Overcast Conditions Research Articles

Short-Term Solar Irradiance Forecasts Using Sky Images and Radiative Transfer Model

In this paper, we propose a novel forecast method which addresses the difficulty in short-term solar irradiance forecasting that arises due to rapidly evolving environmental factors over short time periods. This involves the forecasting of Global Horizontal Irradiance (GHI) that combines prediction sky images with a Radiative Transfer Model (RTM). The prediction images (up to 10 min ahead) are produced by a non-local optical flow method, which is used to calculate the cloud motion for each pixel, with consecutive sky images at 1 min intervals. The Direct Normal Irradiance (DNI) and the diffuse radiation intensity field under clear sky and overcast conditions obtained from the RTM are then mapped to the sky images. Through combining the cloud locations on the prediction image with the corresponding instance of image-based DNI and diffuse radiation intensity fields, the GHI can be quantitatively forecasted for time horizons of 1–10 min ahead. The solar forecasts are evaluated in terms of root mean square error (RMSE) and mean absolute error (MAE) in relation to in-situ measurements and compared to the performance of the persistence model. The results of our experiment show that GHI forecasts using the proposed method perform better than the persistence model.

Evaluation of sea-surface photosynthetically available radiation algorithms under various sky conditions and solar elevations.

In this study, we report on the performance of satellite-based photosynthetically available radiation (PAR) algorithms used in published oceanic primary production models. The performance of these algorithms was evaluated using buoy observations under clear and cloudy skies, and for the particular case of low sun angles typically encountered at high latitudes or at moderate latitudes in winter. The PAR models consisted of (i) the standard one from the NASA-Ocean Biology Processing Group (OBPG), (ii) the Gregg and Carder (GC) semi-analytical clear-sky model, and (iii) look-up-tables based on the Santa Barbara DISORT atmospheric radiative transfer (SBDART) model. Various combinations of atmospheric inputs, empirical cloud corrections, and semi-analytical irradiance models yielded a total of 13 (11 + 2 developed in this study) different PAR products, which were compared with in situ measurements collected at high frequency (15min) at a buoy site in the Mediterranean Sea (the "BOUée pour l'acquiSition d'une Série Optique à Long termE," or, "BOUSSOLE" site). An objective ranking method applied to the algorithm results indicated that seven PAR products out of 13 were well in agreement with the in situ measurements. Specifically, the OBPG method showed the best overall performance with a root mean square difference (RMSD) (bias) of 19.7% (6.6%) and 10% (6.3%) followed by the look-up-table method with a RMSD (bias) of 25.5% (6.8%) and 9.6% (2.6%) at daily and monthly scales, respectively. Among the four methods based on clear-sky PAR empirically corrected for cloud cover, the Dobson and Smith method consistently underestimated daily PAR while the Budyko formulation overestimated daily PAR. Empirically cloud-corrected methods using cloud fraction (CF) performed better under quasi-clear skies (CF<0.3) with an RMSD (bias) of 9.7%-14.8% (3.6%-11.3%) than under partially clear to cloudy skies (0.3<CF<0.7) with 16.1%-21.2% (-2.2%-8.8%). Under complete overcast conditions (CF>0.7), however, all methods showed larger RMSD differences (biases) ranging between 32% and 80.6% (-54.5%-8.7%). Finally, three methods tested for low sun elevations revealed systematic overestimation, and one method showed a systematic underestimation of daily PAR, with relative RMSDs as large as 50% under all sky conditions. Under partially clear to overcast conditions all the methods underestimated PAR. Model uncertainties predominantly depend on which cloud products were used.

Discrimination of herbicide-resistant kochia with hyperspectral imaging

A hyperspectral imager was used to differentiate herbicide-resistant versus herbicide-susceptible biotypes of the agronomic weed kochia, in different crops in the field at the Southern Agricultural Research Center in Huntley, Montana. Controlled greenhouse experiments showed that enough information was captured by the imager to classify plants as either a crop, herbicide-susceptible or herbicide-resistant kochia. The current analysis is developing an algorithm that will work in more uncontrolled outdoor situations. In overcast conditions, the algorithm correctly identified dicamba-resistant kochia, glyphosate-resistant kochia, and glyphosate- and dicamba-susceptible kochia with 67%, 76%, and 80% success rates, respectively.

