Effects of roughness blocks on atmospheric boundary layer flow over a two-dimensional low hill with/without sudden roughness change

Wind tunnel experiments were carried out to investigate the effects of roughness blocks on the atmospheric boundary layer flow over a two-dimensional low hill with maximum slope 0.21. Roughness blocks whose heights were one eighth of the hill height were arranged in a staggered pattern with a roughness density of 4.1% to model the rough conditions. Four situations with/without roughness change in the flow direction, i.e. a smooth hill in smooth flow, a rough hill in rough flow, a smooth hill in rough flow and a rough hill in smooth flow, were considered. The effects of the roughness blocks were clarified by comparing the flow characteristics over hill models, with emphasis on speedup ratio and turbulence structure. Experimental results were compared with the predictions obtained from linear models. The results show that speedup ratio depends strongly on the surface condition in the middle layer, the inviscid but rotational part of the outer layer defined by HLR theory [Hunt, J.C.R., Leibovich, S., Richards, K.J., 1988. Turbulent shear flow over low hills. Quart. J. Roy. Meteorol. Soc. 114, 1435–1471]. Adding or removing roughness blocks on the hill surface or inflow area changes the velocity deficit and creates a completely different turbulence structure in the wake.