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What is the Atmospheric Boundary Layer?

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Tristan is a student at the Wageningen University with its main focus on atmospheric science.

The Atmospheric Boundary Layer

The atmospheric boundary layer is the first layer of air on top of the earth surface. It is the layer of air that we as humans breathe in every day and night. Not only humans, but also animals and plants are affected by the conditions of the boundary layer.

Based on this definition, it seems like the boundary layer's maximum height is several tens of meters. However, in reality the boundary layer can become more than 4 km high. Therefore I will start by introducing a more formal and definitive definition of the boundary layer.

• Is the boundary layer a static thing? Does it develop? Does it change during the day?
• Do we as humans impact the atmospheric boundary layer?

The Formal Definition

The atmospheric boundary layer is the layer of air above the earth's surface that is impacted by the earth's surface.

The Practical Definition of the Atmospheric Boundary Layer

The formal definition seems not very definitive at first sight. However, it tells you to find a way to determine the height up to which the earth surface influences the atmosphere. That height is considered to be the atmospheric boundary layer height.

A solution to this riddle is the wind, because the wind is impacted by surface friction. The friction only affects the wind close to the surface, meaning that the wind is not impacted anymore by the surface friction at a certain height. This height can be considered to be the atmospheric boundary layer height.

The Wind Profile

The wind profile has two extremes, one at the earth's surface and one at the boundary layer height.

At the surface, the no-slip condition applies. This no-slip condition means that at the earth surface, the wind speed equals 0 m/s. Therefore there is no wind at the surface. Mathematically this is expressed as: For z = 0 m, u = 0 m/s, with z being the height of the boundary layer and u being the wind speed.

The second extreme is found at the top of the boundary layer. At that height, the boundary layer wind equals the geostrophic wind. The geostrophic wind is the wind present in the free troposphere, the layer above the atmospheric boundary layer. The most important property of the geostrophic wind is that it is not impacted by the surface anymore. In other words, when the wind in the boundary layer equals the geostrophic wind, the wind in the boundary layer is not impacted anymore by the surface. Therefore, if the wind in the boundary layer equals the geostrophic wind, we reached the top of the boundary layer.

Mathematically, this second extreme in the wind profile can be expressed as: For z = h, u = ug, with h being the atmospheric boundary layer height and ug being the geostrophic wind.

The Diurnal Cycle of the Atmospheric Boundary Layer

The boundary layer knows a strong diurnal cycle. During day time, the boundary layer encounters strong growth due to the incoming solar radiation. During night time, the boundary layer strongly decreases in height due to the strong radiative cooling at the earth's surface.

As a result of this strong relationship between the boundary layer height and the radiation, the boundary layer's diurnal cycle is similar to that of the sun. The main important thing to remember is that there are two types of boundary layers: The convective boundary layer (CBL) and the stable boundary layer (SBL).

The two layers can be found in the figure below and will be elaborated separately below the figure.

The Convective Boundary (CBL)

The CBL is the boundary layer type that occurs during day time. It is still a boundary layer, but one with convective air. This means that the air is rising from the surface to higher altitudes within the convective boundary layer.

The reason for the rising air in the CBL is the incoming radiation from the sun. The incoming radiation heats up the surface of the earth, which in turn heats up the air just above the surface. When air heats up, its density decreases, meaning that the air becomes lighter. The effect of the lower density is that the air starts to rise, because it is lighter than the air on top. That is the reason why during daytime, the atmospheric boundary layer is called the convective boundary layer.

The rising air is also why the boundary layer can grow during day time, meaning that the top of the boundary layer becomes higher. This also impacts the wind profile in the boundary layer. When we state that the boundary layer grows, it means that we indirectly say that the impact of the surface friction on wind is sensible at higher altitudes. This is nicely illustrated by the fact that, with a higher boundary layer, the wind equals the geostrophic wind at higher altitudes.

The explanation for this lies in the rising air parcels; you could say that the rising air parcels transfer the information from lower altitudes to higher altitudes. Therefore the friction induced by the earth's surface can be felt at higher altitudes, and thus the wind equals the geostrophic wind at a higher altitude in a convective boundary layer.

The other important impact of the rising air is that it mixes the air in the CBL. Therefore, the CBL is considered to be a well-mixed atmospheric boundary layer. This means that the moisture content is constant throughout the atmosphere. The fact that the CBL is a well-mixed layer is mainly important for scientists who try to model/calculate certain processes in the atmosphere.

The Stable Boundary Layer (SBL)

The stable boundary layer occurs during the night, that is why it is also called the 'nocturnal boundary layer'. An important aspect of the night is that there is no incoming solar radiation from the sun. The lack of incoming solar radiation results in the cooling of the earth's surface. This means that the air on top of the earth's surface will cool as well. Colder air is denser air, meaning that it cannot rise anymore during the night.

The fact that the air does not rise anymore also means that the height up to which the wind is impacted by the surface is lower. Meaning that also the boundary layer height is lower.

No rising air means that there will be no mixing of the air in the stable boundary layer. This means that moisture content will change with height making it harder for scientists to model processes in the stable boundary layer.

Human impact on the Atmospheric Boundary Layer

The main way in which humans impact the boundary layer is by pollution. The pollution caused by humans is called anthropogenic pollution and causes the release of particle into the atmosphere. These particles are commonly referred to as aerosols.

The aerosols do not directly impact the boundary layer. However, they do directly impact the radiation balance. Research proved that more aerosols lead to more reflection of the incoming solar radiation. This would mean that there is less solar radiation that reaches the surface and causes the warming of the air. Therefore, the main indirect impact of the aerosols is that they limit the boundary layer growth since the aerosols limit the possibilities for air parcels to rise by reducing the warming of the air parcels.

An important note to make is that this limiting effect on the boundary layer growth by the aerosols only occurs once there is strong pollution.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.