Journal of Advances in Modeling Earth Systems
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 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 E during the warm season: for the day following above-average precipitation, E was enhanced at midday by ≈40 W m−2 (relative to dry conditions), and nocturnal E 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 E on a dry day following a wet day. Stable nighttime conditions were problematic for CLM4.5. Nocturnal CLM E 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 E; however, this caused CLM 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 E. Such phenomena would not be properly captured by topographically blind land-surface models, such as CLM.
Burns, S. P., Swenson, S. C., Wieder, W. R., Lawrence, D. M., Bonan, G. B., Knowles, J. F., & Blanken, P. D. (2018). A comparison of the diel cycle of modeled and measured latent heat flux during the warm season in a Colorado subalpine forest. Journal of Advances in Modeling Earth Systems, 10. https://doi.org/10.1002/2017MS001248