Noah land surface model modifications to improve snowpack prediction in the Colorado Rocky Mountains

Simulated snowpack by the Noah land surface model (LSM) shows an early depletion due to excessive sublimation and too early onset of snowmelt. To mitigate these deficiencies, five model modifications are tested to improve snowpack performance: (1) time-varying snow albedo, (2) solar radiation adjustment for terrain slope and orientation, (3) reducing the surface exchange coefficient for stable boundary layers, (4) increase of fresh snow albedo, and (5) adjusting surface roughness length when snow is present. The Noah LSM is executed from 1 November 2007 to 1 August 2008 for the headwater region in the Colorado Rocky Mountains with complex terrain, and its results are evaluated against 1 km Snow Data Assimilation System (SNODAS) output and individual Natural Resources Conservation Service Snowpack Telemetry (SNOTEL) sites. The most effective way to improve magnitude and timing of seasonal maximum snow water equivalent (SWE) is the introduction of the time-varying albedo formulation and the increase in fresh snow albedo. Minor improvement is obtained by reducing nighttime sublimation through adjusting the stable boundary layer surface exchange coefficient. Modifying the surface roughness length over snow surfaces and adding a terrain slope and orientation adjustment for radiation has little effect on average SWE conditions for the entire modeling domain, though it can have a significant effect in certain regions. The net effect of all changes is to improve the magnitude and timing of seasonal maximum SWE, but the snow period end is now somewhat too long. Adding the terrain slope and orientation effects does have an effect on local surface energy flux components depending on the cell slope and orientation.