Coupling of Turbulence in and above a Forest at a steep slope During Nighttime

A micrometeorological field experiment was carried out on the westward facing slope of the Riviera valley close to Bellinzona, Southern Switzerland. The measurements were taken as part of the MAP-Riviera project (Mesoscale Alpine Project). From end of July to beginning of October 1999 five towers, located between 800 and 1000 m on the slope, were equipped with micrometeorological instrumentation. This presentation will focus on the measurements from one tower at 1000m, which was stood in a mixed forest (birch, chestnut, average height 14m) at a relatively steep slope (37°). The 22m high tower supported six levels with ultrasonic anemometers (sonics) which were mounted parallel to the slope. Raw data (20 Hz) of fluctuations of the wind vector and the acoustic temperature were stored continuously (on top also humidity fluctuations). Profiles of temperature and humidity (psychrometers at six levels) and profiles of wind speed (cup anemometers at eight levels) were sampled as 1 min averages. Additionally high resolution temperature profiles (2 Hz, 18 levels) were measured using unshielded thermocouples (75 mirco m) and on top a net radiometer was installed. Wave-like structures are observed in the temperature profiles , especially during night time. There are intermittent turbulent mixing events which reduce the built-up inversion. Cold air drainage and local flushing of part of the cold air layer, which was built up by radiative cooling above the foliage, show an interplay which will be examined by a combined analysis of sonic and thermocouple measurements. Time scales of coherent structures are investigated using spectral and wavelet analysis.