Orographically-induced waves over Minnesota and Lake Superior
GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed the formation of a standing wave cloud along the Minnesota shoreline of Lake Superior, followed by the development of a sequence of standing waves over western Lake Superior on 01 November 2020. This cloud and subsequent wave features were formed by a vertically-propagating internal gravity wave that resulted from the interaction of strong post-frontal northwesterly flow with the topography of the shoreline — the terrain quickly drops from an elevation of about 2000 feet above sea level (over northeastern Minnesota) to about 600 feet above sea level (over Lake Superior) in a very short distance.
A northwest-to-southeast oriented cross section of RAP40 model fields along line segment A-A’ (below) showed a deep pocket of positive Omega (upward vertical motion, yellow to red colors) that aided in development of the cloud band along the Minnesota Lake Superior shoreline. Note that this Omega feature was vertically tilted in an “upshear” direction (toward the northwest), and extended upward to the 350-400 hPa pressure level. There was also an increasing upward component of the ageostrophic vertical circulation, which was likely the initial forcing mechanism leading to formation of the standing wave cloud and standing waves seen on Water Vapor imagery. As the boundary layer wind speeds diminished during the day, the magnitude of the upward forcing also began to decrease.