The daily courses of the heat balance components over the vegetatively active and inactive surfaces differ substantially. The largest outgoing flux over the inactive surface is the sensible heat flux, used for heating the air. Over the active surfaces (evaporating areas), the largest outgoing flux is always the latent heat flux. This leads to the conclusion that the plant cover and its plant development stage plays a double role in the process of shaping the structure of heat balance of ecosystems. Growing plants intensify the transpiration process, which consumes the larger part of energy from net radiation, the energy which could potentially heat air or soil. On the other hand, when plants are fully developed and ripened, they fulfil an entirely different function. By insulating the influx of heat to soil and reducing evaporation from the surface of the soil (they do not transpire themselves), they become the least evaporating element of the landscape and, at the same time, an element which devotes most of its accumulated heat for the sensible heat flux and consequently for heating the air.
Abstract The Tibetan Plateau and nearby surrounding area (the Third Pole area) dramatically impacts the world’s environment and especially controls climatic and environmental changes in China, Asia and even in the Northern Hemisphere. Supported by the Chinese Academy of Sciences (CAS) and some international organizations, the Third Pole Environment (TPE) Programme is now under way. First, the background of the establishment of the TPE, the establishment and monitoring plans on long-term for the TPE and six comprehensive observation and study stations are introduced. Then the preliminary observational analysis results on atmosphere−land interaction are presented. The study on the regional distribution of land surface heat fluxes is of paramount importance over the heterogeneous landscape of the Third Pole area. A parameterization methodology based on satellite and in situ data is described and tested for deriving the regional surface heat fluxes (net radiation flux, soil heat flux, sensible heat flux and latent heat flux) over the heterogeneous landscape. As a case study, the methodology was applied to the whole Tibetan Plateau area. Eight images of MODIS data and four images of AVHRR data were used for the comparison among winter, spring, summer and autumn, and the annual variation analyses. The derived results were also validated by using the ‘‘ground truth’’ measured in the stations of the TPE. The results show that the derived surface heat fluxes in the four different seasons over the Tibetan Plateau area are in good agreement with the ground measurements. The results from AVHRR were also in agreement with MODIS. It is therefore concluded that the proposed methodology is successful for the retrieval of surface heat fluxes using the MODIS data, AVHRR data and in situ data over the Tibetan Plateau area.
Precipitation changes are primarily constrained by the availability of precipitable water that follows from the at- mospheric water balance equation (e.g. Hartman, 1994; Alessandri et al., 2007). On the other hand, including consideration of the atmospheric energy balance can fur- ther aid analysis of observed and projected precipitation changes (Andrews et al., 2009, 2010). Previous research has shown that precipitation also responds to the change in atmo- spheric radiative imbalance caused by the presence of forcing agents such as greenhouse gases (GHGs) and aerosols (e.g. Liepert et al., 2004; Andrews et al., 2010; Feichter et al., 2004). From the static stability point of view, the heating of the atmosphere by GHGs and the related water-vapor positive feedback leads to a more stable atmosphere, which may decrease convection and rainfall occurrence (Trenberth, 2011). That is, any perturbation to the atmospheric radia- tive cooling may compete or be balanced by a change in precipitation (Andrews et al., 2010). Previous studies have shown that hydrological sensitivity is larger for solar radia- tion forcing compared to GHG effects (e.g. Andrews et al., 2009). Therefore, absorption and reflection of solar radiation by aerosols are particularly effective in reducing global-scale precipitation (Trenberth, 2011; Wentz et al., 2007; Feichter et al., 2004). In this respect, Liepert and Previdi (2009) ex- plicitly showed that the precipitation in coupled GCM can be more than three times more sensitive to aerosols compared to GHGs forcing. Furthermore, Liepert and Previdi (2009) ap- plied a method to thermodynamically constrain global pre- cipitation changes and showed that they are linearly related to the changes in the atmospheric radiative imbalance. The strength of this relationship is controlled by the ratio of the change in global surface sensible heat flux to the change in latent heat flux (Liepert and Previdi, 2009).
