Inmeteorology, theplanetary boundary layer (PBL), also known as theatmospheric boundary layer (ABL) orpeplosphere, is the lowest part of theatmosphere and its behaviour is directly influenced by its contact with aplanetary surface.[1] On Earth it usually responds to changes in surfaceradiative forcing in an hour or less. In this layer physical quantities such asflow velocity, temperature, and moisture display rapid fluctuations (turbulence) and vertical mixing is strong. Above the PBL is the "free atmosphere",[2] where the wind is approximatelygeostrophic (parallel to the isobars),[3] while within the PBL the wind is affected by surfacedrag and turns across theisobars (seeEkman layer for more detail).
Typically, due toaerodynamicdrag, there is a wind gradient in the wind flow ~100 meters above the Earth's surface—thesurface layer of the planetary boundary layer. Wind speed increases with increasing height above the ground, starting from zero[4] due to theno-slip condition.[5] Flow near the surface encounters obstacles that reduce the wind speed, and introduce random vertical and horizontal velocity components at right angles to the main direction of flow.[6]Thisturbulence causes verticalmixing between the air moving horizontally at one level and the air at those levels immediately above and below it, which is important in dispersion ofpollutants[7] and insoil erosion.[8]
The reduction in velocity near the surface is a function of surface roughness, so wind velocity profiles are quite different for different terrain types.[5] Rough, irregular ground, and man-made obstructions on the ground can reduce thegeostrophic wind speed by 40% to 50%.[9][10] Over open water or ice, the reduction may be only 20% to 30%.[11][12] These effects are taken into account when sitingwind turbines.[13][14]
Forengineering purposes, the wind gradient is modeled as asimple shear exhibiting a vertical velocity profile varying according to apower law with a constantexponential coefficient based on surface type. The height above ground where surface friction has a negligible effect on wind speed is called the "gradient height" and the wind speed above this height is assumed to be a constant called the "gradient wind speed".[10][15][16] For example, typical values for the predicted gradient height are 457 m for large cities, 366 m for suburbs, 274 m for open terrain, and 213 m for open sea.[17]
Although the power law exponent approximation is convenient, it has no theoretical basis.[18] When the temperature profile is adiabatic, the wind speed should varylogarithmically with height.[19] Measurements over open terrain in 1961 showed good agreement with thelogarithmic fit up to 100 m or so (within thesurface layer), with near constant average wind speed up through 1000 m.[20]
Theshearing of the wind is usually three-dimensional,[21] that is, there is also a change in direction between the 'free' pressure gradient-driven geostrophic wind and the wind close to the ground.[22] This is related to theEkman spiral effect. 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.[12]
After sundown the wind gradient near the surface increases, with the increasing stability.[23]Atmospheric stability occurring at night withradiative cooling tends to vertically constrain turbulenteddies, thus increasing the wind gradient.[8] The magnitude of the wind gradient is largely influenced by theweather, principally atmospheric stability and the height of any convective boundary layer orcapping inversion. This effect is even larger over the sea, where there is much less diurnal variation of the height of the boundary layer than over land.[24]In the convective boundary layer, strong mixing diminishes vertical wind gradient.[25]
The planetary boundary layer is different between day and night. During the dayinversion layers formed during the night are broken up as a consequence of the turbulent rise of heated air.[26] The boundary layer stabilises "shortly before sunset" and remains so during the night.[26] All this make up a daily cycle.[26] During winter and cloudy days the breakup of the nighttime layering is incomplete and atmospheric conditions established in previous days can persist.[26][27] The breakup of the nighttime boundary layer structure is fast on sunny days.[27] The driving force is convective cells with narrow updraft areas and large areas of gentle downdraft.[27] These cells exceed 200–500 m in diameter.[27]
AsNavier–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 asurface layer – constitutes about 10% of the total PBL depth. Above the surface layer the PBL turbulence gradually dissipates, 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. The PBL depth can be 4000 m or higher in late afternoon over desert.
In addition to the surface layer, the planetary boundary layer also comprises the PBLcore (between 0.1 and 0.7 of the PBL depth) and the PBL top orentrainment layer orcapping inversion layer (between 0.7 and 1 of the PBL 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. (This is also known as having CAPE orconvective available potential energy; seeatmospheric convection.) A convective boundary layer is typical in tropical and mid-latitudes during daytime. Solar heating assisted by the heat released from the water vapor condensation could create such strong convective turbulence that thefree convective layer comprises the entire troposphere up to thetropopause (the boundary in the Earth's atmosphere between thetroposphere and thestratosphere), which is at 10 km to 18 km in theIntertropical convergence zone).
The SBL is a PBL when negative buoyancy flux at the surface damps the turbulence; seeConvective inhibition. An SBL is solely driven by the wind shear turbulence and hence the SBL cannot exist without the free atmosphere wind. An SBL is typical in nighttime at all locations and even in daytime in places where the Earth's surface is colder than the air above (i.e. an inversion). SBL is dominated in high latitudes (e.g., the Arctic region), with lower temperature of sea-ice surface, which significantly suppresses turbulent motions.[28][29]
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. To deal with this complexity, the whole array ofturbulence modelling has been proposed. However, they are often not accurate enough to meet practical requirements. Significant improvements are expected from application of alarge eddy simulation technique to problems related to the PBL.
Perhaps the most important processes,[clarification needed] which are critically dependent on the correct representation of the PBL in the atmospheric models (Atmospheric Model Intercomparison Project), are turbulent transport of moisture (evapotranspiration) and pollutants (air pollutants).Clouds in the boundary layer influencetrade winds, thehydrological cycle, and energy exchange.
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.
Therefore the vertical gradient of mean wind speed (dū/dz) is greatest over smooth terrain, and least over rough surfaces.
...both the wind gradient and the mean wind profile itself can usually be described diagnostically by the log wind profile.
In the bulk of the convective boundary layer, strong mixing diminishes vertical wind gradient...
