Investigation of the Influence of Temperature Inversions and Turbulence on Land-atmosphere Interactions for Rolling Terrain
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Authors
Osibanjo, Olabosipo O.
Issue Date
2016
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Thesis
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Abstract
The surface-atmosphere exchange of trace gases and energy is important as it impacts weather, climate, and air quality. The models used for estimating surface fluxes such as heat, moisture, and carbon dioxide (CO2) were designed to work best in horizontally uniform, flat terrain. This is a challenge for measuring surface fluxes in non-uniform, complex terrain as these models break down especially during stable atmospheric conditions. The complex terrain generates its own thermal circulation such as the drainage flow at nighttime. Surface heterogeneity is common in complex terrain, which is one of the factors that leads to erroneous surface fluxes estimation. The concentrations of atmospheric greenhouse gases are increasing, leading to changes in atmospheric boundary layer (ABL) dynamics as a result of the changing surface energy balance. The ABL processes are important to characterize because they are difficult to parameterize in global and regional scale atmospheric models. Empirical data can be collected using eddy covariance micrometeorological methods to measure turbulent fluxes (e.g., sensible heat, moisture, and CO2) and quantify the land-atmosphere exchange. The objectives of this work are to calculate surface fluxes for rolling terrain using observational data collected during one week in September 2014 from a monitoring site in Echo, Oregon and to investigate the log law in the ABL. The site is located in the Columbia Basin with rolling terrain, irrigated farmland, and over 100 wind turbines. The 10 m tower was placed in a small valley depression to isolate nighttime temperature inversions. This thesis presents observations of momentum, sensible heat, moisture, and CO2 fluxes from data collected at a sampling frequency of 10Hz at four heights. Results show a strong correlation between temperature inversions and CO2 flux. The log layer could not be achieved as the value of the estimated von Karman constant (~0.62) is not close to that of the accepted value of 0.41. The impact of the irrigated farmland near the measurement site was observed in the latent heat flux, where the advection of moisture was evident in the tower moisture gradient. A strong relationship was also observed between fluxes of sensible heat, latent heat, CO2, and atmospheric stability. The average nighttime CO2 concentration observed was ~407 ppm, and daytime ~388 ppm compared to the 2013 global average CO2 concentration of 395 ppm. The maximum CO2 concentration (~485 ppm) was observed on the strongest temperature inversion night. There are few uncertainties in the measurements. The manufacturer for the eddy covariance instruments (EC 150) quotes uncertainty of ± 0.1°C for temperature between -0°C-40°C. Error bars were generated on the estimated surface sensible heat flux using the standard deviation and mean values. Under the most stable atmospheric conditions, uncertainty (assumed to be the variability in the flux estimates) was close to the minimum (~± 5 W m^(-2)).
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