Daytime Latent Heat Research Articles

Characterization of the Energy Balance of Wheat Grown under Irrigation in the Hot, Arid Environment of Sudan

An analysis of the crop microclimate is essential for assessing the climate’s appropriateness for cultivation. Here, the Bowen ratio (BR) was used to characterize the energy balance in an irrigated wheat field in a hot, arid environment in Sudan. The hourly BR was calculated using micrometeorological data, including net radiation (Rn) and soil heat flux (G), observed in the 2021–2022 and 2022–2023 growing seasons (December–March) and used to compute hourly daytime latent heat (LE) and sensible heat (H) fluxes during the days before and after irrigation. In both seasons, the observed significant evaporative cooling effect of irrigation led to a daily maximum temperature difference of 2.5–5.7 °C between the wheat field and a nearby meteorological station in a non-vegetated desert area. The energy balance calculation results showed that LE was dominant over H and G. Because BR tended to be negative, H was negative; thus, LE was larger than Rn because of sensible heat advection from the surrounding area. Further, a decrease in G after irrigation indicated an alteration in the soil’s thermal properties. These results provide insights into the micrometeorological response of irrigated wheat to a hot, arid environment and useful information for understanding soil–plant–atmosphere interactions in hot, dry environments.

Investigating the Impacts of Daytime Boundary Layer Clouds on Surface Energy Fluxes and Boundary Layer Structure During CHEESEHEAD19

AbstractStudies of land‐atmosphere interactions under a clear sky and low cumulus cloud conditions are common from long‐term observatories like at the southern great plains. How well the relationships and responses of surface radiative and turbulent heat fluxes determined from these investigations hold for more heterogeneous surfaces in other climate regimes, however, is uncertain. In this study, detailed observations of the surface energy budget and daytime boundary layer properties are analyzed using measurements from the Chequamegon Heterogenous Ecosystem Energy‐Balance Study Enabled by a High‐Density Extensive Array of Detectors 2019 (CHEESEHEAD19) field campaign, July‐October 2019, across a heterogeneous forested landscape of northern Wisconsin. A cloud regime framework is employed to classify consecutive periods of clear skies from lower atmosphere stratiform and cumulus clouds. A seasonal transition from low cumulus to low stratiform periods occurred, together with a diurnal pattern in cloudy or clear sky period dominance. Radiative forcing was highly dependent on sky conditions, leading to changes in the redistribution efficiency of radiative energy by the surface turbulent heat fluxes. During CHEESEHEAD19, small Bowen ratios dominated with daytime latent heat fluxes three times as large as sensible heat fluxes for all sky conditions studied; the forested region, therefore, falls within an energy‐limited regime. The depth of the daytime mixed layer depended upon the sky condition and thermodynamic setting; deeper mixed layers occurred during periods of low cumulus and not clear skies. Profiles of vertical velocity were found to have enhanced variance under low cumulus compared to clear sky periods, suggesting potential for cloud feedbacks on boundary layer structure and surface energy fluxes.

On the cooling potential of urban heating mitigation technologies in a coastal temperate city

We assess the impact of advanced heat-mitigation technologies in a coastal temperate city under heatwave conditions. For the first time urban-heating mitigation scenarios that refer to ‘cool/reflective’ roofs and roads, ‘green/living’ roofs and shading by replacing low urban vegetation with deciduous broadleaf trees are considered at the highly dense-populated city of Athens (Greece). Numerical simulations are performed for a typical see-breeze and a heatwave day with the Weather Research and Forecasting (WRF) model coupled to an urban-canopy model. Highresolution data on vegetation and urban land use, derived from satellite image analysis, are considered. All scenarios show a cooling effect, with the maximum mean daytime temperature reduction in the case of ‘cool/reflective’ roofs and roads. During daytime, the mean ambient-temperature reduction reaches up to 1 °C while for the surface-temperature up to 9.5 °C and 11.5 °C, on the see-breeze and heatwave day respectively. In the case of ‘green/living’ roofs, the mean daytime latent-heat flux is increased (e.g. up to 140 W/m2 on the heatwave day) due to increased evapotranspiration while the surface temperature is more affected during nighttime. Both scenarios result in a sea-breeze attenuation of 0.5–1 m/s. The presence of deciduous broadleaf street trees has a minor impact on mean ambient temperature but an evident reduction in surface temperature. The mean urban-heating reduction ranges from 0.1 °C to 0.8 °C and from 0.3 °C to 1.7 °C during the sea breeze and heatwave day respectively, with the maximum reduction shown in ‘cool/reflective’ roofs and roads and the minimum in ‘shading trees’ scenarios.

