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

Journal Article · Thu Mar 08 00:00:00 EST 2018 · Journal of Advances in Modeling Earth Systems

DOI:https://doi.org/10.1002/2017MS001248· OSTI ID:1424803

Burns, Sean P.  ^[1]; Swenson, Sean C. ^[2]; Wieder, William R.  ^[3]; Lawrence, David M.  ^[2]; Bonan, Gordon B. ^[2]; Knowles, John F.  ^[4]; Blanken, Peter D.  ^[5]

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1. Department of Geography University of Colorado Boulder CO USA, National Center for Atmospheric Research Boulder CO USA
2. National Center for Atmospheric Research Boulder CO USA
3. National Center for Atmospheric Research Boulder CO USA, Institute of Arctic and Alpine Research, University of Colorado Boulder CO USA
4. Institute of Arctic and Alpine Research, University of Colorado Boulder CO USA, School of Geography and Development University of Arizona, the College of Social and Behavioral Sciences Tucson AZ USA
5. Department of Geography University of Colorado Boulder CO USA

Abstract 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.

• Sponsoring Organization: USDOE
• OSTI ID: 1424803
• Alternate ID(s): OSTI ID: 1424805
• Journal Information: Journal of Advances in Modeling Earth Systems, Journal Name: Journal of Advances in Modeling Earth Systems Vol. 10 Journal Issue: 3; ISSN 1942-2466
• Publisher: American Geophysical Union (AGU)
• Country of Publication: United States
• Language: English