Impact of Evapotranspiration on Dry Season Climate in the Amazon Forest*

2014 ◽  
Vol 27 (2) ◽  
pp. 574-591 ◽  
Author(s):  
Anna Harper ◽  
Ian T. Baker ◽  
A. Scott Denning ◽  
David A. Randall ◽  
Donald Dazlich ◽  
...  
Keyword(s):  
Heat Flux ◽  
Water Stress ◽  
Sensible Heat Flux ◽  
Bowen Ratio ◽  
Ecosystem Model ◽  
Dry Season ◽  
Amazon Forest ◽  
Moisture Recycling ◽  
Drying Conditions ◽  
Biosphere Model

Abstract Moisture recycling can be an important source of rainfall over the Amazon forest, but this process relies heavily upon the ability of plants to access soil moisture. Evapotranspiration (ET) in the Amazon is often maintained or even enhanced during the dry season, when net radiation is high. However, ecosystem models often over predict the dry season water stress. The authors removed unrealistic water stress in an ecosystem model [the Simple Biosphere Model, version 3 (SiB3)] and examined the impacts of enhanced ET on the dry season climate when coupled to a GCM. The “stressed” model experiences dry season water stress and limitations on ET, while the “unstressed” model has enhanced root water access and exhibits strong drought tolerance. During the dry season in the southeastern Amazon, SiB3 unstressed has significantly higher latent heat flux (LH) and lower sensible heat flux (SH) than SiB3 stressed. There are two competing impacts on the climate in SiB3 unstressed: cooling resulting from lower SH and moistening resulting from higher LH. During the average dry season, the cooling plays a larger role and the atmosphere is more statically stable, resulting in less precipitation than in SiB3 stressed. During dry season droughts, significantly higher LH in SiB3 unstressed is a necessary but not sufficient condition for stronger precipitation. The moistening effect of LH dominates when the Bowen ratio (BR = SH/LH) is >1.0 in SiB3 stressed and precipitation is up to 26% higher in SiB3 unstressed. An implication of this analysis is that forest conservation could enable the Amazon to cope with drying conditions in the future.

scholarly journals Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

2017 ◽  
Vol 21 (7) ◽  
pp. 3401-3415 ◽  
Author(s):  
Nobuhle P. Majozi ◽  
Chris M. Mannaerts ◽  
Abel Ramoelo ◽  
Renaud Mathieu ◽  
Alecia Nickless ◽  
...  
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Surface Energy ◽  
Latent Heat ◽  
Surface Energy Balance ◽  
Sensible Heat Flux ◽  
Dry Season ◽  
Energy Balance Closure ◽  
Net Radiation ◽  
Wet Season

Abstract. Flux towers provide essential terrestrial climate, water, and radiation budget information needed for environmental monitoring and evaluation of climate change impacts on ecosystems and society in general. They are also intended for calibration and validation of satellite-based Earth observation and monitoring efforts, such as assessment of evapotranspiration from land and vegetation surfaces using surface energy balance approaches. In this paper, 15 years of Skukuza eddy covariance data, i.e. from 2000 to 2014, were analysed for surface energy balance closure (EBC) and partitioning. The surface energy balance closure was evaluated using the ordinary least squares regression (OLS) of turbulent energy fluxes (sensible (H) and latent heat (LE)) against available energy (net radiation (Rn) less soil heat (G)), and the energy balance ratio (EBR). Partitioning of the surface energy during the wet and dry seasons was also investigated, as well as how it is affected by atmospheric vapour pressure deficit (VPD), and net radiation. After filtering years with low-quality data (2004–2008), our results show an overall mean EBR of 0.93. Seasonal variations of EBR also showed the wet season with 1.17 and spring (1.02) being closest to unity, with the dry season (0.70) having the highest imbalance. Nocturnal surface energy closure was very low at 0.26, and this was linked to low friction velocity during night-time, with results showing an increase in closure with increase in friction velocity. The energy partition analysis showed that sensible heat flux is the dominant portion of net radiation, especially between March and October, followed by latent heat flux, and lastly the soil heat flux, and during the wet season where latent heat flux dominated sensible heat flux. An increase in net radiation was characterized by an increase in both LE and H, with LE showing a higher rate of increase than H in the wet season, and the reverse happening during the dry season. An increase in VPD is correlated with a decrease in LE and increase in H during the wet season, and an increase in both fluxes during the dry season.

