Cloudy Periods Research Articles

Novel feedwater preheating system for parabolic trough solar power plant

Several improvements in the referenced parabolic trough power plant (PTPP) model (Andasol II) have been implemented by APROS software. The main advantage of this novel feedwater preheating system is to improve electricity production and evening operation hours at a low cost for modifying an existing PTPP in Spain. This in turn leads to less dependence on fossil fuels during the evening and cloudy periods. The reference model is improved by an increase in the collector loops in the reference solar field (SF) model to 208 loops rather than 156 loops to increase the absorbed thermal power, increasing the capacity of the thermal storage system (TSS) from 1017 MWth h to 1,360 MWth h to improve the night operating period, replacing the Feedwater/Steam circuit with a Feedwater/HTF circuit using five shell and tube counter-current heat exchangers that are used instead of condenser heat exchangers. The HTF enters the fifth low-pressure preheater with 128 kg/s and 393 °C in the daytime and 377 °C at night. The main advantage of this improvement is to use the entire generated steam within the steam turbine and not to heat the feedwater in the feedwater circuit. In addition, the control circuits are modified and added new controllers with new limitations for controlling the new operating conditions. This development is the first attempt in the field of research to use a Feedwater/HTF circuit in the PTPP in order to enhance performance. This improvement (optimization 1) in turn leads to a raise in the total HTF mass flow and the quantity of stored molten salt, as well as increasing the mass flow of HTF to the power block (PB). According to optimization 1, no steam will be taken out of the HP and LP-turbine toward the feedwater circuit. Therefore, the feedwater fed through new preheaters is heated by thermal oil instead of steam extractions. The predictions obtained from this optimization are compared to the simulated results that were previously extracted from the reference model on clear and cloudy days for evaluation of the behaviour of the optimized PTPP. It should be mentioned here that it can be used the same turbine and generator in the actual power plant to achieve this increase in the performance of the power plant, according to the technical data of manufacturers. Finally, an economic study of the cost was conducted for referenced and the optimized PTPP depending on the levelized energy cost (LEC). The findings reveal that the levelized energy cost of the optimized PTPP with storage energy is about 10.5% lower than that of the referenced PTPP.

Estimation of high-resolution solar irradiance data using optimized semi-empirical satellite method and GOES-16 imagery

Semi-empirical satellite method is widely used in estimating global horizontal irradiance (GHI), where various clear-sky models, cloud index (CI) and clear-sky index (CSI) derivation methods are available. This study aims to optimize the semi-empirical satellite model for 5-minute GHI estimation from four aspects: satellite-bands, CI and CSI derivation methods, and clear-sky models. The results show that it achieves better GHI estimates using the blue band, CI derived from monthly fixed upper and lower bounds, and a piecewise CI-to-CSI function. There is no significant difference in all-sky GHI estimation for the clear-sky models regarding normalized root mean squared error (nRMSE, 25.19%–25.53%), which is comparable with the referenced physical model. Clouds cause the largest uncertainty, where the nRMSE is in the range of 37.60%–38.36% in cloudy days and 31.12%–31.54% in cloudy periods. In the application of semi-empirical method with different clear-sky models, Ineichen–Perez has the highest bias of -4.62% in clear days and -3.93% in cloudless periods. REST2 outperforms McClear with slightly lower nRMSE and normalized mean bias error (nMBE) under all sky conditions. McClear is recommended due to its global availability. Modified Ineichen–Perez produces the lowest nRMSE and nMBE using clear-sky GHI as the GHI estimates for clear periods, therefore has the potential for improvements in physical methods.

Sky Radiation Decreases Thermal Mass Requirements to Achieve 100% Ambient Cooling in Hot US Climates