Shade tolerance: an additional factor affecting the distribution of mountain beech and silver beech in New Zealand?

Increased shade tolerance and a lower thermal optimum for photosynthesis may give silver beech a photosynthetic advantage over mountain beech in cloudy regions of New Zealand, and possibly the understory. The geographic distributions of silver beech (Lophozonia menziesii) and mountain beech (Fuscospora cliffortioides) in New Zealand roughly correspond with precipitation gradients, with F. cliffortioides dominating in sunny and dry regions, and L. menziesii either dominating or co-dominating with F. cliffortiodes in regions that are more cloudy and moist. Previous research suggests that greater drought tolerance may account for dominance of F. cliffortioides in sunny/dry regions, while greater tolerance of low soil nutrients may render a competitive advantage to L. menziesii in wetter habitats vulnerable to nutrient leaching. In this short communication, we propose and provide preliminary evidence for an additional hypothesis—that greater shade tolerance and a lower thermal optimum for photosynthesis confers a photosynthetic advantage to L. menziesii relative to F. cliffortioides under heavy cloud cover, and possibly in the forest understory. Sunlight measurements were recorded under clear sky, partly cloudy, overcast, and understory conditions at four field sites in New Zealand where the species co-occur, and integrated into previously published photosynthetic light response curves to estimate predicted carbon gain at 10, 20, and 25 °C. Photosynthetic gas exchange measurements were sampled in the field to corroborate expected results. Consistent with our hypothesis, predicted carbon gain was significantly greater in L. menziesii under cool (10–20 °C), low-light ( 20 °C. We conclude that relative tolerance of cloud and canopy shade may be an additional physiological factor, in combination with those previously described in the literature, affecting geographic and ecological distribution of mountain and silver beech in New Zealand.

Comparison of methodologies for cloud cover estimation in Brazil - A case study

Clouds are the major modulator of the shortwave and longwave radiation components of the Earth's energy balance and, as such, help to regulate the planet's temperature. In the energy sector, clouds are a source of instability in the generation of energy using solar technologies. This study aims at comparing three approaches to get cloud cover information in the Southeastern region of Brazil during the period of approximately three months. The first method, assumed as reference, uses all-sky camera pictures for the cloud cover estimation. The other two methodologies use downward longwave radiation with surface meteorological data and geostationary satellite data. Both methods presented good agreement with the camera for clear sky and overcast conditions, with probabilities of detection of 92.8% and 80.7% for the longwave method and 93.3% and 87.6% for the satellite method, respectively. The major problem occurs with the broken-clouds sky scenario, with probabilities of detection above 38%, where each method has its own specificity, such as, longwave emissivity of the atmosphere, spatial resolution and view geometry. The long-wave method has the minor R correlation with the camera (87%) when compared with the satellite method (93%) and requires a daily normalization, which make it not usable for instantaneous measurements. Regarding the satellite method, the most important issue is the spatial resolution, which has the major impact on the broken-clouds sky scenarios. The cloud masking works properly for large clouds with, at least, the size comparable to the satellite image pixel. Furthermore, the method using the all-sky camera also needs to be improved, because it presented some deficiencies, like very bright areas around the sun, sometimes identified as clouds, leading to cloud cover overestimation.

Blowing snow detection from ground-based ceilometers: application to East Antarctica

