Observed Structure of the Atmospheric Boundary Layer

(2)

Review of last lecture• Vertical structure of the atmosphere and

definition of the boundary layer• Vertical structure of the boundary layer • Definition of turbulence and forcings

generating turbulence• Static stability and vertical profile of virtual

potential temperature: 3 cases. Richardson number

• Boundary layer over ocean

Boundary Layer depth:Lidar

Boundary Layer depth:Lidar

Turbulence inside the boundary layer

Boundary Layer depth:Effects of diurnal forcing over land

• Daytime convective mixed layer + clouds (sometimes)• Nocturnal stable boundary layer + residual layer

Convective mixed layer (CML):Growth

The turbulence (largely the convectively driven thermals) mixes (entrains) down potentially warmer, usually drier, less turbulent air down into the mixed layer

Convective mixed layer (CML):Vertical profiles of state variables

Well-mixed (constant profile)

Super-adiabatic

Nearly adiabatic

Strongly stable lapse rate

Nocturnal boundary layer over land: Vertical structure

• The residual layer is the left-over of CML, and has all the properties of the recently decayed CML. It has neutral stability.

• The stable boundary layer has stable stability, weaker turbulence, and low-level (nocturnal) jet.

Strongly stable lapse rate

Nearly adiabatic

Weakly stable lapse rate

Comparison between day and night

Kaimal and Finnigan 1994

• Subtle difference between convective mixed layer and residual layer: Turbulence is more vigorous in the former

Super-adiabatic

Nearly adiabatic

Strongly stable lapse rate

Strongly stable lapse rate

Nearly adiabatic

Weakly stable lapse rate

Summary• Daytime: convective mixed layer + clouds

(sometimes)• Nocturnal: stable boundary layer + residual

layer