Global Photosynthetically Active Radiation Research Articles

Mapping long-term and high-resolution global gridded photosynthetically active radiation using the ISCCP H-series cloud product and reanalysis data

Abstract. Photosynthetically active radiation (PAR) is a fundamental physiological variable for research in the ecological, agricultural, and global change fields. In this study, we produced a 35-year (1984–2018) high-resolution (3 h, 10 km) global gridded PAR dataset using an effective physical-based model. The main inputs of the model were the latest International Satellite Cloud Climatology Project (ISCCP) H-series cloud products, MERRA-2 aerosol data, ERA5 surface routine variables, and MODIS and CLARRA-2 albedo products. Our gridded PAR product was evaluated against surface observations measured at 7 experimental stations of the SURFace RADiation budget network (SURFRAD), 42 experimental stations of the National Ecological Observatory Network (NEON), and 38 experimental stations of the Chinese Ecosystem Research Network (CERN). Instantaneous PAR was validated against SURFRAD and NEON data; mean bias errors (MBE) and root mean square errors (RMSE) were, on average 5.8 and 44.9 W m−2, respectively, and the correlation coefficient (R) was 0.94 at the 10 km scale. When upscaled to 30 km, the errors were markedly reduced. Daily PAR was validated against SURFRAD, NEON, and CERN data, and the RMSEs were 13.2, 13.1, and 19.6 W m−2, respectively, at the 10 km scale. The RMSEs were slightly reduced when upscaled to 30 km. Compared with the well-known global satellite-based PAR product of the Earth's Radiant Energy System (CERES), our PAR product was found to be a more accurate dataset with higher resolution. This new dataset is now available at https://doi.org/10.11888/RemoteSen.tpdc.271909 (Tang, 2021).

Retrieving high-resolution surface photosynthetically active radiation from the MODIS and GOES-16 ABI data

Incident photosynthetically active radiation (PAR) is a key parameter in plant physiological, biological and physical process-based terrestrial ecosystem models. Deriving highly accurate and spatially continuous PAR from remote sensing data can fill in the gaps of insufficient ground-based measurements. In this study, we provide an efficient approach for estimating PAR by employing look-up table (LUT) method using observations from both MODIS and GOES-16. The GOES-16 is the first satellite of the new-generation US Geostationary Operational Environmental Satellite R-series (GOES-R), which can capture dynamic changes in the atmosphere every 5 min. First, the diurnal variation of surface bidirectional reflectance is characterized assuming that the surface characteristics and solar zenith angles in tropical and middle latitudes at a given time of day do not change dramatically within a month. Next, the aerosol optical depth (AOD) and cloud optical depth (COD) are retrieved from the GOES-16 visible observations under both clear-sky and cloudy conditions. Finally, PAR is estimated via the retrieved AOD and COD using the LUT method and a MODIS-based surface albedo product. The estimated PAR was compared against ground-based measurements collected from seven SURFace RADiation budget network (SURFRAD) sites and 23 National Ecological Observatory Network (NEON) sites at instantaneous, hourly and daily scales. The instantaneous PAR validation results achieved R2 values of 0.97 and 0.82, and root mean square errors (RMSEs) of 18.1 and 51.1 W m‐2, for clear and cloudy skies, respectively. The RMSEs of the hourly averaged PAR were reduced to 16.8 W m−2 and 40.4 W m−2, respectively, for clear-sky and cloudy conditions. The daily mean PAR evaluation for all sky conditions shows an overall mean bias error (MBE) of 0.97 W m−2 and an RMSE of 10.5 W m−2. Compared with other global PAR datasets, the estimated PAR values in this study have lower RMSEs and higher spatial-temporal resolution. Uncertainties in the retrieved PAR caused by topography and solar-cloud-geometry effects (SCGE) can be reduced by spatial and temporal averaging. Based on an evaluation of retrievals at different spatial-temporal scales, we recommend that a 20 × 20 km2 domain-averaged PAR be utilized for instantaneous PAR validation, whereas hourly and daily averaged PAR values are nearly independent of spatial scale.

