High‐resolution simulations of the night‐time stable boundary layer over snow

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AbstractThe nocturnal stable boundary layer (SBL) over snow was studied via 1‐D clear‐sky simulations at 67oN. Three wind regimes could be identified. A windy SBL was relatively well‐mixed, turbulent cooling rates being moderate and long‐wave (LW) cooling rates weak as gradients of temperature and moisture (T,q) remained small. In calm and near‐calm cases the surface temperature Ts dropped rapidly in the evening, leading to a strong shallow inversion dominated by strong LW cooling. During weak winds (geostrophic wind Vg ∼ 3 m s−1), Ts decreased fairly rapidly in the evening with T2m closely coupled. The nocturnal inversion was moderately strong with strong turbulent cooling in the surface layer. The T,q‐profiles induced moderate LW cooling at mid‐inversion levels but LW heating in the lowest metres, which resisted the strong turbulent cooling. Thus the heating/cooling rates were fairly sensitive to the wind, especially near the surface.The snow surface was coldest after a calm night and warmer at windy sunrises, whereas the coldest air temperatures were obtained during weak winds, provided that the model's stability functions were of the ‘short tails’ type. A maximum in the downward sensible heat flux was associated with this. When the model's multilayer snow scheme was replaced by a coarse scheme, the nocturnal Ts and T2m stayed too warm. The use of old snow properties instead of typical snow produced relatively warm nocturnal temperatures whereas fresh snow led to quite cold temperatures. These aspects may have relevance to the SBL warm biases of large‐scale models.