A long-term Climatology of Planetary Boundary Layer Height over the Tibetan Plateau revealed by ERA5

2020 ◽  
Author(s):  
Nils Slättberg ◽  
Deliang Chen
Keyword(s):  
Heat Flux ◽  
Monsoon Season ◽  
Sensible Heat Flux ◽  
Dry Season ◽  
The Tibetan Plateau ◽  
Climate Variables ◽  
Sensible Heat ◽  
Boundary Layer Height ◽  
Surface Sensible Heat Flux

<p>The Planetary Boundary Layer Height (PBLH) is important for the exchange of energy, water, and momentum between the surface and the free atmosphere, making it a significant factor in studies of surface climate and atmospheric circulation. Over the Tibetan Plateau (TP) - a vast elevated heat source exerting significant influence on the Asian monsoon systems - the climate is changing rapidly. Among the many climate variables expected to change as global temperatures rise is the PBLH which, in addition to temperature profile, mechanical turbulence production, vertical velocity, and horizontal advection, is highly dependent on the surface sensible heat  fluxes. Our understanding of PBLH over the TP is very limited, although scattered estimates has indicated that it sometimes reach unusual heights – up to the vicinity of the tropopause. Long-term assessment of PBLH covering the whole TP is hampered by the fact that observations are scarce in time and space. This study takes advantage of a recently available high-resolution reanalysis (ERA5) for 1979-2018 to create a multi-decadal climatology of PBLH over the TP, and assess the seasonality, interannual variation and long-term trend of PBLH in relation to other climate variables such as tropopause height and surface sensible heat flux as well as large-scale atmospheric circulation. </p><p>The results show that the most prominent feature of the PBLH trend is a large region of decline in the central TP during the monsoon season. Notably, this is a region where the temperature increase is smaller than in the rest of the region, and the precipitation shows a statistically significant increasing trend. Increased cloudiness may therefore have decreased the surface heating and thus the sensible heat flux and PBLH. Assessing the spatially averaged trends for the first and second halves of the period separately reveals that the monsoon season PBLH does in fact increase during the first half of the period. In the dry season in contrast, the spatially averaged PBLH decreases by almost 30 meter per decade during the first half of the period and increases slightly in the second. Although none of the spatially averaged PBLH trends are statistically significant at the 95% level, it can be noted that the shift from decreasing to increasing PBLH for the dry season is in accordance with a recent study of spring sensible heat flux over the TP. The aforementioned study found that although the sensible heat flux has been declining because of wind speed decreases, it has recently started to recover in response to an increased difference between the ground surface temperature and the air temperature. Given that the PBLH is highly dependent on the surface sensible heat flux, this decline and recovery may very well have produced the PBLH trends for the dry season. In the monsoon season, with cloudy conditions and less solar radiation reaching the ground, other factors are likely of greater importance for the PBLH.</p>

scholarly journals Seasonal and inter annual variability of energy exchange above a boreal Scots pine forest

2010 ◽  
Vol 7 (4) ◽  
pp. 6441-6494 ◽  
Author(s):  
S. Launiainen
Keyword(s):  
Heat Flux ◽  
Scots Pine ◽  
Seasonal Cycle ◽  
Sensible Heat Flux ◽  
Bowen Ratio ◽  
Pine Forest ◽  
Surface Conductance ◽  
Sensible Heat ◽  
Annual Variability ◽  
Scots Pine Forest

Abstract. Twelve-years of eddy-covariance measurements conducted above a boreal Scots pine forest in Hyytiälä, Southern Finland, were analyzed to assess the seasonal and inter-annual variability of surface conductance (gs) and energy partitioning. The gs had distinct annual course, driven by the seasonal cycle of the Scots pine. Low gs (2–3 mm s−1 in April) restricted transpiration in springtime and caused the sensible heat flux to peak in May–June while evapotranspiration takes over later in July–August when gs is typically 5–7 mm s−1. Hence, during normal years Bowen ratio decreases from 4–6 in April to 0.7–0.9 in August. Sensitivity of gs to ambient vapor pressure deficit (D) was relatively constant but the reference value at D=1 kPa varied seasonally and between years. Only two drought episodes when volumetric soil moisture content in upper mineral soil decreased below 0.15 m3 m−3 occurred during the period. Below this threshold value transpiration was strongly reduced, which promoted sensible heat exchange increasing Bowen ratio to 3–4. Annual evapotranspiration varied between 218 and 361 mm and accounted between 50% and 90% of equilibrium evaporation. The forest floor contributed between 16 and 25% of the total evapotranspiration on annual scale. The fraction stayed similar over the observed range of environmental conditions including drought. The inter-annual variability of evapotranspiration could not be linked to any mean climate parameter while the summertime sensible heat flux and net radiation were well explained by global radiation. The energy balance closure varied annually between 0.66 and 0.95 and had a distinct seasonal cycle with worse closure in spring when large proportion of available energy is partitioned into sensible heat.

