Latent Heat Flux Measurements Research Articles

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.

Carbon dioxide and latent heat flux measurements in a windbreak-sheltered orchard

An eddy correlation system for carbon dioxide and latent heat flux measurement comprising a twin-channel, closed-path infra-red gas analyzer and sonic anemometer is described. Its performance was examined by comparing latent heat fluxes measured concurrently with an open-path sensor over windbreak-sheltered kiwifruit ( Actinidia deliciosia var. deliciosia). Measurements were carried out at the height of surrounding windbreaks (6 m). Excellent average agreement was found between latent heat fluxes measured by either system with no evidence of bandwidth limiting by the closed-path components. This agreement gives confidence that carbon dioxide fluxes were also correctly measured. It also provides validation of the density corrections for eddy fluxes which differ significantly for closed- and open-path sensors. Carbon dioxide fluxes were closely related to solar radiation, reaching 800 μg m −2 s −1 towards the canopy early in the afternoon. Water-use efficiency was approximately 7 mg of CO 2 per g of water evaporated and showed little dependence on either radiation or water use. Night-time fluxes were erratic in low wind conditions showing infrequent large excursions which are attributed to the action of occasional gusts sweeping out high levels of carbon dioxide accumulated from canopy and under-storey respiration. During windier conditions, night-time fluxes were typically 200 μg m −2 s −1 away from the canopy. The role of windbreaks in isolating the canopy from the free airstream was evident in the large diurnal variation in ambient carbon dioxide concentrations (640–800 mg m −3) observed 3–4 m above the height of the crop.

A simple system for automated long-term Bowen ratio measurement

The Bowen ratio method is widely used to estimate fluxes of sensible heat and evaporation between a crop and the atmosphere. The most critical points of this method are the measurements of vertical humidity gradients, and long-term operation. We propose a simple system, using a capacitive hygrometer and alternate sampling of air at two levels with pumps to measure humidity gradients. It is well adapted to long-term measurements: low power consumption, low charge of maintenance, and low cost. It has been tested by comparison of hourly sensible and latent heat flux measurements performed by both eddy correlation and Bowen ratio, over a bare soil field and a fully evaporating canopy. Results are satisfactory over a large range of fluxes (0–400 W m −2) under daytime conditions. The main sources and magnitude of errors were investigated under the contrasted situations of bare soil and fully evaporating canopy. The system was shown to give good flux estimates even with very low humidity gradients.

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.

Field-scale application of flux measurement by conditional sampling

Increasing concerns about atmospheric transport of agricultural chemicals and the diffuse emission of greenhouse gases are motivation for development of accurate, straightforward methods for measurement of trace gas fluxes. A recently proposed hypothesis suggests that the flux of a substance is directly proportional to the product of the standard deviation of the vertical wind velocity ( σ w ) and the difference between mean concentrations of that substance in upward- and downward-moving eddies ( ΔC). Reported simulations indicate that the coefficient of proportionality (β) is unaffected by atmospheric stability and may be similar for heat, water vapor, and, presumably, other scalars. This research was conducted to evaluate the validity of this hypothesis and the utility of the method at the farm field scale. The system we developed samples air from the immediate vicinity of a one-dimensional sonic anemometer. There are two sample lines; one is open only when vertical velocity ( w) is positive, and the other is open only when w is negative. These lines are routed to a differential gas analyzer, which simultaneously measures ΔC for both CO 2 and H 2O. A data-logger samples w at 10 Hz, controls the solenoid valves, and computes σ w . It also samples a fine-wire thermocouple to allow computation of sensible heat flux by both eddy correlation and conditional sampling. Field tests were conducted in a soybean ( Glycine max) field at Rosemount, MN. Two data sets of sensible heat flux, each with more than 250 observations, both produced estimates of β = 0.56, comparable with the previously reported estimate of 0.6 ± 10%. We show that this is not an altogether empirical value, but is derivable from statistical considerations. The value of 0.56 was used in subsequent measurements of latent heat flux (LE) and carbon exchange rate (CER) with an IR gas analysis-based system. There are discretization errors in the sample system that caused systematic underestimation of ΔC, for which we derived an approximate correction. Even with this correction, measurements of LE still tended to be somewhat lower than residual energy balance estimates of LE. In one 4 day trial, the slope of measured LE vs. residual-estimated LE was 0.96, whereas in a second trial the slope was only 0.86. Although such results indicate that further research is necessary, they also provide encouragement that such work will be worthwhile.

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.

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.

