Generation of free convection due to changes of the local circulation system 1Department of Micrometeorology, University of Bayreuth, Bayreuth, Germany
*now at: Institute for Meteorology and Climate Research – Atmospheric Environmental Research (IMK-IFU), Karlsruhe Institut of Technology, Garmisch-Partenkirchen, Germany
Received: 31 March 2009 – Published in Atmos. Chem. Phys. Discuss.: 07 May 2009 Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS
(Convective and Orographically-induced Precipitation Study) field campaign
2007 are used to investigate the generation of near-ground free convection
conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany.
The measured high-quality turbulent flux data revealed that FCCs are initiated
near the ground in situations where moderate to high buoyancy fluxes and a
simultaneously occurring drop of the wind speed were present. The minimum in
wind speed – observable by the Sodar measurements through the whole vertical
extension of the valley atmosphere – 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. Buoyancy then dominates over shear within the
production of turbulence kinetic energy near the ground. 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 FCCs using wavelet transform shows the
occurrence of large-scale turbulence structures. Regarding the entire COPS
measurement period, FCCs in the morning hours occur on about 50% of all days.
Enhanced surface fluxes of latent and sensible heat are found on these days.
Revised: 29 October 2009 – Accepted: 05 November 2009 – Published: 12 November 2009
Citation: Eigenmann, R., Metzger, S., and Foken, T.: Generation of free convection due to changes of the local circulation system, Atmos. Chem. Phys., 9, 8587-8600, doi:10.5194/acp-9-8587-2009, 2009.