Using a paired tower approach and remote sensing to assess carbon sequestration and energy distribution in a heterogeneous sclerophyll forest

The critically endangered Cumberland Plain woodland within the greater Sydney metropolitan area hosts a dwindling refuge for melaleuca trees, an integral part of Australia's native vegetation. Despite their high carbon stocks, melaleucas have not explicitly been targeted for studies assessing their carbon sequestration potential, and especially little is known about their energy cycling or their response to increasing climate stress, precluding a holistic assessment of the resilience of Australia's forests to climate change.To improve our understanding of the role of melaleuca forest responses to climate stress, we combined forest inventory and airborne LiDAR data to identify species distribution and associated variations in forest structure, and deployed flux towers in a melaleuca-dominated (AU-Mel) and in a eucalypt-dominated (AU-Cum) stand to simultaneously monitor carbon and energy fluxes under typical growing conditions, as well as during periods with high atmospheric demand and low soil water content.We discovered that the species distribution at our study site affected the vertical vegetation structure, leading to differences in canopy coverage (75% at AU-Cum vs. 84% at AU-Mel) and plant area index (2.1 m2 m−2 at AU-Cum vs. 2.6 m2 m−2 at AU-Mel) that resulted in a heterogeneous forest landscape. Furthermore, we identified that both stands had comparable net daytime carbon exchange and sensible heat flux, whereas daytime latent heat flux (115.8 W m−2 at AU-Cum vs 119.4 W m−2 at AU-Mel, respectively) was higher at the melaleuca stand, contributing to a 0.3 °C decrease in air temperature and reduced vapor pressure deficit above the melaleuca canopy. However, increased canopy conductance and higher latent heat flux during moderate VPD or when soil moisture was low indicated a lack of water preservation at the melaleuca stand, highlighting the potential for increased vulnerability of melaleucas to projected hotter and drier future climates.

Modeling the Impacts of Urbanization on Summer Thermal Comfort: The Role of Urban Land Use and Anthropogenic Heat

AbstractUrban land use and anthropogenic heat (AH) emission can considerably influence the human thermal comfort during extreme heat events. In this study, a spatially heterogeneous AH emission data and updated urban land use data are integrated into the Weather Research and Forecasting model to simulate the physical processes of urban warming during summer. Simulations conducted in the Yangtze River Delta (YRD) of east China suggest that the mean urban heat island intensity reaches 1.49 °C in urbanized areas during summer, with AH emission making a considerable contribution. The warming effect due to urban land use is intensified during extremely hot days, but in contrast, the AH effects are slightly reduced. Urban development increases the total thermal discomfort hours by 27% in the urban areas of YRD, with AH and urban land use contributing nearly equal amount. By limiting the daytime latent heat release, urban land use reduces the daily maximum heat stress particularly during extremely hot days; however, such alleviations can be offset by the AH emission. Strategies for mitigation of urban heat island effect and heat stress in cities should therefore include measures to reduce AH emission.

Surface energy exchange: Urban and rural forest comparison in a tropical montane cloud forest

Topical urban expansion has undergone a great increase, affecting land cover and altering ecosystems. It is important to know physical processes alterations such as surface energy balance in ecosystems surrounded by expanding cities in order to make predictions, planning and management of urban forests. This study determines the surface energy balance of a fragment of tropical montane cloud forest surrounded by a city (urban forest) in the central region of eastern Mexico, and compares it to values obtained in a large matrix of continuous forest adjacent to the city (rural forest) with similar topography. Surface energy exchange was measured by the eddy covariance technique. The urban forest presented values of daytime latent heat flux 4% higher compared to the rural forest (equivalent to 24.7 W/m2), and values of surface heat flux 4% lower (equivalent to 6.4 W/m2). Daily values ​​of energy closure ratio​ (ECR, 0.98) and energy balance ratio (EBR, 0.96) indicated that a greater amount of energy is partitioned from the system by sensible and latent heat fluxes of a urban forests of 30 ha. Also, the urban forest presented a higher diurnal air temperature (mean differences of 0.5 °C and maximum of 1.5 °C) and evapotranspiration rate (mean differences of 0.06 mm/h and maximum of 0.12 mm/h). We believe our work is the first one to compare forests in such contrasting situations, increasing our understanding on the behavior of surface energy exchange of an urban tropical cloud forest, and the intensity of the effect of urbanization on this type of ecosystem.

