American Meteorlogical Society Symposium on Boundary Layers and Turbulence Dallas, TX, Jan 10-15, 1999.

Near-Shore, Boundary Layer Structure over Lake Michigan in Winter.

E. W. Eloranta, R. E. Kuehn, S. D. Mayor, P. Ponsardin
University of Wisconsin
1225 W. Dayton St.
Madison, Wisconsin 53706


The wintertime flow of cold air over warm water produces a vigorous growing convective boundary layer along the upwind shore of Lake Michigan. This boundary layer, which increases in depth with distance from the upwind shore, provides an attractive setting in which to observe the development of convective structures. The water surface provides a lower boundary with nearly uniform temperature and flat topography to facilitate model calculations.

This paper presents observations gathered by the University of Wisconsin Volume Imaging Lidar (VIL). The lidar was deployed on the lake shore at Sheboygan, WI, as part of the Lake Induced Convection Experiment (Lake-ICE). Data were acquired on 9 days between December 5, 1997 and Jan 22, 1998. Supplementary local data were collected by an NCAR integrated sounding system (ISS) located 10-km west of the lidar and by the National Data Buoy Center's SGNW3 weather station located 3/4-km north of the lidar.

The lidar observations were designed to acquire data to test Large Eddy Simulation (LES) models. Azimuthal scans with the lidar aimmed horizontally over the lake provided high resolution images of structures with 15-m spatial and approximately 15-second temporal resolution. These were analyzed to provide the horizontal wind field and its temporal evolution. Typically, winds could be derived with 1-km or better spatial resolution and 2-minute or shorter temporal resolution. At times, the signal to noise ratio was sufficient to provide wind measurements over the entire 250-square-km scan area. Time-lapse animations of these scans also show the horizontal structure of the convective field and its evolution with distance from the shore. Of particular interest are images showing open cells with downward motion in the center, upward motion in narrow cell walls, and approximately hexagonal cross sections. Vivid images of a land-breeze front and its temporal evolution were also recorded.

Volume scans provided information on both the 3-dimensional structure and temporal evolution of the boundary layer. These data were also analyzed to provide vertical profiles of the horizontal wind speed and direction. A typical scan recorded over 10 million data points in a volume bounded by a 50-degree azimuth sector and a maximum elevation angle of 15 degrees. Signals were recorded to a maximum range of 18 km and the scans were repeated at intervals of approximate 2 minutes. Volume scans recorded during a land-breeze are of particular interest. These data provide a detailed description of the flow of cold dense air out over the water in the face of an on-shore synoptic flow. Animations of the lidar data showing the surface outflow, the elevated return flow, gravity waves on the return flow boundary, the fluctuating frontal boundary and the eventual collapse of the front were recorded.

A extented abstract is avaliable in the conference proceedings

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