Planetary boundary layer

In this layer physical quantities such as flow velocity, temperature, and moisture display rapid fluctuations (turbulence) and vertical mixing is strong.

[13][14] 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.

The cross-isobar angle of the diverted ageostrophic flow near the surface ranges from 10° over open water, to 30° over rough hilly terrain, and can increase to 40°-50° over land at night when the wind speed is very low.

[23] Atmospheric stability occurring at night with radiative cooling tends to vertically constrain turbulent eddies, thus increasing the wind gradient.

[8] The magnitude of the wind gradient is largely influenced by the weather, principally atmospheric stability and the height of any convective boundary layer or capping inversion.

The balance between the rate of the turbulent kinetic energy production and its dissipation determines the planetary boundary layer depth.

Four main external factors determine the PBL depth and its mean vertical structure: A convective planetary boundary layer is a type of planetary boundary layer where positive buoyancy flux at the surface creates a thermal instability and thus generates additional or even major turbulence.

Solar heating assisted by the heat released from the water vapor condensation could create such strong convective turbulence that the free convective layer comprises the entire troposphere up to the tropopause (the boundary in the Earth's atmosphere between the troposphere and the stratosphere), which is at 10 km to 18 km in the Intertropical convergence zone).

An SBL plays a particularly important role in high latitudes where it is often prolonged (days to months), resulting in very cold air temperatures.

Physical laws and equations of motion, which govern the planetary boundary layer dynamics and microphysics, are strongly non-linear and considerably influenced by properties of the Earth's surface and evolution of processes in the free atmosphere.

This movie is a combined visualization of the PBL and wind dynamics over the Los Angeles basin for a one-month period. Vertical motion of the PBL is represented by the gray "blanket". The height of the PBL is largely driven by convection associated with the changing surface temperature of the Earth (for example, rising during the day and sinking at night). 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.
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.
A shelf cloud at the leading edge of a thunderstorm complex on the South Side of Chicago that extends from the Hyde Park community area to over the Regents Park twin towers and out over Lake Michigan
Interactions between the carbon (green), water (blue) and heat (red) cycles in the coupled land–ABL system. As the atmospheric boundary layer decreases in height due to subsidence, it experiences an increase in temperature, a reduction in moisture, and a depletion of CO 2 . This implies a reaction of the land surface ecosystem that will evapotranspire (evaporation from the soil and transpiration from plants) more, to compensate for this loss of moisture in the lower layer, but gradually causing a drying of the soil. (Source: Combe, M., Vilà-Guerau de Arellano, J., Ouwersloot, H. G., Jacobs, C. M. J., and Peters, W.: Two perspectives on the coupled carbon, water and energy exchange in the planetary boundary layer, Biogeosciences, 12, 103–123, .https://doi.org/10.5194/bg-12-103-2015, 2015)