Radiation Balance Components Research Articles

Snowmelt Evolution Mapping Using an Energy Balance Approach over an Alpine Terrain

A computer model simulating snowmelt evolution and the spatial snowmelt pattern using an energy balance approach over an alpine terrain was developed. With a digital elevation model (DEM), surface characteristics information and meteorological data as input, all radiation balance components, turbulent fluxes, precipitation, and finally snowmelt were modeled on a daily basis. Special emphasis was given to snow redistribution. The model was applied to an area of 35 km2 in the Schilthorn Massif (Bernese Oberland, Switzerland) for 1996–97. The model calculations are compared with a snowmelt evolution map, which was produced by combining seven scenes of aerial photographs taken in the Bernese Alps during the melting season 1997 (March–September). Both the temporal comparison of the snowmelt evolution and the spatial comparison of simulated and observed snowmelt patterns show a good accordance: at any of the compared dates, spatial coincidence is equal to or better than 78%. It can therefore be concluded that the model supplies a quite realistic reproduction of the energy exchange processes taking place at the ground snow-cover/atmosphere interface during winter and spring.

Comparative studies on energy balance components and their inter relations on soybean crop and bare soil in kharif season at Pune situated in semi-arid tract

Comparative studies of radiation balance components at different growth stages on soybean crop and bare soil were made at Central Agrometeorological Observatory (CAgMO), Pune. Continuous measurements of net, reflected and global solar radiations were made over cropped field as well as over bare soil all throughout the growth phases in kharif season of 1995. Net and reflected radiations and albedo over canopy were higher by 7, 26 and 25 per cent respectively than bare soil. The net short wave (absorbed) radiation and net long wave (out-going) radiation evaluated over the canopy were less than those over bare soil by 5 and 20 per cent respectively.
 
 The mean daily net, reflected, net short wave and net long wave (out-going) radiation were 9.86, 3.86, 15.35 and 5.49 MJm-2 respectively and the albedo was 20 per cent over soybean canopy whereas for bare soil they were 9.23, 3.07, 16.15 and 6.91 MJm-2 and 16 per cent respectively. The mean daily global solar radiation during the crop growing season was 19.20 MJm-2. The highest albedo (26 per cent) of the crop recorded in the 10th week after sowing (WAS) was in correspondence to maximum LAI (5.9) observed at pod formation stage.

Effects of abandonment on the energy balance and evapotranspiration of wet subalpine grassland

Abandonment of grasslands is an important form of land-use change in subalpine regions. It leads to marked changes in the vegetation cover, which, in turn, may affect the radiative properties of the canopy and exchange processes between the ecosystem and the atmosphere. The aim of this study was to quantify the differences in net radiation ( R n), albedo ( α) and energy balance components between a managed and an abandoned wet grassland field in relationship to biomass and necromass production. The study in the Rotenbach watershed on the northern slope of the Swiss Alps (1400 m above sea level) was based on continuous measurements of the energy and radiation balances during two consecutive seasons. The managed site, which was mown once during the autumn, was characterized by a larger number of species, but in the abandoned field, the dominating species ( Ranunculus aconitifolius) was much more abundant. Annual production of aboveground plant biomass was similar in both fields, but litter accumulation and the amount of standing necromass was larger in the abandoned field. Early in the season, the litter layer at the abandoned site had only a minor influence on R n, but the larger amount of standing necromass in the course of the season decreased α and increased R n. This effect was most pronounced during mid-season and compensated for the higher ratio of sensible to latent heat fluxes, i.e., the Bowen ratio of the abandoned field. The resulting difference in total water vapour flux (evapotranspiration, ET) between the fields was small. Largest reduction in ET occurred early and late in the season, and this was partly due to decreased soil evaporation. Although the effect of abandonment on the cumulative ET during the two measuring periods was small, it was sufficient to increase the soil water content in the abandoned field, particularly during the later part of the season. It is concluded that abandonment causes a change in canopy structure which leads to reduced ET and temporarily increased soil water content.

