P4.5 Analysis of the turbulent kinetic energy budget in the planetary boundary layer by large eddy simulation

Monday, 2 August 2010
Castle Peak Ballroom (Keystone Resort)
Franciano S. Puhales, UFSM, Santa Maria, Rio Grande do Su, Brazil; and O. C. Acevedo, G. A. Degrazia, U. Rizza, and O. L. L. Moraes

The study of the planetary boundary layer (PBL) by computational simulations, such as large eddy simulations (LES), is widespread and increasingly driven by the constant improvement of the computational resources. This numerical (computational) approach has some advantages over other techniques because it describes the complete vertical extension of the PBL. However, it also has its limitations, imposed by simplifications of the equations solved and by numerical approximations. The underlying technique employed in the conception of the LES model consists in using volume averages to solve the equation of motion for a fluid particle. The equations are expected to be solved for large eddies, which contain most of the turbulent kinetic energy. We call this scale as resolved. The remainder of the scales, in other words the smallest scales of the turbulent flow, are approximated by a subgrid or subfilter model. The separation between the resolved and subfilter scales is realized by a filtering process. The width of this filter is directly associated to the computational grid resolution. In physics and meteorology, a description of the system energy variation is important for understanding a given process. When the system concerned is a turbulent flow, it is important to know how turbulent kinetic energy is producted, transported and dissipated. In the specific case of the PBL, we must also understand how this process takes place in relation to stability conditions and at different vertical levels. In this work Moeng's LES was employed to apprise the turbulent kinetic energy budget during the daily cycle of PBL. The results obtained show the satisfactory description of the terms of the turbulent kinetic energy, even with the limitations inherent to the simulation of turbulence at the stable boundary layer. A major advantage of the LES model is the ability to represent the whole PBL vertical profile with a high temporal and spatial resolution. This same type of analysis is quite complicated and expensive when an observational point of view is carried out, and this is one of the factors by which the LES is so widely used by the scientific community. A unique point of this paper is to present the profile of the turbulent kinetic energy transport through pressure correlations. Such term is quite difficult to be obtained experimentally. The LES model, solving the pressure field with high resolution, is capable of giving this description, which together with the description of the other transport terms has the expected characteristics of the turbulent kinetic energy transport in the PBL.

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