Sensible Heat Flux Measurements Research Articles

Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes

Line-averaged measurements of the structure parameter of refractive index (C 2 ) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net radiation. The underlying vegetation comprised in the first instance a horizontally homogeneous, pasture sward well-supplied with water, and in the second experiment, a sparse thyme canopy in a semi-arid environment. Atmospheric stability ranged between near neutral and strongly unstable (−2≤ζ≤0). The temperature structure parameterC 2 computed from the optical measurements over four decades from 0.001 to 2 K2 m−2/3 agreed to within 5% of those determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sensor positioned near the midway position of the 100m optical path and at the beam propagation height (1.5m). With the inclusion of cup anemometer measurements, rule-of-thumb assumptions about surface roughness, and Monin-Obukhov similarity theory, path-averaged optical scintillations allow calculation of surface fluxes of sensible heat and momentum via a simple iterative procedure. Excellent agreement was obtained between these fluxes and those measured directly by eddy correlation. For sensible heat, agreement was on average close to perfect over a measured range of 0 to 500 W m−2 with a residual standard deviation of 30 W m−2. Friction velocities agreed within 2% over the range 0–0.9 m s−1 (residual standard deviation of 0.06 m s−1). The results markedly increase the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical foundation are briefly discussed.

Energy budget measurements using eddy correlation and Bowen ratio techniques at the Kinosheo Lake tower site during the Northern Wetlands Study

Fluxes of heat and water vapor were measured on a 20‐m tower at Kinosheo Lake in the Hudson Bay lowlands using eddy correlation and Bowen ratio energy balance techniques. The study period was June 25 to July 28, 1990. Measurements were made over a peat bog consisting of a mixture of sphagnum moss and lichen hummocks and black pools. About 200 m west of the tower were several shallow ponds. The hummocks had a dry, insulating surface and were underlain by an ice layer near 50 cm depth until mid‐July. At the beginning of the period the black pools were covered with water, and although the free water gradually disappeared over the study period, they remained saturated to the end of July. The depth of peat near the tower was about 3 m. Despite the ice layer under the hummocks, their daytime surface temperatures were high, near 35°C, and after the middle of July, above 40°C. Inspection of temperature, precipitation, and radiation data showed that the midsummer period of 1990 was warmer, drier, and sunnier than usual at Moosonee and so by inference at Lake Kinosheo. When all the data were combined to yield average diurnal energy balance components, the eddy correlation fluxes accounted for 90% of the available energy. Latent heat flux averaged 46% of the total available energy and the sensible heat flux averaged 34%. Daytime Bowen ratios were near 1 for the experimental period, suggesting that the bog behaved more like a dryland than a wetland. Eddy correlation measurements of sensible heat and latent heat flux were less than those measured using the Bowen ratio energy balance technique, the average ratios being 0.81 and 0.86, respectively. These differences were possibly due to the difficulty in measuring energy balance components of net radiation and ground heat flux over the mosaic surface.

Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour

This is the second paper describing a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. Two eddy correlation units were operated in the daytime to measure the fluxes of sensible heat and water vapour and other turbulence statistics at various heights within and above the stand. Net radiation was measured above the overstory using a stationary net radiometer and beneath the overstory using a tram system. Supplementary measurements included soil heat flux, humidity above and beneath the overstory, profiles of wind speed and air temperature, and the spatial variation of sensible heat flux near the forest floor. The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. On some days, energy budget closure was far better than on others. The average value of the Bowen ratio was 2.1 above the stand and 1.4 beneath the overstory. The mid-morning value of the canopy resistance was 150–450 s/m during the experiment and mid-day value of the Omega factor was about 0.20. The daytime mean canopy resistance showed a strong dependence on the mean saturation deficit during the two-week experimental period. The sum of sensible and latent heat fluxes beneath the overstory accounted for 74% of the available energy flux beneath the overstory. One of the reasons for this energy imbalance was that the small number of soil heat flux plates and the short pathway of the radiometer tram system was unable to account for the large horizontal heterogeneity in the available energy flux beneath the overstory. On the other hand, good agreement was obtained among the measurements of sensible heat flux made near the forest floor at four positions 15 m apart. There was a constant flux layer in the trunk space, a large flux divergence in the canopy layer, and a constant flux layer above the stand. Counter-gradient flux of sensible heat constantly occurred at the base of the canopy. The transfer of sensible heat and water vapour was dominated by intermittent cool downdraft and warm updraft events and dry downdraft and moist updraft events, respectively, at all levels. For sensible heat flux, the ratio of the contribution of cool downdrafts to that of warm updrafts was greater than one in the canopy layer and less than one above the stand and near the forest floor.

