Planetary boundary layer

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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. PBLimage.jpg
Depiction of where the planetary boundary layer lies on a sunny day.

In meteorology, the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. [1] On Earth it usually responds to changes in surface radiative forcing in an hour or less. In this layer physical quantities such as flow 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 approximately geostrophic (parallel to the isobars), [3] while within the PBL the wind is affected by surface drag and turns across the isobars (see Ekman layer for more detail).

Contents

Cause of surface wind gradient

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. Light pollution and the planetary boundary layer over Berlin.jpg
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 ~100 meters above the Earth's surface—the surface layer of the planetary boundary layer. Wind speed increases with increasing height above the ground, starting from zero [4] due to the no-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] This turbulence causes vertical mixing between the air moving horizontally at one level and the air at those levels immediately above and below it, which is important in dispersion of pollutants [7] and in soil 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 the geostrophic 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 siting wind turbines. [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 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 vary logarithmically with height. [19] Measurements over open terrain in 1961 showed good agreement with the logarithmic fit up to 100 m or so (within the surface layer), with near constant average wind speed up through 1000 m. [20]

The shearing 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 the Ekman 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 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. 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]

Nocturnal and diurnal conditions

The planetary boundary layer is different between day and night. During the day inversion 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]

Constituent layers

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 20120629 atmospheric thermocline.JPG
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

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 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 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.

Principal types

Atmospheric boundary layer.svg

Convective planetary boundary layer (CBL)

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 or convective available potential energy; see atmospheric 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 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).

Stably stratified planetary boundary layer (SBL)

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 CO2. 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., Vila-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) Interactions-between-the-carbon-green-water-blue-and-heat-red-cycles-in-the-coupled-landABL-system.png
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 CO2. 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)

The SBL is a PBL when negative buoyancy flux at the surface damps the turbulence; see Convective 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. 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. To deal with this complexity, the whole array of turbulence modelling has been proposed. However, they are often not accurate enough to meet practical requirements. Significant improvements are expected from application of a large 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 influence trade winds, the hydrological cycle, and energy exchange.

See also

Related Research Articles

<span class="mw-page-title-main">Troposphere</span> Lowest layer of Earths atmosphere

The troposphere is the lowest layer of the atmosphere of Earth. It contains 75% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is 18 km in the tropics; 17 km in the middle latitudes; and 6 km in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km.

<span class="mw-page-title-main">Tropopause</span> The boundary of the atmosphere between the troposphere and stratosphere

The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometres (11 mi) above the equatorial regions, and approximately 9 kilometres (5.6 mi) above the polar regions.

<span class="mw-page-title-main">Wind shear</span> Difference in wind speed or direction over a short distance

Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.

In common usage, wind gradient, more specifically wind speed gradient or wind velocity gradient, or alternatively shear wind, is the vertical component of the gradient of the mean horizontal wind speed in the lower atmosphere. It is the rate of increase of wind strength with unit increase in height above ground level. In metric units, it is often measured in units of meters per second of speed, per kilometer of height (m/s/km), which reduces inverse milliseconds (ms−1), a unit also used for shear rate.

In atmospheric science, geostrophic flow is the theoretical wind that would result from an exact balance between the Coriolis force and the pressure gradient force. This condition is called geostrophic equilibrium or geostrophic balance. The geostrophic wind is directed parallel to isobars. This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation. Geostrophic flow in air or water is a zero-frequency inertial wave.

<span class="mw-page-title-main">Surface layer</span> Layer of a turbulent fluid affected by interaction with a surface

The surface layer is the layer of a turbulent fluid most affected by interaction with a solid surface or the surface separating a gas and a liquid where the characteristics of the turbulence depend on distance from the interface. Surface layers are characterized by large normal gradients of tangential velocity and large concentration gradients of any substances transported to or from the interface.

<span class="mw-page-title-main">Thermal wind</span> Vector difference of geostrophic wind movement at high and low altitudes

In atmospheric science, the thermal wind is the vector difference between the geostrophic wind at upper altitudes minus that at lower altitudes in the atmosphere. It is the hypothetical vertical wind shear that would exist if the winds obey geostrophic balance in the horizontal, while pressure obeys hydrostatic balance in the vertical. The combination of these two force balances is called thermal wind balance, a term generalizable also to more complicated horizontal flow balances such as gradient wind balance.

<span class="mw-page-title-main">Ekman layer</span> Force equilibrium layer in a liquid

The Ekman layer is the layer in a fluid where there is a force balance between pressure gradient force, Coriolis force and turbulent drag. It was first described by Vagn Walfrid Ekman. Ekman layers occur both in the atmosphere and in the ocean.

