A vertical profile of four measurements points water vapour, air temperature is available at these towers giving the opportunity to compare estimates of storage terms of different locations. Grey circle denotes main tower and black circles denote additionally installed towers. Only study-relevant setup is shown. The measurement of net radiation has been subject of several studies e.
Brotzge and Duchon ; Duchon and Wilk ; Field et al. The evaluation of the accuracy of the net radiation measurements is complicated as there is no accepted standard for net radiation measurements e. Kohsiek et al. More recently, it was recommended to measure the individual components separately Brotzge and Duchon ; Halldin ; Kohsiek et al. Net radiation could be referenced to independent measurements of the four individual components at TH following a procedure as found in, e. Vogt et al. All investigated net radiometers are regularly maintained visual inspection, cleaning, changing of domes if necessary although at different intervals.
However, we performed some simple tests to check for gross errors in the available measurements of net radiation at least and focussed on shortwave radiation as the main component of net radiation. First global radiation hourly daytime values was modelled for selected clear sky days using the radiative transfer model Streamer Key All simulations were done for cloudless conditions using a standard profile of the atmosphere optimised for mid-latitude summer combined with a rural standard model of aerosols. In a second step, the measurements of global radiation mean daily values were compared with global radiation of nearby stations, and thirdly, the ratio half-hourly values of net radiation R net to global radiation RG was inspected for daytime conditions as net radiometers are primarily calibrated in the shortwave band.
Information found in the literature Halldin and Lindroth ; Kohsiek et al. The chosen approach is a combination of two methods. The soil heat flux G Z is measured by heat flux plates at depth z , and calorimetry is used to account for the storage between the heat flux plate and soil surface. This combination of methods is expected to perform well, if the heat flux plates are installed deep enough e. Mayocchi and Bristow ; Liebethal et al. Heat flux plates alone are often reported to underestimate the ground heat flux and the importance of G S is underlined e.
Mayocchi and Bristow ; Ochsner et al. However, most of the studies were performed at agricultural sites or sites with bare ground where the soil heat flux is quite large compared to forest sites, but soil heat flux under almost complete crown cover is regarded to be small due to smaller shortwave fluxes and reduction by litter e. Baldocchi et al. The average of the heat flux plates at each site was used to capture effects of soil heterogeneity and variable sun spots to some degree.
As information about soils was limited, a simplified approach was chosen to account for G S using published values of C s and estimated soil temperatures between soil surface and heat flux plate. Use of linearly extrapolated temperature causes that time shift and damping of temperature with depth cannot be reproduced correctly.
However, as the storage term G S is expected to be small, this effect is expected to have a small impact. C s was taken as constant with a value of 1. This is a crucial point because C s changes with changing soil moisture considerably.
Thus in this approach, G S is overestimated for drier conditions and underestimated for wetter conditions. The storage term J due to storage in canopy air and biomass can be written as follows Bernhofer et al. As storage terms become small for daily sums, they are often neglected for longer integration periods. Despite small magnitudes of most storage terms, several studies report a decrease in the imbalance of the energy budgets if storage is included Aston ; Gu et al.
Therefore, storage terms should be carefully estimated. This storage term is caused by temperature changes in the canopy air and can be evaluated according to Eq. Storage change was calculated for each layer and summed up in order to obtain total storage change for every half hour. This procedure might give larger maximum and minimum values in relation to a calculation procedure using only one level Moderow et al. McCaughey ; Oliphant et al.
The storage change of latent heat was calculated in the same way as J H. Usually J E is calculated for the whole canopy using one measurement of humidity at a representative level in the canopy e. Oliphant et al. For reasons of consistency, J E was calculated in the same way as J H on half-hourly basis, i. This term often receives less attention as it is small at least for low canopies and more extensive measurements as well as information about the structure of the canopy are necessary.
Description
Bole temperatures and information about wet biomass were available for all sites of this study, but no detailed information about the contributions of boles, branches or foliage and their temperatures for every site; therefore, J veg was calculated on a half-hourly basis in the following way:. A critical point of this procedure is that variations of temperatures in the different parts of the canopy are not adequately represented and corresponding time lags are blurred.
Gay et al.
A careful estimation of wet biomass is indicated because this parameter can easily change the magnitude of J veg Eq. It should be kept in mind that the magnitude of wet biomass is influenced by the method used for estimation. Additionally, one estimate of wet biomass cannot reflect the situation throughout a heterogeneous canopy, despite a careful selection of a representative sample.
Thus, the wet biomass already bears an uncertainty itself. Radiation energy is absorbed during the process of photosynthesis by plants at daytime according to. Some of the energy fixed by photosynthesis is steadily released by respiration and is already accounted for in sensible heat storage change J H and heat storage change of biomass J veg Blanken et al.
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J C adds energy to the storage and should be therefore positive during day and is zero during periods without photosynthesis, i. Information found in literature was used to estimate the uncertainty ranges. The applied net radiometer types have been subject of several inter-comparison studies Brotzge and Duchon ; Halldin and Lindroth ; Kohsiek et al.
Taking the findings of Kohsiek et al. This estimate was then slightly enlarged to account for the fact that the net radiation measurements could not be compared to a reference. The inspection of the ground heat fluxes G Z did not reveal any hint for large errors for any of the sites. The measurements of the different heat flux plates used to calculate the final ground heat flux G Z were compared with each other. In general, they reflected the expected heterogeneity of ground heat flux below a heterogeneous canopy quite well except for the site with only two heat flux plates and the average of the measurements of these heat flux plates were taken as a good approximation for the true mean value of ground heat flux at the respective site.
Changes between shade and sun were more apparent for sites with very shallow inserted heat flux plates TH, WS and maxima were shifted towards late afternoon Fig. The mean daily variation was clearly changed if G S was added to G Z. The maximum was shifted towards the morning hours and the amplitudes were increased.
G S was positive at WS as a period in spring and early summer was investigated and heat was added to the soil.
Error bars denoting standard deviations were omitted for clarity. However, the scatter was enlarged and the daily maximum was shifted towards the morning. The latter one is more physical as the extrapolated temperatures were obtained without consideration of any time shift. The relative size of G Z and G S in relation to R net was evaluated on the basis of the mean diurnal variation of both terms Fig. It should be noted that during transition periods, G S , G Z and R net are small quantities and the latter one also changes its sign.
Thus, a determination of a ratio may prone to be erroneous, as absolute values are small, and additionally, these terms are not necessarily in phase. This indicates that G S might be of some importance at least during night. However, an importance of G S , as reported for agricultural sites e. These independent estimates of the storage term J H were in good agreement with each other at each site. The weaker correlation at RE can be ascribed to the greater heterogeneity of this site very uneven aged stand with irregularly distributed gaps in relation to the other sites. Thus, storage change of sensible heat does not strongly depend on the placement of the tower within canopies with moderate heterogeneity.
Comparison of sensible heat storage term J H half-hourly differences , calculated for towers B, C, D in relation to tower A at Wetzstein. Mean diurnal patterns of J H were similar at all sites with maximum positive values during the morning and maximum negative values in the evening Fig. Maximum positive values of the mean diurnal variation were slightly reduced if storage change was calculated using one layer only. This also holds for the maximum negative values in the evening but less strong. These results confirm the statement of Aston who noted that J E and J H may be underestimated if not the measurements of the full profile are used.
However, mean values of both approaches were quite similar and differed by a maximal value of 0.