Abstract Ambient House is a building that maintains indoor temperature within a comfortable range by controlling gains and losses from ambient sources and utilizing thermal mass to moderate temperature changes when the sources are unavailable. Previously, necessary building characteristics were determined for passive solar as the heating source and ventilation as the cooling source in 11 US climate zones. (Sharp, M.K., 2012, “Indoor Comfort Achieved Exclusively from Ambient Sources Across US Climates,” ASME J. Sol. Energy Eng. 143, (6), p. 061005.) It was noted that in hot climates, such as Phoenix, AZ, there are long periods during which outdoor temperature is too warm for cooling, necessitating large thermal mass to avoid indoor overheating. In this article, thermal mass requirements are compared between sky radiation and nighttime ventilation cooling in all 16 US climate zones, including marine subzones 3C and 4C and very cold and subarctic zones 7 and 8. It is shown that sky radiation provides shorter intervals of cooling unavailability and allows much smaller thermal mass to achieve year-round indoor comfort in the hot climates of Las Vegas, Miami, New Orleans, and Phoenix, while it provides no significant benefits in cool climates, where thermal mass is dictated more by the need to slow the decrease in indoor temperature during cloudy periods in the winter. In Fairbanks, AK (zone 8), in particular, the lack of significant solar gains for almost 3 months during the winter requires large thermal mass to maintain indoor comfort. Minimal thermal mass is needed to meet the small summer cooling demand, and both sky and ventilation cooling are sufficient.

Investigating the Impacts of Daytime Boundary Layer Clouds on Surface Energy Fluxes and Boundary Layer Structure During CHEESEHEAD19

AbstractStudies of land‐atmosphere interactions under a clear sky and low cumulus cloud conditions are common from long‐term observatories like at the southern great plains. How well the relationships and responses of surface radiative and turbulent heat fluxes determined from these investigations hold for more heterogeneous surfaces in other climate regimes, however, is uncertain. In this study, detailed observations of the surface energy budget and daytime boundary layer properties are analyzed using measurements from the Chequamegon Heterogenous Ecosystem Energy‐Balance Study Enabled by a High‐Density Extensive Array of Detectors 2019 (CHEESEHEAD19) field campaign, July‐October 2019, across a heterogeneous forested landscape of northern Wisconsin. A cloud regime framework is employed to classify consecutive periods of clear skies from lower atmosphere stratiform and cumulus clouds. A seasonal transition from low cumulus to low stratiform periods occurred, together with a diurnal pattern in cloudy or clear sky period dominance. Radiative forcing was highly dependent on sky conditions, leading to changes in the redistribution efficiency of radiative energy by the surface turbulent heat fluxes. During CHEESEHEAD19, small Bowen ratios dominated with daytime latent heat fluxes three times as large as sensible heat fluxes for all sky conditions studied; the forested region, therefore, falls within an energy‐limited regime. The depth of the daytime mixed layer depended upon the sky condition and thermodynamic setting; deeper mixed layers occurred during periods of low cumulus and not clear skies. Profiles of vertical velocity were found to have enhanced variance under low cumulus compared to clear sky periods, suggesting potential for cloud feedbacks on boundary layer structure and surface energy fluxes.

Improving Representation of Tropical Wetland Methane Emissions With CYGNSS Inundation Maps

AbstractWetlands are the single largest source of methane to the atmosphere and their emissions are expected to respond to a changing climate. Inaccuracy and uncertainty in inundation extent drives differences in modeled wetland emissions and impacts representation of wetland emissions on inter‐annual and seasonal time frames. Existing wetland maps are based on optical or NIR satellite data obscured by clouds and vegetation, often leading to underestimates in wetlands extent, especially in the Tropics. Here, we present new inundation maps based on the Cyclone Global Navigation Satellite System (CYGNSS) satellite constellation, operating in L‐band that is not impacted by clouds or vegetation, providing reliable observations through canopy and cloudy periods. We map the temporal and spatial dynamics of the Pantanal and Sudd wetlands, two of the largest wetlands in the world, using CYGNSS data and a computer vision algorithm. We link these inundation maps to methane fluxes via WetCHARTs, a global wetland methane emissions model ensemble. We contrast CYGNSS‐modeled methane emissions with WetCHARTs standard runs that use monthly rainfall data from ECMWF re‐analysis (ERA5), as well as the commonly used SWAMPS wetland maps. We find that the CYGNSS‐based inundation maps modify the methane emissions in multiple ways. The seasonality of inundation and methane emissions is shifted by two months because of the lag in wetland recharge following peak rainfall. Both inundation and methane emissions also respond non‐linearly to wet‐season precipitation totals, leading to large interannual variability in emissions. Finally, the annual magnitude of emissions is found to be greater than previously estimated.

