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Transcript
Midterm Review
What have we discussed?
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Importance of the atmospheric boundary layer
Surface energy balance
Surface water balance
Vertical structure of the ABL
Modeling the ABL
Startocumulus-topped boundary layer
Boundary layer of shallow and deep convection
Land surface processes
Air pollution
Climate feedbacks for global warming
Ocean-atmosphere interaction
Abrupt climate change
Importance of the ABL
• The mission of meteorology is to understand and predict weatherrelated disasters (e.g. tornados, hurricanes, winter storms) and
climate-related disasters (e.g. El Nino and global warming).
• The modern climatology (meteorology) was born in the 1940s (a very
young science!), but has been growing very fast! Now we have a
global observational network with many satellites, ships, radars and
surface stations, as well as very comprehensive prediction models
running on the world’s fastest supercomputers.
• The current status of weather and climate predictions: (1) weather
prediction good to 10 days, (2) tropical cyclone prediction good in
track but not in intensity, (3) climate prediction good to two seasons,
(4) climate change projections have a 3-fold difference in magnitude.
• The main reasons of the difficulties: (1) Teleconnection problem, (2)
Feedback problem, and (3) Subgrid-scale problem.
• Importance of the ABL: (1) interface between atmosphere and
ocean/land/ice - flux transfer and feedback, (2) the human beings are
living in the ABL and change the environment, (3) a basic subgridscale process
Surface energy balance
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What is energy? 3 methods of energy transfer
The names of the 6 wavelength categories in the electromagnetic
radiation spectrum. The wavelength range of Sun (shortwave) and
Earth (longwave) radition
Earth’s energy balance at the top of the atmosphere.
Incoming shortwave = Reflected Shortwave + Emitted longwave
Earth’s energy balance at the surface.
Incoming shortwave + Incoming longwave = Reflected shortwave
+ Emitted longwave + Latent heat flux + Sensible heat flux
+ Subsurface conduction
What is sensible heat flux? What is latent heat flux?
Bowen ratio B= SH/LH = Cp(Tsurface - Tair) / L(qsurface - qair) provides
a simple way for estimating SH and LH when the net radiative flux
Fr is available LH=Fr/(B+1), SH=Fr B/(B+1)
Other heat sources: precipitation, biochemical, anthropogenic
The Electromagnetic Spectrum
The limitations of
the human eye!
Summary: Surface energy balance
Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave
+ Latent heat flux + Sensible heat flux + Subsurface conduction
SWdn
=Scos
SWup
=SWdn 
LWdn
LWup
=Tair4 =Ts4
LH=CdLV(qsurface- qair)
SH=CdCpV(Tsurface- Tair)

dT/dt
Fc = -  dT/dz
Surface water balance
• Global water cycle
• Surface water balance
• Soil moisture: Increases with depth
• Palmer drought severity index (PDSI): uses
temperature and rainfall
• Desertification
The global water cycle
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 ABL
• 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
Vertical Structure of the Atmosphere
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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
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
Static Stability
• 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
Diurnal variation of boundary layer
over land
• Daytime convective mixed layer + clouds (sometimes)
• Nocturnal stable boundary layer + residual layer (leftover of
daytime convective mixed layer)
Modeling the ABL
• Reynolds averaging: Separation of mean and turbulent
components u = U + u’, < u’ > = 0
• Intensity of turbulence: turbulent kinetic energy (TKE)
TKE = ‹ u’ 2 + v’ 2 + w’ 2 › /2
• Eddy fluxes
Fx = - <u’w’>/z
• The turbulent closure problem: Number of unknowns >
Number of equations
• Surface layer: related to gradient
• Mixed layer:
Local theories (K-theory): < w’a’ >= - Ka dA/dz
always
down-gradient
Non-local theories: organized eddies filling the entire BL,
could be counter-gradient
Reynolds averaging
(1) Separate mean and turbulent components
Assume you are given a time series of
zonal wind speed u for a period of one
hour, the zonal wind speed can be
decomposed into two components:
u = U + u’
where U = < u > is the time average (< >
means time average, over one hour here)
and is called the time mean component,
while u’ is the fluctuation around U, i.e.
u’ = u - U
and is called the turbulent component.
(2) Do time average
<U>=U
< u’ > = 0
< A u’ > = A < u’ > = 0
Only cross terms <a’b’> are left. They are
also called non-linear terms.
Intensity of turbulence:
Turbulent kinetic energy (TKE)
Mean kinetic energy
MKE = (U2 + V2 + W2)/2
Turbulent kinetic energy TKE = ‹ u’ 2 + v’ 2 + w’ 2 › /2
‹›
Time evolution (diurnal)
represents time average
Vertical profile
The turbulence closure problem
• For large-scale atmospheric circulation, we have six
fundamental equations (conservation of mass, momentum, heat
and water vapor) and six unknowns (p, u, v, w, T, q). So we can
solve the equations to get the unknowns.
• When considering turbulent motions, we have five more
unknowns (eddy fluxes of u, v, w, T, q)
• We have fewer fundamental equations than unknowns when
dealing with turbulent motions. The search for additional laws to
match the number of equations with the number of unknowns is
commonly labeled the turbulence closure problem.
Mixed layer theory I: Local theories
• K-theory: In eddy-diffusivity (often called K-theory)
models, the turbulent flux of an adiabatically conserved
quantity a (such as θ in the absence of saturation, but
not temperature T, which decreases when an air parcel
is adiabatically lifted) is related to its gradient:
< w’a’ > = - Ka dA/dz
• The local effect is always down-gradient (i.e. from high
value to low value)
• The key question is how to specify Ka in terms of known
quantities.