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Abstract. We conducted two types of validation for the sim- ulations by MATCRO-Rice developed by Masutomi et al. (2016). In the first validation, we compared simulations with observations for latent heat flux (LHF), sensible heat flux (SHF), net carbon uptake by crop, and paddy rice yield from 2003 to 2006 at the site where model parameters are parame- terized. In the second validation, we compared the observed and simulated paddy rice yields over Japan from 1991 to 2010 between observations and simulations. The 4-year av- erage root mean square errors (RMSEs) of the first valida- tion for LHF and SHF were 18.20 and 15.47 W m −2 , respec- tively. These values for errors are comparable to those re- ported in earlier studies. The comparison of biomass growth during growing periods from 2003 to 2006 at the parame- terization site shows that the simulations were in agreement with the observations, indicating that the model can repro- duce the net carbon uptake by crops well. The 4-year aver- age RMSE of the first validation for crop yield in the same period was 410.6 kg ha −1 , which accounted for 8.1 % of the mean observed yields. The error of the second validation for crop yield was 16.7 % and the correlation of crop yields be- tween observations and simulations from 1991 to 2010 was significant at 0.663 (P < 0.01). These results indicate that MATCRO-Rice has high ability to accurately and consis- tently simulate LHF, SHF, net carbon uptake by crop, and crop yield.
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Figure A1. Schematic representation of the one-dimensional de- scription of STIC1.2. In STIC1.2, a feedback is established between the surface-layer evaporative fluxes and source/sink height mixing and coupling, and the connection is shown in dotted arrows between e 0 , e 0 ∗ , g A , g C , and λE. Here, r A and r C are the aerodynamic and canopy (or surface in the case of partial vegetation cover) resis- tances, g A and g C are the aerodynamic and canopy conductances (reciprocal of resistances), e ∗ S is the saturation vapor pressure of the surface, e ∗ 0 is the saturation vapor pressure at the source/sink height, T 0 is the source/sink height temperature (i.e., aerodynamic temper- ature) that is responsible for transferring the sensible heat (H ), e 0 is the source/sink height vapor pressure, e S is the vapor pressure at the surface, z 0 is the roughness length, T R is the radiometric surface temperature, T SD is the source/sink height dew-point temperature, M is the surface moisture availability or evaporation coefficient, R N and G are net radiation and ground heat flux, T A , e A , and D A are temperature, vapor pressure, and vapor pressure deficit at the refer- ence height (z R ), λE is the latent heat flux, and H is the sensible heat flux, respectively.
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Abstract. Surface fluxes are important boundary condi- tions for climatological modeling and Asian monsoon sys- tem. The recent availability of high-resolution, multi-band imagery from the ASTER (Advanced Space-borne Thermal Emission and Reflection radiometer) sensor has enabled us to estimate surface fluxes to bridge the gap between local scale flux measurements using micrometeorological instruments and regional scale land-atmosphere exchanges of water and heat fluxes that are fundamental for the understanding of the water cycle in the Asian monsoon system. A parameteriza- tion method based on ASTER data and field observations has been proposed and tested for deriving surface albedo, sur- face temperature, Normalized Difference Vegetation Index (NDVI), Modified Soil Adjusted Vegetation Index (MSAVI), vegetation coverage, Leaf Area Index (LAI), net radiation flux, soil heat flux, sensible heat flux and latent heat flux over heterogeneous land surface in this paper. As a case study, the methodology was applied to the experimental area of the Coordinated Enhanced Observing Period (CEOP) Asia- Australia Monsoon Project (CAMP) on the Tibetan Plateau (CAMP/Tibet), located at the north Tibetan Plateau. The ASTER data of 24 July 2001, 29 November 2001 and 12 March 2002 was used in this paper for the case of summer, winter and spring. To validate the proposed methodology, the ground-measured surface variables (surface albedo and
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To understand the detailed vertical structure of cloud and rainfall conditions, we produced a longitude–vertical (in pressure coordinates) cross section of total hydrom- eteors (both liquid and solid clouds and precipitation) and specific humidity along the 18°N latitudinal band (Fig. 8). The vertical structure of the PSMsens of hydrom- eteors showed an increase in the lower troposphere and a decrease in the mid and upper troposphere (Fig. 8a). This indicated an increase in low-level clouds and a decrease in deep convections due to the increase in soil moisture. In the lower troposphere, an increase in water vapor was also distinct (Fig. 8b). Because an increase in soil moisture induced an increase in latent heat flux and a decrease in sensible heat flux, changes in vertical structure of hydrom- eteors and water vapor are explained by the decrease in planetary boundary layer height due to the decrease in sensible heat flux. In other words, the increases in low- level clouds and water vapor are due to weakening of vertical mixing. In addition, we confirmed that sensitivity in planetary boundary layer height was clearly negative to an increase in soil moisture (Fig. 9).
