Factors leading to the observed structures of roll convection observed by Lake Ice as revealed by cloud resolving numerical siimulation

Cloud resolving simulations of the convective boundary layer observed on 10 January, 1998 during Lake-ICE (Lake-Induced Convection Experiment) were conducted to study the physical mechanisms leading to the formation of cloud roll structures. Simulations were first performed employing a series of two-way nests embedded in a regional scale prediction based on the Eta Data Assimilation System's (EDAS's) analysis 14 hours prior to the event. Although these experiments yielded generally good agreement with observations, there were shortcomings attributable to the limited area of the fine cloud resolving nest. For this reason and to enable the design of a series of experiments that could isolate the physical processes producing the simulated banded structures, as series of idealized experiments were designed.

The set of idealized simulations were based on a hypothesis derived from the real data tests that the banded structures were connected with the deep tropospheric stability and shear. The experiments were designed to have periodic boundaries in the along lake direction and open boundaries in the across lake direction. The domain extended sufficiently upwind over the land to allow for the formation of land based rolls in the shear driven boundary layer and sufficiently down wind from the lake to allow for the transition back to shear based roll structures in the neutral boundary layer. An upper level absorbing layer was used to damp vertically propagating gravity waves in the stratosphere.

Results of these experiments suggest that the simulated lake based roll structures were consistent with a gravity wave mode having a harmonic resonant with the wind component flowing across the band. It is suggested that the bands become oriented at a direction relative to the low-level flow that maximized resonance with the upper level gravity wave oscillation. Analysis also suggests that the oscillation was enhanced by wave trapping induced by the dynamical and thermodynamical structures of the post frontal atmosphere aloft.