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 lambda E 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 lambda E during the warm season: for the day following above-average precipitation, lambda E was enhanced at midday by approximate to 40 W m(-2) (relative to dry conditions), and nocturnal lambda E increased from approximate to 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 lambda E on a dry day following a wet day. Stable nighttime conditions were problematic for CLM4.5. Nocturnal CLM lambda E had only a small (approximate to 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 degrees C less than those measured at the site. Using observed u(*) as input to CLM increased lambda E; however, this caused CLM lambda E to be increased during both wet and dry periods. We suggest that sloped topography and the ever-present drainage flow enhanced nocturnal u(*) and lambda E. Such phenomena would not be properly captured by topographically blind land-surface models, such as CLM.U.S. DOE, Office of Science through the AmeriFlux Management Project (AMP) at Lawrence Berkeley National Laboratory [7094866]; NIFA/USDA grant [2015-67003-23485]; NCAR Mesoscale and Microscale Meteorology (MMM) Laboratory; NSF [DEB 1637686]; University of Colorado Boulder Libraries Open Access Fund; NSFOpen access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu