Measurements Of Net Radiation Research Articles

An analysis of the circulation characteristics and energy budget of a dry, asymmetric, east‐west urban canyon. II. Energy budget

AbstractThis study presents partial energy budget evaluations for a dry east‐west oriented urban canyon in Columbus, Ohio, based on canyon‐top measurements of net radiation and the turbulent and circulation‐related components of the sensible heat flux. While net radiation data were similar to those reported in previous studies, the sensible heat exchanges were small in absolute magnitude, showed a “flat” diurnal regime and were uncorrelated with the net radiation. These results suggest a minimal degree of coupling by heat exchange between the canyon air space and the urban boundary layer above roof level. Speculations are offered on the nature of the unmeasured residual in the canyon energy budget. Evidence suggests that both a substrate heat flux and a significant advective term (through the canyon ends and top) may be instrumental in accounting for the residual. However, definitive conclusions were precluded by the absence of simultaneous evaluations of all the energy budget components. The need for such studies at canyon sites with diverse structures and fabrics is emphasized.

Field Comparisons of Direct and Component Measurements of Net Radiation under Clear Skies

Abstract Accurate measurements of net radiation are basic to all studies of the surface energy budget. In preparation for an energy budget experiment significant differences were found between direct and component measurement of net radiation, which prompted this investigation of their cause. The instruments involved were an all-black single-dome Fritschen-type net pyrradiometer, two Eppley model 8–48 pyranometers, and an Eppley model PIR pyrgeometer. Each had recently been calibrated. The accuracy of the component instruments was considered first. Comparisons of about one hour on each of three nights between the pyrgeometer and five empirical formulas showed that the average departure over all formulas from the pyrgeometer average was −1%. Other comparisons between the pyrgeometer and an infrared thermometer viewing the surface yielded similar results. Alternate shading and unshading of the pyrgeometer looking upward during daytime resulted in a formula that was used to correct the downward longwave radi...

Mean meridional transport of energy in the earthatmosphere system using NMC global analyses and ERBE radiation data

Recently processed data from the Earth Radiation Budget Experiment (ERBE) provides an incentive to continue investigations addressing the energy transport of the earth-atmosphere system. Within this study, we compare the top-of atmosphere ERBE net radiation measurements and derived energy transports thereof with computed transports derived from the daily global analyses of the National Meteorological Center (NMC). The period of study is for the year March 1985–February 1986, and the results are divided into the annual and seasonal averages. The basic result is that most features depicted in the calculation of meridional energy transports within the atmosphere are physically realistic. Mean motions dominate in the tropics, with contributions by both the Hadley and Ferrel Cells evident. Comparisons between these ERBE/NMC results and published results of similar studies aid the evaluation process. The greatest differences are found in the tropics, where the mean transports derived from NMC analyses are somewhat smaller than expected due to a weak Hadley Cell circulation. DOI: 10.1034/j.1600-0870.1994.t01-2-00001.x

Energy transports by ocean and atmosphere based on an entropy extremum principle. Part II: Two-dimensional transports

The maximum entropy production (MEP) principle used in Part J has been extended to separate the two-dimensional required energy transports determined from Nimbus 7 satellite net radiation measurements into atmospheric and oceanic components. In terms of the meridional component of the ocean transport vectors, results show northward ocean transports throughout the entire Atlantic ocean from southern hemisphere high latitudes to northern hemisphere polar regions, southward transports throughout the entire Indian Ocean, and poleward transports separated at approximately 10°S in the Pacific Ocean. The ocean transport patterns are consistent with well-known features concerning heat transport within the three ocean basins. However, uncertainty remains in the magnitudes of the transports. Because of the large remaining discrepancies between published estimates based on direct measurements and indirect estimates derived from energy budget methods, assessing the accuracy of the magnitudes is difficult, although there is evidence that the limited model resolution leads to synergistic biases in the North Atlantic and North Pacific. In terms of the crossmeridional energy transport component, results suggest that most of the net energy transfer in the tropics takes place within the ocean. In the southern hemisphere high latitudes, the Pacific and Indian Oceans export heat cross-meridionally to the Atlantic Ocean through the passages below Cape Horn and the Cape of Good Hope, although the magnitudes of these inter-ocean heat exchanges are small. Another important aspect of the southern hemisphere results is that poleward transports are dominated by the atmospheric component with strong zonal asymmetry. By contrast, in the northern hemisphere, atmospheric transports over the ocean are generally weaker than the corresponding southern hemisphere terms, indicating that the northern hemisphere oceans are relatively more effective in transferring heat poleward. Finally, poleward atmospheric transports over the continental areas exceed those over the ocean at equivalent latitudes as a result of the generally greater energy deficits over the land areas.