Direct Radiative Effect of Aerosols on Net Ecosystem Carbon Exchange in the Pearl River Delta Region

The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of urban agglomeration on the net ecosystem carbon exchange (NEE) is restricted. In 2009-2010, an observation of the aerosol optical property and CO₂ flux was carried out at the Dongguan Meteorological Bureau Station (DMBS) using a sun photometer and eddy covariance systems. The different components of photosynthetically active radiation (PAR), including global PAR (GPAR), direct PAR (DPAR), and scattered PAR (FPAR), were calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The effects of PAR on the NEE between land-atmosphere systems were investigated. The results demonstrated that during the study period the aerosol optical depth (AOD) reduced the DPAR by 519.28±232.89 μmol photons · m⁻² s⁻¹, but increased the FPAR by 324.93±169.85μmol photons · m⁻² s⁻¹, ultimately leading to 194.34±92.62 μmol photons·m⁻² s⁻¹ decrease in the GPAR. All the PARs (including GPAR, DPAR, and FPAR) resulted in increases in the NEE (improved carbon absorption), but the FPAR has the strongest effect with the light use efficiency (LUE) being 1.12 times the values for the DPAR. The absorption of DPAR by the vegetation exhibited photo-inhibition in the radiation intensity > 600 photons · m⁻² s⁻¹; in contrast, the absorptions of FPAR did not exhibit apparent photo-inhibition. Compared with the FPAR caused by aerosols, the DPAR was not the primary factor affecting the NEE. On the contrary, the increase in AOD significantly increased the FPAR, enhancing the LUE of vegetation ecosystems and finally promoting the photosynthetic CO₂ absorption.

Global radiation, photosynthetically active radiation, and the diffuse component dataset of China, 1981–2010

Abstract. Solar radiation, especially photosynthetically active radiation (PAR), is the main energy source of plant photosynthesis, and the diffuse component can enhance canopy light use efficiency, thus increasing ecosystem productivity. In order to predict the terrestrial ecosystem productivity precisely, we not only need global radiation and PAR as driving variables, but also need to treat diffuse radiation and diffuse PAR explicitly in ecosystem models. Therefore, we generated a series of radiation datasets, including global radiation, diffuse radiation, PAR, and diffuse PAR of China from 1981 to 2010, based on the observations of the China Meteorology Administration (CMA) and the Chinese Ecosystem Research Network (CERN). The dataset should be useful for the analysis of the spatiotemporal variations of solar radiation in China and the impact of diffuse radiation on terrestrial ecosystem productivity based on ecosystem models. The dataset is freely available from Zenodo on the following website: https://zenodo.org/record/1198894#.Wx6–C_MwWo (https://doi.org/10.11922/sciencedb.555, Ren et al., 2018).

A long-term time series of global and diffuse photosynthetically active radiation in the Mediterranean: interannual variability and cloud effects

Abstract. Measurements of global and diffuse photosynthetically active radiation (PAR) have been carried out on the island of Lampedusa, in the central Mediterranean Sea, since 2002. PAR is derived from observations made with multi-filter rotating shadowband radiometers (MFRSRs) by comparison with a freshly calibrated PAR sensor and by relying on the on-site Langley plots. In this way, a long-term calibrated record covering the period 2002–2016 is obtained and is presented in this work. The monthly mean global PAR peaks in June, with about 160 W m−2, while the diffuse PAR reaches 60 W m−2 in spring or summer. The global PAR displays a clear annual cycle with a semi amplitude of about 52 W m−2. The diffuse PAR annual cycle has a semi amplitude of about 12 W m−2. A simple method to retrieve the cloud-free PAR global and diffuse irradiances in days characterized by partly cloudy conditions has been implemented and applied to the dataset. This method allows retrieval of the cloud-free evolution of PAR and calculation of the cloud radiative effect, CRE, for downwelling PAR. The cloud-free monthly mean global PAR reaches 175 W m−2 in summer, while the diffuse PAR peaks at about 40 W m−2. The cloud radiative effect, CRE, on global and diffuse PAR is calculated as the difference between all-sky and cloud-free measurements. The annual average CRE is about −14.7 W m−2 for the global PAR and +8.1 W m−2 for the diffuse PAR. The smallest CRE is observed in July, due to the high cloud-free condition frequency. Maxima (negative for the global, and positive for the diffuse component) occur in March–April and in October, due to the combination of elevated PAR irradiances and high occurrence of cloudy conditions. Summer clouds appear to be characterized by a low frequency of occurrence, low altitude, and low optical thickness, possibly linked to the peculiar marine boundary layer structure. These properties also contribute to produce small radiative effects on PAR in summer. The cloud radiative effect has been deseasonalized to remove the influence of annual irradiance variations. The monthly mean normalized CRE for global PAR can be well represented by a multi-linear regression with respect to monthly cloud fraction, cloud top pressure, and cloud optical thickness, as determined from satellite MODIS observations. The behaviour of the normalized CRE for diffuse PAR can not be satisfactorily described by a simple multi-linear model with respect to the cloud properties, due to its non-linear dependency, in particular on the cloud optical depth. The analysis suggests that about 77 % of the global PAR interannual variability may be ascribed to cloud variability in winter.

