) are weak, and the simplified atmospheric correction scheme could be applied. The ice grain size, snow specific surface area, and snow spectral and broadband albedos were retrieved using single-view EnMAP measurements. In addition, we propose a technique to retrieve trace gas concentrations (e.g., water vapor and ozone) from EnMAP observations over the snow surfaces. A close correspondence of satellite and ground-measured parameters was found. [ABSTRACT FROM AUTHOR]

The Sentinel-2 Level 2 Prototype Processor (SL2P) is made available to users for the retrieval of vegetation biophysical variables including leaf area index (LAI) from Multispectral Instrument (MSI) data within the Sentinel Application Platform (SNAP). A limited number of validation exercises have indicated SL2P LAI retrievals frequently meet user requirements over agricultural environments, but perform comparatively poorly over heterogeneous canopies such as forests. Recently, a modified version of SL2P was developed, using the directional area scattering factor (DASF) to constrain retrievals as an alternative to regularisation (SL2P-D). Whilst SL2P makes use of prior information on expected canopy conditions, SL2P-D is trained using uniform distributions of input parameters to define radiative transfer model (RTM) simulations. Using in situ measurements available through the Copernicus Ground Based Observations for Validation (GBOV) service, we performed an extensive validation of SL2P and SL2P-D LAI retrievals over 19 sites throughout the United States. For effective LAI (LAI e), SL2P demonstrated good overall performance (RMSD = 0.50, NRMSD = 31%, bias = −0.10), with all LAI retrievals meeting the Sentinels for Science (SEN4SCI) uncertainty requirements over homogeneous canopies (cultivated crops, grasslands, pasture/hay and shrub/scrub), whilst underestimation occurred over heterogeneous canopies (deciduous forest, evergreen forest, mixed forest, and woody wetlands). SL2P-D retrievals demonstrated reduced bias, slightly improving overall performance when compared with SL2P (RMSD = 0.48, NRMSD = 30%, bias = −0.05), indicating its retrieval approach appears to offer some advantages over regularisation using prior information, especially at LAI e > 3. Additionally, SL2P-D resulted in 32% more valid retrievals than SL2P, with the largest differences observed at LAI e < 1. Validation against in situ measurements of LAI as opposed to LAI e yielded similar patterns but poorer performance (RMSD = 1.08 to 1.13, NRMSD = 49% to 52%, bias = −0.64 to −0.68) because the RTM used by SL2P and SL2P-D does not account for foliage clumping. In addition to the retrievals themselves, we examined the relationship between predicted uncertainties and observed differences in retrieved and in situ LAI. With respect to LAI e , SL2P's predicted uncertainties were conservative, underestimating observed differences in only 35% of cases, whilst those for LAI were unbiased. [ABSTRACT FROM AUTHOR]

A method based on Spinning Enhanced Visible and Infrared Imager (SEVIRI) measured reflectance at 0.6 and 3.9 µm is used to retrieve the cloud optical thickness (COT) and cloud effective radius (re) over the Iberian Peninsula. A sensitivity analysis of simulated retrievals to the input parameters demonstrates that the cloud top height is an important factor in satellite retrievals of COT and re with uncertainties around 10% for small values of COT and re; for water clouds these uncertainties can be greater than 10% for small values of re. The uncertainties found related with geometries are around 3%. The COT and re are assessed using well-known satellite cloud products, showing that the method used characterize the cloud field with more than 80% (82%) of the absolute differences between COT (re) mean values of all clouds (water plus ice clouds) centred in the range from ±10 (±10 µm), with absolute bias lower than 2 (2 μm) for COT (re) and root mean square error values lower than 10 (8 μm) for COT (re). The cloud water path (CWP), derived from satellite retrievals, and the shortwave cloud radiative effect at the surface (CRESW) are related for high fractional sky covers (Fsc >0.8), showing that water clouds produce more negative CRESW than ice clouds. The COT retrieved was also related to the cloud modification factor, which exhibits reductions and enhancements of the surface SW radiation of the order of 80% and 30%, respectively, for COT values lower than 10. A selected case study shows, using a ground-based sky camera that some situations classified by the satellite with high Fsc values correspond to situations of broken clouds where the enhancements actually occur. For this case study, a closure between the liquid water path (LWP) obtained from the satellite retrievals and the same cloud quantity obtained from ground-based microwave measurements was performed showing a good agreement between both LWP data set values. [ABSTRACT FROM AUTHOR]