Effect of nitrogen supply on crop conductance, water- and radiation-use efficiency of wheat

Water-use efficiency (WUE DM) is directly related to radiation-use efficiency (RUE) and inversely related to crop conductance ( g c). We propose that reduced WUE DM caused by shortage of nitrogen results from a reduction in RUE proportionally greater than the fall in conductance. This hypothesis was tested in irrigated wheat crops grown with contrasting nitrogen supply; treatments were 0, 80 and 120 kg N ha −1 in 1998 and 0, 80, 120 and 160 kg N ha −1 in 1999. We measured shoot dry matter, yield, intercepted solar radiation and soil water balance components. From these measurements, we derived actual evapotranspiration (ET), soil evaporation and transpiration, WUE DM (slope of the regression between dry matter and ET), WUE Y (ratio between grain yield and ET), RUE (slope of the regression between dry matter and intercepted radiation), and g c (slope of the regression between transpiration and intercepted radiation). Yield increased from 2.3 in unfertilised to an average 4.7 t ha −1 in fertilised crops, seasonal ET from 311 to 387 mm, WUE DM from 23 to 37 kg ha −1 mm −1, WUE Y from 7.6 to 12.4 kg ha −1 mm −1, RUE from 0.85 to 1.07 g MJ −1, while the fraction of ET accounted for soil evaporation decreased from 0.20 to 0.11. In agreement with our hypothesis, RUE accounted for 60% of the variation in WUE DM, whereas crop conductance was largely unaffected by nitrogen supply. A greater fraction of evapotranspiration lost as soil evaporation also contributed to the lower WUE DM of unfertilised crops.

Application of the Canadian land surface scheme to a full canopy crop during a drying cycle

The Canadian Land Surface Scheme (CLASS), a land surface parametrization scheme for use in large scale climate models, was assessed using the observed surface climate data from a full canopy crop in southern Ontario during a 10 day drying cycle period. Half‐hourly and daily modelled radiation and energy balance components and other surface climate parameters were compared to field observations. Different simulations using CLASS were employed and each contained different degrees of realism in model initialization. In general, CLASS simulations of radiation and energy balance components and surface climate parameters were good. Further, only modest improvements in model performance were achieved using more stringent initializations when compared to CLASS default specifications. It was also demonstrated that routine meteorological observations from an Atmospheric Environment Service network station could be used as meteorological input to CLASS without a loss in model output quality. From the research findings, it is apparent that full crop canopy conditions can be successfully modelled and that the more complex aspects of seasonal crop development and the wider range in the surface control of physical climatology should now be addressed using the CLASS framework. However, to ensure overall confidence in modelling approaches, the need to compare model results with quality field observations of surface climate processes remains important.

Mapping daily and monthly radiation components using METEOSAT data

The aim of the study is to present actual daily and climatically representative monthly maps of the surface radiation balance components derived from METEOSAT data. The encrypted digital images are now available in every three hours in Hungary. Hence, the first problem to solve is the estimation of diurnal mean radiation components from the archived 1–3 observations of the day between 8 and 15 UTC (+1–2 hours in local time). This computation is performed by Lagrange-interpolation. Since the skill of these day-to-day estimations is not at all obvious, the estimation of monthly averages is performed in two steps. First, the hourly observations from the 1992–1996 period are averaged for each hour separately, and these, more regularly varying values are smoothed by the Lagrange-interpolation, in the second step. For the monthly radiation maps, our purpose was also to improve their climatic representativity by using stratified sampling, based on macrosynoptic classification, first described by Mika et al. (1994). This classification considers the surface pressure patterns in Central Europe and the reference climatology for them is originated from the 1961–1990 period. The differences between sample means and stratified means are in some cases considerable, as presented in the parallel figures of some months and components presented in the paper.

Energy and radiation balance components for three grass surfaces near Kursk, Russia