Abstract. Blowing snow impacts Antarctic ice sheet surface mass balance by snow redistribution and sublimation. However, numerical models poorly represent blowing snow processes, while direct observations are limited in space and time. Satellite retrieval of blowing snow is hindered by clouds and only the strongest events are considered. Here, we develop a blowing snow detection (BSD) algorithm for ground-based remote-sensing ceilometers in polar regions and apply it to ceilometers at Neumayer III and Princess Elisabeth (PE) stations, East Antarctica. The algorithm is able to detect (heavy) blowing snow layers reaching 30 m height. Results show that 78 % of the detected events are in agreement with visual observations at Neumayer III station. The BSD algorithm detects heavy blowing snow 36 % of the time at Neumayer (2011–2015) and 13 % at PE station (2010–2016). Blowing snow occurrence peaks during the austral winter and shows around 5 % interannual variability. The BSD algorithm is capable of detecting blowing snow both lifted from the ground and occurring during precipitation, which is an added value since results indicate that 92 % of the blowing snow is during synoptic events, often combined with precipitation. Analysis of atmospheric meteorological variables shows that blowing snow occurrence strongly depends on fresh snow availability in addition to wind speed. This finding challenges the commonly used parametrizations, where the threshold for snow particles to be lifted is a function of wind speed only. Blowing snow occurs predominantly during storms and overcast conditions, shortly after precipitation events, and can reach up to 1300 m a. g. l. in the case of heavy mixed events (precipitation and blowing snow together). These results suggest that synoptic conditions play an important role in generating blowing snow events and that fresh snow availability should be considered in determining the blowing snow onset.

Microclimate differences above ground-layer vegetation in lichen-dominated pine forests of north-central British Columbia

Lodgepole pine forests of north-central British Columbia have patchy ground-layer vegetation, typically dominated by either fruticose lichens, feathermosses, or ericaceous vascular plants; this patchy structure has been shown to correspond with environmental variables that likely moderate the ground-layer microclimate. To investigate the potential role of microclimate on patterns of dominance of ground-layer functional groups, we recorded temperature and relative humidity above the ground-layer vegetation during 25 summer days over patches dominated by mat-forming lichens, feathermosses, or vascular plants. Data were summarized for raw microclimate attributes and daily water potential of the air, and in terms of modelled equilibrium water content of moss or lichen thalli. Analysis of variance revealed significant differences in the water potential of air above the three patch types under sunny conditions, but not under overcast conditions. Differences in vegetation cover were only associated with differences in atmospheric moisture when using data from sunny periods during the daytime. These data confirm that lichens occupy microclimatic niches that are distinctly drier than those of feathermosses or vascular plants, and corroborate the suggested mechanism by which canopy or soil properties influence these types of ground-layer vegetation.

Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica

Abstract. The optical-equivalent snow grain size affects the reflectivity of snow surfaces and, thus, the local surface energy budget in particular in polar regions. Therefore, the specific surface area (SSA), from which the optical snow grain size is derived, was observed for a 2-month period in central Antarctica (Kohnen research station) during austral summer 2013/14. The data were retrieved on the basis of ground-based spectral surface albedo measurements collected by the COmpact RAdiation measurement System (CORAS) and airborne observations with the Spectral Modular Airborne Radiation measurement sysTem (SMART). The snow grain size and pollution amount (SGSP) algorithm, originally developed to analyze spaceborne reflectance measurements by the MODerate Resolution Imaging Spectroradiometer (MODIS), was modified in order to reduce the impact of the solar zenith angle on the retrieval results and to cover measurements in overcast conditions. Spectral ratios of surface albedo at 1280 and 1100 nm wavelength were used to reduce the retrieval uncertainty. The retrieval was applied to the ground-based and airborne observations and validated against optical in situ observations of SSA utilizing an IceCube device. The SSA retrieved from CORAS observations varied between 27 and 89 m2 kg−1. Snowfall events caused distinct relative maxima of the SSA which were followed by a gradual decrease in SSA due to snow metamorphism and wind-induced transport of freshly fallen ice crystals. The ability of the modified algorithm to include measurements in overcast conditions improved the data coverage, in particular at times when precipitation events occurred and the SSA changed quickly. SSA retrieved from measurements with CORAS and MODIS agree with the in situ observations within the ranges given by the measurement uncertainties. However, SSA retrieved from the airborne SMART data slightly underestimated the ground-based results.