The log wind profile is a semi-empirical relationship commonly used to describe the vertical distribution of horizontal mean wind speeds within the lowest portion of the planetary boundary layer. The relationship is well described in the literature.

In fluid dynamics, the von Kármán constant, named for Theodore von Kármán, is a dimensionless constant involved in the logarithmic law describing the distribution of the longitudinal velocity in the wall-normal direction of a turbulent fluid flow near a boundary with a no-slip condition. The equation for such boundary layer flow profiles is:

<span class="mw-page-title-main">Horizontal convective rolls</span>

Horizontal convective rolls, also known as horizontal roll vortices or cloud streets, are long rolls of counter-rotating air that are oriented approximately parallel to the ground in the planetary boundary layer. Although horizontal convective rolls, also known as cloud streets, have been clearly seen in satellite photographs for the last 30 years, their development is poorly understood, due to a lack of observational data. From the ground, they appear as rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind. Research has shown these eddies to be significant to the vertical transport of momentum, heat, moisture, and air pollutants within the boundary layer. Cloud streets are usually more or less straight; rarely, cloud streets assume paisley patterns when the wind driving the clouds encounters an obstacle. Those cloud formations are known as von Kármán vortex streets.

<span class="mw-page-title-main">Atmospheric convection</span> Atmospheric phenomenon

Atmospheric convection is the result of a parcel-environment instability in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day expands the height of the planetary boundary layer, leading to increased winds, cumulus cloud development, and decreased surface dew points. Convection involving moist air masses leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

<span class="mw-page-title-main">Atmospheric instability</span> Condition where the Earths atmosphere is generally considered to be unstable

Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air. Because it is warmer, it is less dense and is prone to further ascent.

Ocean dynamics define and describe the flow of water within the oceans. Ocean temperature and motion fields can be separated into three distinct layers: mixed (surface) layer, upper ocean, and deep ocean.

In oceanography, Ekman velocity – also referred as a kind of the residual ageostrophic velocity as it deviates from geostrophy – is part of the total horizontal velocity (u) in the upper layer of water of the open ocean. This velocity, caused by winds blowing over the surface of the ocean, is such that the Coriolis force on this layer is balanced by the force of the wind.

The convective planetary boundary layer (CPBL), also known as the daytime planetary boundary layer, is the part of the lower troposphere most directly affected by solar heating of the earth's surface.

<span class="mw-page-title-main">Alpine planetary boundary layer</span>

The alpine planetary boundary layer is the planetary boundary layer (PBL) associated with mountainous regions. Due to its high spatial and temporal variability, its behavior is more complex than over a flat terrain. The fast changing local wind system directly linked to topography and the variable land cover that goes from snow to vegetation have a significant effect on the growth of the PBL and make it much harder to predict.

Atmospheric lidar is a class of instruments that uses laser light to study atmospheric properties from the ground up to the top of the atmosphere. Such instruments have been used to study, among other, atmospheric gases, aerosols, clouds, and temperature.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

<span class="mw-page-title-main">Conditional symmetric instability</span>

Conditional symmetric instability, or CSI, is a form of convective instability in a fluid subject to temperature differences in a uniform rotation frame of reference while it is thermally stable in the vertical and dynamically in the horizontal. The instability in this case develop only in an inclined plane with respect to the two axes mentioned and that is why it can give rise to a so-called "slantwise convection" if the air parcel is almost saturated and moved laterally and vertically in a CSI area. This concept is mainly used in meteorology to explain the mesoscale formation of intense precipitation bands in an otherwise stable region, such as in front of a warm front. The same phenomenon is also applicable to oceanography.