Comparison of Penman‐Monteith, Shuttleworth‐Wallace, and Modified Priestley‐Taylor Evapotranspiration Models for wildland vegetation in semiarid rangeland

Eddy correlation measurements of sensible and latent heat flux are used with measurements of net radiation, soil heat flux, and other micrometeorological variables to develop the Penman‐Monteith, Shuttleworth‐Wallace, and modified Priestley‐Taylor evapotranspiration models for use in a sparsely vegetated, semiarid rangeland. The Penman‐Monteith model, a one‐component model designed for use with dense crops, is not sufficiently accurate (r2 = 0.56 for hourly data and r2 = 0.60 for daily data). The Shuttleworth‐Wallace model, a two‐component logical extension of the Penman‐Monteith model for use with sparse crops, performs significantly better (r2 = 0.78 for hourly data and r2 = 0.85 for daily data). The modified Priestley‐Taylor model, a one‐component simplified form of the Penman potential evapotranspiration model, surprisingly performs as well as the Shuttle worth‐Wallace model. The rigorous Shuttleworth‐Wallace model predicts that about one quarter of the vapor flux to the atmosphere is from bare‐soil evaporation. Further, during daylight hours, the small leaves are sinks for sensible heat produced at the hot soil surface.

Positional variation in the soil energy balance beneath a row-crop canopy

When crops are grown in a row configuration, differential shading of the soil, coupled with other soil-canopy micrometeorological interactions, may result in large positional variations in the soil energy balance. Experiments were conducted near Manhattan, KS, to examine positional variations in the soil energy balance beneath a soybean ( Glycine max) canopy during periods of partial cover. Continuous measurements of net radiation ( Rn) and soil heat flux ( G) were obtained at four equally spaced positions between one pair of plant rows. Net radiation was calculated from radiation, temperature, and emissivity measurements of the soil, canopy, and sky. Soil heat flux was determined at each location from detailed heat flux and subsurface temperature measurements. When the soil surface was dry, sensible heat flux ( H) was estimated as a residual of the heat balance by assuming that latent heat flux was negligible. Measurements of H and soil-air temperature gradients were used to estimate aerodynamic transport coefficients at each position. Results indicate that the magnitude and pattern of the soil heat balances are strongly dependent on location beneath the canopy. The patterns of G, Rn, and surface temperature at each measurement position were associated with the patterns of shortwave soil irradiance, which were functions of sun-canopy geometry. Soil irradiance between the plant rows sometimes exceeded global irradiance because of reflected radiation from the canopy. Daily Rn between the rows was five to ten times greater than that measured directly beneath the canopy, depending on environmental conditions. Large positional variations in G were also documented. Temperature differences between the sunlit and shaded portions of the surface exceeded 25°C when the surface was dry. Air temperatures, measured at 5 cm above each position, were also dependent on position with respect to the plant rows. When the surface was dry, the majority of sensible heat flux occurred from the soil directly between the plant rows, exceeding 400 W m −2 under certain conditions. Estimates of local aerodynamic transfer coefficients for the soil surface ranged from 1 to 50 mm s −1, but were highly variable and not correlated with above-canopy windspeeds or position beneath the canopy. Results suggest that positional variation in aerodynamic transport from the soil cannot be discerned with the methods used in this study.

The surface energy balance in Mexico City

Results of a field observation programme to investigate the surface energy balance of Mexico City, D.F., are presented. Direct measurements of net all-wave radiation and the turbulent sensible heat flux were conducted from a 28 m tower located about 4 km from the city centre. Heat storage in the urban fabric was parameterized and the turbulent latent heat flux was found by the residual in the energy balance equation. Most of the data were gathered in the dry season from February to the end of March 1985. The results from this tropical highland city resemble those from temperate cities in respect of the magnitude of the Bowen ratio and the diurnal trend in the amount of the net radiation partitioned into turbulent sensible heat. On the other hand, the role of heat storage is much larger than in the temperate case so that the partitioning of the sensible heat between turbulent and conductive transfer seems to be more nearly equal. Nevertheless evaporation appears to be an important term in both the surface water and energy balances. The nature of the energy balance may help explain some of the previously observed features of the heat island and humidity regimes of Mexico City.

Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree

In this paper we describe a device for measuring total radiation absorbed by the canopy of a single tree. The net influx of all-wave and photosynthetically active radiation (PAR) are found by integrating measurements over a sphere enclosing the tree. This is achieved by using radiometers mounted at equi-latitudinal intervals on a rotating circular frame. We describe the device, the placement of the radiometers on it, and the integration method used to compute total absorbed radiation. Data are presented from measurements taken around a Robinia pseudoacacia tree of 3.8 m height with a total leaf area of 10.6 m 2. The diurnal course of radiation absorbed by the tree is compared with the flux densities of PAR and net all-wave radiation measured over a level, grassed area nearby. On an overcast day, total radiation absorbed by the Robinia tree was nearly equal to the radiation absorbed by 8 m 2 of grass. However, on a clear day the same tree absorbed notably more radiation than 8 m 2 of grass when the sun was low on the horizon, and a little less near midday. This appears to be the first time that such a direct measurement of radiation absorption of an isolated tree has been made. The device described should be useful in testing models of radiation absorption by trees as well as providing a reliable field device for experimental studies of tree energy balances and photosynthesis.

Regional surface fluxes under nonuniform soil moisture conditions during drying

The fifth intensive field campaign of FIFE (First ISLSCP Field Experiment (International Satellite Land Surface Climatology Project)) was conducted in northeastern Kansas during late summer 1989, in order to study the dry down of a surface with prairie vegetation prior to senescence and dormancy. As precipitation became sparse and rare, the surface developed nonuniform features at the scales of the rainfall events. Nevertheless, lower boundary layer measurements used together with remotely sensed surface temperatures on the basis of Monin‐Obukhov similarity allowed the estimation of the flux of sensible heat for a region with characteristic dimensions of 104 m. In contrast, under these conditions it was difficult to aggregate an array of local flux measurements into a true regional mean. On account of the same problem in the aggregation of local measurements of net radiation and ground heat flux, the reliability of the estimation of regional evaporation by the energy budget method was hard to assess.

Evaluating canopy temperature-based indices for irrigation scheduling

Since the development of commercial versions of infrared sensors, they have been increasingly used to determine canopy temperature and schedule irrigations. However, some shortcomings of the technique have been identified, among them the sensitivity of canopy temperature measurements to weather fluctuations. Based on field and computer simulated data, an analysis of the suitability of crop water stress indices (CWSI's) developed from canopy temperature under variable weather conditions was done. Important day to day fluctuations of CWSI values determined using an empirical baseline (empirical CWSI) appeared common for nonstressed crops, particularly under low vapor pressure deficit conditions. These fluctuations generate uncertainty in the use of this empirical index to determine needs for irrigation. The use of an improved index (theoretical CWSI) requiring measurements of net radiation, soil heat flux and wind speed, and estimates of aerodynamic and canopy resistances reduced but did not eliminate these fluctuations. Results using a simulation model showed that the empirical CWSI provided late indication of irrigation needs, after some water stress has developed, which may limit its application for crops sensitive to water stress. These simulations also indicated that the theoretical CWSI was able to track the development of water stress and provide reasonable indication of irrigation needs. However, this result may not be fully realized in field applications where the determination of CWSI may be affected by various sources of variability which are not accounted for by the model.

RADIATION CLIMATOLOGY THROUGH THE TRADE-WIND INVERSION ON THE LEE SLOPE OF HALEAKALA, MAUI, HAWAII

Two years of global and net radiation measurements in Hawaii were used to analyze annual and diurnal radiation patterns between 950 and 3000 m on a tropical island mountain. This layer includes the usual altitude of a subsidence inversion over the islands. Maximum average monthly depletion of solar irradiance ranged from between 38% in December to 52% in April at 1650 m, just below the average level of the inversion. It reached a minimum of 6% in May at 3000 m, well above the average inversion level. Climatological average values strongly reflect the diurnal weather pattern on the mountain. Attenuation of solar irradiance peaked near 1300 to 1400 hours in summer and later, near 1500 to 1600 hours, in winter. The coefficient of variation in hourly solar irradiance values was greatest (50% to 60%) at 1650 m in the afternoon in summer. In winter, the coefficient of variation was nearly equal for sites between 950 and 2130 m, ranging from 40% to 60%. Net radiation followed the solar irradiance pattern during ...