A Comparison Study on the Improved Operation Strategy for a Parabolic trough Solar Power Plant in Spain

The present work focuses on the development of a detailed dynamic model of an existing parabolic trough solar power plant (PTSPP) in Spain. This work is the first attempt to analyse the dynamic interaction of all parts, including solar field (SF), thermal storage system (TSS) and power block (PB), and describes the heat transfer fluid (HTF) and steam/water paths in detail. Advanced control circuits, including drum level, economiser water bypass, attemperator and steam bypass controllers, are also included. The parabolic trough power plant is modelled using Advanced Process Simulation Software (APROS). An accurate description of control structures and operation strategy is necessary in order to achieve a reasonable dynamic response. This model would help to identify the best operation strategy due to DNI (direct normal irradiation) variations during the daytime. The operation strategy used in this model has also been shown to be effective compared to decisions made by operators on cloudy periods by improving power plant performance and increasing operating hours.

Simulation of the Effect of the Angle of Incidence on the Solar Panel for Three Solar Photovoltaic (pv) Systems in the Bogotá Savanna– Subparamo

The planet's pollution from greenhouse gases resulting from fossil energy sources' inefficient use, has made the scientific community reflect and has led it to search for new alternative and renewable energy sources to supply increasing energy needs. One of the sources that have had significant growth worldwide in the last decade is photovoltaic solar energy. However, one of the drawbacks with the use of photovoltaic technology is conversion efficiency. For this, alternatives are sought, such as the study of new materials with structures that facilitate the quantum photodetection of a given compound; minimize losses due to transient and harmonic in the regulation devices and inverters, and losses in the storage and transport of the energy generated. One proposal to increase the collection efficiency is solar tracking systems, making the panels remain approximately perpendicular to the direct light beam (direct radiation), from dawn to sunset. In this research work, three photovoltaic solar energy capture systems are compared using a simulation. The first case corresponds a fixed system in a horizontal position; the second case is a system fixed with an inclination of 10 degrees south latitude, and the third, a two-axis tracker. The information collected is taken in the municipality of Sibate, the coordinates 4 ° 30 ' 09.5 N, 74 ° 15 ' 20.2 W in the Savanna of Bogota. The purpose of this study is to use clean electrical energy to obtain bio-hydrogen using electrolysis. In the simulation, two periods, cloudy and clear, are determined based on a threshold taken experimentally in the panels and thus comparing the three systems. The results show that in clear periods the solar tracker has an energy conversion gain of 5 %. On the other hand, in cloudy periods, the three systems' panels showed similarity in the energy measurements captured.

A simplified modelling framework for real-time assessment of conservative pollutants in ungauged rivers during cloudy periods

Recently, the optical remote sensing technique is effectively applied to monitor real-time water quality parameters at finer spatiotemporal scales that are mostly based on the surface reflectance of satellite images. However, during the rainy season due to cloudy or hazy satellite images, it is a great challenge to obtain the surface reflectances and to estimate the pollutant concentration. This study is specially focused on developing a novel approach to estimate the daily-scale pollutant concentrations in ungauged rivers during cloudy days. The developed approach integrates the simplified physically-based VPMM-AD(ΨDc) solute transport model with the remote sensing (RS)-based approach for assessing the non-reactive river pollutants in real-time. This integrated VPMM-AD(ΨDc)-RS approach is tested for simulating the in-situ heavy metal (Fe, Zn, Cu, Cr, Pb, and Cd) and total suspended solid (TSS) concentrations in the Brahmani River during the tropical monsoon (rainy) seasons of the typical years 2010–2013. The study results reveal that the proposed integrated approach performed reasonably well with acceptable accuracy for real-time estimation of pollutant concentrations in the considered ungauged river reach during the cloudy period.

