Horizontal Roll and Boundary-Layer Interrelationships Observed over Lake Michigan

Abstract

On 7, 9 and 10 January 1981 some 35 hours of organized horizontal roll convection in the boundary layer over lake Michigan were probed with two airplanes and two radars from the University of Chicago Lake Snow Project. During this time, roll wavelength varied from 1.5 to 13.7 km, roll depth from 0.9 to 2.1 km, and roll aspect ratio from 1.0 to 9.1; the roll axes changed orientation to remain within ±10° of the inversion-level geostrophic wind direction, and the maximum transverse wind speed varied from 1 to 2 m s?1. Boundary-layer parameters such as wind speed, wind direction, over-water fetch, surface-layer instability, and upstream stability, also varied over the same period. The roll orientations, and the variations of roll wavelength and maximum roll transverse wind speed with Rayleigh number and Richardson number, are examined with reference to theoretical models of roll convection. The observations suggest that roll orientation, roll wavelength, and roll transverse wind speed depend primarily on shear instability, with important modification by thermal instability.