On the turbulence structure over complex mountainous terrain as observed in the i-Box

Abstract:

'i-Box' stands for ‘Innsbruck Box’ and refers to a longtime (but not permanent) observational and modeling initiative focused on turbulence and exchange processes in complex terrain. In the framework of this project a number of research flights were performed within and above the Inn Valley in order to assess the three-dimensional turbulence structure. At the same time additional (to the usual program) surface-based observations using scintillometers were performed. Wavelet analysis (as well as traditional Fourier analysis) was used to assess the airborne 100 Hz data. The surface-based turbulence data underwent the usual post-processing and quality control with special attention required due to complex terrain. Good agreement between wavelet and Fourier-derived turbulence integral statistics is obtained.

In this contribution we present first results on the three-dimensional flow and thermodynamic structure of the valley atmosphere under predominantly fair-weather (convective) conditions on three days in the late summer of 2013. The morning flights are mostly characterized through a relatively shallow (some few hundred meters) turbulent boundary layer while in the afternoon the boundary layer top is higher - but does not reach the height of the surrounding crests.

The spatial characteristics of turbulent fluxes (heat and momentum) as well as those of TKE are presented. Due to the east-west orientation of the valley the northern (south-facing) slopes experience stronger heat fluxes and TKE production throughout the day. The same applies to the surface heat fluxes. If the latter (from a surface site on the northern slope) are used to scale vertical TKE profiles throughout the valley atmosphere reduced scatter is obtained when compared to employing the true surface value, i.e. that at the foot of the actual profile. This is similar, albeit less pronounced, as reported from the Riviera Valley in southern Switzerland.