Observed Structure of the
Atmospheric Boundary Layer
Many thanks to: Nolan Atkins,
Chris Bretherton, Robin Hogan
Review of last lecture: Surface water balance
The changing rate of soil moisture S
dS/dt = P - E - Rs - Rg + I
Precipitation
(P)
Evaportranspiration
(E=Eb+Ei+Es+TR)
Irrigation
(I)
Runoff
(Rs)
dS/dt
(PDSI, desertification)
Infiltration
(Rg)
Vertical Structure of the Atmosphere
• Definition of the boundary layer: "that part of the troposphere that is
directly influenced by the presence of the earth's surface and
responds to surface forcings with a time scale of about an hour or
less.”
• Scale: variable, typically between 100 m - 3 km deep
Difference between boundary layer
and free atmosphere
The boundary layer is:
•
•
•
•
More turbulent
With stronger friction
With more rapid dispersion of pollutants
With non-geostrophic winds while the free
atmosphere is often with geostrophic winds
Vertical structure of the boundary layer
From bottom up:
• Interfacial layer (0-1 cm): molecular transport, no turbulence
• Surface layer (0-100 m): strong gradient, very vigorous turbulence
• Mixed layer (100 m - 1 km): well-mixed, vigorous turbulence
• Entrainment layer: inversion, intermittent turbulence
Turbulence inside the boundary layer
Definition of Turbulence:
The apparent chaotic
nature of many flows,
which is manifested in
the form of irregular,
almost random
fluctuations in velocity,
temperature and scalar
concentrations around
their mean values in
time and space.
Generation of turbulence in the boundary
layer: Hydrodynamic instability
“Hydrodynamically unstable” means that any small
perturbation would grow rapidly to large perturbation
• Shear instability: caused by change of mean wind in
space (i.e. mechanical forcing)
• Convective instability: caused by change of mean
temperature in the vertical direction (i.e. thermal
forcing)
Shear instability
Shear: Change of wind speed and/or direction in space
Example: Kelvin-Helmholtz instability
Shear instability within a fluid or between two fluids with different density
Lab experiment
Real world
(K-H clouds)
Video: Kelvin-Helmholtz
instability
• https://www.youtube.com/watch?v=HZII
9-OUJrE
Video: Kelvin-Helmholtz Clouds
Over the Alps
• https://www.youtube.com/watch?v=QV
NO6lyAcZc
Convective instability
• Static stability – refers to atmosphere’s susceptibility
to being displaced
• Stability related to buoyancy  function of temperature
• The rate of cooling of a parcel relative to its surrounds
determines its ‘stability’ of a parcel
• For dry air (with no clouds), an easy way to determine
its stability is to look at the vertical profile of virtual
potential temperature
v =  (1 + 0.61 r )
Where
 = T (P0/P)0.286 is the potential temperature
r is the water vapor mixing ratio
Three cases:
(1) Stable (sub-adiabatic): v increases w/ height
(2) Neutral (adiabatic): v keeps constant w/ height
(3) Unstable (super-adiabatic): v decreases w/ height
Stable or
sub-adiabatic
Neutral or
adiabatic
Unstable or
super-adiabatic
Boundary layer stability conditions Richardson number
• The Richardson number is a convenient means of
categorizing atmospheric stability in the boundary
layer:
Where g is acceleration due to gravity,  is mean
temperature, U is mean wind speed, z is height,
and Ri is a dimensionless number.
Ri >0 stable
=0 neutral
<0 unstable
Lewis Fry Richardson
(11 October 1881 - 30
September 1953) was an
English mathematician,
physicist, meteorologist,
psychologist and pacifist
who pioneered modern
mathematical techniques
of weather forecasting,
and the application of
similar techniques to
studying the causes of
wars and how to prevent
them.
Forcings generating temperature
gradience and wind shear, which affect
the boundary layer depth
• Heat flux at the surface and at the top of the
boundary layer
• Frictional drag at the surface and at the top
of the boundary layer
Boundary layer depth:
Effects of ocean and land
• Over the oceans: varies more slowly in
space and time because sea surface
temperature varies slowly in space and time
• Over the land: varies more rapidly in space
and time because surface conditions vary
more rapidly in space (topography, land
cover) and time (diurnal variation, seasonal
variation)
Boundary layer depth:
Effect of highs and lows
Near a region of high pressure:
• Over both land and oceans,
the boundary layer tends to be
shallower near the center of
high pressure regions. This is
due to the associated
subsidence and divergence.
• Boundary layer depth
increases on the periphery of
the high where the subsidence
is weaker.
Near a region of low pressure:
• The rising motion associated
with the low transports
boundary layer air up into the
free troposphere.
• Hence, it is often difficult to find
the top of the boundary layer in
this region. Cloud base is often
used at the top of the boundary
layer.
Boundary Layer depth:
Effects of diurnal forcing over land
• Daytime convective mixed layer + clouds (sometimes)
• Nocturnal stable boundary layer + residual layer
Video: Convective boundary
layer observed by lidar
• https://www.youtube.com/watch?v=_lvQ
-uyttuo
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
Strongly stable
lapse rate
Nearly
adiabatic
Super-adiabatic
Well-mixed (constant profile)
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.
Weakly stable lapse rate
Nearly adiabatic
Strongly stable lapse rate
Boundary layer over land:
Comparison between day and night
Strongly stable lapse rate
Nearly
adiabatic
Super-adiabatic
Kaimal and Finnigan 1994
Weakly stable lapse rate
Nearly
adiabatic
Strongly stable lapse rate
•
Subtle difference between convective mixed layer and residual layer:
Turbulence is more vigorous in the former
Summary
• 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 over land: diurnal variation