The Effect of the Vertical Canopy Structure on Snow Processes Scenarios: Simulations of the Vertical Resolved Energy Fluxes and Snow Using a Higher-Order Closure Multi-Layer Soil-Vegetation-Atmospheric Model

Snow cover is a critical driver of the Earth’s surface energy budget, climate change, and water resources. Variations in snow cover not only affect the energy budget of the land surface but also represent a major water supply source. In California, US estimates of snow depth, extent, and melt in the Sierra Nevada are critical to estimating the amount of water available both California agriculture, urban and environmental users. However, accurate estimates of snow cover and snowmelt processes in forested areas still remain a challenge. Canopy structure influences the vertical and spatiotemporal distribution of snow, and therefore ultimately determines the degree and extent by which snow alters both the surface energy balance and water availability in forested regions. In this study we use the Advanced Canopy Atmosphere Soil algorithm (ACASA), a multilayer soil vegetation atmosphere numerical model, to simulate the effect of different snow covered canopy structures on the energy budget, water budget, temperature and other scalar profiles within different forest types in the Sierra Nevada, California. ACASA incorporates a higher order turbulence closure scheme which allows the detailed simulation of turbulent fluxes of heat and water vapor as well as the CO2 exchange of the within canopy layers. As such ACASA can capture the counter-gradient fluxes within canopies that may occur frequently, but are typically unaccounted for, in most snow hydrology models. Four different canopy types were modeled ranging from bottom heavy canopies with most of the biomass located near the ground to top heavy canopies with most of the biomass located near the top of the canopy. Preliminary results indicate that the canopy stand structure associated with the different canopy types fundamentally influence the vertical scalar profiles (including those of temperature, moisture, and wind speed) in the canopy and thus alter the interception and snow melt dynamics in the forested land surfaces. The differences in the vertical scalar profiles resulted in over one week difference in the complete melt of the seasonal below canopy snowpack between canopy types in high-snow scenarios. In addition, the turbulent transport dynamics, including counter-gradient fluxes are discussed in the context of the snow energy balance and potential misattribution of water sources within the vertical canopy that could occur when counter-gradient fluxes are not considered.  (KEYWORDS:  snow, canopy interception, turbulence, counter-gradient fluxes)