The energy and radiation balance components were determined over three grass surfaces, representing different levels of productivity, located on the Streletskaya steppe of the Bio‐spheric Reserve near Kursk, Russia, for 21 days during July 1991. The Bowen ratio energy balance method was used to determine the sensible and latent heat flux densities using six computer controlled systems. A total of 126 variables including global, diffuse and reflected solar radiation, long wave radiation (up and down), net radiation, photosynthetically active radiation above and below the vegetation, infrared surface temperatures, soil temperature and heat flow, air temperature and vapor pressure at two levels, wind speed and direction, and precipitation were sampled at 30 s intervals and averaged for 15 min periods. The day time results from July 20, 1991 (a reasonably clear day) for Site 12 (mowed in 1990), Site 13 (mowed in 1991) and Site 14 (preserve) were: net radiation, 13.50, 12.6, and 13.3MJm−2; soil heat flow, ‐0.11, ‐0.87, and ‐0.09 MJ m−2; latent heat flux density, ‐8.79, ‐7.46 and ‐9.97 MJm−2; and for sensible heat flux density, 4.46, ‐4.26, and ‐3.28 MJm−2, respectively. The ranking of the sites from greatest to smallest for net radiation and latent heat flux density were preserve, mowed in 1990, and mowed in 1991. The ranking of the sites from greatest to smallest for sensible heat flux density were mowed in 1990, mowed in 1991, and preserve.

Estimation of the longwave radiation balance components: KUREX‐91 and FIFE studies∗

Estimates of incoming longwave radiation flux densities (R1?) obtained using several empirical and theoretical algorithms were compared to flux densities measured during the FIFE‐87 and FIFE‐88 experiments. Statistical analysis showed that all models gave values that were greater than the measured values with biases ranging from 25 to 55Wm−2. The incoming longwave radiation models were corrected to reduce or eliminate these biases and applied to independent data sets obtained during FIFE‐89 and KUREX‐91. With these adjustments, the models showed significant improvement in their ability to accurately estimate Rl?. The algorithms of Brunt and Brutsaert performed the best of all models and gave consistently good results for the FIFE‐89 and KUREX‐91 studies. Compared to the Brunt equation, the Brutsaert equation had a higher d value and smaller random and systematic errors. Both models had mean relative errors of less than 2% for the KUREX‐91 study and about 5% for the FIFE‐89 study. Outgoing (emitted) longwave radiation (Rl?) flux densities were estimated from remotely sensed surface temperature data during FIFE‐89 and KUREX‐91. Compared to measured outgoing longwave radiation flux densities (Rl?), estimated Rl? values had mean relative errors of approximately 1% and 2% for KUREX‐91 and FIFE‐89, respectively.

Estimating net radiation with remotely sensed data: Results from KUREX‐91 and FIFE studies∗

Net radiation (Rn) is the major source of energy for evaporating water, heating the soil and air, and photosynthesis. The objective of this study is to estimate this important parameter with various models that have been developed to estimate the radiation balance components with remotely sensed data, and readily available meteorological data. Data used in this paper were collected over grassland vegetation during the FIFE‐87, ‐88, ‐89 studies and the KUREX‐91 study. For all studies estimated values of Rn were within about 10% of measured Rn. For the KUREX‐91 study, measured and estimated Rn agreed to within about 1%. Improvement in a model(s) to estimate the reflected shortwave flux would provide an even better estimate of Rn since in all studies the reflected radiation stream was overestimated compared to the measured values. There was no clear trend for under or over‐estimation of incoming short‐wave radiation from study to study. Components of the long‐wave balance were estimated with low mean relativ...

Observations of the micrometeorology of two forests in eastern Canada. 1. Interannual variations in summer radiation and energy balance

Measurements of radiation and energy balances were made over two forests, 12.8 km apart, in eastern Ontario during three summers, 1989-1991. The two sites were AECL, a primarily deciduous forest with fine sandy soils, and PNFI, a mixed forest over a thin sandy-loam soil. The objective of the study was to investigate spatial heterogeneity in energy fluxes in a mixed forest landscape by examining interannual variations in radiation and energy balance components between the sites. The two sites showed small differences in albedo and net radiation. Total heat storage was not different between the two sites, yet there were some differences in the relative magnitudes of the storage components. The largest between-site differences were in energy partitioning between the convective fluxes of sensible heat, QH, and latent heat, QE. The QH was usually larger at PNFI and QE was usually larger at AECL. Thus, Bowen ratios were almost always smaller at AECL. The magnitude of these differences was controlled by soil moisture stress, with moisture stress frequently observed at PNFI but not at AECL. The implications of these results for regional-scale surface-vegetation-atmosphere climate modelling are discussed.