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The second source for underestimation relates to the steady-state assumption that is being made in the derivation of the limit. In this steady state, power equals dissipation, and in the maximum power limit the driving temperature gradi- ent is in balance with the heat flux. This may not always be the case, particularly on the diurnal timescale of atmospheric boundary layer growth, where these aspects may not have reached such a steady state. Yet, such dynamics would still be exposed to a thermodynamic limit, but this limit would need to account for the diurnal variations in boundary layer development, in which the flux and the depletion of the driv- ing gradient may be temporally offset. One approach where this has been done to some extent is given in Konings et al. (2012), where thermodynamics has been applied to diurnal boundary layer development. However, their study did not consider the feedback on the depletion of the driving gradient that sets the maximum power limit. To describe such tempo- ral dynamics and how the maximum power limit would apply to such dynamics would obviously require us to go beyond the steady-state condition that we used here (see also below). A source for the bias in the partitioning between sensi- ble and latent heat relates to the use of annual means. Our estimate of net radiation is simply linear in absorbed solar radiation (cf. Eq. 8), so that temporal variations average out and the estimate of net radiation would not seem to be much affected by this simplification. The partitioning into sensible and latent heat is also proportional to absorbed solar radia- tion, but is modified by the factors γ /(γ + s) and s/(γ + s). As these factors depend on surface temperature, which in turn depends on absorbed solar radiation, these factors de- pend indirectly on absorbed solar radiation. This covariation between the factors and solar radiation causes a non-linearity in the expressions, and diurnal and seasonal variations would not average out. In fact, since temperatures at high values of solar radiation are generally higher than at low values of solar radiation, our use of annual averages would tend to underes- timate the latent heat flux and overestimate the sensible heat flux. This effect is likely to explain at least in part our low bias that we identified in the comparison of the estimates for evaporation rates.
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observations of reflectance and radiance. The soil heat flux can be parameterized by a simple relation with net radia- tion and canopy structure properties although the coefficients involved need to be calibrated and evaluated for different types of canopies. As a consequence, the aerodynamic re- sistance becomes a very important parameter when estimat- ing sensible heat flux and latent heat flux using remote sens- ing measurements by the energy balance method as shown in Eq. (1). Based on the Monin-Obukhov similarity theory, many authors, such as Monteith (1973), Brown and Rosen- berg (1973), Verma et al. (1976), Louis (1979), Louis (1982), Itier (1980), Riou (1982), Hatfield et al. (1983), Mahrt and Ek (1984), Choudhury et al. (1986), Xie (1988), Byun (1990), Viney (1991), Lee (1997) and Yang et al. (2001), have proposed different parameterizations to estimate aero- dynamic resistances to heat transfer. These parameteriza- tions could be grouped into three categories. One group fol- lows the Monin-Obukohv similarity theory, the other group is the empirical method. The third group includes so-called semi-empirical parameterizations. While these parameteri- zations have been evaluated and given an acceptable agree- ment with data upon which the methods were developed, de- viations were found when applying them to other data.