Subdaily to Seasonal Change of Surface Energy and Water Flux of the Haihe River Basin in China: Noah and Noah-MP Assessment

The land surface processes of the Noah-MP and Noah models are evaluated over four typical landscapes in the Haihe River Basin (HRB) using in-situ observations. The simulated soil temperature and moisture in the two land surface models (LSMs) is consistent with the observation, especially in the rainy season. The models reproduce the mean values and seasonality of the energy fluxes of the croplands, despite the obvious underestimated total evaporation. Noah shows the lower deep soil temperature. The net radiation is well simulated for the diurnal time scale. The daytime latent heat fluxes are always underestimated, while the sensible heat fluxes are overestimated to some degree. Compared with Noah, Noah-MP has improved daily average soil heat flux with diurnal variations. Generally, Noah-MP performs fairly well for different landscapes of the HRB. The simulated cold bias in soil temperature is possibly linked with the parameterized partition of the energy into surface fluxes. Thus, further improvement of these LSMs remains a major challenge.

Effects of snow cover on urban surface energy exchange: Observations in Harbin, China during the winter season

The surface energy balance over Harbin, one of the major cities in severe cold regions of China, is examined during the snow‐covered and snow‐melt periods (January 11 to April 20, 2017) using radiation and eddy covariance measurements. Comparisons of radiation and energy fluxes from fresh snow, cold snow, melting snow, and snow‐free periods revealed that the presence of snow cover has a considerable impact on the radiation and energy balances. The daytime net radiation was mostly dissipated as sensible heat flux with a fraction of 43% during the fresh snow periods. During the cold snow periods, the daytime fraction of sensible heat flux decreased to 31% and most available energy was stored in the urban surfaces. During the melting snow periods, since the snowmelt process consumed a considerable portion of available energy, the daytime fraction of sensible heat flux decreased about 20% when compared to the cold snow periods, while the daytime fraction of latent heat flux increased about 50% due to the evaporation of melt water. After the snow melted, the ratio of daytime latent heat flux decreased because vegetation was not yet active during the snow‐free periods and the Bowen ratio increased from 2.3 (melting snow periods) to 5.9 (snow‐free periods).

The importance of wetlands in the energy balance of an agricultural landscape

Energy fluxes, including net radiation, latent heat flux and sensible heat flux were determined on clear days during the vegetative period in four types of land cover: wet meadow, pasture, arable field, and an artificial concrete surface. The average net radiation ranged between 123 W m−2 at the concrete surface and 164 W m−2 at the wet meadow. The mean maximum daytime latent heat ranged between 500 and 600 W m−2, which corresponds to an evapotranspiration rate of about 0.2 g m−2 s−1 under the prevailing conditions of the wet meadow. The results demonstrated that the wet meadow dissipated about 30 % more energy through evapotranspiration than the field or the pasture, and up to 70 % more energy than the concrete surface. The evaporative fraction indicated that more than 100 % of the energy released by the wet meadow was dissipated through evapotranspiration; this was attributed to local heat advection. Wetland evapotranspiration thus contributes significantly to the cooling of agricultural landscapes; the energy released can reach several 100 MW km−2. Wetland evapotranspiration has a double ‘air conditioning’ effect through which it equalises temperature differences: (1) surplus solar energy is bound into water vapour as latent heat; (2) The vapour moves towards cooler portions of the atmosphere where the energy is released. The air-conditioning effect of wetlands plays an important role in mitigating local climate extremes; this ecosystem service tends to be disregarded in relation to other better-known wetland functions such as nutrient retention and provision of high biodiversity.