scholarly journals Uncertainty analysis of computational methods for deriving sensible heat flux values from scintillometer measurements

2009 ◽  
Vol 2 (3) ◽  
pp. 1383-1417 ◽  
Author(s):  
P. A. Solignac ◽  
A. Brut ◽  
J.-L. Selves ◽  
J.-P. Béteille ◽  
J.-P. Gastellu-Etchegorry ◽  
...  
Keyword(s):  
Heat Flux ◽  
Classical Method ◽  
Sensible Heat Flux ◽  
Bowen Ratio ◽  
Heat Fluxes ◽  
Eddy Correlation ◽  
Time Averaging ◽  
Sensible Heat ◽  
Ratio Measurement ◽  
The Impact

Abstract. The use of scintillometers to determine sensible heat fluxes is now common in studies of land-atmosphere interactions. The main interest in these instruments is due to their ability to quantify energy distributions at the landscape scale, as they can calculate sensible heat flux values over long distances, in contrast to Eddy Correlation systems. However, scintillometer data do not provide a direct measure of sensible heat flux, but require additional data, such as the Bowen ratio (β), to provide flux values. The Bowen ratio can either be measured using Eddy Correlation systems or derived from the energy balance closure. In this work, specific requirements for estimating energy fluxes using a scintillometer were analyzed, as well as the accuracy of two flux calculation methods. We first focused on the classical method (used in standard software). We analysed the impact of the Bowen ratio according to both time averaging and ratio values; for instance, an averaged Bowen ratio (β) of less than 1 proved to be a significant source of measurement uncertainty. An alternative method, called the "β-closure method", for which the Bowen ratio measurement is not necessary, was also tested. In this case, it was observed that even for low β values, flux uncertainties were reduced and scintillometer data were well correlated with the Eddy Correlation results.

scholarly journals Study of energy balance components at different growth stages of green gram grown in new alluvial zone of West Bengal

MAUSAM ◽  
2021 ◽  
Vol 71 (2) ◽  
pp. 315-320
Author(s):  
MONDAL SOUMEN ◽  
BANERJEE SAON ◽  
CHAKRABORTY SHAON ◽  
SAHA SALIL ◽  
MUKHERJEE ASIS
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
West Bengal ◽  
Sensible Heat Flux ◽  
Bowen Ratio ◽  
Green Gram ◽  
Growth Stages ◽  
Sensible Heat ◽  
Net Radiation ◽  
Ground Heat Flux

An experiment was conducted in the experimental farm of Bidhan Chandra KrishiViswavidyalaya, Nadia, West Bengal to study the radiation pattern and its balance over green gram (Vignaradiata var. Samrat). The BREB method was used to determine the sensible heat flux and latent energy. The net radiation was measured through net radiometer and the ground heat flux was measured using Fourier's law. Both the diurnal and seasonal variation of net radiation were studied. Similarly, the energy balance components were studied regularly for different crop growth stages as well as on diurnal basis. It is observed that the net radiation varies from 6.32 Wm-2 to 606.43 Wm-2. The latent heat flux constitutes more than 50% of the net radiation for all growth stages as depicted by energy balance partitioning. The sensible heat flux is partitioned into 10% to 20% of total net radiation throughout the growth stages of green gram, which is the lowest in magnitude among all three energy fluxes. The relationship between Bowen ratio and Vapour pressure deficit (VPD), Bowen ratio and Canopy air temperature difference (CATD) was studied. It was found that Bowen ratio is negatively correlated with VPD but positively correlated with CATD. This study enables to monitor ET pattern through latent heat flux and microclimatic characteristics through sensible and ground heat flux.

scholarly journals On the use of the post-closure methods uncertainty band to evaluate the performance of land surface models against eddy covariance flux data

