A case study of cold-season emergent orographic convection and its impact on precipitation. part II: high-resolution LES analysis of convective cell evolution and precipitation processes
Afrifa, F., Geerts, B., Xue, L., Chen, S., Hohman, C., et al. (2026). A case study of cold-season emergent orographic convection and its impact on precipitation. part II: high-resolution LES analysis of convective cell evolution and precipitation processes. Monthly Weather Review, doi:https://doi.org/10.1175/mwr-d-25-0157.1
| Title | A case study of cold-season emergent orographic convection and its impact on precipitation. part II: high-resolution LES analysis of convective cell evolution and precipitation processes |
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| Genre | Article |
| Author(s) | F. Afrifa, Bart Geerts, Lulin Xue, Sisi Chen, C. Hohman, C. Grasmick, K. Friedrich, J. French, Sarah A. Tessendorf, T. J. Zaremba, R. M. Rauber |
| Abstract | Part I of this study demonstrated how terrain-induced gravity waves triggered elevated convection, with tops up to 6–7 km above sea level, in a potentially unstable layer during a winter storm event over the Idaho Central Mountains on 7 February 2017. Herein, this case is explored further with a large-eddy simulation (LES) at 100-m grid spacing to examine the detailed structure and evolution of convective cells emergent from shallow stratiform clouds, their interaction with complex terrain, and the resulting precipitation processes. The 100-m LES produced fine-scale precipitation structures similar in depth and width to radar observations, with vertical velocity distributions and cloud microphysical properties matching airborne observations. The 100-m LES confirmed the role of vertically propagating gravity waves over the highest terrain ridges in providing the initial lift necessary to release potential instability. Unlike coarser-resolution simulations, the 100-m LES produced clusters of convective towers, ∼2 km wide, roughly matching observations, although they were more regularly spaced than observed. Cospectral analysis of these towers confirms their convective nature. The small-scale convective updrafts, locally exceeding 2 m s −1 and mostly within the −10° to −20°C temperature zone, enabled snow particles to grow rapidly through depositional growth and riming, and a significant fraction of the simulated precipitation fell as graupel, according to the LES model. Precipitation from this emergent convection occurred primarily in the lee of the main terrain ridge on account of the strong flow above mountain top level. Cumulatively, the LES produced 18% more precipitation than non-LES models in this case. Significance Statement This study advances our understanding of cold-season precipitation processes over complex terrain. The key physical mechanisms identified herein are gravity wave–driven potential instability release, multiscale interactions between terrain and convection, and enhanced mixed-phase precipitation growth. This study demonstrates that large-eddy simulations with a grid spacing of ∼100 m are needed to properly capture the observed small convective towers and the snowfall they produce over and downwind of mountain ridges. In this case study, the large-eddy simulations (≤300-m grid spacing) produced slightly more precipitation than less-resolved convection-permitting simulations with parameterized eddy exchanges. A more systematic analysis of precipitation from small-scale convection in winter storms is warranted since orographic precipitation has important implications for water resource management in the western United States. |
| Publication Title | Monthly Weather Review |
| Publication Date | May 1, 2026 |
| Publisher's Version of Record | https://doi.org/10.1175/mwr-d-25-0157.1 |
| OpenSky Citable URL | https://n2t.net/ark:/85065/d7zg6xtv |
| OpenSky Listing | View on OpenSky |
| RAL Affiliations | HAP, RALAO |