Comments on ?correction of errors associated with measurement of net all-wave radiation with double-domed radiometers? by Oliver and Wright (1990)

In their note on correction of measurements with the REBS Q*4 net radiometer, Oliver and Wright discuss a problem of high general interest, namely that of wavelength-dependent responsivities in net radiometry. They suggest that old measurements can be corrected by a simple addition of 40% of the net longwave radiation. It is shown that this is only the first of two necessary steps needed for a good correction. The 40% value suggested by Oliver and Wright leads to night-time radiation values that are typically 20–25% too negative. It is also suggested that their 40% correction varies with time and place of measurement as well as with the way the radiometer was originally calibrated. This percentage should be replaced by a correction factor based on a design-dependent responsivity difference of around 63% multiplied by a factor depending on the relative importance of longwave and shortwave radiation components prevailing for the measurements to be corrected. The second step in the correction procedure should be a “recalibration” of the signal achieved after equalizing responsivity for negative and positive values of net radiation. It is finally suggested that the cause of the malfunction of the double-dome radiometer has to do with the transmission properties of the polyethylene shield and not with properties of the heat-absorbing surface or entrapped hot air.

Correction of errors associated with measurement of net all-wave radiation with double-domed radiometers

Comparisons between measurements of net all-wave radiation made with double and single-domed radiometers have confirmed the existence of a significant offset between the two types of instrument. The availability in the UK and Niger, West Africa, of independent instruments to measure all the radiation components indicated that the double-domed instrument was in error and permitted the determination of a correction factor which is shown to be dependent on net long-wave radiation balance. It is also shown that this factor can be expressed in terms of the more often available net short-wave radiation balance so that retrospective correction of net radiation data is possible.

Comparison of Energy Source Estimates Derived from Atmospheric Circulation Data with Satellite Measurements of Net Radiation

Abstract The distributions of the net sources of atmospheric dry and latent energy are evaluated by the residual technique using the reanalyzed ECMWF FGGE level IIIb data for February and July 1979. Their sum (i.e., the residual estimate of the source of total energy) is compared to simultaneous Nimbus-7 ERB estimates of the net radiation at the top of the atmosphere. Over land the estimated total energy source should be nearly equal to the net radiation. The best agreement is found in July 1979 over the Northern Hemisphere middle latitudes, where monthly averages over large land areas agree to within 10 W m−2. Elsewhere, differences of up to 60 W m−2 occur for corresponding averages. Over low latitude continents, the residuals are generally too small over convectively active regions and too large over subsidence regions. A reason for this may be known deficiencies in the radiation parameterizations of the forecast model used in the data assimilation. The distribution of latent heat sources, obtained from...

Preliminary measurements of net radiation and evaporation over bare soil and fallow bushland in the Sahel

AbstractNet radiation and evaporation are compared over two contrasting land surfaces, fallow bushland and bare soil, in Niger, West Africa. Data are presented for 6 days, before and after a large rainstorm (39 mm), which illustrate how evaporation from the bush vegetation changed little in comparison with the larger change in evaporation observed over the bare soil. Net radiation over the bush vegetation was 20 per cent greater than that over the dry bare soil, but only 12 per cent greater than that over wet bare soil. These differences are consistent with the expected difference in albedo and surface temperature of the two surfaces.

Reducing Dew and Frost on the Dames of Net Pyrradiometers

Abstract The objective of this study was to improve the accuracy of net radiation (Rn) measurements under conditions conducive to dew or frost deposition. Two nonventilated net pyrradiometers were mounted over grass during November and December 1986. A heating element was located on the supporting arm of each radiometer so that heal would be conducted to the sensing head. Heat was applied to one radiometer for a few days, followed by a period during which no heat was applied. The procedure was repeated, alternating between radiometers throughout the experiment. Heating the radiometers successfully averted the deposition of dew and frost on the domes, which produced errors in Rn as high as 54 W m−2. The effect of heating alone was slightly asymmetric and resulted in a significant decrease in the mean Rn of 8 W m−2 relative to the unheated radiometer. This effect can be compensated for in the calibration of the radiometer.