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started in November 1998 as described in Monson et al. (2002) and Turnipseed et al. (2002, 2003). The tree den- sity around the NWT Tower is ≈ 0.4 trees m −2 with a leaf area index (LAI) of 3.8–4.2 m 2 m −2 and tree heights of 12– 13 m (Turnipseed et al., 2002). In winter, NWT is a dry and windy place. Between November–February, the 30-min av- erage 21.5 m wind speed (U ) is around 7 m s −1 (standard de- viation ≈ 4.5 m s −1 ) with a maximum near 20 m s −1 . Typi- cal wintertime mid-day sensible heat flux values are on the order of 200 W m −2 , while latent heat flux is usually less than 40 W m −2 (Turnipseed et al., 2002). On top of Niwot Ridge (i.e., above tree-line), blowing snow is common (Berg, 1986) and snow/ice particles are often blown downslope over the forest. More information on NWT is available on-line at http://public.ornl.gov/ameriflux/.
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Abstract. The Yongxing air–sea flux tower (YXASFT), which was specially designed for air–sea boundary layer ob- servations, was constructed on Yongxing Island in the South China Sea (SCS). Surface bulk variable measurements were collected during a 1-year period from 1 February 2016 to 31 January 2017. The sensible heat flux (SHF) and latent heat flux (LHF) were further derived via the Coupled Ocean– Atmosphere Response Experiment version 3.0 (COARE3.0). This study employed the YXASFT in situ observations to evaluate the Woods Hole Oceanographic Institute (WHOI) Objectively Analyzed Air–Sea Fluxes (OAFlux) reanalysis data products.
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The Tibetan Plateau (TP) is one of regions with strong land- atmosphere interactions, due to strong solar heating over the Plateau. TP land processes are generally characterized by three features. The first is apparent diurnal variations due to strong solar radiation and low air density. The so- lar irradiance over the Plateau is often observed to exceed 1200 W m −2 near noon (Ma et al., 2005), which results in very strong diurnal change of the surface energy budget and near-surface meteorological variables. For instance, the di- urnal range of the surface skin temperature can exceed 60 K. The second is the distinct seasonal march of the surface wa- ter and energy budget in the central and eastern TP. Before the onset of the monsoon (about the end of May to the mid- dle of June), the surface is relatively dry and the sensible heat flux dominates the surface energy budget; after the onset, the land surface becomes wet due to frequent rainfall events and it is the latent heat flux that dominates the energy budget un- til the withdraw of the monsoon in September. The third is the contrast between the dry western region and the wet east- ern region. Annual precipitation amount is about 400 mm or more in most of central and eastern TP (CE-TP), while it is around 100 mm or less in the western TP (W-TP). Under the unique Plateau climate, the land surfaces are typically char- acterized by alpine meadows and grasslands in CE-TP while by alpine deserts in W-TP.
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Net radiation was measured using a CNR1 four- component net radiometer (Kipp and Zonen, Delft, The Netherlands) at a mounting height of 2.5 m above the ground. Soil heat flux was calculated using a Fourier analysis on mea- sured soil temperatures, combined with estimates of thermal diffusivity and heat capacity, the so-called analytical or exact method (see e.g. Verhoef, 2004). The soil temperatures were acquired with type-NT 10 k thermistors (RS, UK) that had been encapsulated with a stainless steel housing, accuracy of ± 0.2 ◦ C, installed at nominal depths of 2 and 5 cm. Soil heat flux at the surface (z=0), i.e. G, was calculated by us- ing a negative z (i.e. − 0.02 m) in the analytical equation of soil heat flux. Thermal diffusivity was calculated using the Arctangent method (Verhoef et al., 1996), using soil temper- ature signals at both depths, and heat capacity was calculated from the soil moisture content at 5 cm depth, measured using a Thetaprobe (Delta-T Devices).