Experimental Study of a Stationary Hot Air Solar Collector Built with Hemispherical Concentrators and Enhanced with Fresnel Lenses

Aims: This paper is about a solar collector made of hemispherical concentrators. This collector is sun tracking free, and used for natural convection. Lenses are used on top of each concentrator to improve its efficiency.
 Study Design: The Solar collector is made of hemispherical concentrators with Fresnel lenses axially-centered to those concentrators and placed on top of each one of them. Those concentrators are covered with a 4 mm glass for a greenhouse effect. The concentrators generate hot spots that heat the inside air. There is no need for receivers at the hot spots.
 Methodology: from Inlet to outlet, temperatures are measured as well as inlet air speed, which allow the efficiency evaluation.
 Results: Although those measurements were conducted in a cloudy period, temperature difference from the inlet to the outlet was around 55°C to 65°C. This result is superior to previous studies of the same system without lenses which gave temperature difference around 35°C to 45°C. It is sharply superior to that of usual black-painted convective system with fined plates that gives a temperature difference of around 20°C to 25°C.
 Conclusion and Perspectives: The global efficiency calculated using measurement values reaches 56%. This is far greater compared to previous black plate systems’ efficiency of 29%, giving an efficiency increase of 93%, but expected knowing that we are using a concentration system for convection.

Prototype of phosphate sludge rotary dryer coupled to a parabolic trough collector solar loop: Integration and experimental analysis

This study presents an experimental analysis of a phosphate sludge drying prototype that uses a Parabolic Trough Collector (PTC) solar technology as an energy source. The interconnection of the system's three components, namely the PTC loop, oil-air heat exchanger (HX), and rotary dryer, is described. Experiments were carried out for each sub-system separately considering three air and Heat Transfer Fluid (HTF) flowrates, a drying temperature up to 135 °C, and a phosphate sludge flowrate of 5 kg/h. Results show that to dry sludge at the target humidity (≤7%), air at 105 °C is sufficient when operating at the average flowrate, this temperature increases to 123 °C at lower flowrate. The overall system was tested under the Moroccan semi-arid conditions for three typical days. All temperature profiles follow similar trends as Direct Normal Irradiation (DNI) variation; this latter influence the final sludge moisture value. During the sunny day, 32 kg of sludge was dried at the target moisture content throughout the day, with a relatively stable temperature (130 ± 4 °C). This amount decreased to 7 kg on the cloudy day while the target moisture content was achieved only 25% of the drying time. A back-up energy source or a thermal storage system is recommended to improve the drying performance during cloudy periods.

Differences in fine particle chemical composition on clear and cloudy days.

Clouds are prevalent and alter PM2.5 mass and chemical composition. Cloud-affected satellite retrievals are often removed from data products, hindering estimates of tropospheric chemical composition during cloudy times. We examine surface fine particulate matter (PM2.5) chemical constituent concentrations in the Interagency Monitoring of PROtected Visual Environments network during Cloudy and Clear Sky times defined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags from 2010-2014 with a focus on differences in particle hygroscopicity and aerosol liquid water (ALW). Cloudy and Clear Sky periods exhibit significant differences in PM2.5 and chemical composition that vary regionally and seasonally. In the eastern US, relative humidity alone cannot explain differences in ALW, suggesting emissions and in situ chemistry exert determining impacts. An implicit clear sky bias may hinder efforts to quantitatively to understand and improve model representation of aerosol-cloud interactions.

Monitoring spring phenology in Mediterranean beech populations through in situ observation and Synthetic Aperture Radar methods

The interest in tree phenology monitoring is increasing because this trait is a robust indicator of the impacts of climate change on natural and managed ecosystems. Different approaches to monitor phenology at different spatial scales, from in situ monitoring to remote sensing, are used to investigate spring and/or autumn phenological changes. In Mediterranean area, most of phenological changes occur during cloudy periods (spring and autumn), leading to a loss of information also for very high temporal resolution satellites. Instead, cloud-uninfluenced sensors, such as radar sensors, can allow to bypass this problem and produce a temporally continuous coverage.In this paper, we analyzed the spring phenology of two European beech (Fagus sylvatica L.) populations, located at different latitudes in Mediterranean area. Weekly in situ monitoring of leaf-out has been correlated with data collected by Synthetic Aperture Radar. Spring phenological phases were monitored in situ following a modified BBCH-code with a 5-scores scale (from 1 - buds closed and covered by scales, to 5 - leaf completely unfolded). The score 3 (young leaves starting to emerge from the bud) was considered the bud break. Different site conditions based on aspect (northern and southern) and altitudinal gradient (high and low altitude) have been considered. The aim was to test and implement a new methodology able to decrease the frequency of the field sampling, using remote data, to extend more detailed information on geographical scale, and to reconstruct past phenology.Results showed a statistically significant different length of the vegetative spring period, spanning from dormant buds, up to leaves completely unfolded, between sites. Through Synthetic Aperture Radar estimation, this study demonstrates that leaf-out can be monitored with an extreme accuracy. The phenophase score 4 and 5 estimation showed the best performance (RMSE < of 4 days), phenophases score 2 and 3 showed promising performances (4 days < RMSE <5 days), while phenophases score 1 seems to be not easily detectable, although it can be extrapolated with an RMSE <6 days.This radar approach fixes the cloud problem typical of multispectral approach and very frequent in phenophase change periods in Mediterranean climate. This study promotes the proposed remote sensing approach as a very useful tool to monitor growing season starting in remote areas, helping to reduce in situ observations and allowing past phenology reconstruction.

