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- On the determination of the ground heat flux in micrometeorology and its influence on the energy balance closure (2005)
- The ground heat flux (heat exchange between the atmosphere and the soil), plays a major role in micrometeorology. This is especially true for bare soils in the morning hours, but also for agricultural sites at any time of the day. Thus, this dissertation focuses on three issues: firstly, to establish a reliable and accurate measurement method for the ground heat flux. Secondly, to assess the quality of parameterisation approaches. And thirdly, to study the impact of the ground heat flux on the energy balance closure at the earth's surface. Regarding the measurement of the ground heat flux, different methods are tested. It is concluded that the safest way to determine the ground heat flux is calorimetry (to calculate the ground heat flux as the temporal change in the soil heat storage). The second best so-lution is to directly measure or to calculate the soil heat flux at several decimeters depth (the deeper the better) and to apply calorimetry to the soil layer above. All of the tested approaches strongly react to errors in soil temperature measurements; hence, it is ge-nerally recommended to calibrate, install and maintain soil thermometers as accurately as possible. The measurement approaches for the ground heat flux also require knowledge about soil properties characterising the heat transport within the soil. These can be de-termined either indirectly (from other soil properties) or directly (using e.g. heated sen-sors). Generally, the direct measurement revealed several difficulties during the tests presented in this thesis. Their application is only recommended with restrictions. Whenever the ground heat flux cannot be measured directly with the methods identified as accurate, parameterising is the second-best choice. Here, six different para-meterisation approaches are tested. The main finding is that acceptable quality of ground heat flux data can only be achieved with parameterisations including at least some measurements made directly in the soil. All other approaches, relying only on at-mospherical data such as the sensible heat flux or net radiation, exhibit severe drawbacks in the comparison. Finally, the impact of ground heat flux determination on the closure of the energy balance at the earth's surface is found to be large. On the one hand, a correct determination of the ground heat flux cannot solve the problem of energy imbalance; even with the highest quality of ground heat flux data, a considerable lack in the energy balance re-mains. On the other hand, this must not lead to the conclusion that an exact determination of the ground heat flux is unimportant. Using data from simplified determination methods results in an additional energy imbalance. Taking into account all the results of this thesis, three main conclusions can be drawn: firstly, a correct determination of the ground heat flux is possible and easily applicable to experimental data sets. Secondly, a parameterisation exclusively relying on meteorological data and delivering high quality data for the ground heat flux could not be found. For an accurate estimation, at least some soil data are required. And thirdly, determining the ground heat flux accurately plays a major role in closing the energy balance with measured data. Still, the ground heat flux alone cannot explain the energy imbalance of experimental data sets.