The main focus of this thesis is the application of process based modelling to esti-mate surface sensible heat fluxes and latent heat fluxes (evapotranspiration) in the specific environment of the Tibetan Plateau. Although several surface model studies already exist on the Tibetan Plateau (see Sect. 2.2), a thorough analysis including eddy-covariance measurements of turbulent fluxes is still missing. Moreover, there is a lack of evaporation measurements above lake surfaces. To my best knowledge, Tobias Biermann and me conducted the first eddy-covariance measurements over a lake sur-face on the Tibetan Plateau. This made it possible to validate a lake sursur-face model.
Based on these preconditions the following research questions have been elaborated.
• Land surface modelling under the specific conditions of the Tibetan Plateau using the land surface scheme SEWAB (Mengelkamp et al., 1999) and validation with eddy-covariance measurements.
• Elaboration of the necessary model parameters: required efforts and impact on model performance.
• Investigation of different methods to correct for the energy balance closure gap and their influence on model performance assessment.
• Potential application of the elaborated model version.
• Specific problems of eddy-covariance data quality on the Tibetan Plateau and mitigation of specific measurement problems occurring with some sonic anemome-ters.
The publications and manuscripts listed on page v contribute to these research ques-tions as follows: Babel et al. (2013, Appendix C) present an adaptation of the land surface scheme SEWAB (Mengelkamp et al., 1999) to the Tibetan Plateau. The adap-tation is designed to consider specific issues of land surface modelling as mentioned in Sect. 2.2. The model performance with respect to turbulent fluxes is investigated by using eddy-covariance measurements above alpine steppe from two nearby sites at Nam Co lake. Parameter sets originating from both standard values and laboratory and in situ measurements are tested in this manuscript. Special emphasis is put on the energy balance closure of turbulent flux measurements and its correction. Besides of using the well known method of distributing the residual according to the Bowen ratio (Twine et al., 2000), a new correction method, suggested by Charuchittipan et al.
(2013, Appendix E), has been applied: The residual is distributed according to the rel-ative contribution of the turbulent fluxes to the buoyancy flux. Charuchittipan et al.
(2013, Appendix E) analyse a comprehensive data set from the LITFASS-2003 cam-paign, Lindenberg, Germany. The influence of the averaging time for eddy-covariance fluxes is intensely studied utilising Ogive analysis, block ensemble averaging, wavelet
1.3. Objectives of the thesis
and quadrant analysis. The manuscript suggests secondary circulations to be responsi-ble for the gap in energy balance closure and attributes a dominant role to the sensiresponsi-ble heat flux. The proposed new closure correction method is based on these experimen-tal findings. As a novel approach, both two correction methods have been applied to the data and the consequences on model performance evaluation is discussed by Babel et al. (2013, Appendix C).
The SEWAB model successfully simulates turbulent fluxes on the Tibetan Plateau, creating several benefits: It can be as a reference for a more simplistic parametrisations as done by Gerken et al. (2012, Appendix B). Therein the simplistic land surface scheme Hybrid is updated with a new soil model, aiming to eliminate a delayed surface response to atmospheric forcing in the original version. SEWAB is successfully utilised for comparisons in example daily cycles. As SEWAB showed no delay in surface response, its simulations have been used to evaluate Hybrid’s responsiveness with cross correlation (Gerken et al., 2012, Appendix B).
Another application is the usage of the simulated timeseries in order to assess land-scape heterogeneity at Nam Co (Biermann et al., 2013, Appendix D). The gappy ob-servations of a wet alpine steppe and a shallow lake could be effectively described by modelled timeseries of SEWAB and a hydrodynamic multilayer model (Foken, 1984) with an extension to shallow water exchange (Panin and Foken, 2005). Turbulent fluxes for both surface types are then compared with the SEWAB simulations of the
“standard” land surface at Nam Co, dry alpine steppe. It could be shown that the differences among land surface types likely exceed the model uncertainty, so the differ-ences are considerable. The effect of this heterogeneity is discussed in terms of using the eddy-covariance data as ground truth for remote sensing.
In order to use eddy-covariance data for model evaluation some issues about data quality should be clarified in advance. Although not included in the thesis Zhou et al.
(2011)1 provide a basis for further usage of eddy-covariance data at the Nam Co Mon-itoring and Research Station for Multisphere Interactions: In addition to standard evaluation of footprint and data quality the occurrence of near-ground free convection events is investigated. It is shown that such events can be created on the Tibetan Plateau already due to changing cloudiness, and their influence on data quality is as-sessed. Furthermore, at one of the CEOP-AEGIS sites the sonic anemometer DAT 600 TR61A probe from Kaijo-Denki is in use. The sensor is not omnidirectional, i.e.
in a certain sector the wind field is disturbed by the sensor structure and irregular friction velocities occur as a consequence. The study by Li et al. (2013, Appendix F) highlights this problem and investigates its influence on scalar fluxes and whether such problems occur also with the commonly used CSAT3, Campbell Scientific Ltd. A sector-wise planar-fit is suggested as an appropriate coordinate rotation to mitigate
such problems.