Evaluation of the thermal performance of a box type solar cooker in the rainy season in a sub-Saharan country

This article deals with the calculation of the performance parameters of a box-type solar cooker in a sub-Saharan country in the rainy season where the solar activity is considerably attenuated because of the numerous cloudy periods. The various tests were carried out at the Labora-tory of Fundamental and Applied Physical Sciences of the École Normale Supérieure (ENS) in Abidjan. During these cooking tests, the illuminance was measured. The temperatures of the ambient air and of various places of the solar cooker were also recorded. Illumination measurements made with an EPPLEY type pyranometer made it possible to determine the average illuminance which oscillates between 636.98 w / m² and 517.91w / m² depending on the day. The average energy efficiency of the cooker has been calculated for the cooking of several foods. It varies between 20.48 ٪ to 26.59%. The overall loss coefficient is between 5.63 and 5.82. The results obtained for these tests are satisfactory and very encouraging, especially since they were carried out in the rainy season.

Swarm Decomposition Technique Based Hybrid Model for Very Short-Term Solar PV Power Generation Forecast

Accurate predictions of solar photovoltaic (PV) power generation at different time horizons are essential for reliable operation of energy management systems. The output power of a PV power plant is dependent on non-linear and intermittent environmental factors, such as solar irradiance, wind speed, relative humidity, etc. Intermittency and randomness of solar PV power effect precision of estimation. To address the challenge, this paper presents a Swarm Decomposition Technique (SWD) based hybrid model as a novel approach for very short-term (15 min) solar PV power generation forecast. The original contribution of the study is to investigate use of SWD for solar data forecast. The solar PV power generation data with hourly resolution obtained from the field (grid connected, 857.08 kWp Akgul Solar PV Power Plant in Turkey) are used to develop and validate the forecast model. Specifically, the analysis showed that the hybrid model with SWD technique provides highly accurate predictions in cloudy periods.

Dynamic simulation and control of solar biomass gasification for hydrogen-rich syngas production during allothermal and hybrid solar/autothermal operation

Solar biomass steam gasification using concentrated sunlight offers an efficient means of storing intermittent solar energy into renewable solar fuels while upgrading the carbonaceous feedstock. Such solar-driven (allothermal) processes have demonstrated the ability and the effectiveness for the production of high quality hydrogen-rich syngas, but they suffer from inherent barriers related to the variability of solar energy caused by cloud passages and shut off at night. The concept of hybrid solar/autothermal gasification appears promising to meet the requirement for stable and continuous operation under fluctuating or intermittent solar irradiation. To date, dynamic modelling to simulate coupled solar/combustion heating and steam gasification using real solar irradiation data has never been proposed and could be used to predict the annual performance of large-scale solar gasification plants. In this study, a dynamic mathematical model of a scaled-up solar gasification reactor was developed. The model was composed of a system of differential equations that were derived from unsteady mass and energy conservation equations. After an experimental validation step with the results from a lab-scale solar reactor, the dynamic model was applied at large scale to determine the reactor temperature and syngas production evolution during continuous day and night operation in both solar-only (allothermal) and hybrid solar/autothermal modes. Different reactants feeding management strategies were proposed and compared with the aim of achieving enhanced syngas productivity and optimized use of solar energy during solar-aided steam gasification. It was shown that the hybrid mode with partial oxy-combustion of the feedstock and dynamic feeding control results in the most stable process operation upon fluctuating solar power input, while ensuring continuous production of H2 and CO at night and during cloudy periods.

