METR2023 - Lecture 24 - Segment 1: Atmospheric Boundary Layer (ABL) Introduction

Summary

This lecture explains the atmospheric boundary layer (ABL), the lowest layer of the Earth's atmosphere. The ABL is directly influenced by the surface, with strong friction and turbulent flow. During the day, it can extend up to 3 kilometers due to solar heating, while at night it contracts to as little as 100-200 meters. Factors such as solar radiation, terrain roughness, and evapotranspiration influence its depth. The lecture covers the dynamics of rising thermals, the entrainment zone, and the effects of temperature fluctuations, offering insight into the ABL's role in weather patterns and atmospheric processes.

Takeaways

• 😀 The atmospheric boundary layer (ABL) is the lowest part of the troposphere, influenced by Earth's surface and characterized by strong friction and turbulent flow.
• 😀 Another term for the atmospheric boundary layer is the planetary boundary layer (PBL), and it is typically about 1 km deep, but can vary with time of day and geographic factors.
• 😀 During the daytime, the boundary layer can expand to 2–3 km or even up to 4–5 km in extreme cases, particularly in regions with intense sunlight and dry air.
• 😀 At night, the boundary layer shrinks significantly, often to depths of 100-200 meters or even lower, due to cooling and stabilization of the surface air.
• 😀 The primary factor influencing the depth of the boundary layer is the amount of solar heating of the Earth's surface, which leads to rising thermals (columns of hot air).
• 😀 Regions with intense solar heating, such as deserts, can lead to exceptionally deep boundary layers due to faster warming of dry air.
• 😀 Terrain features like mountains, valleys, forests, and urban areas contribute to a deeper boundary layer because they increase turbulent flow at the surface.
• 😀 Smooth landscapes like plains or oceans tend to have a shallower boundary layer due to less turbulent mixing.
• 😀 Evapotranspiration, the process of moisture being evaporated by plants, increases the moisture content in the atmosphere, leading to more buoyant air parcels and a deeper boundary layer.
• 😀 The boundary layer is defined by the interaction between rising hot air and cooler air above, with turbulence being strongest at this interface, often referred to as the entrainment zone.
• 😀 At night, the boundary layer shrinks because the surface air cools and becomes stable, preventing upward motions and causing the layer to contract towards the ground.
• 😀 In some cases, turbulent mixing, such as from a passing cold front, can temporarily bring warmer air from aloft down to the surface, leading to an increase in temperature during the night.

Q & A

What is the atmospheric boundary layer (ABL)?

The atmospheric boundary layer (ABL) is the lowest layer of the Earth's atmosphere, typically extending up to about 1 kilometer above the surface. It is characterized by strong friction and turbulent air flow, and it is directly influenced by the Earth's surface.

What is the difference between the atmospheric boundary layer (ABL) and the planetary boundary layer (PBL)?

The atmospheric boundary layer (ABL) and the planetary boundary layer (PBL) are essentially the same. The term PBL is often used interchangeably with ABL, but some may use 'boundary layer' alone without referring to 'atmospheric' or 'planetary.' Both terms refer to the layer of the atmosphere closest to the Earth's surface influenced by friction and turbulent air flow.

Why does the depth of the atmospheric boundary layer vary between day and night?

The depth of the ABL is influenced by the Sun’s heating. During the day, strong solar heating warms the Earth’s surface, causing air near the ground to rise, which deepens the boundary layer. At night, radiational cooling causes the surface to cool, stabilizing the air near the ground, which causes the boundary layer to shrink.

What factors influence the depth of the atmospheric boundary layer?

The depth of the ABL is influenced by factors like solar heating (diabatic heating), terrain (rough terrain leads to deeper boundary layers), and evapotranspiration (moisture evaporation from plants and soil, which increases the buoyancy of rising air parcels).

How does solar heating affect the boundary layer during the day?

During the day, solar heating warms the ground, which in turn heats the air near the surface. This creates buoyant, rising thermals (columns of warm air) that push the boundary layer upward. The stronger the heating, the deeper the boundary layer becomes.

Why does the boundary layer tend to be deeper during the summer?

The boundary layer tends to be deeper during the summer due to stronger solar heating, which causes more intense rising thermals and greater upward motion of air. In regions with dry air, like deserts, this process is even more pronounced because dry air heats up more quickly.

How does terrain influence the depth of the atmospheric boundary layer?

Rough terrain, such as mountains, forests, or urban areas, increases turbulent flow near the surface, leading to a deeper boundary layer. In contrast, smooth terrains, like grasslands or oceans, experience less turbulence and have shallower boundary layers.

What is evapotranspiration, and how does it influence the boundary layer?

Evapotranspiration is the process by which moisture is evaporated from plants and soil into the atmosphere. This increases the moisture content of the air, which enhances the buoyancy of rising air parcels, helping the boundary layer to deepen.

What is the entrainment zone, and where is it located?

The entrainment zone is the interface between the atmospheric boundary layer and the free atmosphere, where turbulent mixing occurs. It is located at the top of the boundary layer, where the turbulent flow decreases and transitions into the calmer free atmosphere.

Can the boundary layer become deeper at night? Under what circumstances?

Normally, the boundary layer shrinks at night due to radiational cooling and stabilized air near the surface. However, under certain conditions, such as during the passage of a cold front or other atmospheric mixing events, the boundary layer can temporarily become deeper as warmer air from aloft is mixed down to the surface.