In Situ and Airborne Doppler Radar Investigations of Lake-Effect Boundary Layer Circulations

Investigations of mesoscale circulations observed in lake-effect convective boundary layers have been hampered by a lack of information on their evolution as the air crosses the lakes. One of the goals of the Lake-Induced Convection Experiment (Lake-ICE) was to utilize the NCAR ELDORA airborne dual-Doppler radar and other measurement platforms to give quantitative information on the evolution of atmospheric boundary layer circulations across Lake Michigan. This presentation will discuss ELDORA observations during the 13 January 1998 case, when meso-g-scale cellular and random convective structures were observed over the lake. These convective patterns developed in response to intense sensible and latent heat fluxes from the surface. Superimposed on the cellular structures, crosswind bands of convection were observed to propagate across the lake, possibly in response to gravity waves above the boundary layer.

In situ observations from the University of Wyoming King Air and the NCAR Electra, and rawinsonde observations from three near-lake sites, are used to determine the atmospheric conditions in which the convection developed. The aircraft observations confirmed the presence of convective patterns with horizontal wavelengths of 3-5 km. The boundary layer increased from a shallow mixed layer near the upwind shore to a convective layer about 0.9 km deep across Lake Michigan (" 90-km fetch). Lake-effect snow was first observed by the aircraft about 15-20 km from the upwind shore, and was observed by radar about 30 km from the upwind shore. The snowfall intensity, and lake-effect cloudiness, increased in magnitude and coverage across the lake. While the cross-lake atmospheric conditions changed rapidly, the temporal evolution of the lake-effect system remained nearly steady-state, with near-lake boundary layer depths nearly constant for about 30-hr. A comparison is made between atmospheric conditions observed here and qualitatively similar conditions accompanying roll convection in past lake-effect studies.