The impact of vertical mixing biases in large‐eddy simulation on nocturnal low clouds

Abstract The use of large‐eddy simulation at a resolution insufficient to resolve the largest turbulent eddies frequently suffers from an overestimation of vertical mixing in the nocturnal boundary layer. The resulting biases in the mean thermodynamic structure – typically an overestimation of the near‐surface temperature, and underestimation of the boundary layer top temperature – could potentially influence the formation of low clouds or fog. Whether this is likely to happen is assessed using both observations, and a newly developed conceptual model. First, observations from Northwestern Europe (Germany and Netherlands) are used to characterize the humidity conditions at night. The results indicate that saturation most likely occurs at the surface, but that the conditions at 200 m height are also often close to saturation. Second, the conceptual model is used to study whether overestimating vertical mixing, and the resulting negative temperature biases at the boundary layer top, could result in a spurious formation of low clouds in large‐eddy simulations. The results convincingly indicate that this is unlikely to happen, as overestimating vertical mixing decreases the relative humidity throughout the nocturnal boundary layer