Solar net radiation over water in a class “A” evaporation pan at Giza, estimated on the base of its close relationship with global radiation

Measurements of net radiation over water in a class “A” pan at Giza during daytime, nighttime, and daily periods for different seasons were compared with the corresponding simultaneous values over short vegetation and to the global radiation. Close linear relationships were obtained in all cases. The close linear relationships between daytime and daily net radiation and global radiation were applied to monthly averages of daily global radiation for a 10-year period to estimate average monthly values of daily and daytime net radiation over pan water. The annual variations of these values were described. Seasonal and annual values of these and other relevant items were given. An idea of the diurnal variation of net radiation over pan water in the different seasons was given by the seasonal variation of the ratio daytime/daily net radiation, as well as by information on the highest and lowest values of net radiation and their times of occurrence, depending on the hourly measurements. The relationships between pan evaporation and potential evapotranspiration, and between net radiation over pan water and pan evaporation were also obtained. The close relationship of radiation and other parameters could be explained by Linacre's formula[1], the energy budget equation, and the characteristics of the surfaces concerned. Such relationships could be applied to other desert climates like Egypt.

Foliage temperature: Effects of environmental factors with implications for plant water stress assessment and the CO2/climate connection

Throughout the summer and fall of 1985, several day‐long sets of foliage temperature measurements were obtained for healthy and potentially transpiring water hyacinth, cotton, and alfalfa plants growing in a sealed and unventilated greenhouse at Phoenix, Arizona, along with concurrent measurements of air temperature, vapor pressure and net radiation, plus, in the case of the water hyacinths, leaf diffusion resistance measurements. Some data for these plants were additionally obtained out of doors under natural conditions, while dead, nontranspiring stands of alfalfa and water hyacinth were also monitored, both out of doors and within the greenhouse. Analyses of the data revealed that plant nonwater‐stressed baselines, i.e., plots of foliage‐air temperature differential versus air vapor pressure deficit for potentially transpiring vegetation, were (1) curvilinear, as opposed to the straight lines which have so often appeared to be the case with much smaller and restricted data sets, and (2) that these baselines are accurately described by basic theory, utilizing independently measured values of plant foliage and aerodynamic resistances to water vapor transport. These findings lead to some slight adjustments in the procedure for calculating the Idso‐Jackson plant water stress index and they suggest that plants can adequately respond to much greater atmospheric demands for evaporation than what has been believed possible in the past. In addition, they demonstrate that the likely net radiation enhancement due to a doubling of the atmospheric carbon dioxide concentration will have little direct effect on vegetation temperatures, but that the antitranspirant effect of atmospheric CO2 enrichment on foliage temperature may be substantial.

Estimation of soil heat flux from net radiation during the growth of alfalfa

Soil heat flux studies have indicated that the instantaneous daytime flux can be estimated as a fraction of the net radiation, the ratio ranging from 0.1 to 0.5, depending on the amount of vegetation present and on the time of day. Soil heat flux and net radiation were measured for an alfalfa crop over two regrowth cycles during the fall growing season. For both sparse alfalfa stubble and full vegetative canopy, the surface soil water content did not significantly affect the fraction of net radiation consumed as soil heat flux. The ratio of soil heat flux to net radiation around midday was found to be a linearly decreasing function of crop height only for heights up to 450 mm. As crop growth continued beyond this height, the ratio remained nearly constant at 0.1. The ratio data were also found to be well-fitted by a linearly decreasing function of a spectral vegetation index (near-IR to Red ratio) over both regrowth cycles. These results indicate that both crop height and spectral vegetation indices can be used to estimate soil heat flux from net radiation measurements.

Critical Evaporative Demands for Differential Stomatal Action in Peanut Grown in Narrow and Wide Row Spacings1

AbstractInduced stomatal closure behavior of irrigated peanut (Arachis hypogeaeL. cv. Comet, a bunch type) as related to row spacing and evaporative demand has been previously demonstrated as real and repeatable. The objective of this research was to determine the magnitude of evaporative demand that induced stomatal closure in narrow‐row plantings. Stomates in wide rows tended to show far less closure on the same days. The differences were not noted on days with low evaporative demand. Concurrent measurements of net radiation, dry‐ and wet‐bulb temperature, and horizontal wind velocity were made every 15 min. Estimates of environmental evaporative demand were made. No distinct peak or cumulative evaporative demand levels were found that separated stomatal differential action days from nonaction days. However, analysis of the cumulative advective energy component of cumulative evaporative demand showed a threshold above which stomatal closure responses were noted. This threshold was 8.5 MJ m−2. Further detailed analysis of the factors involved in the cumulative advective energy component indicated stomatal closure was dominated by neither horizontal wind velocity, integrated over time, nor integrated vapor pressure deficit.