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The objective of this work is to evaluate the feasibility of moderate resolution satellite data estimating the surface heat balance in a tropical hydroelectric reservoir. Each component of the heat flux balance was computed using the MODIS (Moderate Resolution Imaging Spectroradiometer) water surface temperature (WST) level 2, 1 km nominal resolution data (MOD11L2, version 5) from 2003 to 2008. The consequence of the heat flux exchange in the water column ther- mal structure is also investigated. The passage of cold front over a region decreases the atmospheric pressure and air temperature, enhancing the relative humidity. The sensible flux presents a small variability but an increase occurs due to a convective turbulence caused by front passage. The latent flux decrease but insufficiently to cause a condensation, just the evaporation decreases. The upwelling events are the responsible to maintain the loss of heat after the cold front pas- sage.
It should be pointed out, however, that although precipitation plays an important role in affecting the SST, effect of sensible and latent heat fluxes cannot be neglected. Actually even during the squall line processes, which last for only a few hours, the magnitudes of sensible and latent heat fluxes are comparable to that of rain drop- induced sensible heat flux. Time series of the maximum heat transferred across the air-sea interface by sensible, latent and rainfall-induced heat fluxes (Equation 4.6) during the simulated squall line process is shown in Figure 4.4. It is clear that at the beginning of the simulation, when the rainfall amount is still small, the sensible and latent heat fluxes are greater than the rainfall induced heat flux. With the increase of rainfall amount, the rainfall-induced heat flux increases rapidly and surpasses the sensible and latent heat fluxes. The accumulated rainfall amount increases until t=140 minutes. With no further rainfall, the maximum rainfall-induced heat flux level s off and subsequently begins to decrease. Although during the rainfall process, the rainfall-induced heat flux is larger than the sensible and latent heat fluxes, they are of comparable magnitudes.
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Abstract. The ACASA (Advanced Canopy–Atmosphere– Soil Algorithm) model, with a higher-order closure for tall vegetation, has already been successfully tested and vali- dated for homogeneous spruce forests. The aim of this paper is to test the model using a footprint-weighted tile approach for a clearing with a heterogeneous structure of the under- lying surface. The comparison with flux data shows a good agreement with a footprint-aggregated tile approach of the model. However, the results of a comparison with a tile ap- proach on the basis of the mean land use classification of the clearing is not significantly different. It is assumed that the footprint model is not accurate enough to separate small- scale heterogeneities. All measured fluxes are corrected by forcing the energy balance closure of the test data either by maintaining the measured Bowen ratio or by the attribution of the residual depending on the fractions of sensible and latent heat flux to the buoyancy flux. The comparison with the model, in which the energy balance is closed, shows that the buoyancy correction for Bowen ratios > 1.5 better fits the measured data. For lower Bowen ratios, the correction probably lies between the two methods, but the amount of available data was too small to make a conclusion. With an assumption of similarity between water and carbon dioxide
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Making surface radiation measurements in the harsh arctic environment is a challenging endeavor. Here we have tried to address problems facing the BSRN working group “Cold Climate Issues” by identifying issues and providing possi- ble solutions. However more work needs to be done. The ultimate goal is to prevent tracker aiming errors and contam- ination of the measurements by frost, ice, snow, and rime at all times. This might be achieved by a carefully designed solar tracker operation strategy, having adequate and well- designed radiometer heating and ventilation, and the use of redundant multi-variable radiometers for better data quality control, and the addition of new heated multi-variable ra- diometers with no moving parts that measure solar compo- nents, albeit with lesser accuracy, to cover periods when the solar tracker is inoperable or during periods of severe rim- ing. Perhaps, the incorporation of new instrument types may be required. Establishing viable data quality control, strict dome cleaning schedules, homogenization of the data, and consistent calibration procedures are also necessary for the success of the radiation measurements in the Arctic. Some of these practices will also benefit BSRN stations worldwide, regardless of their location. Last, the addition of upwelling irradiance measurements at stations where they do not exist, and surface latent, sensible, and ground heat flux measure- ments to arctic stations would lead to a better understanding of how residual energy at the surface is utilized. These valu- able additions, in turn, would give modelers of arctic surface processes the information they need to improve their models and ultimately lead to better understanding and forecasts.
