Roll vortices and their impact on the mean flow in the atmospheric boundary layer

Zhitao Yu, University of Rhode Island

Abstract

In existing tropical cyclone research and forecast models, the description of the boundary layer (BL) is typically based on local gradients of mean wind and potential temperature and vertical diffusivity coefficients. In the commonly used K theory, the turbulence is represented by an eddy-diffusivity coefficient, K, which is typically parameterized as a function of the local Richardson number. The most fundamental limitation of K theory is that it doesn't account for transfer of heat, moisture, and momentum caused by convective motion or roll vortices, which are known to be common feature of the atmospheric BL. In this study numerical experiments are performed to investigate the BL roll vortices using a two and a half dimensional (2.5D) BL model. This model, first introduced by Khain et al. (1986), explicitly calculates a two-way interaction between the mean flow and convective motion and thus serves as a suitable tool to study the impact of BL convection on the mean flow. The model results show that the Coriolis force can affect BL roll vortices through mean flow. There are more energetic rolls in the presence of Coriolis force. Model results also show that roll vortices trigger internal gravity waves in the stable inversion layer when taking into account the effect of the Coriolis force on the mean flow. The interaction between the BL roll vortices and the internal gravity waves increases the aspect ratio of roll vortices, promotes stronger rolls, enhances the entrainment process, decreases the roll-induced momentum and heat fluxes and weakens the non-local mixing generated by roll vortices. The wave frequency and wave number of the internal gravity waves in the stable inversion layer increase with the increase of the inversion layer stability. Convective motion generated by clouds compensates the condensation and evaporation process by transporting heat from the cloud base to the cloud top. Latent heat release can increase the aspect ratio of roll vortices, suppress BL roll vortices, and increase the near surface air temperature. Strong clouds tend to dampen the internal gravity waves in the stable inversion layer.

Subject Area

Atmospheric sciences

Recommended Citation

Zhitao Yu, "Roll vortices and their impact on the mean flow in the atmospheric boundary layer" (2010). Dissertations and Master's Theses (Campus Access). Paper AAI3451841.
https://digitalcommons.uri.edu/dissertations/AAI3451841

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