Three commonly used approaches:
(1) First-order closure
(2) 1.5-order closure or TKE closure
(3) K-profile
Mixed layer theory II: Non-local theories
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Any eddy diffusivity approach will not be entirely accurate if most of the
turbulent fluxes are carried by organized eddies filling the entire boundary
layer.
The non-local effect could be counter-gradient.
Consequently, a variety of ‘nonlocal’ schemes which explicitly model the
effects of these boundary layer filling eddies in some way have been
proposed.
A difficulty with this approach is that the structure of the turbulence
depends on the BL stability, baroclinicity, history, moist processes, etc.,
and no nonlocal parameterization proposed to date has comprehensively
addressed the effects of all these processes on the large-eddy structure.
Nonlocal schemes are most attractive when the vertical structure and
turbulent transports in a specific type of boundary layer (i. e. neutral or
convective) must be known to high accuracy.
Stratocumulus-topped Boundary Layer
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Definition of stratocumulus clouds
Global distribution
Importance for global warming
Vertical structure and formation mechanism of
STBL
• Modeling of STBL: non-local forced by surface
heating and cloud-top cooling
Global distribution of stratocumulus clouds
Distributed over eastern part of subtropical oceans
Vertical structure and formation mechanism of
stratocumulus-topped boundary layer (STBL)
• Intense longwave
radiative cooling at
cloud top drives eddies
in BL
• Eddies pick up
moisture and maintain
cloud
• Eddies also entrain
warm, dry air from
above the inversion
• Entrainment lifts the
cloud, large-scale
subsidence lowers it
Boundary layer of shallow and deep convection
• Global distribution of shallow and deep convection
• Vertical structure of trade wind cumulus (shallow
convection)
• Vertical structure of deep convection. Four components:
convective updraft, convective downdraft, mesoscale
updraft, mesoscale downdraft
• Differences between shallow convection and deep
convection: change of T, q and h in the boundary layer
• Self-suppression processes in deep convection: Overly
stabilized state after deep convection
• Problems in current global climate models: lack of selfsuppression processes
Hadley circulation and cloud types
Tropics
L
Deep convection
H
Trade wind cumulus
Stratocumulus
Trade wind cumulus
• Cloud top height generally
below 4 km
• Often associated with light rain
• Sometimes topped by
stratocumulus
Vertical structure and self-suppression processes in
deep convection
Convective updrafts
Mesoscale updrafts
Mesoscale
downdrafts
Convective downdrafts
Onion sounding
Differences in boundary layer between
shallow convection and deep convection
Shallow: T decreases
Deep: T decreases
Shallow: q increases
Deep: q decreases
Shallow: h keeps const
Deep: h decreases
Effects of human activities
Human beings are living in the BL and affect the BL in
three different ways:
• Change land cover (deforestation and afforestation)
• Release or cleanse pollutants (aerosols)
• Release or cleanse greenhouse gases
Land surface processes
• Effects of different surface types: desert, city, grassland,
forest, sea. Deeper heat/water reservoir, decreased
Bowen ratio, thinner BL and enhanced convective
instability.
• Effects of vegetation: (1) makes heat/water reservoir
deeper, (2) enhance evaporation, (3) grows and dies in
response to environmental conditions
• Heat island effect. 7 causes
• Community land model (CLM). 4 components:
biogeophysics, hydrological cycle, biogeochemistry,
dynamical vegetation
Effects of different surface types
BL depth decreases
Convective instability increases
Deeper heat reservoir (smaller T change)
Deeper water reservoir (Wetter surface)
Bowen ratio decreases (More LH contribution)
Effects of vegetation
• Makes water/heat reservoir deeper (transport deep water
out of soil)
• Enhances evaporation (leafs increase evaporation area)
• Dependent on vegetation type
The heat island effect
• Nighttime: City warmer than surrounding rural area
• Daytime: City has same air temperature as rural area
Causes of the heat island effect
• Increased SW absorption caused
by canyon geometry (increased
area and multiple reflection)
• Decreased LW loss caused by
canyon geometry
• Increased greenhouse effect
caused by air pollution
• Anthropogenic heat source
• Increased sensible heat storage
caused by construction materials
• Decreased latent heat flux caused
by change of surface type
• Decrease sensible and latent heat
fluxes caused by canyon geometry
(reduction of wind speed)
“Canyons” between buildings
Air pollution
• Air pollution. 2 categories
• 6 types of major pollutants: particulates, carbon oxides,
sulfur dioxides, nitrogen oxides, volatile organic
compounds, ozone
• Air quality index
• Atmospheric Conditions and Air Pollution (1) effect of
wind, (2) effect of stability and turbulence
• History of air pollution: The Medieval pollution, The 16th19th centuries, The 20th century, The 21st century
Inversions
• Inversions are absolutely stable and free of eddies
• Inversions can trap pollutants near the Earth’s surface.
Low level inversion
Upper level inversion
(most dangerous)
Climate feedbacks for global warming
• Large spread in projected temperature change comes from
uncertainties in climate feedbacks
• Main climate feedbacks for global warming: albedo, lapse
rate, water vapor, cloud, aerosol, carbon cycle
• Feedback strength in climate models: cloud feedback causes
the largest uncertainty
Ocean-Atmosphere Interaction
• Mean state: The two basic regions of SST? Which region has
stronger rainfall? What is the Walker circulation? Two types of
ocean upwelling
• Mean state: ocean-atmosphere feedback
• ENSO: Which region has warm SST anomaly during El Nino? 4year period.
• Existing ENSO theories
• AMO and thermohaline circulation
Abrupt Climate Change
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Past abrupt climate change
Tipping points
Future abrupt climate change
Mitigation: International (Kyoto Protocol), Green economy
(Renewable energy, Sustainable transportation, Green
buildings, Energy-efficient industry and carbon capture,
Land management, afforestation, waste management)
Important
• Please remember to bring your calculator
to the exam!