Diurnal temperature range variation and its causes in a semiarid region from 1957 to 2006

ABSTRACTThe diurnal temperature range (DTR) is an important indicator of climate change, and it has decreased worldwide since the 1950s, particularly over arid and semiarid regions. This study analyses the effect of meteorological and anthropogenic factors on DTR variation to investigate the possible causes of DTR decreases in semiarid climates. The study region is located in northeast China, and the study period is from 1957 to 2006. There are three main results. First, the rate of decrease in the DTR is −1.24 K per 50 years. This decrease is mainly attributed to the increasing daily minimum temperature rate (Tmin, 2.24 K per 50 years), which is greater than the change in the daily maximum temperature (Tmax, 1.00 K per 50 years). Second, sunshine duration (SD) appears to be the most significant meteorological factor that determines the DTR through downward shortwave radiation (Rsw,d) and surface soil moisture (SM). The effect of Rsw,d is larger for Tmax than for Tmin; therefore, the decrease in Rsw,d results in a smaller increase in Tmax than in Tmin. On the other hand, the increase in SM can strengthen daytime latent heat release, and the increase in Tmax is then slowed because of the cooling effect of evaporation. The precipitation values and the leaf area index show a negative correlation with the DTR, whereas the cloud amount and the relative humidity appear not to be main causes of the DTR decrease in this region. Finally, atmospheric aerosols can reduce the SD by 0.27 h year–1 by decreasing atmospheric transparency, as indicated by an analysis of the Total Ozone Mapping Spectrometer Aerosol Index from 1979 to 2005. The decrease in direct solar radiation is the main cause of decreases in Rsw,d. These findings will provide references for DTR variation studies in similar climates.

ENERGY BALANCE AND CO2 EXCHANGE BEHAVIOUR IN SUB-TROPICAL YOUNG PINE (Pinus roxburghii) PLANTATION

Abstract. A study was conducted to understand the seasonal and annual energy balance behaviour of young and growing sub-tropical chir pine (Pinus roxburghii) plantation of eight years age in the Doon valley, India and its coupling with CO2 exchange. The seasonal cycle of dekadal daytime latent heat fluxes mostly followed net radiation cycle with two minima and range between 50–200 Wm-2 but differed from the latter during the period when soil wetness and cloudiness were not coupled. Dekadal evaporative fraction closely followed the seasonal dryness-wetness cycle thus minimizing the effect of wind on energy partitioning as compared to diurnal variation. Daytime latent heat fluxes were found to have linear relationship with canopy net assimilation rate (Y = 0.023X + 0.171, R2 = 0.80) though nonlinearity exists between canopy latent heat flux and hourly net CO2 assimilation rate . Night-time plant respiration was found to have linear relationship (Y = 0.088 + 1.736, R2 = 0.72) with night-time average vapour pressure deficit (VPD). Daily average soil respiration was found to be non-linearly correlated to average soil temperatures (Y = -0.034X2 + 1.676X – 5.382, R2 = 0.63) The coupled use of empirical models, seasonal energy fluxes and associated parameters would be useful to annual water and carbon accounting in subtropical pine ecosystem of India in the absence high-response eddy covariance tower.

A Numerical Study of the Hydrometeorological Dryline in Northwest India During the Monsoon

