Diurnal Variation of Wind Profiles Across Mountainous Terrain during and Air Stagnation Period.

The diurnal variation of wind profiles across mountainous terrain during an air stagnation period was evaluated for 7 days in July, 1957. The study was conducted across the North-Central Appalachian Mountains, an area of heavy pollution concentration it was divided into easterly 16-18 July and westerly 19-22 July flow across the mountains. Examination over the 7 days showed a diurnal variation in boundary layer winds on the eastern side of the mountain range with a maximum amplitude of about 3 to 4 msec at 1000-1500 m MSL in both the easterly and westerly flows. On the western side of the mountain range, a diurnal variation with a maximum amplitude of about 4 msec at 600-1100 m MSL occurred in both flows. This oscillation in the lower levels showed the presence of a low-level jet, which was unexpected in that this study was conducted during an air stagnation period. The low-level jet in the easterly flow across the mountains reaches a maximum wind speed at approximately 0600 GMT at about 300 m above ground level. In the westerly flow, the low-level jet occurs at approximately 1200 GMT at 600-800 m above the ground. This low-level jet is due to an inertial type oscillation driven by the diurnal variation of the frictional forces aided by thermal forcing.