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.

Estimation of surface heat and moisture fluxes over a prairie grassland: 3. Design of a hybrid physical/remote sensing biosphere model

This is the third in a series of papers describing a measurement program and modeling approach to the estimation of surface heat and moisture fluxes over a tallgrass prairie. We describe the design and formulation of an experimental biosphere model (Ex‐BATS) which follows the development of Dickinson's biosphere‐atmosphere transfer scheme (BATS). Ex‐BATS has been designed to incorporate in situ measurements and satellite parameterizations of certain canopy variables which are slowly varying in the course of a growing season. All components of a multistage biosphere process model used to simulate the exchange of heat and moisture between the canopy and the atmosphere are presented here. The remote sensing aspects of the model are described in a companion paper. The procedures used to optimize the model and the validation of the model against First ISLSCP Field Experiment (FIFE) observations taken during 1987 are described. Validation was carried out for three of the four intensive field campaigns (IFCs): 1, 2 and 3. The validation intercomparison shows that the Ex‐BATS model reproduces the diurnal behavior of the surface fluxes very closely. The rms differences between in situ measurements of sensible and latent heat fluxes and their model counterparts are of the order of 35 W m−2. Biases over the three IFCs, which ranged in duration from 10 to 17 days, are typically less than 20 W m −2. The overall bias for the 44 days within the first three IFCs is less than 10 W m −2. The biases and rms differences are of the same order as the accuracy and precision uncertainties of the measured fluxes, therefore the model provides a useful experimental tool to explain radiative, hydrological, and physiological controls on surface fluxes over vegetated canopies and the explicit sources of water flux to the atmosphere from transpiration, foliage evaporation, and soil evaporation.

Comparisons of surface flux measurement systems used in FIFE 1989

Several types of Bowen ratio energy balance and eddy correlation systems were used to estimate sensible and latent heat fluxes during FIFE. To provide information on the ability of these systems to make representative measurements of sensible and latent heat fluxes at individual sites, several systems were compared by operating them side by side. The different Bowen ratio systems produced estimates that were usually in close agreement; most of the discrepancies were minor and were due to differing estimates of Rn and G. Comparisons between results of Bowen ratio systems and eddy correlation systems at sites 906 (2133‐ECA) and 926 (8739‐ECB), however, were less satisfactory. The discrepancies were occasionally large and seemed to vary in complex terrain with local conditions such as wind direction, upwind slope aspect, and solar zenith angle.

Energy budgets and resistances to energy transport in sparsely vegetated rangeland

Partitioning available energy between plants and bare soil in sparsely vegetated rangelands will allow hydrologists and others to gain a greater understanding of water use by native vegetation, especially phreatophytes. Standard methods of conducting energy budget studies result in measurements of latent and sensible heat fluxes above the plant canopy which therefore include the energy fluxes from both the canopy and the soil. One-dimensional theoretical numerical models have been proposed recently for the partitioning of energy in sparse crops. Bowen ratio and other micrometeorological data collected over phreatophytes growing in areas of shallow ground water in central Nevada were used to evaluate the feasibility of using these models, which are based on surface and within-canopy aerodynamic resistances, to determine heat and water vapor transport in sparsely vegetated rangelands. The models appear to provide reasonably good estimates of sensible heat flux from the soil and latent heat flux from the canopy. Estimates of latent heat flux from the soil were less satisfactory. Sensible heat flux from the canopy was not well predicted by the present resistance formulations. Also, estimates of total above-canopy fluxes were not satisfactory when using a single value for above-canopy bulk aerodynamic resistance.

