WIND-TUNNEL STUDY OF TURBULENT-FLOW STRUCTURE IN THE CONVECTIVE BOUNDARY-LAYER CAPPED BY A TEMPERATURE INVERSION

Experiments on simulating the atmospheric convective boundary layer (C BL), capped by a temperature inversion and affected by surface shear, were carried out in the thermally stratified wind tunnel of the Instit ute of Hydrology and Water Resources, University of Karlsruhe. The tun nel is of the closed-circuit type, with a test section 10 m long, 1.5 m wide, and 1.5 m high. The return section of the tunnel is subdivided into 10 layers, each driven by its own fan and heating system. By thi s means, velocity and temperature profiles can be preshaped at the inl et of the test section, which allows for the reproduction of developed CBL over comparatively short fetches. The bottom heating is controlle d to produce the constant heat flux through the floor of the test sect ion. The flow velocity components in the tunnel are measured with a la ser Doppler system; for temperature measurements, the resistance-wire technique is employed. A quasi-stationary, horizontally evolving CBL w as reproduced in the tunnel, with convective Richardson numbers Ri(Del ta T) and Ri(N) up to 10 and 20, respectively, and the shear/buoyancy dynamic ratio u()/w(*) in the range of 0.2-0.5. Within the employed m odeling approach, means and other statistics of the flow were calculat ed by temporal averaging. Deardorff mixed-layer scaling was used as a framework for processing and interpreting the experimental results. Th e comparison of the wind tunnel data with results of atmospheric, wate r tank, and numerical studies of the CBL shows the crucial dependence of the turbulence statistics in the upper part of the layer on the par ameters of entrainment, as well as the modification of the CBL turbule nce regime by the surface shear.