2015 ◽  
Vol 12 (8) ◽  
pp. 2311-2326 ◽  
Author(s):  
J. Ingwersen ◽  
K. Imukova ◽  
P. Högy ◽  
T. Streck
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Eddy Covariance ◽  
Land Surface ◽  
Sensible Heat Flux ◽  
Land Surface Model ◽  
Bowen Ratio ◽  
Surface Model ◽  
Flux Data ◽  
Uncertainty Band

Abstract. The energy balance of eddy covariance (EC) flux data is normally not closed. Therefore, at least if used for modelling, EC flux data are usually post-closed, i.e. the measured turbulent fluxes are adjusted so as to close the energy balance. At the current state of knowledge, however, it is not clear how to partition the missing energy in the right way. Eddy flux data therefore contain some uncertainty due to the unknown nature of the energy balance gap, which should be considered in model evaluation and the interpretation of simulation results. We propose to construct the post-closure methods uncertainty band (PUB), which essentially designates the differences between non-adjusted flux data and flux data adjusted with the three post-closure methods (Bowen ratio, latent heat flux (LE) and sensible heat flux (H) method). To demonstrate this approach, simulations with the NOAH-MP land surface model were evaluated based on EC measurements conducted at a winter wheat stand in southwest Germany in 2011, and the performance of the Jarvis and Ball–Berry stomatal resistance scheme was compared. The width of the PUB of the LE was up to 110 W m−2 (21% of net radiation). Our study shows that it is crucial to account for the uncertainty in EC flux data originating from lacking energy balance closure. Working with only a single post-closing method might result in severe misinterpretations in model–data comparisons.

scholarly journals Technical note: Using distributed temperature sensing for Bowen ratio evaporation measurements

2018 ◽  
Vol 22 (1) ◽  
pp. 819-830 ◽  
Author(s):  
Bart Schilperoort ◽  
Miriam Coenders-Gerrits ◽  
Willem Luxemburg ◽  
César Jiménez Rodríguez ◽  
César Cisneros Vaca ◽  
...  
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Solar Radiation ◽  
Sensible Heat Flux ◽  
Bowen Ratio ◽  
Temperature Sensing ◽  
Sensible Heat ◽  
Distributed Temperature Sensing ◽  
Available Energy ◽  
Air Column

Abstract. Rapid improvements in the precision and spatial resolution of distributed temperature sensing (DTS) technology now allow its use in hydrological and atmospheric sciences. Introduced by ) is the use of DTS for measuring the Bowen ratio (BR-DTS), to estimate the sensible and latent heat flux. The Bowen ratio is derived from DTS-measured vertical profiles of the air temperature and wet-bulb temperature. However, in previous research the measured temperatures were not validated, and the cables were not shielded from solar radiation. Additionally, the BR-DTS method has not been tested above a forest before, where temperature gradients are small and energy storage in the air column becomes important. In this paper the accuracy of the wet-bulb and air temperature measurements of the DTS are verified, and the resulting Bowen ratio and heat fluxes are compared to eddy covariance data. The performance of BR-DTS was tested on a 46 m high tower in a mixed forest in the centre of the Netherlands in August 2016. The average tree height is 26 to 30 m, and the temperatures are measured below, in, and above the canopy. Using the vertical temperature profiles the storage of latent and sensible heat in the air column was calculated. We found a significant effect of solar radiation on the temperature measurements, leading to a deviation of up to 3 K. By installing screens, the error caused by sunlight is reduced to under 1 K. Wind speed seems to have a minimal effect on the measured wet-bulb temperature, both below and above the canopy. After a simple quality control, the Bowen ratio measured by DTS correlates well with eddy covariance (EC) estimates (r2 = 0.59). The average energy balance closure between BR-DTS and EC is good, with a mean underestimation of 3.4 W m−2 by the BR-DTS method. However, during daytime the BR-DTS method overestimates the available energy, and during night-time the BR-DTS method estimates the available energy to be more negative. This difference could be related to the biomass heat storage, which is neglected in this study. The BR-DTS method overestimates the latent heat flux on average by 18.7 W m−2, with RMSE = 90 W m−2. The sensible heat flux is underestimated on average by 10.6 W m−2, with RMSE = 76 W m−2. Estimates of the BR-DTS can be improved once the uncertainties in the energy balance are reduced. However, applying, for example, Monin–Obukhov similarity theory could provide independent estimates for the sensible heat flux. This would make the determination of the highly uncertain and difficult to determine net available energy redundant.

scholarly journals Estimation of Water Stress on Rice Crop Using Ecological Parameters.

