Atmospheric Flow Development and Associated Changes in Turbulent Sensible Heat Flux over a Patchy Mountain Snow Cover

In this study, the small-scale boundary layer dynamics and the energy balance over a fractional snow cover are numerically investigated. The atmospheric boundary layer flows over a patchy snow cover were calculated with an atmospheric model (Advanced Regional Prediction System) on a very high spatial resolution of 5 m. The numerical results revealed that the development of local flow patterns and the relative importance of boundary layer processes depend on the snow patch size distribution and the synoptic wind forcing. Energy balance calculations for quiescent wind situations demonstrated that well-developed katabatic winds exerted a major control on the energy balance over the patchy snow cover, leading to a maximum in the mean downward sensible heat flux over snow for high snow-cover fractions. This implies that if katabatic winds develop, total melt of snow patches may decrease for low snow-cover fractions despite an increasing ambient air temperature, which would not be predicted by most hydrological models. In contrast, stronger synoptic winds increased the effect of heat advection on the catchment's melt behavior by enhancing the mean sensible heat flux over snow for lower snow-cover fractions. A sensitivity analysis to grid resolution suggested that the grid size is a critical factor for modeling the energy balance of a patchy snow cover. The comparison of simulation results from coarse (50 m) and fine (5 m) horizontal resolutions revealed a difference in the spatially averaged turbulent heat flux over snow of 40%-70% for synoptic cases and 95% for quiescent cases.

How do patchy snow covers affect turbulent sensible heat fluxes? – Numerical analysis and experimental findings

Michael Lehning, Sebastian Schlögl

The surface energy balance of a snow cover significantly changes once the snow cover gets patchy. The substantial progress in knowledge about the surface energy balance of patchy snow covers is a mandatory requirement to reduce biases in flux parameterizations in larger scale meteorological or climatological models. The aim of this project was to numerically improve energy balance calculations late in the melting season when the spatial variability of turbulent fluxes is especially high owing to the complex feedback between bare/snow-covered areas and the atmosphere above. In order to account for the feedback between the atmosphere and the patchy snow-cover we calculated three-dimensional air temperature and wind velocity fields with the non-hydrostatic atmospheric model ARPS for an idealized flat test site initialized with different snow distributions and atmospheric conditions. The physics-based surface process model Alpine3D has been forced with these atmospheric fields close to the snow surface in order to resolve the small-scale spatial variability. We further initialized the model with atmospheric fields above the blending height as a reference case. The numerical analysis shows that for simulations initialized with fully-resolved atmospheric fields below the blending height, turbulent sensible heat fluxes are up to 50 W m-2 larger than for calculations forced without resolved atmospheric fields. This difference in turbulent sensible heat fluxes over snow increase with increasing number of snow patches and decreasing snow-cover fraction. This is mainly attributed to an increase in the mean near-surface air temperature over snow due to horizontal and vertical exchange processes induced by the heterogeneous land-surface. We used flux footprint estimations to analyse turbulence data measured during three ablation periods in the Dischma valley (Switzerland). This fundamental theory was deployed for eddy-covariance measurements revealing the origin of the measured turbulence as a function of the measurement height, atmospheric stability and wind speed. For patchy snow covers with flux footprints larger than the fetch distance, the observed turbulence is typically recorded as a combination of the turbulence over snow and the turbulence over the adjacent bare ground. These conditions were observed in the upper Dischma valley at the end of the melting period for a small snow cover fraction (< 30 %). The strong influence of the bare ground on the heat exchange over snow leads to much smaller measured heat fluxes than calculated in the physics-based one-dimensional snow model SNOWPACK. We show that at least two eddy-covariance measurements in different vertical heights and a snow cover mask to detect the fetch distance are required to clearly separate the turbulence over snow and the turbulence over the adjacent bare ground. Using the footprint technique turbulent sensible heat fluxes towards the snow patch significantly increase by this separation. This approach will allow an improved parametrization of turbulent heat fluxes over patchy snow-covers in larger-scale energy balance models.

2017

Boundary layer processes over a patchy mountain snow cover and associated changes in the energy balance

Michael Lehning

Heterogeneous land-covers induce the formation of secondary flows and small-scale boundary layer processes. While turbulent fluxes over a continuous snow cover are mostly governed by the spatial variability of wind velocity and solar radiation, local advection of sensible heat and boundary layer decoupling become additionally important for the surface energy exchange over patchy snow covers. Furthermore, evolving katabatic and anabatic winds strongly change the mean flow patterns over snow free and snow covered areas causing positive and negative buoyancy production. In this study we use an atmospheric model (Advanced Regional Prediction System) to investigate boundary layer processes over patchy snow covers in an alpine catchment. The model is initialized with low to high snow cover ratios based on field measurements of snow distribution. We particularly analyse the effect of heat advection, boundary layer decoupling and changing patterns of secondary flows on the energy balance at the local and catchment scale. The numerical results show that the changing land-cover controls the length scale of internal boundary layers, while secondary flows drive the strength of boundary layer decoupling and heat advection. The relative importance of boundary layer processes depend on the snow patch size distribution and the synoptic wind. Stronger synoptic winds and low snow cover ratios increase the effect of heat advection and decrease the impact of boundary layer decoupling on the catchment`s melt behaviour. The typical lengths of stable internal boundary layers coincide with snow patch size distribution, which is consistent with typical length scales of snow accumulation patterns. As those length scales are in the order of tens of meters for the studied alpine catchment, very small grid sizes (below 10 m) are required to adequately model boundary layer processes which are shown to significantly alter the energy balance over patchy snow covers.

2013

Large-Eddy Simulation of the Convective Atmospheric Boundary Layer over Heterogeneous Land Surfaces

Chad Higgins, Daniel Nadeau, Marc Parlange

The parameterization of the atmospheric boundary layer is crucial for accurate numerical weather predictions. Over heterogeneous terrain, several open challenges remain regarding the growth of internal boundary layers, the determination of mixing layer heights and the spatial distribution of heat and momentum fluxes. The large-eddy simulation (LES) code we apply is based on the very robust Lagrangian scale-dependent dynamic subgrid-scale model. The flow is driven by a mean pressure gradient expressed in terms of horizontal geostrophic winds. The code is pseudo-spectral with spectral decomposition in the horizontal dimensions. Land surface heterogeneities take the form of distributed patches of surface temperature and momentum roughness derived from satellite imagery and land use analysis. Although recent developments in the code have enabled the simulation of the complete diurnal cycle, the focus here is restricted to convective conditions. Data from LITFASS 2003 (standing for Lindenberg Inhomogeneous Terrain Fluxes between Atmosphere and Surface: a long-term Study) are utilized to validate the simulation results. The LES domain covers a 99-m meteorological tower with turbulent measurements and a few surface micrometeorological stations. We investigate the presence of large-scale turbulent structures that are typical for daytime conditions with a particular interest in the blending height. Overall, this study illustrates how important is the accurate parameterization of heterogeneous terrain in studies of the atmospheric boundary layer.