This objective of this study is to elucidate the processes that govern the space-time persistence of the hydrometeorological dryline in Northwest India. The working hypothesis is that orographic forcing and land-atmosphere interactions via soil moisture and vegetation processes lock the hydrometeorological line to the Aravalli range and the Thar Desert (a.k.a. the Great Indian Desert). For this purpose, simulations of active and break phases of the 2001 monsoon season were conducted using a mesoscale model (MM5). During the active phases of the monsoon, southeasterly depressions from the Bay of Bengal propagate over northern India, maintaining sustained convergence of moist available energy east of the Aravalli range, leading to increased rainfall and cloudiness patterns consistent with deep convective activity. Drier air originating from the Arabian Sea in the Western Indian Ocean is constrained to the west. During monsoon break phases, moisture convergence from the Bay of Bengal to the Northern India Convergence Zone (NICZ) decreases dramatically, weakening regional circulations east of the Aravalli range. This allows ventilation of the central portion of the NICZ through penetration of westerly dry air, leading to reduced rainfall, lower soil wetness, decrease of latent heat fluxes, and finally lower CAPE and humidity in the lower troposphere. Whereas the inland propagation of monsoon depressions from the Bay of Bengal triggers the onset (demise) of active and break periods, the sustainability of either regime requires strong feedbacks between humidity and stability in the lower troposphere and the surface energy balance: negative in the case of monsoon breaks, positive in the case of active periods. This study shows that, albeit relatively modest (<600 m average elevation), the Aravalli provides sufiicient lift (upwind) and descend (downwind) to organize the spatial distribution of updrafts westward (active phase) and eastward (break phase) of the topographic divide in such a way that low level updrafts are nearly suppressed over the Thar Desert. Sensitivity experiments with modified soil and vegetation cover show that daytime latent heat fluxes (and evapotranspiration) play an important role in the spatial orgnization of CAPE and in triggering light rainfall processes in the semi-arid regions of northwest India, whereas the occurrence of heavy rainfall to the east of the Aravalli range is controlled by large-scale monsoon dynamics.

Spectral Characteristics and Correction of Long-Term Eddy-Covariance Measurements Over Two Mixed Hardwood Forests in Non-Flat Terrain

We present turbulence spectra and cospectra derived from long-term eddy-covariancemeasurements (nearly 40,000 hourly data over three to four years) and the transferfunctions of closed-path infrared gas analyzers over two mixed hardwood forests inthe mid-western U.S.A. The measurement heights ranged from 1.3 to 2.1 times themean tree height, and peak vegetation area index (VAI) was 3.5 to 4.7; the topographyat both sites deviates from ideal flat terrain. The analysis follows the approach ofKaimal et al. (Quart. J. Roy. Meteorol. Soc. 98, 563–589, 1972) whose results were based upon 15 hours of measurements atthree heights in the Kansas experiment over flatter and smoother terrain. Both thespectral and cospectral constants and stability functions for normalizing and collapsingspectra and cospectra in the inertial subrange were found to be different from those ofKaimal et al. In unstable conditions, we found that an appropriate stabilityfunction for the non-dimensional dissipation of turbulent kinetic energy is of the form Φe(ζ) = (1 - b-ζ)-1/4 - c-ζ, where ζ representsthe non-dimensional stability parameter. In stable conditions, a non-linear functionGxy(ζ) = 1 + bxyζc xy (cxy < 1) was found to benecessary to collapse cospectra in the inertial subrange. The empirical cospectralmodels of Kaimal et al. were modified to fit the somewhat more (neutraland unstable) or less (stable) sharply peaked scalar cospectra observed over forestsusing the appropriate cospectral constants and non-linear stability functions. Theempirical coefficients in the stability functions and in the cospectral models varywith measurement height and seasonal changes in VAI. The seasonal differencesare generally larger at the Morgan Monroe State Forest site (greater peak VAI) andcloser to the canopy. The characteristics of transfer functions of the closed-path infrared gas analysersthrough long-tubes for CO2 and water vapour fluxes were studied empirically. This was done by fitting the ratio between normalized cospectra of CO2 or watervapour fluxes and those of sensible heat to the transfer function of a first-order sensor.The characteristic time constant for CO2 is much smaller than that for water vapour. The time constant for water vapour increases greatly with aging tubes. Three methods were used to estimate the flux attenuations and corrections; from June through August, the attenuations of CO2 fluxes are about 3–4% during the daytime and 6–10% at night on average. For the daytime latent heat flux (QE), the attenuations are foundto vary from less than 10% for newer tubes to over 20% for aged tubes. Correctionsto QE led to increases in the ratio (QH + QE)/(Q* - QG) by about 0.05 to0.19 (QH is sensible heat flux, Q* is net radiation and QG is soil heat flux),and thus are expected to have an important impact on the assessment of energy balanceclosure.

