Planetary Boundary Layer Concepts

Entrainment Zone (EZ)

• The layer at the top of the mixed layer, marked by a stable layer and temperature inversion, where air from the mixed layer is entrained into the free atmosphere.
• Air from the mixed layer is entrained into the free atmosphere due to temperature inversion.

Virtual Potential Temperature Profiles

• Surface heating creates a superadiabatic lapse rate in the surface layer, a nearly isothermal mixed layer, and an entrainment zone at the top during daytime.
• Surface cooling leads to a surface inversion and a stable boundary layer at night, which deepens as the night progresses.
• On sunny days with strong winds, the virtual potential temperature remains nearly constant with height.
• A stable layer atop a convective boundary layer restricts further vertical mixing referred to as a capping inversion.

Planetary Boundary Layer (PBL)

• The PBL is the lowest part of the atmosphere directly influenced by the Earth's surface.
• Surface friction slows down the wind in the PBL, creating turbulence.
• Turbulence mixes air within the PBL, helping transport heat, moisture, and momentum vertically.
• The PBL's height varies by time of day, season, and location, ranging from hundreds of meters to several kilometers.
• Turbulence within the PBL controls vertical transport, whereas molecular processes are negligible outside the PBL due to low viscosity.

Sublayers of the PBL

• The surface layer: Closest to the ground (tens of meters deep), in direct contact with the Earth's surface.
• Features of the surface layer include strong wind shear, a superadiabatic lapse rate during the day, and a logarithmic wind-speed profile.
• The mixed layer: Above the surface layer (hundreds of meters deep), characterized by well-mixed conditions with nearly constant potential temperature and wind speed independent of height.

Turbulent Kinetic Energy (TKE)

• Measures turbulence intensity, defined as half the sum of squared velocity fluctuations.

Flux Richardson Number ($R_f$)

• Indicates the balance between buoyancy production and shear production of turbulence:
• $R_f$ = B/MP, where B is the buoyancy flux and MP is the shear flux.

Eddy Stress

• This is the force exerted by turbulent eddies on the mean flow.
• It is responsible for vertical momentum transport and is given by: $τ = -ρ\overline{u'w'}$, where ρ is the air density.

Theories and Models of the PBL

• Similarity Theory: Assumes a similar PBL structure at different heights.
• K-Theory: Assumes turbulent fluxes proportional to mean variable gradients, applied in neutrally or stably stratified boundary layers.
• Ekman Theory: Describes the wind's balance between pressure gradient force, Coriolis force, and turbulent drag, leading to the Ekman spiral.

Ekman Layer and Spiral

• The Ekman Layer: A theoretical layer where wind is affected by Coriolis force and friction, producing the Ekman spiral.
• Modified Ekman Layer: Combines the logarithmic surface layer profile with the Ekman spiral, offering a more accurate wind profile representation.

Importance of PBL Sub-Layers

• Surface Layer: Facilitates exchange of momentum, heat, and moisture with the surface.
• Upper Layer: Influences large-scale weather patterns.

Communication within Boundary Layers

• Entrainment: Transfer of air from the mixed layer to the free atmosphere at the top of the mixed layer.
• Secondary Circulations: Vertical motions driven by horizontal temperature gradients, transporting heat and moisture globally.
• Turbulent Mixing: Main mechanism for vertical transport of heat, moisture, and momentum.

Key Takeaways

• The PBL's height, structure, and turbulence are essential in atmospheric dynamics.
• Virtual potential temperature and turbulence help define stability and transport within the PBL.
• The Ekman layer and modified Ekman spiral offer theoretical insights into wind profiles.
• Air moves between boundary and upper layers via entrainment, secondary circulations, and turbulent mixing.

Explore the essential concepts of the Planetary Boundary Layer (PBL) including the Entrainment Zone (EZ) and virtual potential temperature profiles. Understand how temperature inversions and surface heating influence air movement and mixing within the atmosphere. This quiz encompasses key atmospheric dynamics crucial for atmospheric sciences.