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Abstract. As a system is moved away from a state of ther- modynamic equilibrium, spatial and temporal heterogeneity is induced. A possible methodology to assess these impacts is to examine the thermodynamic entropy budget and assess the role of entropy production and transfer between the sur- face and the atmosphere. Here, we adopted this thermody- namic framework to examine the implications of changing vegetation fractional cover on land surface energy exchange processes using the NOAH land surface model and eddy co- variance observations. Simulations that varied the relative fraction of vegetation were used to calculate the resultant en- tropy budget as a function of fraction of vegetation. Results showed that increasing vegetation fraction increases entropy production by the land surface while decreasing the overall entropy budget (the rate of change in entropy at the surface). This is accomplished largely via simultaneous increase in the entropy production associated with the absorption of solar ra- diation and a decline in the Bowen ratio (ratio of sensible to latent heat flux), which leads to increasing the entropy export associated with the latent heat flux during the daylight hours and dominated by entropy transfer associated with sensible heat and soil heat fluxes during the nighttime hours. Eddy covariance observations also show that the entropy produc- tion has a consistent sensitivity to land cover, while the over- all entropy budget appears most related to the net radiation at the surface, however with a large variance. This implies that quantifying the thermodynamic entropy budget and en- tropy production is a useful metric for assessing biosphere- atmosphere-hydrosphere system interactions.
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The implementation details of the hydrological model used in this study, MIKE SHE SW-ET, are presented in Stisen et al. (2011a) and Overgaard and Rosbjerg (2005). Briefly, the model couples ground-water and surface-water modules together with an ET module (Overgaard and Rosbjerg, 2005). The SW-ET module, based on the two-source model of Shut- tleworth and Wallace (1985), uses hydrological modules’ outputs of soil moisture, soil heat flux and fraction of soil and leaf covered by ponded water. Besides these parameters, meteorological observations of air temperature and humidity, wind speed and incoming shortwave radiation and maps of albedo, LAI and land cover are used to solve a set of 10 lin- ear equations for the temperature and humidity of dry and wet soil, dry and wet leaf and inter-canopy air (see Appen- dices A and D in Overgaard and Rosbjerg, 2005, for more details). With those parameters it is possible to estimate the effective soil and leaf temperatures as well as the radiometric surface temperature (LST) and the latent and sensible heat fluxes. Since the model simulates LST, it is possible to cal- ibrate the model against remotely sensed LST in addition to hydrological variables such as hydraulic head or stream outflow. The model used in this study was calibrated for the Skjern river catchment against the above mentioned hy- drological variables, LST taken from the MYD11A1 Aqua- MODIS product, evapotranspiration measured at three flux tower sites placed within the catchment area and soil mois- ture measurements from a distributed sensor network. The calibration methodology will be a topic of a subsequent pa- per.
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above the ocean in the atmosphere. Assuming a constant-flux layer in the near-surface part of the atmospheric boundary layer, this flux equals the exchange flux between ocean and atmosphere. The purpose of this paper is the comparison of long-term flux measurements at two different heights above the Baltic Sea to investigate this assumption. The results are based on a 1.5-year record of quality-controlled eddy-covariance measurements. Concerning the flux of momentum and of sensible and latent heat, the constant-flux layer theory can be confirmed because flux differences between the two heights are insignificantly small more than 95 % of the time. In contrast, significant differences, which are larger than the measurement error, occur in the CO 2 flux about 35 % of the time.