Comparative energy exchange from cowpeas ( Vigna unguiculata (L.) Walp cvs. TC-9-6 and M-28-6-6) with differences in canopy architectures and growth durations at the Orinoco llanos

This study analyzes the eddy correlation measurements of mass and energy taken simultaneously over two contrasting cowpeas ( Vigna unguiculata (L.) Walp cvs. TC-9-6 and M-28-6-6). Here, flux densities were measured over an early-maturing and postrate-canopy (TC-9-6) and a medium-maturity and erect-canopy cowpea (M-28-6-6). The results indicates that at maximum crop growth stage, leaf area index was similar in the two cultivars (i.e. 2.13 and 2.15 m 2 m −2 for TC-9-6 and M-28-6-6, respectively), and also that different architectures of the canopy had a minor effect on the flux densities of net radiation as well as the partitioning of available energy into sensible and latent heat. In Tejero, the phenological averaged daytime latent heat flux (LE) to available energy ratio throughout the vegetative and reproductive stages was 0.73 and 0.82, respectively. Over the erect M-28-6-6 canopy, this ratio was 0.75 and 0.86, respectively. As the senescence stage proceeded, the ratio in the early-maturing TC-9-6 (i.e. 0.72) was significantly lower than that in the medium-maturing M-28-6-6 canopy (i.e. 0.81). As consequence of this difference in leaf delayed senescence as well as seasonal variations in daily Bowen ratio, the accumulated seasonal LE over the TC-9-6 (14.8 mm) was 8% lower than that measured over M-28-6-6 (161 mm). Results indicate that the phenological trend of the daily LE was controlled by the leaf area index (LAI) development. When LAI reached its maximum value at the flowering and pod-filling stages, LE was controlled mainly by the available energy and temperature. Consequently, LE was related to equilibrium LE, and the Penman evapotranspiration model for equilibrium was most appropriate for modeling LE. During this stage, the LE/LE L ratio increased linearly with increases of surface conductance. By contrast, during the vegetative and senescence stages, the partial exposed soil surface by the sparsely canopies and the low surface conductance were able to decrease LE. Although the reported results are relevant only to similar agricultural sites in the Orinoco lowlands, the processes and controls described here are representative of cowpeas growing during savanna wet seasons, which are subject to short rainless periods.

EVAPOTRANSPIRATION FROM A BULRUSH‐DOMINATED WETLAND IN THE KLAMATH BASIN, OREGON1

ABSTRACT: Growing‐season evapotranspiration and surface energy and water balances were investigated for an extensive, bulrush‐dominated wetland in the Upper Klamath National Wildlife Refuge of south‐central Oregon, a semi‐arid region with competing demands for scarce water resources. Turbulent fluxes of sensible and latent heat were measured by eddy covariance for 1.2 to 1.9 days during each of four site visits during late‐May to mid‐October 1997. Mean daytime latent heat flux and the Bowen ratio ranged from 148 to 178 W m−2 and from 0.38 to 0.51, respectively, during late May, mid‐July, and late August site visits. By mid‐October, when the plant canopy had senesced, daytime latent heat flux and the Bowen ratio averaged 46 W m−2 and 2.8, respectively. An hourly Penman‐Monteith (PM) model that was fitted to the surface‐flux data provided values for the surface resistance to water‐vapor diffusion that ranged from 78 s m−1 during late August to 206 s m−1 during mid‐October. Similarly, a Priestley‐Taylor (PT) model provided values for the PT multiplier (a) that ranged from 0.96 during late August to 0.37 during mid‐October. The PM and PT models predicted evapotranspiration totals of 560 and 480 mm, respectively, for May 28 to October 12, 1997.

Estimating daytime latent heat flux and evapotranspiration in Jamaica

The suitability of the two-parameter scheme of De Bruin and Holtslag (1982) to estimate daytime latent heat flux and evapotranspiration in Jamaica was examined using meteorological data collected over fields of short grass. The study covered three different periods, January to February of 1994, January to February of 1990, and August to September of 1989. The fields were irrigated regularly (three times a week) to provide enough water to evaporate. The daytime (07:00–18:00 hours) 20 min and hourly latent heat flux values, average evaporation for the earlier mentioned three periods and evaporation on individual days were estimated using De Bruin and Holtslag scheme for two situations. One was with the parameter α′=0.95 and β=20 W m −2 and the other was with α′=1.05 and β=20 W m −2. The analysis of the latent heat flux and evaporation led to the following results. (i) The values of latent heat flux and evaporation estimated using α′=0.95 and β=20 W m −2 compare well with the corresponding measured values for time steps of the order of 20 min to several days. (ii) The level of agreement between estimated and measured hourly values of latent heat flux seen in our work was very close to that reported in the study of De Bruin and Holtslag, implying the consistency of the skill of that scheme at higher average latent heat flux levels in Jamaica. (iii) The choice of 1.05 for α′ applies better for our data. These results indicate that the scheme of De Bruin and Holtslag with α′=0.95 and β=20 W m −2 can satisfactorily be used to estimate daytime latent heat flux and evaporation from well watered grass surfaces in regions having environmental conditions similar to those encountered in Jamaica, but appears better with α′=1.05.