Observations of equilibrium evaporation from a windbreak-sheltered kiwifruit orchard

Eddy correlation measurements of sensible heat and evaporation fluxes from a kiwifruit orchard confirm earlier, less comprehensive measurements that the spatially-averaged evaporation rate from well-watered and well-sheltered orchard blocks in the northern part of New Zealand occurs at the equilibrium rate ( E q . In this humid coastal climate the rapid onset of dewfall after sunset prevents night-time water use, which contrasts with other work showing that nocturnal transpiration may represent 25% of the daily water economy of a kiwifruit vine. Sensible heat fluxes measured at both shelter height ( h) and 2.25 h agreed within 5%, implying the existence of a constant flux region above shelter height. Under the low windspeed conditions common in mid-summer, the aerodynamic resistance to scalar transport between the canopy and the height of the windbreaks is shown to be of comparable magnitude (about 65 s m −1) to both the canopy ‘stomatal’ and boundary-layer resistances. One consequence of this is that transpiration will be less sensitive to stomatal regulation. Independent of imposed wind and radiative conditions, the effect of the close-spaced windbreaks and crop geometry is to create an orchard environment that is an efficient absorber of radiation but which is relatively poorly coupled with overhead conditions through turbulent exchange.

Bowen ratio, eddy correlation, and portable chamber measurements of sensible and latent heat flux over irrigated spring wheat

Measurements of the latent ( LE) and sensible ( H) heat flux density in the atmospheric boundary layer of irrigated crops have applications for understanding processes in agriculture and meteorology and for water management. The objective of this research was to compare measured Bowen ratios and calculated LE and H from four Bowen ratio systems (BR1–BR4) of different design with each other and with fluxes measured by three sets of eddy correlation instrumentation ( H and LE) and a portable chamber ( LE). Measurements were made on 9 and 10 April 1989 in an irrigated wheat field at the Maricopa Agricultural Center near Maricopa, Arizona. The Bowen ratio system designs varied in terms of temperature and humidity sensors and measurement arm movement. Bowen ratios were lower (more negative) on 9 April for all of the systems. The range of the four Bowen ratios was greatest in the early morning and late afternoon ( ±0.1) and least around noon ( +0.02). Measured net radiation and soil heat flux density were constant in the Bowen ratio LE calculations. The range of daytime LE from the four systems on 9 April and from three on 10 April was 11% and 1% of the mean LE, respectively. The three eddy correlation H measurements were essentially equal to each other. The average eddy correlation H was 82% and 69% of Bowen ratio H on 9 and 10 April, respectively whilst the eddy correlation LE was 77% and 67% of Bowen ratio LE on the two days. On 9 and 10 April, portable chamber LE was greater than Bowen ratio LE during periods of southerly winds owing to the effect of advected energy to the southern field edge where chamber measurements were made. On 10 April, portable chamber LE was 125% of Bowen ratio LE. This study has shown that: (1) Bowen ratios from instrumentation of different designs were similar; (2) eddy correlation H from three systems were similar to each other and were slightly less than Bowen ratio H; (3) eddy correlation LE was consistently and significantly less than Bowen ratio LE; (4) measurements of portable chamber LE on the edge of a field were affected by surrounding conditions.

