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Abstract

The boundary-layer development and convection-pattern transition typically occurring in cold-air outbreaks is studied using three-dimensional simulations. The simulations include the secondary-flow transition starting with the relatively small-scale boundary-layer rolls developing during the initial phase and ending with mesoscale cellular convection patterns. The application of a computational grid, whose horizontal mesh size enables the resolution of the small-scale initial patterns and whose domain size is large enough to capture mesoscale convection patterns, overcharges even state-of-the-art supercomputers. In order to bypass the computer storage problem, the horizontal size of the model domain and the horizontal resolution of the computational grid are adjusted to the scale of the dominant convective structures. This enables the simulation of convection cells whose horizontal scales increase up to values exceeding the size of the initial model domain.
The model is applied to conditions of a cold-air outbreak observed during the ARKTIS 1991 experiment. The most important characteristics of the observed situation are revealed by the model. Sensitivity studies are performed in order to investigate the relation between cell broadening and various physical processes. The artificial cutoff of liquid-water formation prevents the enlargement of convective scales. Latent heating due to condensation and especially radiative cloud-top cooling are identified as processes leading to cell broadening. We propose a conceptual model that elucidates the mechanism by which cloud-top cooling may generate larger aspect ratios.