Yang Qiong

University of Wyoming

Dept of Atmospheric Science 

Vertical velocity and buoyancy characteristics of echo plumes in the convective boundary layer over Lake Michigan during a cold-air outbreak

Cloud streets have long been observed during cold-air outbreaks over open water, e.g. over Lake Michigan in winter. These cloud streets visualize the spatial organization of convection within the convective boundary-layer that deepens with fetch from the upstream shore. The linear alignment of convection is believed to be due to a weak but persistent secondarily circulation normal to the streets and the mean wind in the CBL. Even on very cold days cloud streets do not form in the boundary layer when the wind is too weak.

During the NASA ROLLS experiment in January 2004 over Lake Michigan, we collected WKA and WCR data both on two days with a distinct cloud street structure, and on a day lacking linear organization. For these three days we describe the topography of the CBL inversion, the vertical structure of the WCR reflectivity and vertical velocity in the CBL, and we examine the thermodynamic characteristics of echo plumes and updraft cores. The echo plumes and especially the updraft plumes have the characteristics of buoyant, entraining thermals. We find no significant differences in updraft and buoyancy characteristics on the day without cloud streets, as compared to the two days with cloud streets.