MODIS-derived global land products of shortwave radiation and diffuse and total photosynthetically active radiation at 5 km resolution from 2000

Incident shortwave radiation (SW), photosynthetically active radiation (PAR), and diffuse PAR (PARdif) at the land surface drive a multitude of processes related to biosphere-atmosphere interactions and play a critical role in the Earth climate system. Previous global solar radiation products were spatially coarse (>50-km resolution) or temporally short (a few years), which hindered scaling-up ground based observations of the land surface processes into regional, continental, and global scales across multiple time scales. Here, we report Breathing Earth System Simulator (BESS) SW, PAR, and PARdif products over the global land surface at a 5km resolution with 4day intervals between 2000 and 2016. We combined an atmospheric radiative transfer model with an artificial neural network (ANN) to compute SW, PAR, and PARdif. A series of MODerate Resolution Imaging Spectroradiometer (MODIS) atmosphere and land products were used as inputs to run the ANN. We test the performance of the products using data from 158 (SW), 77 (PAR), and 22 (PARdif) stations collected in the Baseline Surface Radiation Network (BSRN) and flux tower networks, which covered a range of climatic zones from polar to tropical zones. BESS had strong linear relationships with in-situ SW data (R2=0.95, relative bias=−2.3%), PAR (R2=0.94, relative bias=1.7%), and PARdif (R2=0.84, relative bias=0.2%). BESS captured the interannual variability of SW at both the site (a majority of long-term BSRN sites) and continental levels. Over the study period, global annual SW, PAR, and PARdif values did not show any dimming or brightening trends, although these trends appeared at regional levels, e.g. dimming in India. Mean annual SW over the global land surface was 184.8Wm−2 (875ZJyr−1, zetta=1021); 46% of SW was partitioned to PAR, which was further split into direct (59%) and diffuse (41%) components. The developed products will be useful in solar energy harvesting research and will improve water, carbon, and energy flux estimates of terrestrial ecosystems from local to the global scales.

Leaf color is fine-tuned on the solar spectra to avoid strand direct solar radiation.

The spectral distributions of light absorption rates by intact leaves are notably different from the incident solar radiation spectra, for reasons that remain elusive. Incident global radiation comprises two main components; direct radiation from the direction of the sun, and diffuse radiation, which is sunlight scattered by molecules, aerosols and clouds. Both irradiance and photon flux density spectra differ between direct and diffuse radiation in their magnitude and profile. However, most research has assumed that the spectra of photosynthetically active radiation (PAR) can be averaged, without considering the radiation classes. We used paired spectroradiometers to sample direct and diffuse solar radiation, and obtained relationships between the PAR spectra and the absorption spectra of photosynthetic pigments and organs. As monomers in solvent, the spectral absorbance of Chl a decreased with the increased spectral irradiance (Wm(-2)nm(-1)) of global PAR at noon (R(2)=0.76), and was suitable to avoid strong spectral irradiance (λmax=480nm) rather than absorb photon flux density (μmolm(-2)s(-1)nm(-1)) efficiently. The spectral absorption of photosystems and the intact thallus and leaves decreased linearly with the increased spectral irradiance of direct PAR at noon (I dir-max), where the wavelength was within the 450-650nm range (R(2)=0.81). The higher-order structure of photosystems systematically avoided the strong spectral irradiance of I dir-max. However, when whole leaves were considered, leaf anatomical structure and light scattering in leaf tissues made the leaves grey bodies for PAR and enabled high PAR use efficiency. Terrestrial green plants are fine-tuned to spectral dynamics of incident solar radiation and PAR absorption is increased in various structural hierarchies.

