Poster, Joint 8th COPS Workshop and CSIP Meeting 2009, Madingley Hall, Cambridge, UK: 2009-10-26 - 2009-10-28
Experimental data of the energy balance, turbulence and Sodar/RASS network installed during the COPS field campaign 2007 are used to investigate the generation and occurrence of near-ground free convection events (FCEs). Special emphasis is put on the valley stations in the Murg, Rench and Kinzig valley (Black Forest Mountains), which are influenced by the frequent occurrence of a strong diurnal valley wind circulation system. The measured high-quality turbulent flux data revealed that free convection is initiated near the ground in situations where high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyant forces then dominate over shear forces within turbulence production. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during the FCE using wavelet transform confirms the large-eddy scale character of the detected plume-like coherent structures.