An Aircraft Investigation of Mesoscale Convection over Lake Michigan during the 10 January 1984 Cold Air Outbreak

Abstract

Surface, upper air, aircraft and satellite observations during the Lake-Effect Snow Studies (LESS) field program have been examined to study the properties of a cloud-topped boundary layer (CTBL) that formed in response to a northerly flow of continental polar air over Lake Michigan on 10 January 1984. An analysis of inversion base heights shows that the depth of the convectively mixed boundary layer ranged from 1.0 to 1.9 km across the central part of the lake from Wisconsin to Michigan, to an uncharacteristically high value of 2.4 km near the south end of the lake in Indiana. Spatial cross sections and streamline analyses show that the polar air was first modified by Lake Superior and then by the full fetch of Lake Michigan. Aircraft datasets of ambient air temperature, virtual potential temperature, specific humidity, liquid water content, and the u, v, and w wind components sampled at a rate of 20 s−1 by the NCAR King Air and Queen Air have been analyzed to determine the structure and interaction of the convectively mixed layer with the capping inversion layer. New findings in this study include the identification of a penetrative convection layer that resides between the top of the fully mixed layer and the unmodified part of the inversion layer. Evidence from the aircraft data analysis shows the characteristics of this layer and the associated fluxes and entrainment. Spectral representations of the convective well-mixed layer contain length scales at approximately 3 and 10 km. These correspond to the onset of convection or basic convective mode (BCM) and a higher convective mode (HCM), which is manifest in GOES satellite imagery as two-dimensional north–south cloud bands over Lake Michigan. The aspect ratio of these convective bands or streets, the most prominent mode, was in the range of 5 to 7.5 (typical for cold air outbreaks at the latitude of Lake Michigan). Also, observational evidence from the raw data and covariance calculations is presented to support the incorporation of an interfacial transition layer between the inversion layer and well-mixed layer in the conceptual model of a Type I CTBL.