The Effect of Differing Vertical Canopy Structures on Energy and Water Budgets in the Forested Sierra Nevada: Simulations Utilizing 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. In California, US estimates of snow depth, extent, and melt from the inland forested Sierra Nevada are critical to estimating the amount of water available for both California agriculture and urban users; however accurate modeling of snow cover and snow melt processes in forested areas remains a challenge. Vertical 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. Yet there are almost no studies explicitly resolving complex canopy processes in multiple vertical layers. The Advanced Canopy-Atmosphere-Soil-Algorithm (ACASA), a multilayer soil-vegetation-atmosphere numerical model, was used to simulate the effect of different vertical snow covered canopy structures on the energy budget, temperature, and other scalar profiles within the forested Sierra Nevada. 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 several layers within the canopy. 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 canopy types were modeled ranging from a fir (e.g. most biomass near the ground) to an umbrella pine (e.g. most biomass near the top of the crown). This study is the first time to our knowledge that the vertical snow canopy structure has been explicitly modeled and resolved in a higher order closure model. The impact of the vertical structure on snow forest processes is examined to help explain the gaps in the snow forest energy and hydrology models. Preliminary results indicate that the vertical canopy shape and structure associated with 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 forested land surfaces, which resulted in significant differences in the onset of spring melt. The turbulent transport dynamics, including counter gradient fluxes and radiation, are discussed in the context of the snow energy balance. The results of the work could help elucidate whether disturbed forests create a reduction in water availability because of more wind/sun exposure or an increased water availability due to the lack of snow interception (i.e., creating decreased canopy snow sublimation). In addition, the results should be considered in the context of future vegetation changes in snow covered landscapes.