EFFECTS OF ATMOSPHERIC CONDITIONS ON WIND PROFILES AND AEOLIAN SAND TRANSPORT WITH AN EXAMPLE FROM WHITE-SANDS-NATIONAL-MONUMENT

Quantifying aeolian sand transport rates relies upon the computation o f the near-surface shear velocity (u) determined from velocity profil es of the wind. While it has been recognized that various conditions, such as saltation, surface roughness, surface slope and atmospheric co nditions, have an effect on the velocity profile, it is commonly assum ed that measurements made above the surface will be representative of the near-surface shear velocity. Airflow and temperature data collecte d over a flat substrate at White Sands National Monument in New Mexico , however, show the significant effects that;atmospheric conditions ha ve on velocity profiles. During the day, when solar insolation is heat ing the surface, atmospheric conditions in the lowest several metres b ecome unstable, resulting in enhanced convection and vertical mixing s o that the velocity gradient changes little with height. As a result, the shear stress in this region of vertical mixing lessens, while the near-surface shear stress is increased because the higher wind speeds are now nearer the surface. At night, the near-surface atmospheric con ditions are stable, thereby reducing convection and vertical mixing, r esulting in stratified airflow and increased shear velocity away from the surface. Unless this atmospheric effect is accounted for, estimate s of sediment transport rates may be in error by as much as a factor o f 15 times when wind speeds are near threshold velocity. At wind speed s approaching 10 m s(-1), at 5m above the surface, this error in compu ting sediment transport is reduced to a factor of only two to three ti mes, and may be within the range of measurement error.