An overview of the results of the FIFE-87 and FIFE-89 campaigns

Results of a major land surface-atmosphere exchange study, the First ISLSCP Field Experiment (FIFE) are summarized. Findings result from nearly 80 days of field measurements over a 15 kilometer square area involving 30 science teams, seven remote sensing aircraft, and 5 satellites which acquired over 1200 low resolution and 35 high spatial resolution satellite images, maps of remotely sensed temperature, vegetation index and soil moisture, surface flux measurements from more than 20 ground stations and airborne sensors, and transects of vegetation, soil moisture and soil chemistry. Relationships between in-situ, airborne, and spaceborne remote measurements and the problem of scaling will be discussed. Energy balance comparisons among sites were made. Diurnal measurements of latent heat fluxes indicated a strong correlation between the evaporative fraction at midday and the daytime average value.

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.

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.

Sensible and Latent Heat Flux Measurements over the Ocean

This papar presents an extensive act of sensible heat (Reynolds flux and dissipation methods) and latent heat (dissipation method) flux measurements from a stable deep water tower and from ships on the deep sea. Operational difficulties associated with ship spray and flow distortion and with sensor calibration, response and contamination are discussed. The influence of atmospheric stability on the dissipation measurements and the bulk transfer coefficients is considered and a parameterization of Z/ L in terms of wind speed and the sea-air potential temperature difference is found to be adequate. Temperature variances, Stanton numbers and w–t cospectra from the Roynolds flux measurements are compared to previous results. The dissipation method is shown to be a viable means of measuring the heal fluxes over the deep sea by comparison with simultaneous Reynolds flux measurements, using our data for the sensible heat and the data of others for the latent heat. The neutral drag coefficient at 10 m height, CDN, because it is relatively well established, is used to check the performance of the shipboard measurements The dissipation sensible and latent heat fluxes are well described, on average, by the neutral transfer coefficients at 10 m height, CTN and CEN, respectively:Previously published results are considered, indicating that 103 CTN = 0.75 may be preferable in stable conditions Some data suggest a slight wind-speed dependency above 10 m s−1, which is mostly accounted for with CTN and CEN proportional to CDN½, as implied by constant roughness lengths A bulk aerodynamic method of estimating the heat fluxes from CDN, CTN and CEN, wind speed, sea temperature, and air temperature and humidity is described and compared to time series of the dissipation method boat fluxes. Potential problem with the data are discussed using the time series.

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.

Two Examples of Planetary Boundary Layer Modification Over the Great Lakes

Abstract Aircraft measurements of sensible and latent heat fluxes, surface and air temperature, mean wind and humidity were used to examine the boundary layer structure over the Great Lakes for two cases in late fall when the water was warmer than the air above. The sensible and latent vertical turbulent heat fluxes at the surface were in the range 5–10 mW em−2, and 6–15 mW em−3, respectively. Estimates of the mean vertical velocity at the interface between the mixed layer (where the equivalent potential temperature lapse rate was ∼1C km−1 and the mixing ratio was effectively constant) and the stable air above were obtained from the equations for the rate of change of sensible plus latent heat and of water vapor in the boundary layer. The values obtained by this method appear to be reasonable when compared with estimates obtained from the wind field. Using this mean vertical velocity, the vertical fluxes of latent and sensible heat and liquid water in the cloud layer just below the top of the mixed layer ...

Some evaluations of drag and bulk transfer coefficients over water bodies of different sizes

Three recent experiments allow evaluation of the bulk transfer coefficients for momentum, water vapour and sensible heat over water bodies of different sizes. As part of a study of evaporation rates from a swamp, measurements of latent and sensible heat fluxes were made over Lake Wyangan in southern N.S.W., Australia. This lake is of several kilometers diameter. In a later experiment, Reynolds stress and sensible heat transfer were measured from a natural-gas platform standing in Bass Strait, south of mainland Australia. The most recent experiment involved the direct measurement of each of these turbulent fluxes from a fixed tower erected in Lake Michigan, U.S.A. Perhaps the most important of the results is the finding that drag coefficients measured over Bass Strait are not significantly different from those over Lake Michigan, despite the obvious differences in depth, fetch, and hence surface wave structure. At both locations, drag coefficients are found to increase slightly with increasing wind speed, while at low wind speeds they are not significantly different from those corresponding to aerodynamic smoothness. Near-neutral bulk transfer coefficients for sensible heat and for water vapour are found to be similar. An average value of about 1.4 × 10−3 is obtained. It is emphasized that stability effects should be considered in any discussion of drag coefficients or bulk transfer coefficients. Large errors can result if near-neutrality is incorrectly assumed.