Planetary boundary layer
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The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behavior is directly influenced by its contact with the planetary surface. It responds to surface forcings in a timescale of an hour or less. In this layer physical quantities such as flow velocity, temperature, moisture etc., display rapid fluctuations (turbulence) and vertical mixing is strong. Above the PBL is the free atmosphere where the wind is approximately geostrophic (parallel to the isobars) while within the PBL the wind is affected by surface drag and turns across the isobars. The free atmosphere is usually non turbulent, or only intermittently turbulent.
As Navier-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. This layer - conventionally called a surface layer - constitutes about 10% of the total PBL depth. Above the surface layer the PBL turbulence gradually dissipate losing its kinetic energy to friction as well as converting the kinetic to potential energy in a density stratified flow. The balance between the rate of the turbulent kinetic energy production and its dissipation determines the planetary boundary layer depth. The PBL depth varies broadly. At a given wind speed, e.g. 8 m/s, and so at a given rate of the turbulence production, a PBL in wintertime Arctic could be as shallow as 50 m, a nocturnal PBL in mid-latitudes could be typically 300 m in thickness, and a tropical PBL in the trade-wind zone could grow to its full theoretical depth of 2000 m.
In addition to the surface layer, the planetary boundary layer also comprises the PBL core (between 0.1 and 0.7 of the PBL depth) and the PBL top or entrainment layer or capping inversion layer (between 0.7 and 1 of the PBL depth).
Four main external factors determine the PBL depth and its mean vertical structure: (1) the free atmosphere wind speed; (2) the surface heat (more exactly buoyancy) balance; (3) the free atmosphere density stratification; (4) the free atmosphere vertical wind shear or baroclinicity.
There are two principal types of the planetary boundary layers: - Convective planetary boundary layer (CBL, see also convection) is the 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. Solar heating assisted by the heat realised from the water vapor condensation could create so strong convective turbulence that the CBL comprises the entire troposphere up to 10 km to 18 km (Intertropical convergence zone). - Stably stratified planetary boundary layer (SBL) is the PBL where negative buoyancy flux at the surface damps the turbulence. The SBL is solely driven by the wind shear turbulence and hence the SBL cannot exist without the free atmosphere wind. The SBL is typical in nighttime at all locations and even in daytime in place where the earth's surface is colder than the air above. Particularly important role, the SBL plays in high-latitudes where the long-lived (days to months) SBLs result in very cold air temperatures.
Physical laws and equations of motions, 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 the processes in the free atmosphere. To deal with this complicity, the whole array of turbulence modelling has been proposed. However, they are often not accurate enough to met practical requests. Significant improvements are expected from application of a large eddy simulation technique to problems related to the PBL.
Perhaps the most important processes, which are critically dependent on the correct representation of the PBL in the atmosperic models (Atmospheric Model Intercomparison Project), are turbulent transport of moisture (evapotranspiration) and pollutants (air pollutants). Clouds in the boundary layer influence trade winds, the hydrological cycle, and energy exchange.