The Sahelian Energy Balance Experiment (SEBEX): Ground based measurements and their potential for spatial extrapolation using satellite data

Large scale changes in vegetation cover in the Sahelian zone of West Africa may produce changes in climate. However, before any reliable forecasts can be made, fundamental data on the physical and biological processes controlling the energy balance of Sahelian vegetation are urgently required for calibrating global circulation models. To provide some of these data the Institute of Hydrology has recently begun a collaborative project with the ICRISAT Sahelian Center called SEBEX; the Sahelian Energy Balance EXperiment. The main objective of this project is to obtain direct measurements of available energy, evaporation and sensible heat flux from contrasting Sahelian land types in order to see how they might be affected by a change in vegetation. To use these data to calibrate climate models it will be necessary to extrapolate them to much larger areas. A second objective of SEBEX is, therefore, to investigate the possibility of using satellite remote sensing to estimate energy fluxes for areas of the size required for making climate forecasts on a regional scale.

Measurements of sensible and latent heat flux in the western equatorial Pacific Ocean

Micrometeorological measurements, including direct eddy‐correlation measurements of heat and moisture fluxes, have been made from shipboard under light‐wind conditions in the western equatorial Pacific warm pool. Air‐sea temperature differences were typically 1.5°–2°C, that is, 1°–1.5°C larger than long‐term averages from merchant ship data. A sea surface “cool skin” of about 0.3°C was observed. Bulk transfer coefficients for both fluxes agree well with the predictions of Liu et al. (1979) in the convective wind speed regime below 4 m s−1. Between 4 and 6 m s−1 values of the neutral exchange coefficients were CEN ‐ 0.89 × 10−3; CHN = 1.03 × 10−3; CDN = 1.16 × 10−3. At zero mean wind speed, latent heat flux is maintained at about 25 W m−2 by convective exchange. Inertial dissipation estimates of the latent heat flux are about 20% lower than the directly measured eddy‐correlation values below 4 m s−1.

A year of Bowen ratios over the frozen Beaufort Sea

The data set obtained during the Arctic Ice Dynamics Joint Experiment (AIDJEX) contains almost a year of 6‐hourly measurements of sensible (Hs) and latent (HL) heat fluxes over multiyear ice in the Beaufort Sea. Here I compute the Bowen ratio, Bo = Hs/HL, for each set of observations and plot histograms of Bo values for the four seasons, spring, summer, autumn, and winter. In every season except summer the Bowen ratio is as likely to be negative as positive; the histograms are, thus, generally bimodal. The Bowen ratio shows a seasonal progression that is related to the seasonally averaged air temperature. In summer, when moisture exchange is most rapid, the peaks in the histogram are near Bowen ratios of 1 and −1. In winter, when the rate of moisture exchange is smallest, the peaks are at Bo>20 and Bo < −20. Spring and autumn are transition seasons with a broader range of values.

Estimation of sensible heat flux from measurements of surface radiative temperature and air temperature at two meters: Application to determine actual evaporation rate

From measurements of sensible heat flux ( H), using the simplified aerodynamic approach, and measurements of surface thermal infrared temperature ( T s), both in arid and semi-arid areas of Tunisia, it is shown that a simple linear relationship can be used to estimate H from the difference ( T s- T a), T a being the air temperature at 2 m. It is also pointed out that this relationship can be used in unstable conditions for various types of surface roughness. Application to determine actual evaporation rate is made using the similarity of the H/ Rn instantaneous values at noon with the average daily values of H/ Rn.

Fetch requirements for bowen ratio measurements of latent and sensible heat fluxes

Simultaneous measurements of the Bowen ratio were made at different fetch-to-height ratios over an aerodynamically smooth plot of bermudagrass which was downwind of an aerodynamically rough field of cotton. The measurements were used to evaluate effects of fetch and measurement height on Bowen ratio estimates of latent and sensible heat fluxes. The measurements indicated that significant boundary-layer adjustment occurred within the first 15 m of fetch. For the conditions encountered, the Bowen ratio and its variability increased with measurement height. However, departure from ideal fetch (measurement within the equilibrium sublayer) had only a small effect on flux estimates. The data suggest that when the Bowen ratio is small, the method can be used successfully at fetch-to-height ratios as low as 20:1, much less than the often-quoted value of 100:1.

