The influence of warm-season precipitation on water cycling and the surface energy budget within and just-above a Colorado subalpine forest in mountainous terrain: Measurements and modeling
PhD: University of Colorado Boulder, 2018.
Environmental towers are used at measure the pulse of the terrestrial biosphere at over 500 sites around the world and form a global network of measurements that is commonly known as FLUXNET (http://fluxnet.ornl.gov/). This thesis uses over a decade of data from the Niwot Ridge Forest AmeriFlux site in a high-elevation subalpine forest to examine how warm-season precipitation a ected the above-canopy diel cycle of net radiation Rnet, ecosystem eddy covariance fluxes (sensible heat H, latent heat lambdaE, and CO2 net ecosystem exchange NEE) and vertical profiles of scalars (air temperature Ta, specific humidity q, and CO2 dry mole fraction Xc). This analysis allowed us to examine how precipitation modified these variables from hourly (i.e., the diel cycle) to multi-day time scales (i.e., typical of a weather-system frontal passage). Even though precipitation caused mean changes on the order of 50-70% to mid-day Rnet, H, and lambdaE, the surface energy balance (SEB) was relatively insensitive to precipitation with mid-day closure values ranging between 90-110%. With respect to measured lambdaE during the warm season we found that during the day following above-average precipitation, lambda E was enhanced at mid-day by approximately 40 W m-2 (relative to dry conditions), and nocturnal lambda E increased from 10 W m-2 in dry conditions to over 20 W m-2 in wet conditions. The ecosystem fluxes were also modeled using the Community Land Model (CLM, version 4.5), and a comparison between the model and observations was presented. With default settings, CLM4.5 did not successfully model the changes in lambda E due to precipitation. However, by increasing the amount of time that rainwater was retained by the canopy/needles, CLM was able to better match the observed mid-day increase in lambda E on a dry day following a wet day. This thesis demonstrates techniques which can be used to compare observed and modeled ecosystem fluxes.