Entrainment into sheared convective boundary layers as predicted by different large eddy simulation codes

Entrainment is a complex physical process that is a driving mechanism of convective boundary-layer (CBL) development. There is no consensus in the boundarylayer research community regarding the role played by wind shears in the entrainment dynamics. Previous numerical studies of sheared CBLs primarily focused on the turbulence regimes associated with quasi-stationary CBL evolution. In these studies, convective entrainment and its influence on CBL growth dynamics were not regarded as main research issues. In the present study, the emphasis is specifically laid on the characteristics of entrainment related to the CBL growth. Some previously suggested theories (e.g., Hunt and Durbin 1999), pointed to possible attenuation of vertical turbulent transport across the sheared entrainment layer that could lead to the reduction of entrainment. On the other hand, most numerical studies conducted to date (e.g., Fedorovich et al. 2001b; Pino et al. 2003) have found overall enhancement of entrainment and CBL growth with surface and/or elevated wind shear, yet others (e.g, Conzemius and Fedorovich 2002) have not. Because of these different results, this comparison exercise seeks to explore any differences in the predicted entrainment in sheared CBLs that may be code-dependent, and with this goal in mind, we have brought together several numerical codes built on the large-eddy simulation (LES) methodology in an attempt to investigate basic features of entrainment dynamics in sheared CBLs as reproduced by LES codes of different architecture, with different numerical algorithms, of different spatial and temporal resolution, and employing different subgridturbulence and surface-layer parameterizations.

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