References

  1. ↑ "Planetary boundary layer | atmospheric science | Britannica". www.britannica.com. Retrieved 28 June 2020.
  2. ↑ "Free atmosphere". glossary.ametsoc.org. Retrieved 21 March 2021.
  3. ↑ "Geostrophic wind level". glossary.ametsoc.org. Retrieved 20 September 2018.
  4. ↑ Wizelius, Tore (2007). Developing Wind Power Projects . London: Earthscan Publications Ltd. p.  40. ISBN   978-1-84407-262-0. The relation between wind speed and height is called the wind profile or wind gradient.
  5. 1 2 Brown, G. Z.; DeKay, Mark (2001). Sun, Wind & Light. New York: Wiley. p. 18. ISBN   0-471-34877-5.
  6. ↑ Dalgliesh, W. A. & D. W. Boyd (1 April 1962). "CBD-28. Wind on Buildings". Canadian Building Digest. Archived from the original on 12 November 2007. Retrieved 30 June 2007. 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.
  7. ↑ Hadlock, Charles (1998). Mathematical Modeling in the Environment . Washington: Mathematical Association of America. ISBN   0-88385-709-X.
  8. 1 2 Lal, Rattan (2005). Encyclopedia of Soil Science. New York: Marcel Dekker. p. 618. ISBN   0-8493-5053-0.
  9. ↑ Oke, Timothy R. (1987). Boundary Layer Climates. London: Methuen. p. 54. ISBN   0-415-04319-0. Therefore the vertical gradient of mean wind speed (dū/dz) is greatest over smooth terrain, and least over rough surfaces.
  10. 1 2 Crawley, Stanley (1993). Steel Buildings. New York: Wiley. p. 272. ISBN   0-471-84298-2.
  11. ↑ Harrison, Roy (1999). Understanding Our Environment . Cambridge: Royal Society of Chemistry. p.  11. ISBN   0-85404-584-8.
  12. 1 2 Thompson, Russell (1998). Atmospheric Processes and Systems . New York: Routledge. pp.  102–103. ISBN   0-415-17145-8.
  13. ↑ Maeda, Takao, Shuichiro Homma, and Yoshiki Ito. Effect of Complex Terrain on Vertical Wind Profile Measured by SODAR Technique. Retrieved on 2008-07-04.
  14. ↑ Lubosny, Zbigniew (2003). Wind Turbine Operation in Electric Power Systems: Advanced Modeling. Berlin: Springer. p. 17. ISBN   3-540-40340-X.
  15. ↑ Gupta, Ajaya (1993). Guidelines for Design of Low-Rise Buildings Subjected to Lateral Forces. Boca Raton: CRC Press. p. 49. ISBN   0-8493-8969-0.
  16. ↑ Stoltman, Joseph (2005). International Perspectives on Natural Disasters: Occurrence, Mitigation, and Consequences. Berlin: Springer. p. 73. ISBN   1-4020-2850-4.
  17. ↑ Chen, Wai-Fah (1997). Handbook of Structural Engineering . Boca Raton: CRC Press. pp.  12–50. ISBN   0-8493-2674-5.
  18. ↑ Ghosal, M. (2005). "7.8.5 Vertical Wind Speed Gradient". Renewable Energy Resources. City: Alpha Science International, Ltd. pp. 378–379. ISBN   978-1-84265-125-4.
  19. ↑ Stull, Roland (1997). An Introduction to Boundary Layer Meteorology. Boston: Kluwer Academic Publishers. p. 442. ISBN   90-277-2768-6. ...both the wind gradient and the mean wind profile itself can usually be described diagnostically by the log wind profile.
  20. ↑ Thuillier, R.H.; Lappe, U.O. (1964). "Wind and Temperature Profile Characteristics from Observations on a 1400 ft Tower". Journal of Applied Meteorology . American Meteorological Society. 3 (3): 299–306. Bibcode:1964JApMe...3..299T. doi: 10.1175/1520-0450(1964)003<0299:WATPCF>2.0.CO;2 . ISSN   1520-0450.
  21. ↑ McIlveen, J. F. Robin (1992). Fundamentals of Weather and Climate. London: Chapman & Hall. p.  184. ISBN   0-412-41160-1.
  22. ↑ Burton, Tony (2001). Wind Energy Handbook . London: J. Wiley. p.  20. ISBN   0-471-48997-2.
  23. ↑ Köpp, F.; Schwiesow, R.L.; Werner, C. (January 1984). "Remote Measurements of Boundary-Layer Wind Profiles Using a CW Doppler Lidar". Journal of Applied Meteorology and Climatology . American Meteorological Society. 23 (1): 153. Bibcode:1984JApMe..23..148K. doi: 10.1175/1520-0450(1984)023<0148:RMOBLW>2.0.CO;2 . ISSN   1520-0450.
  24. ↑ Johansson, C.; Uppsala, S.; Smedman, A.S. (2002). "Does the height of the boundary layer influence the turbulence structure near the surface over the Baltic Sea?". 15th Conference on Boundary Layer and Turbulence. 15th Conference on Boundary Layer and Turbulence. American Meteorological Society.
  25. ↑ Shao, Yaping (2000). Physics and Modelling of Wind Erosion. City: Kluwer Academic. p. 69. ISBN   978-0-7923-6657-7. In the bulk of the convective boundary layer, strong mixing diminishes vertical wind gradient...
  26. 1 2 3 4 Foken 2017, p. 7.
  27. 1 2 3 4 Foken 2017, p. 8.
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