A Comparison of the Diel Cycle of Modeled and Measured Latent Heat Flux During the Warm Season in a Colorado Subalpine Forest

Precipitation changes the physiological characteristics of an ecosystem. Because land-surface models are often used to project changes in the hydrological cycle, modeling the effect of precipitation on the latent heat flux λE is an important aspect of land-surface models. Here we contrast conditionally sampled diel composites of the eddy-covariance fluxes from the Niwot Ridge Subalpine Forest AmeriFlux tower with the Community Land Model (CLM, version 4.5). With respect to measured λE during the warm season: for the day following above-average precipitation, λE was enhanced at midday by ≈40 W m⁻² (relative to dry conditions), and nocturnal λE increased from ≈10 W m⁻² in dry conditions to over 20 W m⁻² in wet conditions. With default settings, CLM4.5 did not successfully model these changes. By increasing the amount of time that rainwater was retained by the canopy/needles, CLM was able to match the observed midday increase in λE on a dry day following a wet day. Stable nighttime conditions were problematic for CLM4.5. Nocturnal CLM λE had only a small (≈3 W m⁻²) increase during wet conditions, CLM nocturnal friction velocity u_∗ was smaller than observed u_∗, and CLM canopy air temperature was 2°C less than those measured at the site. Using observed u_∗ as input to CLM increased λE; however, this caused CLM λE to be increased during both wet and dry periods. We suggest that sloped topography and the ever-present drainage flow enhanced nocturnal u_∗ and λE. Such phenomena would not be properly captured by topographically blind land-surface models, such as CLM.