CEE Seminar Insights from Turbulence Anisotropy on flows over vegetated canopies

Jan 26

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Friday, January 26, 2024 – 12:00PM to 1:00PM

Presenter

Marc Calaf, Associate Professor, Department Mechanical Engineering, University of Utah

About 25% of the Earth's land-surface is covered by forests, however the accuracy in which these are represented in Earth System Models (ESMs, e.g. Numerical Weather Predictions and Climate models) is quite limited. Given the generally coarse resolution employed in ESMs (e.g. Δ~O(10^2 )m or larger) trees cannot be individually represented. Instead, their integral effect on the mean flow and turbulence statistics near the surface is imposed through universal functions. However, these universal functions were initially developed for atmospheric flows over flat and homogenous conditions with the goal to characterize the so-called inertial sublayer. It is only later that they were adjusted to include the representation of vegetated canopies through changes in the so-called surface roughness parameter and displacement height. While this approach had the advantage of being computationally efficient and provided a simple recipe for representing near-surface flows over all types of land surface covers, it was later shown to underestimate the turbulent exchanges taking place within and above the vegetated canopies. This is because it neglects the more complex turbulence mixing induced by the vegetated cover, characteristic of the so-called roughness sublayer. To better capture the enhanced momentum, energy, and mass exchanges characteristic of vegetated canopies, additional correction functions for implementation in ESMs had to be developed that would therefore better represent the hand-sake between the near-surface and canopy process with the inertial sublayer above. In this work we further explore the characteristics of the flow within the roughness sublayer where these new roughness sublayer correction functions have been defined. For this purpose, we study the flow over a series of vegetated canopies with different degrees of cover heterogeneity and forcing conditions. For the analysis, we leverage some recent developments where the metric of turbulence anisotropy has been introduced as an additional non-dimensional parameter in traditional atmospheric surface layer scaling relations as a means to generate new insight on the turbulence mixing processes immediately above the forested canopies. Results illustrate that while some type of turbulence exhibit the traditional roughness sublayer structure, other types of turbulence exhibit a departure from it.