2019 ◽  
Vol 01 (01) ◽  
Author(s):  
Muhammad Saifullah ◽  
Bilal Islam ◽  
Saif-ul Rehman ◽  
Muhmmad Shoaib ◽  
Ehsan ul Haq ◽  
...  
Keyword(s):  
Heat Flux ◽  
Water Stress ◽  
Water Scarcity ◽  
Sensible Heat Flux ◽  
Heat Fluxes ◽  
Rice Crop ◽  
Growth Stages ◽  
Sensible Heat ◽  
Area Index ◽  
Ecological Parameters

About half of world’s population intake rice as a staple food. As being water baby, rice need surplus of water to get targeted yield. Water scarcity has become a global issue therefore it has become a need to enhance the rice yield with reduced amount of water. In this research we used ecological parameters e.g., temperature, pressure, actual vapor pressure, sunshine hours and the extraterrestrial radiation to compute net radiations, ground and sensible heat fluxes on daily basis. Net shortwave radiations were observed as 23087 w/m2 in comparison to net longwave radiations which were 4387 w/m2 for the complete Rice Growth Period (RGP). The soil heat flux Go was observed as 3104 w/m2. Go was observed dependent upon the Leaf Area Index (LAI) with inverse relationship between them. Sensible heat flux (H) was measured as 1771 w/m2 throughout the RGP. H was observed dependent upon net radiations with a direct relationship between them. Rn, Go and H were used as input parameters to compute water stress which determines the excess of water in early growth stages of rice crop and water scarcity in the ripening stage. The flow of methodology is easily applicable at domestic level to determine water stress in rice fields.

Quantifying the controls of Peruvian glacier response to climate

2021 ◽  
Author(s):  
Catriona L. Fyffe ◽  
Emily Potter ◽  
Stefan Fugger ◽  
Andrew Orr ◽  
Simone Fatichi ◽  
...  
Keyword(s):  
At Risk ◽  
Heat Flux ◽  
Energy Balance ◽  
Mass Loss ◽  
Sensible Heat Flux ◽  
Dry Season ◽  
Shortwave Radiation ◽  
Sensible Heat ◽  
Wet Season ◽  
Cordillera Blanca

<p>Peruvian glaciers are important contributors to dry season runoff for agriculture and hydropower, but they are at risk of disappearing due to climate warming. Their energy balance and ablation characteristics have previously been studied only for individual glaciers, with no comparisons between regions. We applied the physically-based, energy balance melt component of the model Tethys-Chloris at five on-glacier meteorological stations: three in the Cordillera Blanca near Huaraz (with glaciers above ~4300 m a.s.l.), and two in the Cordillera Vilcanota east of Cusco (with glaciers above ~ 4800 m). The climate of these regions is strongly seasonal, with an austral summer wet season and winter dry season. </p><p>Our results revealed that at most sites the energy available for melt is greatest in the wet season. This is a consequence of the dry season energy losses from the latent heat flux and net longwave radiation which counter-balance the high dry season net shortwave radiation, which otherwise dominates the energy balance. The sensible heat flux is a relatively small contributor to melt energy in both seasons. Comparison of the five sites suggests that there is more energy available for melt at a given elevation in the Cordillera Vilcanota compared to the Cordillera Blanca. At three of the sites the wet season snowpack was discontinuous, forming and melting within a daily to weekly timescale. Albedo and melt are thus highly variable in the wet season and closely related to the precipitation dynamics. At the highest site, in the accumulation zone of the Quelccaya Ice Cap, 81% of ablation was from sublimation. Sublimation was less important at the lower sites, but it reduces dry season melt. </p><p>Correlation of the NOAA Oceanic El Niño Index (ONI) to the outputs of the two sites with the longest records revealed that the warmer wet season temperatures characteristic of a positive ONI were associated with a decreased albedo, greater net shortwave radiation, a more positive sensible heat flux and increased melt rates.  Air temperature and precipitation inputs were also perturbed at all five sites to understand their sensitivity to climate change. Enhanced mass loss was predicted with a static increase of 2°C or more, even with a +30% precipitation increase, with the lower elevation Cordillera Blanca sites at risk of the greatest mass loss due to warming.</p>

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