Atmospheric boundary layer
The atmospheric boundary layer (ABL), also referred to as the planetary boundary layer (PBL), is the lowest portion of the troposphere, typically comprising the bottom 10-20 percent of this atmospheric layer. It is characterized by significant interaction with the Earth's surface, resulting in turbulence driven by thermal convection and wind shear. The ABL is crucial for maintaining a stable atmospheric composition, which supports life on Earth, as it helps to distribute heat and energy inputs evenly. Structurally, the ABL consists of three layers: the surface layer, the core, and the entrainment layer, which influences weather patterns and convective activity.
In urban environments, the ABL can be altered by human activities, leading to surface heating and the trapping of pollutants, thereby impacting air quality and climate. Understanding the dynamics of the ABL is vital for meteorologists, as it plays a key role in weather forecasting and climate models. The exchange of energy within this layer is essential for regulating temperatures and preventing drastic climate changes. Consequently, research on the ABL is instrumental in developing strategies to mitigate environmental impacts and improve urban planning.
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Atmospheric boundary layer
Definition
Also known as the planetary boundary layer (PBL), the atmospheric boundary layer (ABL) is the lowest 10-20 percent of the troposphere, the lowest layer of the atmosphere. Its contact with a planetary surface directly influences its behavior. The ABL contains a disproportionately large amount of the mass and the kinetic energy of the atmosphere. It is the most dynamically active of the layers of Earth. The phrase “boundary layer” originates in the study of boundary layers in fluid flows: The boundary layer is the layer of fluid that is most influenced by friction with the Earth’s surface. A defining characteristic of the ABL is turbulence caused by thermal convection, due to thermal buoyancy, and wind shear, due to frictional forces. The atmospheric boundary layer has three layers: the surface layer, the core, and the entrainment layer, also called the capping inversion layer.
Significance for Climate Change
The atmospheric boundary layer is important for ensuring that Earth’s atmospheric composition remains relatively homogeneous throughout, despite external heat and energy inputs. Consequently, it is important for ensuring that life can be sustained on Earth. ABL considerations are particularly important in the area of the urban environment. The development of large cities has changed the ABL in those areas, resulting in surface heating, and artificial boundary layers have developed that trap pollutants. Calculations of the boundary layer can help architects develop urban environments in such a way as to minimize impact on the boundary layer. ABL research can help improve weather forecasts, especially long-term forecasts and forecasts of longtime climate models. Without a properly constituted boundary layer, Earth would lose the unique conditions that make it hospitable to human existence.
The ABL is important meteorologically in the area of assessing convective instability. The entrainment zone (at the top of the ABL) acts as a lid on rising air parcels attributable to temperature inversion. If that entrainment layer is broken, capped air parcels can rise freely, resulting in vigorous convection that produces severe thunderstorms. When sunlight enters the atmosphere, a part of it—known as the albedo—is immediately reflected back to space; the remainder penetrates the atmosphere, and the Earth’s surface absorbs it. This energy is then reemitted by the Earth back into the atmosphere in the form of longwave radiation. Carbon dioxide and water molecules absorb this energy, then emit much of it back toward Earth again. This delicate exchange of energy between the Earth’s surface and the atmosphere is what keeps the average global temperature from changing drastically from year to year. When this exchange is disrupted, climate problems result.
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