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