Effect of Clouds on the Diurnal Evolution of the Atmospheric Boundary-Layer Height Over a Tropical Coastal Station

The growth of the daytime convective atmospheric boundary layer, which plays a pivotal role in the vertical mixing and dispersal of water vapour and pollutants, is modulated by cloud radiative effects. Assessment of this cloud effect is sparse in any geographical region and non-existent over tropical coastal regions. We investigate the effect of clouds on the diurnal evolution of boundary-layer height over tropical coastal location Thumba (8.5°N, 77°E) during onshore and offshore flow using multi-year (2010–2016) microwave radiometer profiler observations. The boundary-layer height during both cloudy and clear-sky periods increases rapidly from 0800 LT (local time = UTC + 5.1 h) to attain a daytime peak around noon (400–1500 m). The seasonal mean noontime boundary layer height during cloudy periods is lower than that during clear-sky periods by > 900 m (> 400 m) when offshore (onshore) flow prevails during winter and pre-monsoon seasons. The forenoon growth rate of the boundary-layer height during clear-sky offshore flow is rapid (> 380 m h−1) compared to that during cloudy offshore (160 m h−1), clear-sky onshore (160–250 m h−1), and cloudy onshore (> 100 m h−1) flow. Effects of shortwave cloud radiative forcing and soil temperature on the noontime boundary-layer height and their interdependencies are presented. These observations reveal the contrasting and significant effect of clouds on the growth of the daytime boundary layer during onshore and offshore flow, and the coupled effects of cloud radiative forcing and soil temperature on boundary-layer height over tropical coastal regions, which provide essential constraints for evaluating model simulations.

Theoretical and Experimental Studies of a Box-Type Solar Cooker in Unfavorable Climatic Conditions

A box-type solar cooker with an inclined surface, equipped with a concentration reflector to allow maximum energy to be collected, enabled cooking tests to be carried out in the rainy season. Different thermocouples were implanted on various places of the cooker. The temperature measurements from these sensors were taken every 10 minutes. The tests presented in this article relate to the preparation of eggs and rice. The absorber temperatures during the tests exceeded 100°C. The cooking times were between 1 h 50 min and 2 h 20 min despite the numerous cloudy periods. The cooker made it possible to reach sufficient temperatures for healthy cooking of food. The results obtained for these first tests are satisfactory and very encouraging.

The influence of aspect ratios and wall heating conditions on flow and passive pollutant exposure in 2D typical street canyons

Deep street canyons and unfavourable meteorological conditions usually induce high pollutant exposure. Validated by experimental data, this paper employs computational fluid dynamic simulations with RNG k-ε model to investigate the flow, and passive pollutant dispersion within scale-model two-dimensional street canyons(H = 3 m). As a novelty, this paper quantifies the impacts of various wall heating scenarios(bottom, leeward/windward wall and all-wall heating), ambient velocity(Uref = 0.5–2 m s−1, Froude numbers Fr = 0.25–4.08, Reynolds numbers Re = 95602–382409) and aspect ratios(building height/street width, AR = 0.5, 0.67, 1, 2, 3) on personal intake fraction for entire streets(<P_IF>). The governing equations are implicitly discretized by a finite volume method (FVM) and the second-order upwind scheme with Boussinesq model for quantifying buoyancy effects. The SIMPLE scheme is adopted for the pressure and velocity coupling.In most isothermal cases, one-main-vortex structure exists as AR = 0.5–3(<P_IF> = 0.43–3.96 ppm and 1.66–27.51 ppm with Uref = 2 and 0.5 m s−1). For non-isothermal cases with Fr = 4.08(Uref = 2 m s−1), wind-driven force dominates urban airflow as AR = 0.5–1 and four heating conditions attain similar <P_IF>(0.39–0.43 ppm, 0.57–0.60 ppm, 0.91–0.98 ppm). As AR = 2, windward and all-wall heating get two-vortex structures with greater <P_IF>(3.18–3.33 ppm) than others(<P_IF> = 2.13–2.21 ppm). As AR = 3, leeward-wall heating slightly reduces <P_IF>(~3.72–3.96 ppm), but the other three produce two-vortex structures with greater <P_IF>(6.13–10.32 ppm). As Fr = 0.25(Uref = 0.5 m s−1), leeward-wall heating always attains smaller <P_IF>(1.20–7.10 ppm) than isothermal cases(1.66–27.51 ppm) as AR = 0.5–3, however the influence of the other three is complicated which sometimes raises or reduces <P_IF>. Overall, smaller background wind speed (Fr = 0.25) with two-vortex structures attains much larger <P_IF>. Special attention is required at night(all-wall heating), noon(bottom-heating) and cloudy period(no-wall heating) as AR = 2–3, while it is during windward-wall heating and cloudy period for AR = 0.5–1.

