Three-dimensional numerical study of cell broadening during cold-air outbreaks

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

Daytime convective development over land: A model intercomparison based on LBA observations

“…These simulations were initiated using high spatial resolution and small computational domain typical of large-eddy simulations (LES) applied in boundary-layer studies. To reach a computational domain large enough for deep convection (both in the horizontal and in the vertical), an approach similar to that applied by Müller and Chlond (1996) in their simulation of the boundary-layer development in cold-air outbreaks was used. In this approach, a remapping procedure was applied three times to increase the computational domain and horizontal/vertical grid length towards those applied in CRM simulations.…”

“…To resolve the range of spatial and temporal scales involved in the development of convective boundary layer and transition from shallow to deep convection, we follow a modelling strategy used by Müller and Chlond (1996) in their simulation of the boundary-layer development in cold-air outbreaks. The strategy involves applying a small computational domain and high spatial resolution (typical of large-eddy simulation approach) to represent the development of a convective boundary layer and shallow convection in the early part of the simulation.…”

“…In Müller and Chlond (1996), a decision when to apply the remapping procedure to increase the domain size and reduce spatial resolution was based on the evolution of the wave-number spectrum of the kinetic energy. Herein, the decision is made by monitoring the increase of the boundary-layer depth and the increase in the horizontal and vertical extent of clouds.…”

“…The domain appropriate for an ensemble of deep convective clouds, i.e. covering the entire troposphere and extending several tens of kilometres in both horizontal directions, is reached by performing three times a remapping procedure similar to that used by Müller and Chlond (1996) (see Fig. 3 for the timeline of benchmark simulations).…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…In a modeling study, Müller and Chlond (1996) showed that removing liquid water formation reduces the broadening of convective cells, leading to smaller aspect ratios. The majority of previous studies of rolls, where the aspect ratio is usually larger than two, are based on convective rolls that produced cloud streets, i.e., where liquid water formation was present (e.g., Miura, 1986).…”