Testing an Alternative Eddy Viscosity Scheme

  • Author / Creator
    Marshall, Ginny A
  • The purpose of this thesis is to evaluate the performance of an alternative eddy viscosity scheme proposed by Wilson (2012) for use in single column models of the atmospheric boundary layer (ABL). More popular schemes parameterize eddy viscosity as a function of turbulent kinetic energy, since turbulent mixing should increase as turbulent kinetic energy increases. However, it is an implication of Taylor’s (1921) Lagrangian theory of dispersion that time and velocity scales relevant to eddy viscosity are functions of vertical velocity statistics. As such, vertical statistics are seemingly a more sensible choice when parameterizing eddy viscosity; thus, the scheme proposed parameterizes eddy viscosity as a function of vertical velocity variance and an empirical time scale. In this thesis, the new scheme has been tested against an experiment that was conducted in Kansas in 1999 during the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES- 99) (Poulos et al., 2002). The protocol for the test is identical to that outlined by Svensson et al. (2011) which has been used to compare the performance of various ABL closure models. The test was run for 59 hours and model results were compared to observations, model results from a scheme proposed by B´elair et al. (1999), and results presented by Svensson et al. (2011). Results show that the alternative scheme performs reasonably well, comparable to many schemes presented by Svensson et al. (2011), and no worse than B´elair’s scheme, although it may not erode the stable surface inversion fast enough. Modeled 10 m wind speed, turbulent kinetic energy, friction velocity and sensible heat flux show a delayed response to the driving surface warming which is evident in the 3-4 hour lag observed in the increase of these quantities during the transition from the stable morning to the turbulent afternoon. Unfortunately the experiment was performed for only one full diurnal cycle, so it cannot be concluded that the problem lies in the model parameterization; therefore, further testing is required.

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.