Gravity waves can directly influence surface-layer wind stress in a convective boundary layer

Atmospheric internal gravity waves (called “gravity waves” here) have been observed at all levels above the atmospheric boundary, where the air is stably stratified. Indeed, a region of stable stratification has been generally considered a pre-requisite for not only the production but also the support of significant wind variations associated with gravity waves. Therefore, it should be difficult for gravity waves to manifest themselves in deep, well-mixed regions of the atmosphere such as within a highly convective atmospheric boundary layer. Here, we show two case studies over Lake Superior that indicate the superposition of terrain-forced gravity waves on a highly convective boundary layer during cold air outbreaks over Lake Superior. This is accomplished using nearly coincident Advanced Very High Resolution Radiometer (AVHRR) and RADARSAT-1 Synthetic Aperture Radar (SAR) imagery. The simultaneous presence of gravity waves and convection is evident in both cloud patterns captured by AVHRR imagery and in lake-surface roughness variations captured by the SAR imagery. Of particular interest is the fact that wind variations associated with the gravity waves manifest themselves not only in the inversion where these features are generated but also as surface wind stress patterns deep within the convective boundary layer, where the atmosphere is well-mixed.