> [!example] Learning goals > 1. Understand what is the atmospheric boundary layer. > 2. Understand why turbulence is important. > 3. Understand why the atmospheric boundary layer is turbulent. ## What is the atmospheric boundary layer? The *atmospheric boundary layer* is the lowest layer of the atmosphere. In this layer, the atmosphere adjusts to its no-slip boundary, the Earth's surface. This adjustment process generally results in the generation of turbulence. Turbulence (see animation below) is a chaotic movement of the air in which motion is organized in structures called eddies.  *This animation shows the streamwise velocity in an atmospheric surface layer generated in a direct numerical simulation.* ## Why is turbulence important? Turbulence results in mixing that is orders of magnitude stronger than the mixing of gradients by viscosity. Consequently, the time scales in the atmospheric boundary layer are short. A typical eddy turnover in an atmospheric surface layer lasts seconds to minutes, which extends up to tens of minutes in a deep convective boundary layer. The fast mixing causes that changes at the surface are quickly felt in the entire boundary layer, and the atmospheric boundary layer therefore has a strong diurnal cycle. This is particularly true over land, where the surface fluxes closely follow the surface solar irradiance. Also, atmospheric constituents released at the surface, such as water vapor through evaporation, or pollutants are mostly contained in the atmospheric boundary layer. ## Why is the atmospheric boundary layer turbulent? If a moving air mass encounters a surface, viscous forces ensure that right above the surface, the air has the velocity of the surface. Consequently, there must be a region in which the velocity profile adjusts from the velocity of the surface to the velocity of the atmosphere outside of the influence of the surface. This layer is the boundary layer. Fluid dynamics books (or the [Wikipedia entry on the boundary layer](https://en.wikipedia.org/wiki/Boundary_layer)) that treat the boundary layer make a distinction between laminar and turbulent boundary layers depending on the characteristic velocity $U$ and depth $L$ of the layer, and the kinematic viscosity $\nu$ of the fluid. From these three quantities, one can derive only one non-dimensional number, the Reynolds number. > [!note] The Reynolds number > $ > Re \equiv \frac{U L}{\nu} > $ The Reynolds number compares the importance of inertia to that of viscous forces and the larger the number, the harder it becomes for viscous forces to remove momentum from the fluid. If the Reynolds number reaches a critical value, the flow transitions from a laminar to a turbulent state. Even a calm atmospheric surface layer ($U = 1$, $L = 100$, $\nu = 10^{-5}$) has a Reynolds number of $10^7$, which exceeds the definitions of typical critical Reynolds number of $\sim 10^5$ of a wide range of flow types. Therefore, we conclude that the atmospheric boundary layer is nearly always turbulent.