Partitioning of radiation and energy balance components in an inhomogeneous desert valley

Radiation and energy balance components are required to validate global, regional, and local scale models representing surface heat flux relationships in the heterogeneous surfaces of the world's arid and desert regions. Research was conducted in north-eastern Nevada, U.S.A., in a Great Basin inhomogeneous semi-arid desert valley located at 40° 44′ N, 114° 26′ W, with an elevation of 1707 m above mean sea level, to study the daily, monthly, and annual mesoscale radiation and energy balance components. We established five radiation stations along with five Bowen ratio systems to measure the incoming (Rsi) and outgoing (Rso) solar (shortwave) radiation, net (Rn) radiation, air temperatures and moisture at 1 and 2 m above-ground, the aggregated (soil + vegetation) surface temperature, soil heat flux at 8 cm (three locations at each station), soil temperatures at 2 and 6 cm above each soil flux plate, wind speed and direction at 10 m, and precipitation (if any) every 5 s averaged into 20 min throughout the valley during the 93–94 water year (beginning 1 October). Our study during the 93–94 water year showed that albedo (Rso/Rsi) ranged from 85% (snow-covered surface) to 10% (cloudy skies with wet surface) among stations. The water year total incoming solar radiation (averaged among stations) amounted to 6·33 × 103MJ·m−2and about 24% of that was reflected back to the atmosphere. The net longwave radiation (Rln= Rlo− Rli) was about 32% of Rsi, where Rloand Rliare the terrestrial (outgoing) and atmospheric (incoming) longwave radiation, respectively. The 93–94 water year average net radiation (Rn) among stations amounted to 2·68 × 103MJ·m−2(about 44% of Rsi). Approximately 85·3% and 14·6% of Rnwere used for the processes of sensible (H) and latent (LE) heat fluxes, respectively. The annual Rncontribution to surface soil heat flux (Gsurf) was almost 0·1%. Monthly and annual relationships among radiation and energy balance components are also reported. Using just one pyranometer and these relationships, one can estimate the daily, monthly, and annual values of Rso, Rn, Rln, LE, H, and Gsurf. We have made a major step toward being able to estimate net radiation of heterogeneous desert landscape based on vegetation cover and irradiance levels. The results can be applied to any similar closed desert basin in Australia, the Great Basin in the U.S.A., the Middle East, and North Africa.

Radiation absorption and use by humid savanna grassland: assessment using remote sensing and modelling

The components of the canopy radiation balance in photosynthetically active radiation (PAR), phytomass and leaf area index (LAI) were measured during a complete annual cycle in an annually burned African humid savanna. Directional reflectances measured by a hand-held radiometer were used to compute the canopy normalized difference vegetation index (NDVI). The fraction f APAR of PAR absorbed by the canopy (APAR) and canopy reflectances were simulated by the scattering from arbitrarily inclined leaves (SAIL) and the radiation interception in row intercropping (RIRI) models. The daily PAR to solar radiation ratio was linearly related to the daily fraction of diffuse solar radiation with an annual value around 0.47. The observed f APAR was non-linearly related to NDVI. The SAIL model simulated reasonably well directional reflectances but noticeably overestimated f APAR during most of the growing season. Comparison of simulations performed with the 1D and 3D versions of the RIRI model highlighted the weak influence of the heterogeneous structure of the canopy after fire and of the vertical distribution of dead and green leaves on total f APAR . Daily f APAR values simulated by the 3D-RIRI model were linearly related to and 9.8% higher than observed values. For sufficient soil water availability, the net production efficiency ϵ n of the savanna grass canopy was 1.92 and 1.28 g DM MJ −1 APAR (where DM stands for dry matter) during early regrowth and mature stage, respectively. In conclusion, the linear relationship between NDVI and f APAR used in most primary production models operating at large scales may slightly overestimate f APAR by green leaves for the humid savanna biome. Moreover, the net production efficiency of humid savannas is close to or higher than values reported for the other major natural biomes.