Estimating the photosynthetically active radiation under clear skies by means of a new approach

Abstract. The k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computationally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an accurate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is −0.4 W m−2 (−0.2%) and −4 W m−2 (−1.3%) and the root mean square error is 1.8 W m−2 (0.7%) and 4.5 W m−2 (1.5%). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 μmol m−2 s−1 (+0.8%) and 1.9 μmol m−2 s−1 (−0.1%) respectively, and the root mean square error is 11.4 μmol m−2 s−1 (0.9%) and 4.0 μmol m−2 s−1 (0.3%). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance.

Generating Global LAnd Surface Satellite incident shortwave radiation and photosynthetically active radiation products from multiple satellite data

Incident shortwave radiation (ISR) at the surface is an essential parameter in the land surface radiation budget and in many land surface process models. Incident photosynthetically active radiation (PAR) is required by nearly all terrestrial ecosystem models. Although several global radiation products from numeric models and satellite observations have been released, their coarse spatial resolutions and low accuracy, especially at high altitude regions, limit their applications in land community. In this study, the Global LAnd Surface Satellite (GLASS) ISR and PAR products are developed based on an improved look-up table method from 2008 to 2010 at a 5km spatial resolution and a 3hour temporal resolution, the first global radiation products at such high resolutions, from multiple polar-orbiting and geostationary satellite data, including the Moderate Resolution Imaging Spectroradiometer (MODIS), the Meteosat Second Generation (MSG2) SEVIRI, the Multi-functional Transport Satellite (MTSAT)-1R, and the Geostationary Operational Environmental Satellite (GOES) Imager. The look-up table algorithm primarily considers the influences of surface elevation, atmospheric water vapor because sensitivity experiments indicate that ISR and PAR are more sensitive to surface elevation, and less sensitive to atmospheric profiles and ozone amount. The water vapor absorption has strong impact on ISR but weak on PAR. Moreover, the surface bi-directional reflectance distribution function (BRDF) is taken into account to retrieve spatially and temporally continuous surface reflectances from the geostationary satellite observations, which is an input parameter in the revised look-up table method. Ground-based measurement data from 34 sites are used to validate the improved algorithm and the GLASS products. The validation results of the instantaneous ISR and PAR products at all validation sites are notably good with coefficient of determination values of 0.83 and 0.84, respectively, and root mean square error values of 115.0Wm−2 and 49.0Wm−2, respectively. We also aggregate the GLASS ISR product for comparing with the corresponding ISCCP and CERES data at 7 SURFRAD sites and demonstrate that the GLASS ISR product is more accurate. The GLASS ISR and PAR products have been made publicly available.

Evaluation of global, photosynthetically active radiation and diffuse radiation transmission of agricultural screens

Transmittance of a material depends on the type of radiation impinging on the material (direct or diffuse), the angle of incidence of the sun’s rays (in direct radiation conditions) and the structure and characteristics of the material itself. The aim of this study was to evaluate the performance of nine agricultural screens of different densities, colours, thread diameters and porosities. A simple metal frame was used to quantify global, diffuse and photosynthetically active radiation (PAR) transmission to determine their transmittance values (as a function of the incidence angle of solar rays in direct radiation conditions) for global radiation and PAR, as well as the diffuse radiation transmitted characteristics of the screens (ratio of diffuse radiation to global radiation transmitted by screens). Non-coloured (translucent) screens contributed to a higher and more efficient proportion of diffuse radiation (Di/Gi ≈ 90%). The green screen behaved similarly to the non-coloured 20×10 type as far as global radiation was concerned, although the PAR transmittance values were lower (up to 12.4% at 15º). Variation in transmission according to diameter and density of the threads was greatest in the black screens. The non-coloured-thread screens may be the best option for maximising the transmission of diffuse radiation and the 6×6 green screen for vegetables cultivation during the summer in inland areas with latitudes close to 36ºN.