On the Summertime Planetary Boundary Layer with Different Thermodynamic Stability in China: A Radiosonde Perspective

AbstractStrongly influenced by thermodynamic stability, the planetary boundary layer (PBL) is key to the exchange of heat, momentum, and moisture between the ground surface and free troposphere. The PBL with different thermodynamic stability across the whole of China, however, is not yet well understood. In this study, the occurrence frequency and spatial distribution of the convective boundary layer (CBL), neutral boundary layer (NBL), and stable boundary layer (SBL) were systematically investigated, based on intensive summertime soundings launched at 1400 Beijing time (BJT) throughout China’s radiosonde network (CRN) for the period 2012 to 2016. Overall, the occurrences of CBL, NBL, and SBL account for 70%, 26%, and 4%, respectively, suggesting that CBL dominates in summer throughout China. In terms of the spatial pattern of PBL height, a prominent north–south gradient can be found with higher PBL height in northwest China. In addition, the PBL heights of CBL and NBL were found to be positively (negatively) associated with near-surface air temperature (humidity), whereas no apparent relationship was found for SBL. Furthermore, clouds tend to reduce the occurrence frequency, irrespective of PBL type. Roughly 70% of SBL cases occur under overcast conditions, much higher than those for NBL and CBL, indicating that clouds govern to some extent the occurrence of SBL. In contrast, except for the discernible changes in PBL height under overcast conditions relative to those under clear-sky conditions, the changes in PBL height under partly cloudy conditions are no more than 170 m for both NBL and CBL types.

SOCIAL BEHAVIOUR DI PESANTREN SALAF

Salaf pesantren is an institution that is still strongly bound by old traditions. By inheriting and maintaincontinuity of Islamic tradition developed by scholars from time to time indefinite periodesasinya, has elementswhich include clerics, students, cottage / hostel, mosque, recitals of classical texts / yellow book, applyingthe method bandongan, sorogan, wetonan, principled firmly on Islamic education material. Pesantren Salafurgently need progress in science and technology. Pesantren Salaf there are many who “failed” to preserve itsown leadership weights, weak in preparing a “succession” Kyai guardians comparable with the foregoing, letalone exceeded. Answering the needs of the era, schools Salaf open formal educational institutions. Sensitivitypesantren Salaf against the phenomena that appear in the society is a social concern and reflexive owned evenare characteristic. So that the aspirations of the people making the boarding school received the developmentand advancement of science and technology. The real step taken is establishing formal educational institutionsfrom kindergarten through college. The character of a scholar is asceticism and totality. On the other hand,people still expect the efficacy of the function and role of pesantren Salaf like period once, without knowing thatit is actually what is expected it is experiencing overcast condition and shrinkage. Expenses are given the addedweight and more and more, but the ability to lift a boarding school in general was declining. Source overcastcondition and depreciation mainly boils down to the weight and quality of the “clerics who brought up”. Both interms of science, wisdom, exemplary, sincerity, and shelter to the people.Keywords : Social behavior, pesantren Salaf

CFD simulation of corn drying in a natural convection solar dryer

ABSTRACTA computational fluid dynamics model was developed to simulate the corn drying process in a solar cabinet dryer. Incident solar radiation was modeled using a dual-band spectrum to simulate the absorption of shortwave radiation by corn and account for the greenhouse effect caused by glazing materials. The performance of the dryer was simulated at fair and overcast weather conditions. The model allowed visualization of temperature, humidity, and air velocity profiles in the dryer. The model was validated with experimental results, which showed an overprediction of temperature (8.5%) and humidity (21.4%). The experimental humidity profile suggests that there was stagnation in the airflow of the dryer, which was accurately predicted by the model. The model was used to simulate the dryer’s performance under overcast conditions, and the predicted moisture removal was 32% less than the simulated fair-weather case.

From Near-Neutral to Strongly Stratified: Adequately Modelling the Clear-Sky Nocturnal Boundary Layer at Cabauw

The performance of an atmospheric single-column model (SCM) is studied systematically for stably-stratified conditions. To this end, 11 years (2005–2015) of daily SCM simulations were compared to observations from the Cabauw observatory, The Netherlands. Each individual clear-sky night was classified in terms of the ambient geostrophic wind speed with a 1hbox { m} hbox { s}^{-1} bin-width. Nights with overcast conditions were filtered out by selecting only those nights with an average net radiation of less than -,30hbox { W }hbox {m}^{-2}. A similar procedure was applied to the observational dataset. A comparison of observed and modelled ensemble-averaged profiles of wind speed and potential temperature and time series of turbulent fluxes showed that the model represents the dynamics of the nocturnal boundary layer (NBL) at Cabauw very well for a broad range of mechanical forcing conditions. No obvious difference in model performance was found between near-neutral and strongly-stratified conditions. Furthermore, observed NBL regime transitions are represented in a natural way. The reference model version performs much better than a model version that applies excessive vertical mixing as is done in several (global) operational models. Model sensitivity runs showed that for weak-wind conditions the inversion strength depends much more on details of the land-atmosphere coupling than on the turbulent mixing. The presented results indicate that in principle the physical parametrizations of large-scale atmospheric models are sufficiently equipped for modelling stably-stratified conditions for a wide range of forcing conditions.

