PhD Thesis

High-resolution modelling of surface atmosphere interactions and convection development at Nam Co Lake, Tibetan Plateau

Tobias Gerken (03/2009-11/2013)

Support: Thomas Foken, Hans-F. Graf, Michael Herzog

The Tibetan Plateau has recently become an area of increased interest for the atmospheric and environmental sciences. Surface-atmosphere interactions and specifically the exchange of momentum, turbulent energy and water vapour as well as the development of convection are not only important for the surface energy balance and local water resources, but also have influence on the evolution of the monsoon system and climate. High-resolution, numerical atmospheric models with a fully coupled surface model are a valuable tool for the systematic investigation of surface-atmosphere interactions. Nam Co Lake, located at the northern extent of the monsoon's influence, was selected as a complex system in order to study the interaction of the land and the lake in the generation of mesoscale circulations and the development from boundary-layer clouds to moist convection. Turbulent fluxes estimated by eddy-covariance and atmospheric profiles measured by radiosondes are used in this work in conjunction with the ATHAM (Active Tracer High-resolution Atmospheric Model) and Hybrid models. Substantial model development is undertaken for both ATHAM and Hybrid. This means a more consistent formulation of tracer and heat transport in ATHAM and improved model stability. Hybrid has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and surface model base temperature is assumed in each layer and extended to the surface. Compared to eddy-covariance measurements and a Surface-Vegetation-Atmosphere Transport (SVAT) Model there is an overall reasonable model performance, when tested on four days for two sites with variable environmental conditions during the 2009 summer monsoon season. At the same time, errors are reduced by 40-60% compared to the unmodified Hybrid. Subsequently, the coupled modelling system is used for 2-dimensional cross-sections through the Nam Co Lake basin with horizontal resolutions of 200 m and at least 150 vertical layers between the surface and the model top located in the lower stratosphere. The 2-dimensional modelling approach has a tendency to overestimate convective strength due to the underestimation of dry air entrainment and cannot reproduce fully realistic flow fields. Nevertheless, it provides a valuable tool for systematic investigations of environmental factors, where 3D simulations are prohibitively expensive. In simulations with several background wind speeds it is found that the model adequately simulates the mesoscale circulation system between the lake and the surrounding mountain chains. Dependent on the geostrophic wind direction there are two different mechanisms for the triggering of convection: Convective triggering, when overflowing topography, and triggering due to convergence between the lake-breeze front and the background wind. It is concluded that coupled modelling setup is capable of reproducing the system's most important dynamics, such as realistic turbulent surface fluxes, mesoscale circulations and cloud evolution. Thereafter, the influence of the atmospheric profiles of temperature and relative humidity and the uncertainty that arises from them is discussed. Simulations are initialised with profiles based on direct measurements (radiosondes), NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on two days during the summer of 2012. The simulated convection from radiosondes compares reasonably well with weather observations for the first day, but less well for the second day, when large-scale synoptic effects, which are not included in the model, gain importance. The choice of vertical profile information leads to strongly differing convection development, causing modifications of the surface energy balance and thus of the energy and water cycle for Nam Co Lake. With respect to precipitation it is found that a large fraction of the precipitation that is generated in the simulations is deposited within the basin and on the slopes of the surrounding mountain chains and thus locally recycled. This also means that a weather station in the centre of the basin is not representative of the system. Furthermore, Nam Co Lake may be of importance as a water supply for the region. Additionally, the choice of profile and the initial water vapour contents determine the amount of precipitation so that there are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles. The findings from the thesis provide an example of the impacts of surface-atmosphere interactions, mesoscale circulations and convective evolution on the Tibetan Plateau. Scaled to the entire plateau these processes are highly relevant to ecosystems, climate and the water cycle.

last modified 2014-02-06