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Subdecks (1)
4.2
ATMOS
41 cards
Cards (104)
Boundary layer
Also called "
Atmospheric Boundary layer
" or "Planetary Boundary Layer", it is the tropospheric layer that is directly influenced by the presence of Earth's surface and responds to surface forcings in an
hour
or less
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Boundary layer
Typically
1
km deep during the day and ~
100
m deep during the night
Above the boundary layer is the
free
atmosphere
View source
Types of atmospheric boundary layer
Convective
boundary layer (
CBL
)
Stable
boundary layer (
SBL
)
Residual
layer (
RL
)
View source
Convective
boundary layer (
CBL
)
A BL that is dominated by buoyant
turbulence
generation and usually forms in the
daytime
View source
Stable boundary layer (SBL)
Forms at nighttime or when warm air moves over a colder surface
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Residual layer
(RL)
May occur during the morning or
evening
when the previous CBL is
disconnected
from the surface by the SBL
View source
Diurnal cycle of atmospheric boundary layer
1. Mixed layer stirred by
solar heating
and
convection
2.
Convective
stirring takes 10-20 minutes to go from
bottom
to top
3. Entrainment zone where
clouds
form
View source
Diurnal evolution of atmospheric boundary layer
1. During
sunset
, no mixing within convective boundary layer, stays above
lower nighttime stable
boundary layer (residual layer)
2. Some mixing still occur during
nighttime
at the
nocturnal
boundary layer
3. Increase of nocturnal layer due to
strong shear
caused by
accelerated wind
at residual layer
4. In the morning,
convection
and
mixing
start again
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Virtual potential temperature (θV)
Temperature of a parcel at a specific pressure level and virtual temperature would have if it were
lowered
or raised to
1000
mb
Important concept in atmospheric boundary meteorology, serves as a
stability criterion
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When θV is constant, the atmosphere is
statically neutral
When it decreases with elevation, the atmosphere is
statically unstable
When it increases with elevation, the atmosphere is
statically stable
View source
Diurnal evolution of atmospheric boundary layer as told by virtual potential temperature profiles
1.
S1
-
late morning
2.
S2
-
just after sunset
3.
S3
-
just before sunrise
4.
S4
-
just after sunrise
5.
S5
-
mid-morning
6.
S6
-
late afternoon
View source
During
morning rush hour
,
vehicle emissions
mix into a shallow boundary layer leading to high pollutant levels
During
evening rush hour
, emissions mix into a larger boundary layer volume so pollutant levels are
less
severe
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Diurnal variation of planetary boundary layer
height
influences our daily lives and health as we live, work, and breathe mostly in this layer
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Virtual potential temperature
Found by replacing temperature in the
formula
for virtual temperature with the
potential
temperature
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Cumulative heating or cooling (QA)
The area under the curve of
heat flux
vs time, more important than instantaneous
heat flux
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ABL structure and evolution on temperature
1. Cumulative
heating
during
daytime
2. Cumulative
cooling
during
nighttime
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Geostrophic wind
(
G)
Theoretical equilibrium wind speed for frictionless conditions
View source
Steady-state winds in the ABL are usually
slower
than
geostrophic
(subgeostrophic) due to frictional and turbulent drag against the surface
View source
Wind profile evolution in the ABL
1.
Shallow
mixed layer in the morning
2.
Deeper
mixed layer with
subgeostrophic
winds in the afternoon
3.
Nocturnal
jet of supergeostrophic winds at
night
4.
Logarithmic
wind profile in the
surface
layer
5.
Linear
wind profile in the
stable
surface layer
6.
Exponential power-law
wind profile in the
convective radix
layer
View source
Drag
Frictional
force between
two
objects
View source
Stress
Force
per
unit
area
View source
Friction velocity
Velocity scale related to
surface drag
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Roughness length
Measure of
surface roughness
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Surface layer
In the bottom 5% of the statically neutral ABL,
wind
speeds increase roughly logarithmically with
height
View source
Statically stable surface layer
Logarithmic profile changes to a more
linear
form
View source
Winds close to the ground
Become
slower
than
logarithmic
, or near calm
View source
Winds just above the surface layer
Often not in steady state, and can temporarily increase to faster than
geostrophic
(supergeostrophic) in a process called an
inertial oscillation
View source
Radix layer (RxL)
The bottom
20%
of the
convective
(unstable) ABL
View source
Winds in the RxL
Have an exponential power-law relationship with
height
View source
RxL
Faster
winds
near
the surface, but slower winds aloft than the neutral logarithmic profile
View source
Stress (τ)
The amount of
friction
force per unit surface contact area, acts
parallel
to the surface
View source
Pressure
Force per unit area that is
perpendicular
to the surface
View source
Stress
is felt by both objects that are
sliding
against each other
View source
Mean wind
Relatively
constant
, but varying
slowly
over the course of hours
View source
Waves
Regular
(linear) oscillations of
wind
, often with periods of ten or longer
View source
Turbulence
Irregular
, quasi-random, non-linear variations or gusts, with duration of
seconds
to minutes
View source
U(t)
direction
component of
wind
at some instant in time, t
View source
ഥ�
�
Mean wind, where the
overbar
denotes an
average
View source
u'
Turbulence
or gust part, obtained by
subtracting
the mean wind from the instantaneous wind
View source
Averaging
time
Typically about
0.5
h
View source
See all 104 cards
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