Burns, S.P., Swenson, S.C., Wieder, W.R., Lawrence, D.M., Bonan, G.B. , Knowles, J.F., Blanken, P.D.
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 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.
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 (3) :617-651 , DOI: 10.1002/2017MS001248
This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1637686. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.
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