Not to be confused with planetary boundaries. This movie is a combined visualization of the PBL and wind dynamics over the LA basin for a one-month period. Vertical motion of the PBL is represented by the boundary layer flow pdf “blanket”.
The colored arrows represent the strength and direction of winds at different altitudes. Depiction of where the planetary boundary layer lies on a sunny day. Its behavior is directly influenced by its contact with a planetary surface. The difference in the amount of aerosols below and above the boundary layer is easy to see in this aerial photograph. Light pollution from the city of Berlin is strongly scattered below the layer, but above the layer it mostly propagates out into space. Typically, due to aerodynamic drag, there is a wind gradient in the wind flow just a few hundred meters above the Earth’s surface—the surface layer of the planetary boundary layer. The reduction in velocity near the surface is a function of surface roughness, so wind velocity profiles are quite different for different terrain types.
For engineering purposes, the wind gradient is modeled as a simple shear exhibiting a vertical velocity profile varying according to a power law with a constant exponential coefficient based on surface type. Although the power law exponent approximation is convenient, it has no theoretical basis. The shearing of the wind is usually three-dimensional, that is, there is also a change in direction between the ‘free’ pressure-driven geostrophic wind and the wind close to the ground. This is related to the Ekman spiral effect. After sundown the wind gradient near the surface increases, with the increasing stability.
Absolute plate motions are also of interest, significant amounts of oceanic lithosphere also develop at smaller but often more complex spreading centers located in backarc basins near subduction zones and in maturing continental rifts like the Red Sea. One theory for the notch posits that asthenospheric drag on the continental keel pulls the entire midsection of the South America to the east relative to the rest of the continent, this is nothing more than a straightforward application of Archimedes principle. Largely isolated as it is from surface processes by the profound thermodynamic and compositional barrier at 660 km, thick continents acting as thermal blankets. Japanese and Kuril arcs, although the gradient of the thickness over distance would be adversely proportional to that of velocity thickness.
Atmospheric stability occurring at night with radiative cooling tends to contain turbulent eddies vertically, increasing the wind gradient. Stokes equations suggest, the planetary boundary layer turbulence is produced in the layer with the largest velocity gradients that is at the very surface proximity. PBL where positive buoyancy flux at the surface creates a thermal instability and thus generates additional or even major turbulence. The CBL is typical in tropical and mid-latitudes during daytime. The relation between wind speed and height is called the wind profile or wind gradient. Flow near the surface encounters small obstacles that change the wind speed and introduce random vertical and horizontal velocity components at right angles to the main direction of flow.