Measurement of the environmental temperature using the sol-air thermometer

Heat flow measurement with a heat flow meter is a standardized method (ISO 9869-1) to estimate thermal transmittance (U-value) of a building element. The heat flow meter is a thin plate mounted on top of the surface of the element, and measures the heat flux q through the plate. The measured q is the product of the difference in temperatures between exterior and interior environment, and the U-value. The heat transferred from the element is based on the radiant and the convective heat transfer.ISO 9869-1 specifies that the environment temperature Te “is a notional temperature" and it "cannot be measured directly” (section A.3.1). The air temperature Ta is proposed as a reasonable approximation for the indoor environment, while overcast conditions and absence of significant solar radiation are specified conditions for replacing Te with Ta for the exterior environment.The sol-air thermometer (SAT) measures the sol-air temperature Tsa, i.e. the equivalent temperature of the convective and the radiative environment. In the absence of solar radiation, Te = Tsa. SAT is a sensor consisting of a thin flat solid plate, of high thermal conductivity. The front side of the sensor is exposed to the environment, whose Tsa is to be measured, and the backside is thermally insulated. The temperature of the SAT-plate equals Tsa.In this work we propose introduction of the measured Te in the existing standard (ISO 9869-1). The method for measurement of Tsa, using the SAT, has been demonstrated experimentally for different periods, without solar radiation present and under stable climatic conditions.

Atmospheric components of the surface energy budget over young sea ice: Results from the N‐ICE2015 campaign

AbstractThe Norwegian young sea ice campaign obtained the first measurements of the surface energy budget over young, thin Arctic sea ice through the seasonal transition from winter to summer. This campaign was the first of its kind in the North Atlantic sector of the Arctic. This study describes the atmospheric and surface conditions and the radiative and turbulent heat fluxes over young, thin sea ice. The shortwave albedo of the snow surface ranged from about 0.85 in winter to 0.72–0.80 in early summer. The near‐surface atmosphere was typically stable in winter, unstable in spring, and near neutral in summer once the surface skin temperature reached 0°C. The daily average radiative and turbulent heat fluxes typically sum to negative values (−40 to 0 W m−2) in winter but then transition toward positive values of up to nearly +60 W m−2 as solar radiation contributes significantly to the surface energy budget. The sensible heat flux typically ranges from +20–30 W m−2 in winter (into the surface) to negative values between 0 and −20 W m−2 in spring and summer. A winter case study highlights the significant effect of synoptic storms and demonstrates the complex interplay of wind, clouds, and heat and moisture advection on the surface energy components over sea ice in winter. A spring case study contrasts a rare period of 24 h of clear‐sky conditions with typical overcast conditions and highlights the impact of clouds on the surface radiation and energy budgets over young, thin sea ice.

Accuracy analysis of computational algorithms for prediction of daylight illuminance in space with shading devices

This study examines the accuracy of annual daylight simulation method (ADSM) in predicting illuminance for spaces with shading device conditions. ADSM algorithms were developed separately for the sun and sky to predict their effect on indoor daylight illuminance. Sun-matching and daylight coefficient methods were developed for the sun, while sky-matching and daylight coefficient methods with one and four sky patches were developed for the sky. The daylight illuminance computed from ADSM under various daylight conditions was compared with those calculated from Radiance and field measurements.Results imply strong linear correlations existed between the predicted daylight illuminance levels by the ADSM and Radiance under diverse sky conditions based on weather data. The predicted illuminance from ADSM was lower than field measurements for all sky conditions. ADSM computations mostly agree with field measurements. For clear and partly cloudy sky conditions, the daylight coefficient approach for sky of ADSM generated a stronger correlation to measured data, but the sky-matching algorithm showed a stronger correlation to field data. The daylight coefficient approach for sky, combined with ADSM computation algorithms for sun, effectively reduced the difference between the predicted and measured illuminance under clear or partly cloudy sky conditions. Under overcast conditions, there was no significant reduction in difference between simulated and measured illuminance.

