Turbulence and exchange processes in highly complex terrain

Mathias W. Rotach, Swiss Federal Institute of Technology – IACETH, Zurich Switzerland

Presently visiting NCAR/MMM, Boulder CO

Little is known at present concerning the turbulence structure and the associated exchange processes in highly complex terrain. In fact, what we essentially know concerning the turbulence structure in, e.g., an Alpine valley, is that commonly used approaches in boundary layer meteorology such as the validity of scaling regimes cannot be expected to hold due to the violation of essentially all prerequisites leading to their derivation. Nevertheless, and due to a lack of better knowledge, numerical models of essentially all spatial resolutions employ in one way or another boundary layer transfer schemes based on idealized, flat and horizontally homogeneous conditions.

Therefore, during the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP) boundary layer processes in highly complex topography were investigated in the Riviera valley in southern Switzerland. In contrast to previous studies the main focus was thereby on the turbulence structure and turbulent exchange processes at the valley surfaces and with respect to the free troposphere. Due to the anticipated spatial inhomogeneity a large number of turbulence probes (sonic anemometers, fast-response hygrometers and scintillometers) were deployed on a cross-section through the valley. Also a light research aircraft with high temporal sampling resolution flew various patterns within the valley to yield information on the turbulence structure within the bulk of the valley. Additional instrumentation such as a radio acoustic sounding system, a passive-microwave temperature profiler and a tethered balloon were operated during selected periods. In this seminar, a short overview is given on the measurements and observational strategies, as well as on specific problems related to turbulence observations in highly complex terrain. Then, some research highlights from the ongoing data analysis and numerical modeling are presented and discussed.