Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Generation of free convection in a valley due to changes of the local circulation system Rafael Eigenmann, Thomas Foken Dept. Micrometeorology, University of Bayreuth 7 th COPS Workshop 27 th -29 th October 2008, Strasbourg

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Content Data basis of the study Local circulation system in the Kinzig valley Near-ground generation of free convection Parameters indicating free convection Characterisation of free convection event days Conclusion Future plans

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Flux calculation with the software package TK2 Footprint analysis Check for possible internal boundary layers Data basis – COPS energy balance network Station near Fussbach (UBT1ETG) in the Kinzig valley (Eigenmann, 2008; diploma thesis) Turbulence systemSodar/RASSRadiation and soil

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Kinzig valley – local circulation system Sodar measurements at COPS IOP8b: –Down-valley winds (S) at night, up-valley winds (N) during the day Wind direction [°]Wind speed [ms -1 ] Strong collapse of the horizontal wind speed in the morning hours: 6:30 – 8:50 UTC; v h < 1.5 ms -1

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Near-ground generation of free convection Detection of free convection events (FCEs) by the surface eddy-covariance system: Free convection for: –small friction velocities u * –high buoyancy fluxes Induced by the wind speed collapse during the wind direction change

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Further parameters indicating free convection (I) Ratio of the Deardorff velocity w * to the friction velocity u * : z i : visual inspection of a secondary maximum in the reflectivity profile Reflectivity [dB] (Beyrich, 1997)

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Further parameters indicating free convection (II) Buoyancy (B) and shear term (S) of the TKE equation: Wind shear : profile mast

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Circulation system regarding all COPS days Visualisation with the persistence P of wind direction: (Lugauer and Winkler, 2005) all days (n=92) event days (n=23) intermittent days (n=19) non-event days (n=37) Values between 0 and 1

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Free convection events regarding all COPS days 23 free convection event days (25%) Mean duration: 1h 24min Adjustment to the annual cycle of sunrise

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Mean diurnal courses of the classified days Above average values on ‘event days’ (Q E : ~100 Wm -2 ; Q H : ~40 Wm -2 ) Influence on temperature and moisture profiles of the ABL Density effects Support of cloud formation (fair-weather cumuli) latent heat Q E [Wm -2 ] sensible heat Q H [Wm -2 ] all days (n=92) event days (n=23) intermittent days (n=19) non-event days (n=37)

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Conclusions The local circulation system in the Kinzig valley is a powerful trigger mechanism for the generation of free convection events (FCEs) Eddy-covariance systems are able to detect FCEs with the stability parameter ζ for ζ < -1 Further parameters indicate FCEs: w * /u * and B/S FCEs may have a not negligible impact on ABL thermodynamics and its structure Contribution to the pre-convective environment of the cell at IOP8b (?)

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Future plans Follow-on project: Turbulent Fluxes and thermal convection in a valley (SALVE) Analysis of the development of free convection situations in the whole Kinzig valley atmosphere with LES modelling Supported by the flux measurements as initial field and boundary conditions Dependence on valley width and land use ? Virtual towers: attempt to close the energy balance by landscape-scale flux averaging (Prof. Th. Foken, Prof. V. Wirth, Dr. N. Kalthoff)

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Thank you for your attention !

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Further flux measurements at IOP8b Fußbach (UBT1ETG) Latent heat flux Q E Bowen ratio BoAvailable energy at the surface: net radiation Q S * minus ground heat flux Q G

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Definition of free convection Stull, R.B., Meteorology for Scientists and Engineers. 2nd edition. BROOKS/COLE, Pacific Grove, 502 pp.

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Mean diurnal courses of the classified days circulation systemtriggered by u * minimum FCEs on both ‘intermittent’ and ‘event days’ Above-average values of Q H and global radiation on ‘event days‘ stability parameter global radiation [Wm -2 ] sensible heat Q H [Wm -2 ] friction velocity u * [ms -1 ]

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Mean, standard deviation and number

Rafael Eigenmann University of Bayreuth – Department of Micrometeorology Bayreuth Center of Ecology and Environmental Research Example for the calculation of the persistence