Aesthetic perception of a small office with different daylighting systems

The quality of a lit environment cannot be deduced solely from the quantity of light. Therefore, daylighting studies should not only be focused on the usage of lighting metrics but should also consider the aesthetic experience. This paper examines the influence of daylighting systems on the aesthetic perception of a small office. One single side lit office was equipped with four different daylighting systems (white blinds, high-reflecting blinds, hybrid light shelf and mirror light shelf) under two sky conditions (clear sky and overcast sky). In total, eight stimuli were captured and presented via stereoscopic images. Fifty participants evaluated the images using the semantic differential scale to rate nine architectural quality attributes. The results from MANOVA indicated that both the daylighting systems and the type of sky had an effect on the aesthetic attributes, and that the significant interaction effect suggested that the aesthetic perception of a daylighting system depends on the type of sky. Subsequent statistical findings showed that the high-reflecting blinds comprised the daylighting system that scored highest in nearly all attributes under both clear and overcast sky conditions.

Nocturnal Hunting by Eleonora's FalconsFalco eleonoraeon Their Breeding and Non-Breeding Grounds

We report on nocturnal hunting by Eleonora's Falcons in their breeding range in the Mediterranean region and in their non-breeding range in Madagascar. Hunting activity of Eleonora's Falcons near floodlights during the breeding season in western Morocco peaked 30–60 min after sunset, but continued into the early morning. Hunting activity and prey capture rates near floodlights were highest during nights with little moonlight or overcast conditions. Fifty-one percent of 73 group capture attempts were successful. Of the migratory prey species identified at the Moroccan study site (26 species), 73% belonged to species mainly migrating at night, whereas 57% of all migratory bird prey species of Eleonora's Falcon reported to date (122 species) migrate predominantly at night; suggesting that hunting near artificial light may increase the proportion of nocturnal migrant species in the diet of falcons. Sylvia and Acrocephalus were the most commonly recorded genera among prey caught after dark. Our direct observations and analysis of satellite transmitter data indicated that Eleonora's Falcons also hunted away from artificial light in Morocco, Italy, and frequently so in Madagascar. Flight activity was detected in 18% of 342 night-time locations of seven satellitetagged Eleonora's Falcons in Madagascar, at an average moon illumination of 60%. We conclude that nocturnal hunting by Eleonora's Falcons is more common than previously assumed and occurs preferably, but not exclusively, at above-average moon illumination on wintering grounds or near artificial lights during the breeding period.

Light distribution in air-supported pneumatic structures: Comparison of experimental and computer calculated daylight factors

This study examines daylight distribution and illumination levels with daylight factors (DFs) inside an air-supported (pneumatic) structure (50.0 m long, 25.0 m wide and 11.0 m high). The DFs values were determined experimentally for 45 different points on the playground floor. Two sets (set1 and set2) of time dependent illuminance measurements were performed at overcast but variable (5400-2400 lx) sky conditions similar to the CIE overcast sky. Results revealed that the DFs values were in the range from 1.1% to 1.7% and were not evenly distributed across the interior, with the greatest values at the centre and the smallest at the edges of the playground floor. Composition of the membrane was studied with SEM micrographs and by measuring the Raman and Infrared spectra. Hemispherical (total) transmittance (TT(λ)) and reflectance (TR(λ)) spectra (300–2500 nm) of the membrane used were measured and averaged over the visible spectral region giving TTvis and TRvis and haze, the latter confirming that fabrics strongly scattered the incoming solar radiation. The surface of the fabrics was characterized by contact angle measurements which indicates that the fabrics were weakly hydrophobic and strongly soiled. Daylight factor values obtained from the DAYSIM computer simulations matched well the experimental ones allowing further studies of daylighting inside the translucent dome. It was found that various colour patterns substantially affected the DFs values. DAYSIM calculations also demonstrated the possible use of aerogel insulation blanket as composite fabrics for air-supported structures.