A comparative study of mass and energy exchange over a closed C 3 (wheat) and an open C 4 (corn) canopy: I. The partitioning of available energy into latent and sensible heat exchange

Eddy correlation measurements of mass and energy were made simultaneously over a closed wheat and an open corn crop. This combination of crops, differing in canopy coverage and stomatal physiology, facilitated an investigation on how surface and environmental variables control the partitioning of net radiation into sensible, latent and soil heat exchange. Contrasts in canopy closure had a minor affect on the flux densities of net radiation ( R n) measured over open corn and closed wheat canopies. On the other hand, differences in canopy closure perturbed the partitioning of net radiation into sensible, latent and soil heat exchange. On average, measurements of latent heat flux densities ( LE) over the sparse corn canopy were 59% less than LE measured over the closed wheat crop. Compensation for this bias caused greater flux densities of sensible ( H) and soil ( G) heat to be generated from the sparse corn canopy. Sensible heat flux densities were, on average, 22% greater over the open corn canopy. During the day, soil heat flux densities were, on average, 64% greater under the corn canopy than under the wheat. Available energy ( A) governed daytime latent heat flux densities over the closed wheat canopy. Consequently, LE was proportional to equilibrium evaporation rates ( LE eq). When the corn canopy was dry, latent heat flux densities were less coupled to available energy ( A). This response occurred because exposed soil and the corn's C 4 physiology increased the corn's canopy resistance to water vapor transfer. When the sparse corn canopy was wet, LE approached values experienced by the closed wheat canopy. A rule of thumb, explaining the surface control of latent heat exchange, was extracted from these data. The ratio between LE and LE eq diminished linearly with logarithmic increases of surface resistance. Different processes controlled canopy evaporation at night. Nocturnal evaporation flux densities measured over the closed wheat crop were independent of available energy, and instead were a function of the atmosphere's vapor pressure deficit. Evaporation flux densities measured over the sparse crop were weakly dependent on available energy.

Seasonal and diurnal variation of energy budget components in coniferous forests

This study was part of the large multidisciplinary ecosystem investigations undertaken in Sweden during the 70's. One of the main aims of the abiotic research was to deepen our understanding of the physical processes governing the local- and microclimate of northern coniferous forests. As part of this, the energy budget of a sparse pine forest in central Sweden was intensively studied. Analysis of Bowen ratio measurements during 1977 and 1978 showed that the fraction of net radiation ( R n) utilized for sensible and latent heat ( LE) fluxes was not constant over the season. The ratio LE R n increased from about 40% in May to about 60% in August and September. The mean daytime latent heat flux from a dry canopy was about 135 W m −2 in May, increasing to 150–160 W m −2 in July and August and then decreasing to 100 W m −2 in September. The sensible heat flux decreased from about 190 W m −2 in May to 55 W m −2 in September. The mean daytime energy storage rate was largest in May and September with about 23 W m −2 and smallest in July with 12 W m −2. The Bowen ratio at noon decreased from about 2 in May to 0.5-1 in August and September. During a period of six days, simultaneous measurements of evapotranspiration from the ground vegetation and the whole forest were made. Averaged over the daytime period, about 59% of the net radiation was used for latent heat flux, 32% for sensible heat flux and 9% for storage. Only 13% of the latent heat flux came from the ground vegetation while more than 50% of the sensible heat flux originated from the ground. Comparison between four European coniferous forests showed that the mean daytime latent heat flux varied relatively little. The mean rate was 135 ± 27 W m −2 and there was no correlation between density of forest and rate of latent heat flux.