Diploma Thesis
Investigation of conditions initiating free convection using energy exchange measurements
Rafael Eigenmann (09/2007-06/2008)
Support: Thomas Foken, Johannes Lüers
Surface energy exchange measurements of two energy balance stations installed from June 1 to August 31, 2007 during the COPS field campaign within the low mountain range of the Black Forest are investigated in this study with regard to the near-ground generation of free convection events. Eddy-covariance turbulence, radiation and soil heat flux measurements were carried out in the Kinzig valley at both of the sites under investigation. In addition, a Sodar/RASS system and a profile mast were set up at one of the both sites. Turbulent fluxes of momentum, sensible and latent heat as well as of CO2 are processed using a comprehensive software package including all necessary flux correction procedures and quality control with tests on stationarity and integral turbulence characteristics. Footprint analysis revealed highly spatial representivity of the flux measurements for the target land use type at both of the sites. The applied forward Lagrangian footprint model, in combination with the quality results and a check for internal boundary layers, identified sectors of reduced data quality and reliability, thus enabling the application of a procedure of data rejection within further analysis. Degraded data quality could be generally observed at both of the sites towards the valley sidewalls. Energy balance considerations showed an average non-closure of 20.4% and 16.2%, respectively for both of the sites. The imbalance can primarily be attributed to the landscape heterogeneity inducing unconsidered low-frequency flux contributions and advective flux components. The energy exchange measurement results led to the detection of buoyantly driven free convection events (FCEs) in the morning hours within a growing convective boundary layer (CBL) environment and during situations of weak synoptic forcing of convection. These FCEs – occurring on about half of the days for both sites – are assumed to have an essential impact on ABL thermodynamics and structure, and were found to be triggered by a change of the local valley wind system frequently observable in the Kinzig valley. During the short transition period in the morning hours when down-valley winds – prevailing at night – cease and winds start to blow in up-valley direction, a secondary minimum of the friction velocity is recorded. Together with simultaneously occurring high sensible heat fluxes, the drop of the wind speed – registered by the Sodar measurements throughout the whole valley atmosphere – facilitates the conditions for the generation of FCEs indicated by the stability parameter for ζ <-1, as buoyant forces (B) then dominate over shear forces (S) within turbulence production. The stability parameter can be derived from the turbulence measurements. Other parameters such as the ratio of the Deardorff velocity to the friction velocity and the ratio of B/S have been deduced and confirm their capability to denote FCEs. A reduction of the averaging interval of the flux measurements revealed the FCEs to be an assemblage of several convective pulses, of only a few minutes duration, releasing moisture and heat into the ABL. The case-study of COPS IOP8b could not satisfactorily relate the surface-induced FCEs to possible cloud formation by investigating satellite imagery and radar data, as FCEs are mainly assumed to trigger subgrid-scale fair weather cumuli. However, a contribution of the FCEs to the pre-convective environment of a single cell, which developed in the close vicinity of the upper Kinzig valley, cannot be excluded. A slight surplus of FCEs at one site compared to the other could not be completely explained by valley width differences. Further investigations applying large eddy simulation (LES) are necessary.