The features of erythemally weighted ( EW ) and short-wave downwelling ( SWD ) solar irradiances, observed during the spring–summer months of 2007–2011 at Johann Gregor Mendel (63°48′S, 57°53′W, 7 m a.s.l.) and Dome Concordia (75°06′S, 123°21′E, 3233 m a.s.l.) stations, placed at the Antarctic coastal region and on the interior plateau respectively, have been analysed and compared to each other. The EW and SWD spectral components have been presented by the corresponding daily integrated values and were examined taking into account the different geographic positions and different environmental conditions at both sites. The results indicate that at Mendel station the surface solar irradiance is strongly affected by the changes in the cloud cover, aerosols and albedo that cause a decrease in EW between 20% and 35%, and from 0% to 50% in SWD component, which contributions are slightly lower than the seasonal SWD variations evaluated to be about 71%. On the contrary, the changes in the cloud cover features at Concordia station produce only a 5% reduction of the solar irradiance, whilst the seasonal oscillations of 94% turn out to be the predominant mode. The present analysis leads to the conclusion that the variations in the ozone column cause an average decrease of about 46% in EW irradiance with respect to the value found in the case of minimum ozone content at each of the stations. In addition, the ratio between EW and SWD spectral components can be used to achieve a realistic assessment of the radiation amplification factor that quantifies the relationship between the atmospheric ozone and the surface UV irradiance. [ABSTRACT FROM AUTHOR]

Abstract: The strong ozone depletion event that occurred in Arctic during spring 2011 was found to cause appreciable reduction in the ozone column (OC) in Europe, even at lower latitudes. The features of this episode have been analysed using the data recorded at 34 ground-based stations located in the European area and compared with the similar events in 2000 and 2005. The results provided evidence that OC as far south as 40°N latitude was considerably influenced by the Arctic ozone loss in spring 2011. The reduction of OC at the northernmost sites was about 40% with respect to the mean value calculated over the previous six-year period, while a similar decrease at the southern extreme ranged between 15 and 18%, and were delayed by nearly two weeks compared to the Arctic region. The ozone distributions reconstructed for the West Europe sector show that the decline of OC lasted from late March to late April 2011. The echo of the Arctic ozone depletion on mid-latitude UV irradiance has been analysed trough model computations that show an increase of the midday erythemal dose by 3–4 SED (1 SED = 100 J m−2) that was slightly higher than at polar regions. On the other hand it was assessed that the biosystems in the northernmost regions were a subject of about 4 times higher UV stress than those at mid-latitudes. Despite indications of an OC recovery, the event examined here shows that the issue of ozone depletion episodes cannot be belittled. [Copyright &y& Elsevier]

Here we present the first multi-year record of nitrate in the atmospheric aerosol (2005-2008) and surface snow (2006-08) from central Antarctica. PM10 and size-segregated aerosol, together with superficial snow, have been collected all year-round at high resolution (daily for all the snow samples and for most of aerosol samples) at Dome C since the 2004/05 field season and analysed for main and trace ionic markers. The suitability of the sampling location in terms of possible contamination from the base is shown in detail. In spite of the relevance of nitrate in Antarctic atmosphere, both for better understanding the chemistry of N cycle in the plateau boundary layer and for improving the interpretation of long-term nitrate records from deep ice core records, nitrate sources in Antarctica are not well constrained yet, neither in extent nor in timing. A recurring seasonal pattern was pointed out in both aerosol and snow records, showing summer maxima and winter minima, although aerosol maxima lead the snow ones of 1-2 months, possibly due to a higher acidity in the atmosphere in mid-summer, favouring the repartition of nitrate as nitric acid and thus its uptake by the surface snow layers. On the basis of a meteorological analysis of one major nitrate event, of data related to PSC I extent and of irradiance values, we propose that the high nitrate summer levels in aerosol and snow are likely due to a synergy of enhanced source of nitrate and/or its precursors (such as the stratospheric inputs), higher solar irradiance and higher oxidation rates in this season. Moreover, we show here a further evidence of the substantial contribution of HNO3/NOx re-emission from the snowpack, already shown in previous works, and which can explain a significant fraction of atmospheric nitrate, maintaining the same seasonal pattern in the snow. As concerning snow specifically, the presented data suggest that nitrate is likely to be controlled mainly by atmospheric processes, not on the daily timescale but rather on the seasonal one. [ABSTRACT FROM AUTHOR]

Variations in total ozone column and sun exposures able to cause erythema and damage the DNA molecules were observed by the narrow-band filter radiometer UV-RAD in Bologna, Italy from 2005 to 2010. The ozone columns determined from the UV-RAD measurements were found to be close to those provided by the satellite Ozone Monitoring Instrument (OMI) showing an average discrepancy of 1 % with standard deviation of ± 6 %. Analysis of the data highlights a well-marked annual cycle of the ozone column variations while the oscillations with periods of 8, 18 and 34 months present much smaller amplitudes. The influence of the frequency of solar irradiance measurements on the accuracy of the evaluated daily exposure dose has been studied and it was found that time intervals no longer than 5-10 min between the measurements of erythema and DNA damage effective UV irradiances provide a satisfactory assessment of the corresponding daily exposures. The latter do not present significant year-to-year variations for the period under study, while their annual distributions show slight changes likely due to the specific cloud cover and ozone column variability for different years. The annual erythemal exposure dose for 2007-2010 varied between 603.7 and 638.1 kJ m, while the corresponding sun exposure affecting DNA changed from 6.38 to 7.91 kJ m. [ABSTRACT FROM AUTHOR]

