Transcript Document

e(T) = es(TDew)
USE
THESE
VALUES.
PRACTICE WITH STABILITY
Atmospheric Structure
From Wallace and Hobbs, 2nd ed, Atmospheric Science, chapter 9.
Atmospheric Boundary Layer Diurnal Variation
E.Z. refers to entrainment zone where energetic
parcels from the surface overshoot and entrain
and mix in air from the free atmosphere, as well
as to mix boundary layer air into the free
atmosphere above.
From Wallace and Hobbs, 2nd ed, Atmospheric Science, chapter 9.
Atmospheric Boundary Layer Diurnal Variation
Potential temperature in the U.S. standard atmosphere, and its change near the
Earth’s surface due to turbulent mixing driven by sunlight.
From Wallace and Hobbs, 2nd ed, Atmospheric Science, chapter 9.
http://weather.uwyo.edu/upperair/sounding.html
Example Soundings for Homework
5:00 am local time
3 September 2010
morning inversion
5:00 pm local time
3 September 2010
top of boundary layer in
afternoon. Mixing height.
super adiabatic
solar heated
surface
5
5
Data Format (see spreadsheet online)
First set of columns.
September
Day 2 0 1 0
GammaI /Ga
H top of
P top of
T top of
mmaD =
H s urfac e aminvers ion am P s urfac e am invers ion am T s urfac e am invers ion am GammaI = - GammaI /9 .7
(m)
(m)
(mb)
(mb)
(C elc ius )
(C elc ius )
dT /dz (K/km) 7 k/km
Second set of columns
P top of
H top of
boundary
H s urfac e pmboundary
P s urfac e pm layer pm
(m)
layer pm (m) (mb)
(mb)
T top of
boundary
T s urfac e pm layer pm
(C elc ius )
(C elc ius )
h=H top - H
s urfac e (m)
Equation for Boundary Layer Height
as a function of time during the day,
and by the day of year.
T0 = surface temperature, P0 = surface pressure, 0 = adiabatic lapse rate
I = environmental lapse rate, typically of inversion and < 0,
 = latitude,  = angle of tilt of Earth’s axis = 23.5 degrees, tR = time of sunrise
in hour of day,
t = time of day in hours, F is the integrated solar irradiance (W m-2) over all
wavelengths at the surface, A is the surface albedo, air is the thermal
diffusivity of the air, and ground= thermal diffusivity of the ground.
Observed Structure of the
Atmospheric Boundary Layer
Many thanks to: Nolan Atkins,
Chris Bretherton, Robin Hogan
Googled using presentation and Atmospheric Boundary layer.
Review of the last lecture
Incoming shortwave + Incoming longwave = Reflected shortwave
+ Emitted longwave + Latent heat flux + Sensible heat flux
• Incoming solar radiation = (Solar constant) cos(Solar zenith
angle)
• Reflected solar radiation = (Incoming solar radiation) x Albedo
• Longwave I=T4
• Sensible heat flux Qh =  Cd Cp V (Tsurface - Tair)
• Latent heat flux Qe =  Cd L V (qsurface - qair)
• Bowen ratio B= Qh/Qe = Cp(Tsurface - Tair) / L(qsurface - qair)
provides a simple way for estimating Qh and Qe when radiation
measurements are available
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 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)
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
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
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
• Boundary layer over ocean
• Boundary layer over land: diurnal variation
Please remember to bring your calculator on
Friday