Expanding Snow Treatment in CESM Vegetation

The CESM land model accounts for precipitation interception, throughfall & drip in the canopy hydrology. Portions of falling snow and rain are intercepted by the canopy and maintained in a water storage term. The remainder falls through to the ground, and intercepted water also drips from the canopy. The Community Land Model uses exposed leaf and stem area indices to determine the throughfall flux and drip flux for liquid and frozen water. The interception by vegetation and the water mass storage term does not differentiate between liquid or frozen precipitation based on the justification that a lower evaporation rate roughly negates the difference. Observations show, however, that leaf capacities for water are about double those of snow. Optical parameters are influenced by the canopy storage and area indices through a wetted fraction of vegetation. Different optical properties are assumed when air temperature is below the freezing temperature of water. This is an efficient method to account for snow in vegetation albedo and radiative flux calculations, but doesn't account for the different morphologies and mechanics of snow. Canopy snow can be blown off by wind, or slide off without wind intervention if the branches provide an unstable or slanting support. We fully separated the liquid and solid terms in CLM's hydrology, creating a canopy snow throughfall and canopy snow storage term. Snow in vegetation can convert to meltwater and vapor. Using these new simulation developments, we are able to quantify radiative and hydrological sensitivity to improved model representation.