Multiscale evaluation of the improvements in surface snow simulation

The downwelling shortwave radiation on the Earth's land surface is affected by the terrain characteristics of slope and aspect. These adjustments in turn impact the evolution of snow over such terrain. In this study, we evaluate the impact of terrain-based adjustments to incident shortwave radiation on snow simulations over two mid-latitude regions, using two versions of the Noah land surface model. The evaluation is performed by comparing the snow cover simulations against the 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) fractional snow cover product. The model simulations are evaluated using categorical measures, such as the probability of detection of ``yes'' events (PODy), which measures the fraction of snow cover presence that was correctly simulated and false alarm ratio (FAR), which measures the fraction of no-snow events that were incorrectly simulated. The results indicate that the terrain-based correction of radiation leads to systematic improvements in the snow cover estimates in both domains and in both LSM versions. The contrast in adjustments to the incident shortwave radiation on north and south-facing slopes is reflected in the snow cover improvements over these slopes, with an increased contribution to PODy and FAR improvements observed over the north and south-facing slopes, respectively. The comparison of model predicted SWE fields against in-situ measurements confirms that the terrain-based corrections provide improvements not only to the snow cover estimates, but also to other snow fields. A multiscale evaluation using a two dimensional discrete Haar wavelet analysis quantifies the decomposition of the improvements observed at 1 km spatial resolution to coarser scales. The wavelet analysis also quantifies the relevant spatial scale where the topographic correction to radiation would be effective in impacting the snow simulations.