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

摘要

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.