Abstract: An updated set of time series of derived aerosol optical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurements performed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature. The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole. [Copyright &y& Elsevier]

Abstract: Diurnal variations in total ozone column were observed to exceed 40DU during the QAARC campaign of late spring 2009 held at the Ny-Ålesund Arctic station in Svalbard. They were correlated to corresponding oscillations of surface UV-B irradiance, assuming that the strong dependence between these two variables ensures the reliability of registered total ozone oscillations. To make the comparison, an approach is proposed that allows the retrieval of diurnal variability in UV-B irradiance due to total ozone column changes, for application to the data collected during the campaign. The UV irradiance features observed with the reference spectroradiometer over a 7-day period were analyzed together with the corresponding ozone column data-sets, simultaneously obtained using two different devices. Power curves were found to provide the best-fit solutions for the retrieved UV-B variations, examined as a function of total ozone variations. Such approximations are characterized by regression coefficients of about −0.94 and by the exponent named in earlier studies as the radiation amplification factor (RAF), which was found to vary between 0.79 and 2.49. These RAF evaluations are in good agreement with previous estimates derived from both field measurements and model evaluations, which confirm the complete dependability of the short-term variations in the ozone column measured at Ny-Ålesund. [Copyright &y& Elsevier]

Abstract: The time-patterns of ground-level solar irradiance during the solar eclipse of 29 March, 2006, were observed at three Italian stations (Lampedusa, Mt. Cimone and Bologna) using different radiometric techniques. The global irradiance measured at the sites was found to reach the minimum at times not coinciding with those predicted by radiative transfer model evaluations, with ahead or lag times depending on the optical characteristics of the surface–atmosphere system in the areas surrounding of the stations. This different behaviour has been mainly attributed to the different influence of the environmental conditions on the diffuse radiance component measured at the observation sites. The present results indicate that the incoming diffuse radiance recorded at the three stations was appreciably affected by contributions arising from extended regions of about 30–100km range from the stations. Such an explanation agrees very well with the theoretical evaluations obtained in earlier studies. The surrounding environmental areas of impact at ultraviolet wavelengths have been found to be wider than those in the visible and near-infrared spectral ranges. [Copyright &y& Elsevier]

This article presents the evaluation of the Copernicus Sentinel-3 Ocean Land Colour Instrument (OLCI) operational terrestrial products corresponding to the green instantaneous Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) and its associated rectified channels. These products are estimated using OLCI spectral measurements acquired at the top of the atmosphere by a physically-based approach and are available operationally at full (300 m) and reduced (1.2 km) spatial resolution daily. The evaluation of the quality of the FAPAR OLCI values was based on the availability of data acquired over several years by Sentinel-3A (S3A) and Sentinel-3B (S3B). The evaluation exercise consisted of several stages: first, an overall comparison of the two S3 platform products was carried out during the tandem phase; second, comparison with an FAPAR climatology derived from the Medium Resolution Imaging Spectrometer (MERIS) provided information on the seasonality of various types of land cover. Then, direct comparisons were made with the same type of FAPAR products retrieved from two sensors, the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sentinel-2 (S2) Multispectral Instrument (MSI), and with several ground-based estimates. In addition, an analysis of the efficiency of the retrieval algorithm with 3D radiative transfer simulations was performed. The results indicated that the consistency between daily and monthly S3A and S3B on a global scale was very good during the tandem phase (RMSD = 0.01 and a correlation R2 of 0.99 with a bias of 0.003); we found an agreement with a correlation of 0.95 and 0.93 (RMSD = 0.07 and 0.09) with JRC FAPAR S2 and JRC FAPAR MODIS, respectively. Compatibility with the ground-based data was between 0.056 and 0.24 in term of RMSD depending on the type of vegetation with an overall R2 of 0.89. Immler diagrams demonstrate that their variances were lower than the total uncertainties. The quality assurance using 3D radiative transfer model has shown that the apparent performance of the algorithm depends strongly on the type of in-situ measurement and canopy type. • First evaluation of Copernicus S3 OLCI green instantaneous FAPAR products. • Seasonality with MERIS climatology provide high temporal confidence of products. • S2 indicated better agreements than with MODIS. • 3D-RT simulations were used for assessing various definition of reference values. [ABSTRACT FROM AUTHOR]