Eddy correlation measurements of CO2, latent heat, and sensible heat fluxes over a crop surface

Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO2 sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities. Diurnal fluxes of sensible heat, latent heat and CO2 were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO2 fluxes were near 1.0 mg m−2 (ground area) s−1. The eddy correlation technique was also employed in this study to measure nocturnal CO2 fluxes caused by respiration from plants, soil, and roots. These CO2 fluxes ranged from - 0.1 to - 0.25 mg m−2s−1. From the data collected over mature soybeans, a relationship between CO2 flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 ΜEi m−2 s−1. The light compensation point was found to be about 160 ΜEi m−2 s−1.

Influence of Water Stress on the Diurnal Exchange of Mass and Energy between the Atmosphere and a Soybean Canopy1

AbstractA micrometeorological‐physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well‐watered and water‐stressed soybean (Glycine max (L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully‐developed. Measurements of CO2, latent heat and sensible heat flux were made using the Bowen‐ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well‐watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two‐thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water‐stressed, stomatal closure appears to have been the primary cause of the reduction in LE.Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO2 exchange appears to have been caused by a combination of high stomatal resistance limiting CO2 diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO2‐water flux ratio) was greater when the crop was well‐watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO2‐water flux ratio.

Indirect measurement of the latent heat flux of meadow grass from the energy balance — The sensible heat flux being measured by using the correlation technique

Direct measurements of net radiation, soil heat and sensible heat fluxes are continuously taken over a grass surface at Schoonheeten, The Netherlands. The data are integrated over 1-hr. intervals and the sensible heat is measured by using an eddy correlation technique. The results show no phase lag between sensible heat and radiation densities and a phase lag of ∼1 hr. between sensible heat and latent heat densities. A comparison is given between the real evaporation and the potential one.

A Classification of the Structure of the Tropical Atmosphere and Related Energy Fluxes

Abstract Meteorological data from the tropics have been classified according to a scheme based on the rate of moist convection obtained indirectly from cloud photos. The degree of convection has been categorized into six modes ranging from severely depressed to severely enhanced. Simultaneous high-resolution measurements of tropospheric static energy, longwave radiation, and sensible and latent heat fluxes across the air-sea interface were categorized accordingly. The specific results clearly reveal many of the physical processes associated with the observed changes in the variables which are discussed. They also emphasize the complex interaction of the various processes and scales of motion, which suggests application of classification techniques in parameterization of variables.

Eddy-correlation measurements of sensible heat fluxes over a grass surface

Direct measurements of sensible heat fluxes were conducted over a grass surface at Ladner, British Columbia, using yaw-sphere-thermometer eddy-correlation systems. The results show that for half-hour averaging periods, there is no phase-lag between sensible heat and net radiation flux densities. Field comparison of two yaw-sphere-thermometer systems gave good and consistent agreement. At a height of 2 m above ground and a horizontal crosswind separation of 1.5 m, less than 5% variability was noted in the measured heat fluxes. For a 19-m horizontal separation, the variability was less than 20%. The aridity index (α) advanced by Priestley and Taylor (1972), is shown to be a potentially useful climatic indicator.

Drag and bulk transfer coefficients associated with a shallow water surface

During September, 1971, measurements of Reynolds stress and sensible heat flux were made over a shallow coral reef off the coast of Papua. The water varied (with the tide) from 0 to 2.5 m in depth. Two independent measurements of the momentum flux confirmed that the surface deviated only slightly from aerodynamically smooth. The independent determinations of the sensible heat flux are consistent with this result, indicating a much lower Stanton number than would be expected over deeper water (viz., 1.1 × 10−3 rather than about 1.5 × 10−3). The near smoothness of the surface appears to be associated with either the attenuation of small surface waves resulting from the presence of a surface film or their inability to grow in the absence of longer wave components. Although it is not clear which of these effects was predominant, the data show that great care should be taken in extrapolating shallow water results to open ocean conditions, particularly when it is conceded that surface films may exist in either situation.