Responses of plastic shading nets to global and diffuse PAR transfer: Optical properties and evaluation

To cover agricultural structures for protecting plants from high solar radiation in hot and sunny regions different types of shading nets are used. In summer, the amount of photosynthetically active radiation (PAR) transmitted through these nets usually fulfils plant growth requirements. However, their applicability in winter is still unknown. So far, the choice of nets has mostly been empirical, because technical data such as the radiometric properties that are used to characterize the different types of nets are not available. This paper presents a simple method for measuring the radiometric properties related to global and diffuse PAR of seven types of shading nets including nets with colours and shading rates that are commonly used in hot regions. A shading system was developed to simulate the diffuse PAR under cloudy conditions. The results show that under clear sunny and cloudy conditions the radiometric properties depended on both net porosity and colour. Net reflectance strongly depended on net colour. Net transmittance and absorptance depended primarily on colour and secondarily on porosity. The radiometric properties of nets under global and diffuse PAR conditions tended to be similar to those of translucent materials so that a plastic net can be considered as translucent material when investigating the equivalent optical parameters (refractive index and absorption coefficient). This can help to predict the radiometric properties of the net without the need of measurements. Under the sunny conditions of winter seasons, the structures covered with the tested nets are suitable for growing crops that can grow at PAR less than 150 W m −2. However, under cloudy conditions very few plants will grow that can grow at PAR less than 30 W m −2.

Row orientation effect on UV-B, UV-A and PAR solar irradiation components in vineyards at Tuscany, Italy

Besides playing an essential role in plant photosynthesis, solar radiation is also involved in many other important biological processes. In particular, it has been demonstrated that ultraviolet (UV) solar radiation plays a relevant role in grapevines (Vitis vinifera) in the production of certain important chemical compounds directly responsible for yield and wine quality. Moreover, the exposure to UV-B radiation (280-320 nm) can affect plant-disease interaction by influencing the behaviour of both pathogen and host. The main objective of this research was to characterise the solar radiative regime of a vineyard, in terms of photosynthetically active radiation (PAR) and UV components. In this analysis, solar spectral UV irradiance components, broadband UV (280-400 nm), spectral UV-B and UV-A (320-400 nm), the biological effective UVBE, as well as the PAR (400-700 nm) component, were all considered. The diurnal patterns of these quantities and the UV-B/PAR and UV-B/UV-A ratios were analysed to investigate the effect of row orientation of the vineyard in combination with solar azimuth and elevation angles. The distribution of PAR and UV irradiance at various heights of the vertical sides of the rows was also studied. The results showed that the highest portion of plants received higher levels of daily radiation, especially the UV-B component. Row orientation of the vines had a pronounced effect on the global PAR received by the two sides of the rows and, to a lesser extent, UV-A and UV-B. When only the diffused component was considered, this geometrical effect was greatly attenuated. UV-B/PAR and UV-A/PAR ratios were also affected, with potential consequences on physiological processes. Because of the high diffusive capacity of the UV-B radiation, the UV-B/PAR ratio was significantly lower on the plant portions exposed to full sunlight than on those in the shade.

Radiação solar em ambiente protegido cultivado com tomateiro nas estações verão-outono do Rio Grande do Sul

O crescimento e desenvolvimento de uma planta dependem da intensidade, qualidade e duração da radiação solar. Por esse fator ser de importância vital às plantas, o presente trabalho objetivou fazer uma avaliação sobre sua variação, bem como sobre a sua disponibilidade no interior do ambiente protegido durante o ciclo do tomateiro nas estações verão-outono em Pelotas, Rio Grande do Sul. O experimento foi conduzido de janeiro a junho de 2003 no Campus da Universidade Federal de Pelotas (latitude 31°52'S; longitude 52°21'W e altitude de 13m), em estufa plástica disposta no sentido Leste-Oeste, com área de 180m². A cultivar utilizada foi "Flora-dade", semeada em 24/01/03, transplantada no dia 28/02/03, sendo a última colheita em 12/06/03. Avaliou-se a radiação solar global externa (Rgext) e interna (Rgint), transmitância, radiação fotossinteticamente ativa (RFA) e o albedo da cultura a partir de sensores eletrônicos conectados a um "datalloger". Durante o ciclo da cultura, o total de Rgext foi 1161,21MJ m-2, enquanto a Rgint foi 881,85MJ m-2. A Rgint e a RFA apresentaram valores médios diários de 8,5MJ m-2 dia-1 e 3,4MJ m-2 dia-1, respectivamente. A transmitância média da cobertura plástica à radiação solar global foi de 76%. O albedo médio diário da cultura foi 0,23, com albedo de 0,17 nos estádios iniciais, 0,26 no período de máximo crescimento e 0,23 no final do ciclo.