Estimation of surface radiation balance components from METEOSAT images: Five years statistics

A set of methods to estimate the surface radiation balance has been developed by using METEOSAT digital image data and a radiative transfer model. Cloud characteristics are determined from a recent method based on brightness temperature and its variance, whereas other necessary parameters are derived from the radiosonde network or its simulation (numerically forecasted ‘pseudo-sounds’). Estimated components of global radiation and radiation blance are analysed for five Aprils in 1992–1996. Daily course of the components is presented for the elaborated 16×25 quadrates around Hungary, demonstrating that in April, at least, climatic analysis of the radiation components in different local times should be performed separately. Traditional statistical estimates of the averages and standard deviations are given for the 09 UTC estimations that have been found important in ecological applications. Stratification of the samples is also performed by employing weather types derived from the macrosynoptic classification by Péczely (1957), with respect to the 1961–1990 period. In the particular month analysed the difference between traditional and stratified averages are not large: 1–2 % of overall increases in both components of the radiation balance can be registrated after applying the stratification. Whereas averages exhibit clear E-W and some S-N gradients in connection with obvious astronomical circumstances, pattems of the standard deviations are rather patchy with no definite gradients.

Radiation balance of an alfalfa crop in Saudi Arabia

Short-wave reflectivity or albedo is an important component of net radiationwhich represents the major determinant of radiation balance of crop surface. This study was conducted on an irrigated alfalfa crop field at Al-Kharj agricultural area in Saudi Arabia, grown according to normal agricultural practices. Data on radiation balance and crop cover were collected over a number of days from March to October 1986, crop albedo varying from 0–4 in early morning to 0–20 at noon, the overall mean value of the crop albedo being estimated at 0–26. The relation between the individual components of radiation balance was studied, and a significant correlation between incident radiation and net radiation was found. Possible causes responsible for changes in crop albedo were discussed.

Variability of surface fluxes within a complex area observed during TABLE 92

A data set obtained in a 3 week intensive field experiment called Tsukuba Atmospheric Boundary Layer Experiment (TABLE) 92 allowed some investigation on the heterogeneity of the surface fluxes within a 16 km by 16 km experimental area at and around Tsukuba city, Japan. The area can be characterized by its generally flat topography and by its complex surface land usages. The land covers include crop fields, pine woods and forests, houses or buildings, grass fields, and paddy fields. Five flux stations were installed, in addition to two existing stations, to determine the energy and radiation balance components at each of the major surface types. The analysis of these data made in the present paper has revealed that the spatial variations of surface fluxes were caused by two independent factors. The first one is the non-uniform nature of the incoming solar energy S d over the area, owing to the inhomogeneous presence of clouds, and the second factor is the variation of the surface cover. The former effect is important only when the sky is partly cloudy. The coefficient of variation for the hourly mean S d , defined by the ratio of the standard deviation and the mean of S d measured simultaneously at different stations, was of the order of V = 0.2, whereas the corresponding values of V for the net radiation, latent and sensible heat fluxes owing to the latter effect alone were around V = 0.3–1.0. Thus the variation of the surface cover had a somewhat larger effect on the inhomogeneity of the surface fluxes in the area.

衛星リモートセンシングを用いたHEIFE領域における諸過程のスケーリング・アップ

The use of satellite remote sensing is indispensable to improving the understanding of the processes of land surface-atmosphere interactions on a scale of 10 000 kmr in HEIFE. Because of their high spatial resolution, Landsat TM observations can be compared directly with surface measurements and be used for this purpose. The relevant data set and a state-of-the-art data processing methodology are discussed. A summer scene of TM was used to produce a set of maps of surface albedo, vegetation index, surface temperature, net radiation and other basic energy balance components of the whole area. Statistical analysis based on these maps revealed some quantitatively significant land surface parameters. Future developments are discussed.

Modeling of Downward Surface Longwave Flux Density for Global Change Applications and Comparison with Pyrgeometer Measurements

Abstract The success of satellite monitoring of global climate change depends on the ability to validate satellite inference methods against accurate “ground truth.” Under a recent World Meteorological Organization—World Climate Research Program activity a baseline surface radiation network is being established for long-term monitoring of surface radiation balance (SPB) components. At present, it is not possible to measure all of the SRB components at known accuracies. In this paper, selected longwave surface irradiance measurements are tested against high-resolution radiative transfer computations. It is shown that the differences between the modeled IR flux density using a line-by-line code and pyrgeometer measurements are within the required accuracy for ground observations.

