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A New Large-Eddy Simulation Model for Simulating Air Flow and Warm Clouds Above Highly Complex Terrain. Part II: The Moist Model and its Application to Banner Clouds

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Abstract

In Part I the dry version of a new large-eddy simulation (LES) model was presented that is specifically designed to simulate air flow and clouds above highly complex terrain. Here the implemented moisture physics are described and a new method for the generation of turbulent inflow conditions for meteorological LES is proposed. As a typical area of application the new model is applied to simulate banner clouds developing downwind of pyramidal mountain peaks. Banner clouds are shown to be primarily a dynamical phenomenon, and form in the lee of steep mountain peaks as a result of dynamically forced lee upslope flow. Due to the highly asymmetric flow field induced by the extreme orography, banner clouds can form even under horizontally homogeneous initial conditions regarding both moisture and temperature. Thus, additional leeward moisture sources, distinct air masses on both windward and leeward sides, or radiation effects are no prerequisite for banner-cloud formation. The probability of banner-cloud formation increases with increasing obstacle height and steepness and is, to a first approximation, independent of the pyramid’s orientation. Simulations with and without moisture physics reveal that, for the set-up chosen, moisture is of only secondary importance for banner-cloud dynamics. The reinforcement of lee upslope flow and corresponding cloud formation due to latent heat release turns out to be almost negligible. Nevertheless moisture physics are shown to induce a dipole-like structure in the vertical profile of the Brunt-Väisälä frequency, which in turn leads to a moderate increase in leeward turbulence.

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  1. Institute for Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany

    D. Reinert & V. Wirth

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  1. D. Reinert
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  2. V. Wirth
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Correspondence to D. Reinert.

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Reinert, D., Wirth, V. A New Large-Eddy Simulation Model for Simulating Air Flow and Warm Clouds Above Highly Complex Terrain. Part II: The Moist Model and its Application to Banner Clouds. Boundary-Layer Meteorol 133, 113–136 (2009). https://doi.org/10.1007/s10546-009-9419-x

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  • DOI: https://doi.org/10.1007/s10546-009-9419-x

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