Numerical simulation of the development of the convective boundary layer during a cold air outbreak

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

Section: Resultssupporting

confidence: 94%

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

Müller¹,

Chlond

1996

“…m Therefore, an additional prognostic equation for the TKE has to be solved, which is discussed in Raasch (1990a) or more generally by Holt and Raman (1988).…”

Section: Les Modelmentioning

confidence: 99%

“…The numerical grid has 76 points horizontally and 58 points vertically. For model details, the reader is referred to Raasch (1988Raasch ( , 1990a. In the following text, this model will be abbreviated by 2DYZ.…”

Section: Les Modelmentioning

confidence: 99%

A comparison of local and non-local turbulence closure methods for the case of a cold air outbreak

Chrobok

Raasch

Etling

1992

Self Cite

Numerical experiments have shown that large-eddy-simulation models (LES) are able to reproduce the common features of convective boundary layers (CBL) quite well. Models which cannot resolve the convective motions due to their grid structure (lD-models or models with coarse horizontal and/or vertical resolution) have to take into account the effects of large eddies within their subgrid diffusion terms. Turbulent fluxes are frequently parameterized through first-order-closure methods (Ktheory). Recently, non-local closure schemes have also been developed. In this paper we compare IDand 2D-models using different local and non-local first-order closure methods. The analysis is carried out for the case of an idealized cold air outbreak (CAO). One of the non-local closures is based on the so-called transilient turbulence theory. The reference states are given by a bulk-model and a 2D-model which resolves the large eddies explicitly. A comparison of the results is presented for characteristic quantities such as evolution of boundary-layer height and surface heat flux as well as mean wind and temperature profiles. It is found that simple local first-order closure does not give good agreement with the reference models. The results of the transilient turbulence model shows that a non-local closure is able to parameterize the effects of the large eddies. Comparable results are produced by a local closure where eddy diffusivities are parameterized by dimensionless gradient-functions,

“…Mason and Sykes (1982), Mason (1985), Etling and Raasch (1987), Chlond (1987), Sykes et al (1988Sykes et al ( , 1990, and Raasch (1990). These studies have provided a better understanding of the physical processes involved, and have given detailed information on roll-scale velocity variances and roll-scale vertical transports.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional simulation of cloud street development during a cold air outbreak

Chlond

1992

A three-dimensional numerical model is used to study boundary-layer eddy structure during a cold air outbreak. The model explicitly represents the large-scale three-dimensional motions, while small-scale turbulence is parameterized; it contains a water cycle with cloud formation and it takes into account infrared radiative cooling in cloudy conditions and the influence of large-scale vertical motions.The model is applied to conditions corresponding to an observed case of cloud street/stratocumulus development which occurred over the Greenland Sea during the ARKTIS 1988 experiment. The boundary layer is found to grow rapidly as the cold air flows off the ice over the relatively warm water. Coherent structures were identified in this boundary layer. It is found that the rolls become increasingly more convective in character with distance from the ice edge. Qualitative and quantitative descriptions of the flow field are given. Additionally, the relative importance of the various physical processes and external parameters in the evolution of the mean field of variables is indicated.