Relationships between photosynthetically active radiation and clearness index at Bloemfontein, South Africa

Relationships between photosynthetically active radiation (PAR) and solar radiation (SR) and between diffuse radiation and global radiation at Bloemfontein, South Africa were investigated. The ratio of PAR to SR (PAR/SR) has been reported from many places in the northern hemisphere, but there were not many PAR/SR measurements documented from the southern hemisphere, including the study region. Based on the present observations, mean PAR/SR in the region (0.48) was not exceptionally different from other reported data (0.45–0.50). The effects of sky condition on PAR/SR were also investigated and confirmed the previous findings. PAR/SR increased as the clearness index (KT = ratio of global to extraterrestrial SR) decreased, as sky conditions varied from clear to cloudy. Thus, PAR/SR can be a simple function of the clearness index (the global PAR model). In this study, PAR/SR was explained by a quadratic function of KT. For the relationship between diffuse and global radiation, a simple linear threshold diffuse PAR model, as a linear function of KT, was proposed for the study region using long-term data of diffuse and global SR. Using the previous findings on the relationship between the ratio of diffuse to global SR (KSR) and the ratio of diffuse to global PAR (KPAR), at high KT, KPAR can be estimated as 30% higher than KSR, while at low KTKPAR is equivalent to KSR.

Spectral composition and quanta‐to‐energy ratio of diffuse photosynthetically active radiation under diverse cloud conditions

Accurate and detailed modeling of vegetation photosynthesis and ecosystem‐atmosphere CO2 exchange, particularly for forests, requires data on the diffuse and global fluxes of photosynthetically active radiation (PAR) (400–700 nm). Such ecosystem process models typically employ PAR data in quantum units (photosynthetic photon flux density, Qp, μmol m−2 s−1), although PAR is often reported in energy units (irradiance, Rp, W m−2). Reliable conversion of diffuse PAR data from energy units (Rp,d, W m−2) to quantum units (Qp,d, μmol m−2 s−1) requires knowledge of how the conversion factor β*p,d (quanta‐to‐energy ratio for diffuse PAR at an arbitrary timescale, μmol J−1) varies across a broad range of sky conditions that includes variations in cloudiness, including broken clouds. This study employs spectral measurements of the diffuse irradiance of PAR to examine the influence of a diverse range of observed sky conditions on the spectral composition of Rp,d and the value of β*p,d at 1‐minute and daily timescales. The results indicate that the enhanced contribution of green‐to‐red (550–700 nm) radiation to the spectral composition of Rp,d, induced by atmospheric scattering and diffuse reflectance by clouds, causes β*p,d at the daily timescale (β′p,d) to increase nonlinearly by up to 7% relative to clean, cloudless sky conditions. The relation is explained well (R = 0.98) by the equation β′p,d = (4.5886 × f′p,d)/(0.010773 + f′p,d), where f′p,d is equal to the diffuse fraction of the daily total irradiation of global PAR. These results may be useful for conversion of diffuse PAR data between energy and quantum units for environments where clouds are the dominant agent of atmospheric scattering. The observed quanta‐to‐energy ratio for global PAR at the daily timescale (β′p,g), while not strictly constant, is relatively insensitive to atmospheric scattering, and a representative value of 4.56 μmol J−1 may be employed for a wide range of cloud conditions with little or no error.

Improved estimation of diffuse photosynthetically active radiation using two spectral models

The efficiency with which incoming photosynthetically active radiation (PAR) is intercepted by a canopy depends on the proportions of diffuse and direct radiation in the incoming PAR. In the present work, we developed a comparison between two spectral cloudless sky parameterization schemes in terms of their capability to provide accurate estimates of global, diffuse and direct incoming PAR. Both methods provide an estimation of the solar spectral irradiance that can be integrated spectrally within the limits of interest. For this purpose, data recorded at two radiometric stations one located at Granada, an inland location, and the other at Almerı́a, a coastal location, were used. After our study, it appears that the information concerning the aerosol radiative effects is fundamental to obtain a good estimation, especially for the direct and diffuse components. In this sense SMARTS2 model offers increased flexibility concerning the selection of different aerosol models included in the code. The original version of SPCTRAL2 code does not offer this flexibility but in this work a modified version that allows the selection between several aerosol models was considered. Our results show that a rather accurate estimation of the global component can be obtained for any of the aerosol models tested in this work, thus indicating the lower influence of the scattering processes in this term. In our analyses, we considered different aerosol models compatible with the features of each one of the analyzed locations. The a priori choice of the aerosol models dictated by the features of each place provided estimation of the different components of PAR with mean bias deviation (MBD) smaller than the experimental error. This is a clear advantage over the simpler broadband parametric models that need local information of all the relevant aerosol properties and some ad hoc modifications to provide accurate estimates of the PAR components.