A set of 1330 radiosoundings was performed at the Italian station of Terra Nova Bay in Antarctica during 12 measurement campaigns from 1987 to 1998. These measurements were separately analyzed for the various 10-day periods from mid-October to mid-February, to determine the mean vertical profiles of air pressure and temperature in the troposphere and lower stratosphere, and those of moisture parameters in the troposphere. The temperature data were corrected for the errors due to radiation and heat exchange processes and for the lag errors of the sensor. Due to temperature dependence and other dry bias effects, the humidity errors were also taken into account. The tropospheric temperature was found to present average values of its vertical gradient varying between -5.4 and -6.3 K/km, while its minimum height associated with the tropopause gradually lowered from 13.7 to 7.9 km. Total water vapor content increased correspondingly from 0.12 to 0.36 g cm-2 during the first two months and decreased to 0.26 g cm-2 in the following period. Stratospheric temperature was observed to increase during the first 2 months by about 15 K in the region below the 24-km height and to appreciably decrease at upper levels, maintaining almost stable features during the subsequent months. [Copyright &y& Elsevier]

A framework is proposed for assessing the physical consistency between two terrestrial Essential Climate Variables (ECVs) products retrieved from Earth Observation at global scale. The methodology assessed the level of agreement between the temporal variations of Leaf Area Index (LAI) and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). The simultaneous changes were classified according to their sign, magnitude and level of confidence, whereby the respective products uncertainties were taken into consideration. A set of proposed agreement metrics were used to identify temporal and spatial biases of non-coherency, non-significance, sensitivity and the overall level of agreement of the temporal changes between two ECVs. We applied the methodology using the Joint Research Center (JRC) Two-stream Inversion Package (TIP) products at 1 km, those provided by the Copernicus Global Land Service (CGLS) based on the SPOT/VGT and Proba-V at 1 km, and the MODIS MCD15A3 at 500 m. In addition, the same analysis was applied with aggregated products at a larger scale over Southern Africa. We found that the CGLS LAI and FAPAR products lacked consistency in their spatial and temporal changes and were severely affected by trends. The MCD15A3 products were characterized by the highest number of non-coherent changes between the two ECVs but temporal inconsistencies were mainly located over the eastern hemisphere. The JRC-TIP products were highly consistent. The results showed the advantages of physically-based retrieval algorithms, in both JRC-TIP and MODIS products, and indicated also that, except for MODIS over forests, aggregated products using an uncertainty-based weighted average led to higher agreement between the ECVs changes. • A new framework to evaluate physical consistency between LAI and FAPAR changes. • Spatial and temporal agreements are assessed considering user confidence level. • Products based on Radiative Transfer Model retrieval show best results. • Results depend on both spatial scale and land cover types. [ABSTRACT FROM AUTHOR]

With a growing number of Earth observation (EO) products available through operational programmes such as the European Union's Copernicus, there is increasing emphasis on product accuracy and uncertainty, necessitating evaluation against in situ reference measurements. Whilst existing reference datasets have proven a valuable resource, they incorporate little data with which products from recent EO instruments can be assessed. A reliance on individual field campaigns has also led to several inconsistencies, whilst limiting the extent to which temporal variations in EO product performance can be captured. Recently established environmental monitoring networks such as the National Ecological Observatory Network (NEON), which collect routine in situ measurements using standardised instruments and protocols, provide a promising opportunity in this respect. The Copernicus Ground Based Observations for Validation (GBOV) service was initiated in recognition of this fact. In the first component of the project, raw observations from existing networks have been collected and processed to provide reference data for a range of EO land products. In this study, we focus on leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR). Raw digital hemispherical photography (DHP) from twenty NEON sites was processed to derive in situ reference measurements, which were then upscaled to provide high spatial resolution reference maps. Using these data, we assess the recently released Copernicus Global Land Service (CGLS) 300 m Version 1 (V1) products derived from PROBA-V, in addition to existing products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Radiometer Suite (VIIRS). When evaluated against reference data, the CGLS 300 m V1 product demonstrated the best agreement (RMSD = 0.57 for LAI and 0.08 for FAPAR), followed by the Collection 6 VNP15A2H and MOD15A2H products (RMSD = 0.81 to 0.89 for LAI and 0.12 for FAPAR). Differing assumptions of the products and in situ reference measurements, which cause them to be sensitive to slightly different quantities, are thought to explain apparent biases over sparse vegetation and forest environments. To ensure their continued utility, future work should focus on updating the GBOV in situ reference measurements, implementing additional corrections, and improving their geographical representativeness. • LAI/FAPAR products evaluated over twenty NEON sites between 2015 and 2018 • In situ measurements upscaled to moderate spatial resolution using Sentinel-2 • CGLS 300 m V1 demonstrates best agreement (RMSD = 0. 57 for LAI and 0.08 for FAPAR) • MOD15A2H/VNP15A2H perform similarly (RMSD = 0.81 to 0.89 for LAI, 0.12 for FAPAR) [ABSTRACT FROM AUTHOR]