On Slope Winds

Li Peng

ABSTRACT

　　The equations of shallow convection are applied to the investigation of the circulations in a stable atmosphere above a V-shaped valley. For simplicity, the motion is assumed to be uniform along the valley and symmetric about the vertical planes passing through valley-and ridge-axis. The heating or cooling of the sloping ground is simulated by prescribing the variations of ground temperature. Two numerical integrations of the equations have been carried out for upslope-and downslope-winds separately. The results show that:
　　a)the peak intensity of the circulation in daytime leads the maximum ground temperature nearly two hours;
　　b)the highest upslope velocity occurs above the upper half of the slope at a definite distance less than fifty meters from the slope and maximum vertical velocity is directly above the ridge;
　　c)the thickness of the upslope winds is within the range of 100 of 200 meters, changing accordingly with the intensity of the circulation;
　　d)in the early stage of the development of the upslope winds, the potential energy of the system increases with time because of subscale diffusion process, but after the slope wind is fully developed, it decreases with time mainly due to subscale diffusion, only a small portion of the reduction is transformed into kinetic energy;
　　e)the nighttime downslope wind is much weaker and shallower and reaches its maximum intensity much faster than the daytime upslope wind;
　　f)the returning flow is much weaker but of greater thickness than the slope wind. These results are in good agreement with observations summarized in Defant’s review article (Defant, 1951) and Geiger’s book (Geiger, 1957)