Assessing the Effects of Dust Loading of Snow on Regional Hydroclimatology Using an Improved Regional Climate Model

Radiative processes play an important role on both global and regional scales. This study focuses on their effects over snow-covered surfaces due to dust loading. Studies have shown that dust emissions from the Colorado Plateau have increased 5-7 fold in the last century and a half due to grazing and agricultural practices, which decreases snow albedo and enhances solar radiation absorption. In an offline study, Painter et al. (2007) have shown that snow cover was shortened by 18 to 35 days due to dust radiative forcing in snow in the San Juan Mountains, Colorado, USA. Our present study will quantitatively assess dust's influence on radiative forcing and runoff timing in mountain snow packs using a physically comprehensive regional climate model. For this study, we employ NCAR's WRF ARW model, which is coupled with a land surface model, Simplified Simple Biosphere version 3 (SSiB3). We have modified the original WRF-SSiB3 framework to include a snow-radiative transfer model, Snow, Ice, and Aerosol Radiative (SNICAR) model. SNICAR considers the effects of snow grain size and aerosol on snow albedo evolution. Snow grain size and growth is important in snow albedo feedbacks, especially when aerosols in snow are considered, because larger snow grains decrease snow albedo, and in the presence of dust, grain growth rates increase, decreasing snow reflectance even further than if the snow was pure. In the original WRF-SSiB3, albedo was empirically adjusted during snow melt. Implementing SNICAR allows us to have a more physically based process to represent changes in albedo due to snow metamorphism as well as those due to impurities in snow, which makes the regional climate model capable of realistically simulating radiative forcing on snow covered areas with aerosol loading. The model was further modified to account for the presence of aerosols in snow in terms of the distribution of these impurities as well as their scavenging by melt water throughout the snow layers We evaluate the ability of our new regional climate model framework to simulate the impact of dust-on-snow on regional surface albedo and snowpack state over North America by conducting a sensitivity test in which constant aerosol deposition is imposed. Results from this study are compared with results from a simulation that does not consider dust effects on radiative forcing over snow-covered surfaces. We also force WRF-SSiB3 with climatology aerosol deposition based on GOCART dataset to test a more realistic scenario for the impact of dust-on-snow on the mountain snowpack, surface albedo, and regional hydrology. The results of the model simulations are validated against observational datasets from ground measurements and satellite observations.