The Diel Cycle of Modeled and Measured Latent Heat Flux in a Colorado Subalpine Forest

Precipitation changes the physical and biological characteristics of an ecosystem. Because land-surface models are often used to project changes in the hydrological cycle, successful modeling of the effect of precipitation on latent heat flux is an important aspect of land-surface models. Here, we use composites of the diel cycle of the ecosystem-scale energy fluxes to compare the Community Land Model (CLM, operated in single-point mode) to above-canopy tower measurements from the Niwot Ridge Subalpine Forest AmeriFlux site (US-NR1). The US-NR1 site is located in complex mountainous terrain approximately 8 km east of the Continental Divide below Niwot Ridge, Colorado. By defining "wet" days as those with above-average rainfall, two specific aspects of the CLM model of latent heat flux related to warm-season rainfall are evaluated: (1) does nocturnal latent heat flux increase in wet conditions, and (2) for a day following a wet day, is latent heat flux enhanced with a concomitant reduction in sensible heat flux. For both (1) and (2), the tower observations show that the presence of liquid water on the canopy and in the soil leads to increased evaporation which increases the latent heat flux (for (2), there is also a reduction to the sensible heat flux following the surface energy balance). The feasibility and pitfalls of performing a CLM-versus-tower-observations intercomparison will also be discussed.