The Time Scale of Shallow Convective Self-Aggregation in Large-Eddy Simulations Is Sensitive to Numerics
M. Janssens (Wageningen University & Research, TU Delft - Atmospheric Remote Sensing)
Jordi Vilà-Guerau De Arellano (Wageningen University & Research)
Chiel C. Van Heerwaarden (Wageningen University & Research)
Bart J.H. van Stratum (Wageningen University & Research)
Stephan De Roode (TU Delft - Atmospheric Remote Sensing)
A. P. Siebesma (Royal Netherlands Meteorological Institute (KNMI), TU Delft - Atmospheric Remote Sensing)
Franziska Glassmeier (TU Delft - Atmospheric Remote Sensing)
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Abstract
Numerical simulations of the tropical mesoscales often exhibit a self-reinforcing feedback between cumulus convection and shallow circulations, which leads to the self-aggregation of clouds into large clusters. We investigate whether this basic feedback can be adequately captured by large-eddy simulations (LESs). To do so, we simulate the non-precipitating, cumulus-topped boundary layer of the canonical “BOMEX” case over a range of numerical settings in two models. Since the energetic convective scales underpinning the self-aggregation are only slightly larger than typical LES grid spacings, aggregation timescales do not converge even at rather high resolutions (<100 m). Therefore, high resolutions or improved sub-filter scale models may be required to faithfully represent certain forms of trade-wind mesoscale cloud patterns and self-aggregating deep convection in large-eddy and cloud-resolving models, and to understand their significance relative to other processes that organize the tropical mesoscales.
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