Satellite remote sensing of surface energy balance: Success, failures, and unresolved issues in FIFE

The FIFE staff science group, consisting of the authors, developed and evaluated process models relating surface energy and mass flux, that is, surface rates, to boundary layer and surface biophysical characteristics, that is, surface states. In addition, we developed and evaluated remote sensing algorithms for inferring surface state characteristics. In this paper we report the results of our efforts. We also look in detail at the sensor and satellite platform requirements (spatial resolution and orbital requirements) as driven by surface energy balance dynamics and spatial variability. We examine also the scale invariance of the process models and remote sensing algorithms, that is, to what degree do the remotely sensed parameters and energy balance relations translate from the patch level where they were developed to the mesoscale level where they are required? Finally, we examine the atmospheric correction and calibration issues involved in extending the remotely sensed observations within a season and between years. From these investigations we conclude that (1) existing formulations for the radiation balance and latent heat components of the surface energy balance equation are valid at the patch level. (2) Many of the surface physiological characteristics that parameterize these formulations can be estimated using satellite remote sensing at both local and regional scales; a few important ones cannot. (3) The mathematical structures relating radiation and surface energy flux to remote sensing parameters are, for the most part, scale invariant over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) study area. The conditions for scale invariance are derived. (4) The precision of satellite remote sensing estimates of surface reflectance, calibrated and corrected for atmospheric effects, is no worse than about 1% absolute. The errors may actually be smaller, but an upper bound of 1% results from sampling variance caused by differences among the satellite and ground sensors in spatial resolution, atmospheric effects, and calibration. (5) Afternoon cumulus in the study area required both the Landsat and the SPOT satellites for monitoring of the vegetation dynamics. This result implies the need for multiple polar orbiters, or geosynchronous satellites in an operational implementation. We found that canopy Fpar, the fraction of incident photosynthetically active radiation absorbed by a canopy, can be estimated with an error of about 10% using remote sensing, provided that regional variability in the reflectance of the canopy substrate is dealt with properly. We also found that spectral vegetation indices (VIs) respond primarily to the photosynthetically active radiation absorbed by the live or green component of the canopy as opposed to its necrotic or dead vegetation. This is of critical importance since radiation absorption by the live part of the canopy is the rate‐limiting process for photosynthesis and other key process rates such as evaporation. We found for the FIFE study area the surface moisture content at O to 10 cm to be another key rate‐limiting variable in photosynthesis and evaporation. At gravimetric soil moisture levels below 20%, photosynthesis and evaporation were strongly attenuated. Only microwave sensors have shown potential for satellite remote sensing of soil moisture and only in the top few centimeters. Hydrological models may also play a critical role in monitoring root zone soil moisture levels, but additional research is needed. From our review of the research of others in FIFE we conclude that downwelling shortwave radiation and surface albedo are also amenable to remote sensing. Unfortunately, from our research we also found that the remote estimation of surface temperature to useful accuracies is problematical; consequently, the use of thermal infrared measurements to infer sensible heat flux is probably not feasible to acceptable accuracies.

BALANÇO DE RADIAÇÃO SOBRE UMA CULTURA DE MILHETO FORRAGEIRO

Em uma cultura de milheto forrageiro (Pennisetum americanum, Leek) cv. pérola, em Eldorado do Sul.RS a 30°05'27"S de latitude, 51°40'18"W de longitude e 40m de altitude, foram quantificados os componentes do balanço de radiação, no ano agrícola de 1988/89. A perda diária de energia por reflexão de ondas curtas foi superior à perda líquida de radiação de ondas longas, porém com valores semelhantes no início da manhã e ao final da tarde. O albedo demonstrou dependência do ângulo zenital da radiação solar, sendo menor próximo ao meio-dia e máximo no início da manhã. O albedo médio diário variou de 0,22 a 0,28, sem relação consistente com o índice de área foliar e com a nebulosidade.

Studies of surface energy balance of sloping terrain

AbstractMeasurements of radiation and soil energy balance components were made for a complete solar season on a south and north facing slope on the edge of the Chilterns, England.The contrasts between the two slopes are illustrated on both the diurnal and seasonal time‐scale. It is shown that a consistent relationship exists between daily solar and net radiation totals for the slopes throughout the year but that soil heat flux cannot be modelled easily. Further, it is shown that it is possible to derive a simple model of solar radiation on to a slope from measurements made on the horizontal.Combining these conclusions points the way towards simple routine calculation of slope radiation balances for use in energy and evaporation models over undulating terrain.