Abstract
Nocturnal cooling processes in simple 2-dimensional small basin were investigated by a numerical method, in which Cartesian co-ordinate system was used and hydro-static apprximation was not used. When radiative heat transfer by the long wave was not considered, no slope wind occurred on the surrounding slope, whose angle was 5.7°. In the calculation cooling of the air by radiative heat transfer near the surface was a trigger which gave rise to nocturnal down slope flow.
There were 3 stages in the slope flow during 6 hours after sun set. Strong down slope flow appeared at only early stage. The temperature profiles were changed especially between 20m level above the bottom of the basin and the ridge of mountain, that is, the air between these levels was more cooled when slope flow existed. The total heat loss in the atmosphere in excess of that by radiative cooling was roughly balanced with the sensible heat absorbed by the wall slope. Judging from the Lagrangean flows in this small basin, pollutants emitted continually at the center of the slope diffused in whole basin in 6 hours in spite of very weak turbulent diffusion. And this means small ascending flow and weak circulation over the center of the basin cooled the air in the whole basin.
