Friction velocity

Definition

Friction velocity, often denoted as u*, is a measure of the turbulent shear stress in the boundary layer and is defined as the square root of the ratio of shear stress to fluid density. It quantifies how wind speed near the ground affects momentum transfer between the atmosphere and the surface. This concept is crucial for understanding boundary layer dynamics, as it influences wind profiles, turbulent mixing, and surface fluxes of heat and moisture.

5 Must Know Facts For Your Next Test

1. Friction velocity is crucial for estimating surface roughness lengths, which help model wind profiles in different terrains.
2. It is often calculated using empirical relationships that involve measurements of wind speed and temperature gradients near the surface.
3. In stable atmospheric conditions, friction velocity decreases, leading to reduced turbulence and a more stable boundary layer.
4. Conversely, in unstable conditions, increased friction velocity enhances turbulence, promoting mixing and energy exchange in the boundary layer.
5. Friction velocity plays a key role in estimating scalar fluxes (like heat and moisture) using methods like eddy covariance.

Review Questions

• How does friction velocity influence the wind profile within the boundary layer?
  • Friction velocity directly affects the wind profile by influencing how momentum is transferred from the atmosphere to the surface. Higher friction velocity indicates stronger turbulence and greater momentum transfer, resulting in a more accelerated wind speed close to the surface. As turbulence increases, it alters vertical wind profiles, making them more logarithmic in nature due to increased mixing effects.
• Discuss how friction velocity can be used to estimate surface fluxes of heat and moisture in boundary layer studies.
  • Friction velocity serves as a key parameter in estimating surface fluxes because it relates to how much momentum is transferred between the atmosphere and the surface. In methods like eddy covariance, researchers use friction velocity alongside measurements of scalar quantities (like temperature and humidity) to quantify how energy and mass are exchanged at the surface. This relationship allows scientists to better understand ecosystem responses and atmospheric processes.
• Evaluate the implications of changes in friction velocity on boundary layer dynamics during varying atmospheric conditions.
  • Changes in friction velocity can significantly alter boundary layer dynamics, especially under different atmospheric stability conditions. For example, an increase in friction velocity typically enhances turbulence, which can lead to better vertical mixing of heat and moisture, impacting local weather patterns. Conversely, during stable conditions with lower friction velocities, turbulence diminishes, resulting in stratification that can trap pollutants and influence thermal structures. These variations have critical implications for weather forecasting and climate models.

Related terms

Boundary Layer: The boundary layer is a thin region of the atmosphere directly adjacent to the Earth's surface where friction and turbulence significantly affect wind speed and direction.

Turbulent Shear Stress: Turbulent shear stress refers to the force per unit area acting parallel to the flow direction, resulting from turbulence in a fluid, affecting momentum transfer.

Monin-Obukhov Similarity Theory: This theory provides a framework for understanding the behavior of turbulent flow in the atmospheric boundary layer by relating height to stability, surface roughness, and friction velocity.