A Tiling Approach to Represent Subgrid Snow Variability in Coupled Land Surface–Atmosphere Models

A mosaic approach to represent subgrid snow variation in a coupled atmosphere–land surface model (WRF– Noah) is introduced and tested. Solid precipitation is scaled in 10 subgrid tiles based on precalculated snow distributions, giving a consistent, explicit representation of variable snow cover and snow depth on subgrid scales. The method is tested in the Weather Research and Forecasting (WRF) Model for southern Norway at 3-km grid spacing, using the subgrid tiling for areas above the tree line. At a validation site in Finse, the modeled transition time from full snow cover to snow-free ground is increased from a few days with the default snow cover fraction formulation to more than 2 months with the tiling approach, which agrees with in situ observations from both digital camera images and surface temperature loggers. This in turn reduces a cold bias at this site by more than 28C during the first half of July, with the noontime bias reduced from 258 to 218C. The improved rep- resentation of subgrid snow variation also reduces a cold bias found in the reference simulation on regional scales by up to 0.88C and increases surface energy fluxes (in particular the latent heat flux), and it resulted in up to 50% increase in monthly (June) precipitation in some of the most affected areas. By simulating individual soil properties for each tile, this approach also accounts for a number of secondary effects of uneven snow distri- bution resulting in different energy and moisture fluxes in different tiles also after the snow has disappeared.