The impact of BL turbulence and shear instability on snow growth: idealized simulations

Houze and Medina (2004) and Medina and Houze (2015) hypothesize that small-scale cellular overturning motions and associated turbulence within a stably stratified elevated shear layer upwind of blocking terrain enhance hydrometeor growth. Another observational study, by Geerts et al. (2011), hypothesizes that turbulence in the shear-driven boundary layer over mountains may contributes to snow growth when strong winds produce a deep BL and the cloud base is near or below mountain top level. The observational studies mentioned above can only speculate that turbulence enhances hydrometeor growth, because they lack detailed cloud-microphysical chain-of-events evidence to demonstrate the significance of this process. The objective of this paper is to examine whether shear-induced overturning in stable blocked flow over a mountain, as well as PBL turbulence, enhance hydrometeor growth in a mixed-phase cloud by means of idealized WRF large eddy simulations (LES). The LES framework is required because vertical exchanges implied by BL parameterizations do not communicate with the cloud microphysics in a model. We show that both KH billows in the free troposphere and PBL turbulence enhance snow growth, and that this mechanism is adequately captured with a 100 m LES resolution

Specifically, we aim to examine (A) how coherent KH overturning and turbulence generated within the shear layer above the PBL impact snow growth and snow fall-out, and (B) whether turbulent eddies in the PBL, above cloud base, increases snow mass and precipitation. We also aim to evaluate (C) what LES resolution is sufficient to resolve the eddies responsible for the snow growth in turbulent PBL.