Fusing optical and SAR time series for LAI gap filling with multioutput Gaussian processes

The availability of satellite optical information is often hampered by the natural presence of clouds, which can be problematic for many applications. Persistent clouds over agricultural fields can mask key stages of crop growth, leading to unreliable yield predictions. Synthetic Aperture Radar (SAR) provides all-weather imagery which can potentially overcome this limitation, but given its high and distinct sensitivity to different surface properties, the fusion of SAR and optical data still remains an open challenge. In this work, we propose the use of Multi-Output Gaussian Process (MOGP) regression, a machine learning technique that learns automatically the statistical relationships among multisensor time series, to detect vegetated areas over which the synergy between SAR-optical imageries is profitable. For this purpose, we use the Sentinel-1 Radar Vegetation Index (RVI) and Sentinel-2 Leaf Area Index (LAI) time series over a study area in north west of the Iberian peninsula. Through a physical interpretation of MOGP trained models, we show its ability to provide estimations of LAI even over cloudy periods using the information shared with RVI, which guarantees the solution keeps always tied to real measurements. Results demonstrate the advantage of MOGP especially for long data gaps, where optical-based methods notoriously fail. The leave-one-image-out assessment technique applied to the whole vegetation cover shows MOGP predictions improve standard GP estimations over short-time gaps (R2 of 74% vs 68%, RMSE of 0.4 vs 0.44 [m2m−2]) and especially over long-time gaps (R2 of 33% vs 12%, RMSE of 0.5 vs 1.09 [m2m−2]). A second assessment is focused on crop-specific regions, clustering pixels fulfilling specific model conditions where the synergy is profitable. Results reveal the MOGP performance is crop type and crop stage dependent. For long time gaps, best R2 are obtained over maize, ranging from 0.1 (tillering) to 0.36 (development) up to 0.81 (maturity); for moderate time gap, R2 = 0.93 (maturity) is obtained. Crops such as wheat, oats, rye and barley, can profit from the LAI-RVI synergy, with R2 varying between 0.4 and 0.6. For beet or potatoes, MOGP provides poorer results, but alternative descriptors to RVI should be tested for these specific crops in the future before discarding synergy real benefits. In conclusion, active-passive sensor fusion with MOGP represents a novel and promising approach to cope with crop monitoring over cloud-dominated areas.

Grain‐filling response of winter wheat (Triticum aestivum L.) to post‐anthesis shading in a humid climate

AbstractTo assess the impacts of clouds during the grain‐filling period on the leaf senescence and yield of wheat (Triticum aestivum L.), we conducted a 4‐year plot shading experiment in a well‐drained Andosol in northern Japan. Plants were shaded after the beginning of anthesis in mid‐June, which corresponds to a naturally cloudy period in the study region. The effect of shading depended on its synchronization with soil drying, which affected leaf senescence. Grain yield was determined by solar radiation at the beginning of the grain‐filling period, except in 2015, which was a dry year. Later anthesis in 2016 and 2017 significantly reduced the number and weight of individual grains in the shading treatments. Moreover, “Kitahonami”, with late anthesis, showed a significantly slower increase in the individual‐grain weight during the grain‐filling process after shading than “Yumechikara”. When the duration between anthesis and shading was short, shading significantly reduced the number and weight of grains, and later anthesis makes it more likely that regional reduced light levels are encountered, resulting in fewer grains; the sensitivity of individual‐grain weight to shading synchronized with an increase in grain protein contents. Our results suggest that in wet years, later anthesis makes it easier to encounter regional reduced light and fewer grains; in addition, insufficient light limits photosynthesis under shade and decreases yield.