An automated method for the evaluation of the pointing accuracy of Sun-tracking devices

Abstract. The accuracy of solar radiation measurements, for direct (DIR) and diffuse (DIF) radiation, depends significantly on the precision of the operational Sun-tracking device. Thus, rigid targets for instrument performance and operation have been specified for international monitoring networks, e.g., the Baseline Surface Radiation Network (BSRN) operating under the auspices of the World Climate Research Program (WCRP). Sun-tracking devices that fulfill these accuracy requirements are available from various instrument manufacturers; however, none of the commercially available systems comprise an automatic accuracy control system allowing platform operators to independently validate the pointing accuracy of Sun-tracking sensors during operation. Here we present KSO-STREAMS (KSO-SunTRackEr Accuracy Monitoring System), a fully automated, system-independent, and cost-effective system for evaluating the pointing accuracy of Sun-tracking devices. We detail the monitoring system setup, its design and specifications, and the results from its application to the Sun-tracking system operated at the Kanzelhöhe Observatory (KSO) Austrian radiation monitoring network (ARAD) site. The results from an evaluation campaign from March to June 2015 show that the tracking accuracy of the device operated at KSO lies within BSRN specifications (i.e., 0.1° tracking accuracy) for the vast majority of observations (99.8 %). The evaluation of manufacturer-specified active-tracking accuracies (0.02°), during periods with direct solar radiation exceeding 300 W m−2, shows that these are satisfied in 72.9 % of observations. Tracking accuracies are highest during clear-sky conditions and on days where prevailing clear-sky conditions are interrupted by frontal movement; in these cases, we obtain the complete fulfillment of BSRN requirements and 76.4 % of observations within manufacturer-specified active-tracking accuracies. Limitations to tracking surveillance arise during overcast conditions and periods of partial solar-limb coverage by clouds. On days with variable cloud cover, 78.1 % (99.9 %) of observations meet active-tracking (BSRN) accuracy requirements while for days with prevailing overcast conditions these numbers reduce to 64.3 % (99.5 %).

Cue-conflict experiments between magnetic and visual cues in dunlin Calidris alpina and curlew sandpiper Calidris ferruginea

Despite our extensive knowledge on various aspects of their lives, there has been limited investigation into the hierarchical relationships among different compass systems in shorebirds. The aim of this study was to investigate the relationship between magnetic and celestial compasses in two species of shorebirds, the curlew sandpiper (Calidris ferruginea; pre-breeding migration) and the dunlin (Calidris alpina; post-breeding migration) using cue-conflict experiments. Birds were captured in a Mediterranean stopover site, after which their magnetic orientation was determined under simulated overcast conditions at sunset using modified Emlen funnels fitted with infrared video cameras. Birds that demonstrated a well-defined directional preference were then exposed over two sunsets to conflicting directional information between the local geomagnetic field and the ±90° shifted band of maximum polarisation. These individuals were tested again for magnetic orientation at sunset in the same conditions as previous test, to determine whether their directional choices had changed after the cue-conflict. Our results showed that individuals from both species did not recalibrate their magnetic compass from visual cues after the cue-conflict, even though at least dunlins did not appear to completely disregard the information derived from celestial cues. This study is one of the few experimental studies on the migratory orientation of Charadriiformes and on the hierarchical relationships between the different compasses used by these birds during their extensive migratory movements. Migrating birds are able to use different compass mechanisms based on geomagnetic or celestial cues, and it seems reasonable to hypothesise that birds calibrate their various compasses to maintain the correct direction especially when the directional information does not agree. The hierarchy among different compasses has been studied largely on night migrating passerines, but it is still poorly understood. We investigated the hierarchy among geomagnetic and celestial cues (band of maximum polarisation) in two species of Charadriiformes by means of cue-conflict experiments. Our result showed that the geomagnetic cues have a dominant role in the orientation mechanisms of the studied species, even though the information derived from celestial cues did not appear to be completely disregarded.