Investigation for Photosynthetically Active Radiation Regime in the Mongolian Grain Farm Region.

This article discusses common features of photosynthetically active radiation (PAR) in the Mongolian grain farm region during the period of 1986 to 1996. The quantitative characteristics of solar radiation, such as direct, diffuse and global radiation are evaluated in the study through the long term trend.The mean pattern of seasonal variation of PAR shows that the yearly minimum global (direct+diffuse) PAR is 68.9±7.6MJ/(m2 month) in December while the maximum is 307.5±10.3MJ/(m2 month) in May. The yearly summation is 2.22±0.07GJ/(m2yr) and is very stable compared with monthly variations.

New empirical relationships for determining global PAR from measurements of global solar radiation, infrared radiation or sunshine duration

Data on global solar radiation (H), global photosynthetically-active radiation (PAR), downward infrared sky radiation (IR) and sunshine duration (n) for a 2-year duration are analysed on monthly (monthly mean of daily values) basis at Ilorin (8°32′N, 4°34′E), Nigeria. The data were taken using a precision spectral pyranometer, quantum sensor, pyrgeometer and Campbell–Stokes recorder, respectively. Two thermistor circuits were built into the pyrgeometer for monitoring the dome and body temperatures so as to take care of the dome heating effect. From monthly values of IR, PAR and H, a correlation model relating the three parameters was established and is of the form: (1) where fp is the ratio of PAR to H. Further analysis showed that the use of maximum likelihood quadratic fit to relate the three parameters does not improve the coefficient of determination. Also from the monthly mean values of PAR and n, equations of the Angstrom–Prescott one-parameter model of linear and quadratic forms are developed for the estimation of global PAR at this location and locations with similar climatic conditions. The advantage of these models over the traditional method of estimating PAR, by assuming it to be of constant ratio to insolation, is that PAR can be estimated even at locations where insolation values are not available since they make use of only solar-related astronomical parameters (extraterrestrial radiation and maximum sunshine duration) and duration of sunshine. The results show that in practice, the simple linear regression model is sufficient and hence the use of a quadratic model is only slightly justifiable. On developing the models based on the two seasons (rainy and dry) encountered here, the rainy season model showed a high correlation coefficient, R, of 0.9793 against the poor R of 0.5075 for the dry season. The poor correlation during the dry season period is probably due to large differences in the characteristics of the sky during this period. Copyright © 2000 Royal Meteorological Society

Global PAR related to global solar radiation for central Nigeria

Measurements of global solar radiation ( R s) and global photosynthetically active radiation (PAR) were made for a period of 12 months at Ilorin, Nigeria in order to find the relationship between them. The average ratio of PAR to R s for the year was 2.08 E MJ −1 with the dry and rainy season values of 2.02 and 2.12 E MJ −1, respectively. The minimum monthly mean daily ratio of 1.92 E MJ −1 was in January, a dry (Harmattan) season month while the maximum was 2.15 E MJ −1 in May, a rainy season month. Generally, the ratio values in the rainy season months and even dry season months (except the Harmattan months) remain constant at about 2.1 E MJ −1. On daily basis, the maximum and minimum ratios were 1.86 and 2.31 E MJ −1, respectively. Hourly values of the ratio increased on average from 2.07 to 2.15 E MJ −1 as the sky conditions changed from ‘clear’ to ‘cloudy’. There was no daylength effect on the ratio.

Technical note Measurement of global solar global photosynthetically-active and downward infrared radiations at Ilorin, Nigeria

Methods of measuring and results of analyses of data spanning a period of two years on global solar radiation, global photosynthetically-active radiation (PAR) and downward infrared radiation at Ilorin, a tropical location, are presented.The results show that there exist seasonal tendencies in the variation of these radiations, especially for global solar radiation and PAR. The distribution and range of variation of global solar radiation show that Ilorin has a high potential for solar energy utilization while those of PAR indicate the availability of this very important radiation throughout the year, perhaps with the exception of the months of July and August. For downward infrared radiation, the results show that it is almost uniform throughout the year with the exception of the relatively small values in the Harmattan months (December and January) of the dry season.