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Generation of free convection in a valley due to changes of the local circulation system

Generation of free convection in a valley due to changes of the local circulation system. Rafael Eigenmann, Thomas Foken Dept. Micrometeorology, University of Bayreuth www.bayceer.uni-bayreuth.de/mm. 7 th COPS Workshop 27 th -29 th October 2008, Strasbourg. Content. Data basis of the study

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Generation of free convection in a valley due to changes of the local circulation system

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  1. Generation of free convection in a valley due to changes of the local circulation system Rafael Eigenmann, Thomas Foken Dept. Micrometeorology, University of Bayreuthwww.bayceer.uni-bayreuth.de/mm 7th COPS Workshop 27th-29th October 2008, Strasbourg

  2. 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

  3. Data basis – COPS energy balance network • Station near Fussbach (UBT1ETG) in the Kinzig valley Turbulence system Radiation and soil Sodar/RASS • Flux calculation with the software package TK2 • Footprint analysis • Check for possible internal boundary layers (Eigenmann, 2008; diploma thesis)

  4. Wind direction [°] Wind speed [ms-1] Strong collapse of the horizontal wind speed in the morning hours: 6:30 – 8:50 UTC; vh < 1.5 ms-1 Kinzig valley – local circulation system • Sodar measurements at COPS IOP8b: • Down-valley winds (S) at night, up-valley winds (N) during the day

  5. 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

  6. Reflectivity [dB] Further parameters indicating free convection (I) • Ratio of the Deardorff velocity w* to the friction velocity u*: zi: visual inspection of a secondary maximum in the reflectivity profile (Beyrich, 1997)

  7. Wind shear : profile mast Further parameters indicating free convection (II) • Buoyancy (B) and shear term (S) of the TKE equation:

  8. Values between 0 and 1 all days (n=92) event days (n=23) intermittent days (n=19) non-event days (n=37) Circulation system regarding all COPS days • Visualisation with the persistence P of wind direction: (Lugauer and Winkler, 2005)

  9. 23 free convection event days (25%) • Mean duration: 1h 24min • Adjustment to the annual cycle of sunrise Free convection events regarding all COPS days

  10. all days (n=92) event days (n=23) intermittent days (n=19) non-event days (n=37) Mean diurnal courses of the classified days latent heat QE [Wm-2] Above average values on ‘event days’ (QE: ~100 Wm-2; QH: ~40 Wm-2) Influence on temperature and moisture profiles of the ABL Density effects Support of cloud formation (fair-weather cumuli) sensible heat QH [Wm-2]

  11. 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 (?)

  12. 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)

  13. Thank you for your attention !

  14. Further flux measurements at IOP8b • Fußbach (UBT1ETG) Latent heat flux QE Bowen ratio Bo Available energy at the surface: net radiation QS* minus ground heat flux QG

  15. Definition of free convection Stull, R.B., 2000. Meteorology for Scientists and Engineers. 2nd edition. BROOKS/COLE, Pacific Grove, 502 pp.

  16. circulation system triggered by u* minimum Mean diurnal courses of the classified days global radiation [Wm-2] stability parameter Above-average values of QH and global radiation on ‘event days‘ FCEs on both ‘intermittent’ and ‘event days’ sensible heat QH [Wm-2] friction velocity u* [ms-1]

  17. Mean, standard deviation and number

  18. Example for the calculation of the persistence

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