Existence of Large Turbulent Eddies in the Early-Morning Boundary Layer Acting as an Effective Mountain to Force Mountain Waves

摘要

Numerical modelling suggests that the turbulent boundary layer can act as an effective mountain forcing mountain waves. In the daytime, convective rolls can cover the mountains, raising the mountain-wave launching height. In non-convective conditions, the nature of the effective mountain is unknown. Here, we investigate if the early-morning boundary layer, moving rapidly across mountains, also contains large eddies of size comparable with convective cells. Temperature profiles from thousands of high-resolution radiosondes show superadiabatic gradients of vertical scale a few hundred metres in the boundary layer, appearing as the boundary-layer wind speed increases. These are explained by the overturning of potential temperature surfaces in large eddies advected with the wind and/or longitudinal rolls. An early-morning satellite image shows longitudinal rolls over mountains up to 1 km height. It is suggested that early-morning fast-moving airflow over mountains, producing mountain waves, also creates a turbulent boundary layer underneath them containing large eddies of scale a few hundred metres, in addition to classic turbulence. These are part of the effective mountain, higher than the actual mountain, which explains the formation of mountain waves.