PhD Thesis
Free convection and turbulent fluxes over complex terrain
Rafael Eigenmann (08/2008-04/2013)
Support: Thomas Foken
The impact of complex terrain on the land-atmosphere exchange is investigated in this thesis. Here, free convection, a very effective vertical transport mechanism as turbulence is predominantly driven by buoyant forces, is explicitly addressed. Recently, it was shown for certain situations over complex terrain that free convective injections of surface layer air masses into the atmospheric boundary layer (ABL) can alter the ABL properties significantly. This study aims at the general identification and description of such situations of near-ground free convection conditions (FCCs) over complex terrain. For this purpose, data obtained during the COPS (Convective and Orographically induced Precipitation Study) field campaign in summer 2007 were used. Within this project, several surface flux measurement stations were installed, mainly in valleys and on mountaintops of the Black Forest, southwestern Germany. Turbulent fluxes were calculated with the eddy-covariance (EC) method and were used to detect FCCs with the help of a stability parameter. The flux measurements were further combined with ABL profiling measurements (Sodar/RASS) and a large-eddy simulation (LES) model in order to investigate the impact of FCCs on ABL properties. The effect of complex terrain on the energy balance closure and on spatial and temporal flux differences was also studied with these flux data.
FCCs were detected on about 25% of the days during the three month COPS experiment. In situations of weak synoptic forcing, thermally driven orographic (e.g. valley winds) or local wind systems developed over the complex terrain due to heating differences. During the adaption of these wind systems to changing heating differences (e.g. during the reversal of the valley wind from down- to up-valley winds in the morning), the horizontal wind vanished. If, at the same time, the buoyancy flux was positive and enhanced, buoyant forces exceeded the usually prevailing shear forces in the surface layer and FCCs were detected. Moreover, it was demonstrated that FCCs are not restricted to the COPS region. Also, a data set of Nam Co station on the Tibetan Plateau showed FCCs during the reversal of a thermally driven land-lake breeze. However, at this high-altitude site, FCCs were more often detected in the afternoon compared to the COPS region due to the frequent change of heating differences during cloud cover periods.
The Sodar/RASS as well as the LES model showed the presence of coherent updraft structures in the developing early-morning convective boundary layer (CBL) in the Kinzig valley (Black Forest) during FCCs. Spectral analysis of the EC data in these situations indicated the existence of large-eddy turbulent scales – typical for thermal updrafts in the CBL – already close to the ground. An ensemble and time mean analysis of the simulated flow field in the valley further confirmed that the Sodar/RASS was located preferably in an updraft region during FCCs. In a CBL over flat homogeneous terrain, the locations of convective structures would occur randomly. However, over the complex orography of the Kinzig valley, the updraft structures were found to develop in quasi-stationary patterns at specific locations relative to the surrounding mountain ridges. The model further showed that the flux through the valley boundary layer is mainly determined by the flux within these coherent updrafts. In combination with the Sodar/RASS observations, the model also showed that these updrafts deeply penetrated into the stably stratified valley boundary layer up to approximately the height of the surrounding mountains leading to an effective upward counter-gradient transport of surface layer air mass properties during FCCs.
The analysis of the turbulent fluxes at the different COPS sites showed that the flux values were strongly determined by varying land surface characteristics. Also an increase of the Bowen ratio with increasing altitude could be detected. These findings are in accordance with former studies in this area. As expected, the energy balance was found to be unclosed on average during the entire COPS period, with values of the residual typical for heterogeneous landscapes. However, regarding only the periods with FCCs, no residual occurred on average. This is due to the fact that the landscape heterogeneity is of minor importance in case of the more vertical oriented exchange regime during FCCs, so that missing advective flux components became strongly reduced in these situations. Moreover, it was found that in comparable periods with no FCCs, flux components were missing with exactly the proportions of the buoyancy flux ratio, thus suggesting a correction of the energy balance according to the buoyancy flux ratio approach. These results support recent publications on the energy balance closure problem.