Parameterizing the Antarctic stable boundary layer: synthesising models and observations

Abstract

The accurate representation of the stable boundary layer (SBL) is a key issue for weather prediction and climate models. The SBL exerts a crucial influence controlling heat, moisture and momentum fluxes between the surface and the rest of the atmosphere. Some of the world's most stably stratified boundary layers develop on the Antarctic continent. The British Antarctic Survey has observed the boundary layer at their Halley Station for the past several decades. Previous work investigating stable boundary layers has tended to take either a purely observational or purely modelling-based approach. In this thesis, a novel three-way methodology has been developed which uses the Halley observations, alongside single-column model (SCM) and large-eddy simulation (LES) techniques to examine two case studies. The LES and observations were first used together to establish the correct initial conditions and forcings for each case study. Very close agreement was generally achieved between the LES and observations, particularly for the first case study. This approach represents a powerful framework for verifying SCM and LES results against a range of in-situ observations. The choice of stability function is an important decision for column-based parameterizations of the SBL. Four schemes were tested in the SCM, providing persuasive evidence for the use of shorter-tailed stability functions. The LES data was also used to extract implied stability functions. These experiments reinforced the conclusion that shorter-tailed stability functions offered improved performance for the Antarctic stable boundary layer. The wind turning angle was defined as the difference between the geostrophic and near-surface wind directions. A slightly larger wind-turning angle was found with the LES and SCM results presented in this thesis, as compared to previous work. This difference might be explained by the shallowness of the boundary layers studied here. Finally, some investigations into the resolution sensitivity of the LES and SCM were conducted. Increases in resolution in the LES generally led to convergence towards the observations, with grid-convergence being qualitatively approached with a grid-length of 2 m. The use of enhanced vertical resolution yielded excellent agreement against the observations, with lower computational expense. Vertical resolution sensitivity tests were also conducted using the SCM. Limited sensitivity was found over the grid-length range explored here, with the main benefits being delivered close to the surface.