Department of Micrometeorology University of Bayreuth Determination of the atmospheric boundary layer height in complex terrain during SALSA 2005 Diploma thesis in Geoecology Katharina Staudt September 2006 I Content Abstract ............................................................................................................................. III Zusammenfassung .............................................................................................................IV 1 Introduction ................................................................................................................. 1 2 Theoretical Background ............................................................................................. 3 2.1 Thermally induced wind systems .......................................................................... 3 2.1.1 Slope winds ................................................................................................... 4 2.1.2 „Alpine Pumping“: circulation between mountains and foreland................. 4 2.2 Sodar/RASS........................................................................................................... 6 2.2.1 Measuring principle sodar............................................................................. 6 2.2.2 Measuring principle RASS............................................................................ 8 2.3 The atmospheric boundary layer (ABL)................................................................ 8 2.3.1 The convective boundary layer (CBL)........................................................ 10 2.3.2 The stable boundary layer (SBL)................................................................. 11 2.3.3 The atmospheric boundary layer in complex terrain................................... 11 2.3.4 The atmospheric boundary layer height ...................................................... 14 2.3.4.1 The ABL height determined from measurements ................................... 15 2.3.4.2 Parameterisations..................................................................................... 16 2.3.4.3 Slab models ............................................................................................. 20 3 Data and methods...................................................................................................... 24 3.1 SALSA 2005 ....................................................................................................... 24 3.1.1 Investigation area......................................................................................... 24 3.1.2 Measuring stations....................................................................................... 26 3.2 Data...................................................................................................................... 27 3.2.1 Meteorological Observatory Hohenpeißenberg (MOHp)............................ 27 3.2.2 Sodar/RASS................................................................................................. 27 3.2.3 Radiosondes................................................................................................. 28 3.2.4 Tethered balloon.......................................................................................... 29 3.2.5 Turbulence measurements........................................................................... 29 3.2.6 Lokal-Modell............................................................................................... 30 3.3 Methods for the determination of the atmospheric boundary layer height.......... 30 3.3.1 Determination from sodar/RASS data......................................................... 30 3.3.2 Determination from radiosonde and tethered balloon data ......................... 33 3.3.3 NWP model output (Lokal-Modell)............................................................ 35 3.4 Input data for the parameterisations and the slab models.................................... 36 3.4.1 Parameterisations for the SBL height.......................................................... 36 II 3.4.2 Slab models for the CBL height.................................................................. 36 3.5 Statistical measures to evaluate model performance........................................... 37 4 Results and discussion............................................................................................... 38 4.1 Weather conditions during the experiment.......................................................... 38 4.2 Thermally induced wind systems ........................................................................ 40 4.2.1 Slope winds ................................................................................................. 40 4.2.2 Alpine pumping........................................................................................... 42 4.3 Characterisation of the three golden days ........................................................... 47 4.4 Determination of the atmospheric boundary layer height ................................... 49 4.4.1 The ABL height determined from radiosonde data..................................... 50 4.4.2 The ABL height determined from sodar/RASS data................................... 53 4.4.2.1 Visual inspection of reflectivity profiles ................................................. 53 4.4.2.2 Automatic detection................................................................................. 57 4.4.3 Comparison of ABL heights inferred from sodar and balloon data ............ 62 4.4.4 Parameterisations of the SBL height........................................................... 66 4.4.5 Slab models for the CBL height.................................................................. 71 4.4.6 The ABL height in the Lokal-Modell.......................................................... 75 5 Conclusions ................................................................................................................ 78 6 References .................................................................................................................. 82 7 Index of figures .......................................................................................................... 92 8 Index of tables............................................................................................................ 96 9 Index of used abbreviations and symbols................................................................ 98 9.1 Abbreviations ...................................................................................................... 98 9.2 Latin symbols ...................................................................................................... 98 9.3 Greek symbols................................................................................................... 100 10 Appendix: Technical specifications of measurement systems ............................. 101 Acknowledgements .......................................................................................................... 105 Eidesstattliche Erklärung ............................................................................................... 106 III Abstract This study investigates the atmospheric boundary layer (ABL) height over complex terrain in the Bavarian Alpine foreland. Data was collected during the field experiment SALSA 2005 at Hohenpeißenberg in August and September of 2005. Sodar/RASS measurements, tethered balloon soundings and turbulence measurements were performed at the foot of the mountain, which overtops the surrounding terrain by about 300 m. In the first step, thermally induced wind systems were identified. The mesoscale circulation system between the mountains and the foreland, referred to as Alpine pumping, was observed on four consecutive fair weather days. On the smallest scale of thermally induced wind systems, slope winds occurred. The ABL height was determined for three selected days that were assumed to represent different weather conditions (easterly winds, westerly winds, Alpine pumping). Emphasis was placed on sodar measurements. Visual inspection of reflectivity profiles as well as automatic detection using profiles of reflectivity and the standard deviation of the vertical wind velocity component were applied to determine the ABL height. The applied automatic method performed reasonably well but could not replace the visual inspection method, which turned out to be the most reliable. Sodar reflectivity profiles frequently revealed a complex ABL structure. Tethered balloon measurements for one of the selected days were compared to sodar data which confirmed this complex structure. ABL heights determined with various methods using profiles of meteorological variables from routine radiosonde ascents at three sites in the vicinity turned out to be of only limited value due to their low temporal and vertical resolution. The applicability of parameterisations of the stable boundary layer (SBL) depth and of slab models for the growth of the convective boundary layer (CBL) using surface measurements was investigated in complex terrain. If the SBL top is assumed to be horizontal and the difference in elevation between measurement sites is taken into account, simple parameterisations reasonably agreed with SBL depths from sodar measurements, whereas more complicated parameterisations underestimated the SBL depth. Results from slab models agreed well with measurements for two of the three days, whereas these models largely overestimated the CBL depth for the