Transcript part-2

Atmospheric Properties II
Martin Visbeck
DEES, Lamont-Doherty Earth Observatory
[email protected]
Outline
Review of the first part of lecture.
Water in the climate system - thermodynamic
properties of moist air.
[Convection Experiment]
Atmospheric Processes
Why?
Properties of dry air
Dry air is air that contains no
water.
The state of a parcel of dry
air is described by three
properties: temperature
(T, expressed in °K,
where 273°K = 0°C),
pressure (p, force per
unit area, expressed in
Newtons/m2) and density
(r, the mass of a unit
volume, in Kg/m3).
Ideal Gas Law
p=rRT
or
r = p / (R T)
Ideal Gas Law
p=rRT
or
r = p / (R T)
Ideal Gas Law
p=rRT
or
r = p / (R T)
Oh..... R was 287 kg? or m N? ...
Oh... I forgot the units of
the gas constant R ....@#! ???
How can I "remember” them or
work them out?
Ideal Gas Law
p=rRT
or
r = p / (R T)
I do know the units of all the other
properties....
R = p / (r T)
p: [Pa or N/m2]
T: [K]
r: [kg/m3]
Ideal Gas Law
p: [Pa]
T: [K]
r: [kg/m3]
=> equation
R = p / (r T)
units [R] = Pa / ( kg/m3 K)
units [R] = Pa K-1 m3 kg-1
R = 287 Pa K-1 m3 kg-1
Ideal Gas Law
R = 287 Pa
-1
K
3
m
-1
kg
But last lecture had:
R of air is constant and equal to 287 Joules/(kg °K)
Hmm... so how are [Pa] and [J] related?
Ideal Gas Law
R = 287 Pa
-1
K
3
m
-1
kg
But last lecture had:
R of air is constant and equal to 287 Joules/(kg °K)
Hmm... so how are [Pa] and [J] related?
287 J /(kg °K) = 287 Pa K-1 m3 kg-1
J = (kg °K) Pa K-1 m3 kg-1
J = Pa m3
Ideal Gas Law
R = 287 Pa
-1
K
3
m
-1
kg
But last lecture had:
R of air is constant and equal to 287 Joules/(kg °K)
Hmm... so how are [Pa] and [J] related?
287 J /(kg °K) = 287 Pa K-1 m3 kg-1
J = (kg °K) Pa K-1 m3 kg-1
J = Pa m3 = Nm-2 m3 = Nm
Thermodynamic properties of
dry air - adiabatic temperature change
DE = - W
for an adiabatic system
a container with insulating flexible walls
Atmosphere under gravity
- hydrostatic balance.
Hydrostatic balance.
To find the expression for the hydrostatic balance, we
first note that atmospheric surface pressure is due to
the weight of the entire atmospheric column above.
As we ascend, there is less of an atmosphere above us,
and hence the pressure drops.
Dp = - r g Dz
where g is the acceleration of gravity = 9.8 m/s2.
Atmosphere under gravity
- hydrostatic balance.
Hydrostatic balance.
Dp = - r g Dz
where g is the acceleration of gravity = 9.8 m/s2.
How do the units work out here?
Atmosphere under gravity
- hydrostatic balance.
Hydrostatic balance.
Dp = - r g Dz
where g is the acceleration of gravity = 9.8 m/s2.
Dp ~ Pressure [ Pa]
r ~ Density [kg m-3]
Dz ~ Length [m]
=> [Pa] = [kg m-3] [m/s2] [m] = [ kg / (m s2)]
Atmosphere under gravity
- hydrostatic balance.
Hydrostatic balance.
Dp = - r g Dz
where g is the acceleration of gravity = 9.8 m/s2.
Dp ~ Pressure [ Pa]
r ~ Density [kg m-3]
Dz ~ Length [m]
=> [Pa] = [kg m-3] [m/s2] [m] = [ kg / (m s2)]
=> [N m-2] = [ kg / (m s2)]
=> [N] = [ kg m /s2]
*m-2
Atmosphere under gravity
- hydrostatic balance.
The drop of pressure
with height
Exponential Function !
Atmospheric Processes
Pressure
Density
Adiabatic cooling of rising air
Gd = - DT /DZ = 9.8
°K/km
Adiabatic cooling of rising air
The stability of dry air - dry convection.
If the environment (the
surrounding atmosphere) is
such that vertically displaced
parcels continue to rise on
their own, even when the
lifting exerted on them stops,
the environment is referred to
as unstable.
If vertically displaced parcels sink
back to their initial elevation
after the lifting ceases, the
environment is stable.
If vertically displaced parcels
remain where they are after
being lifted, the environment
is neutral.
The stability of dry air - dry convection.
The stability of dry air - dry convection.
Ok this was dry enough.....
Lets talk about.....
Water
Water in the atmosphere thermodynamic properties of moist air
Importance of water in the climate system.
Water exists in the atmosphere in
all three phases:
gas (vapour, mixed with other gasses),
liquid (cloud droplets), and
ice (ice crystals as clouds).
Water in the atmosphere thermodynamic properties of moist air
Atmospheric water plays an extremely important role
in the climate system due to three outstanding
properties:
1. Water vapour is an absorber of infrared
radiation:
2. Water vapour acts like a reservoir of heat:
3. In its condensed phase in the atmosphere, as
water droplets which form clouds, water
absorbs infrared radiation and, more
Water cycles through the
atmosphere-ocean-land system
Water cycles
through the
atmosphereocean-land
system
Reservoirs:
Atmosphere:
0.001%
Land: 2.43%
Ocean: 97.57 %
Water cycles
through the
atmosphereocean-land system
A
361
99
62
324
O
Fluxes in
1012 m3/year
37
L
Total reservoir is
1383 1015 m3
Cycle time is
~1000 years
Water in the atmosphere thermodynamic properties of moist air
Describing amounts of water vapor in the atmosphere.
There are a few ways to measure the concentration of water
vapor in the atmosphere.
1. Vapor pressure (denoted e): is the partial pressure of
water vapor molecules in the atmosphere.
2. Relative humidity: is the ratio of actual vapor pressure to
saturation vapor pressure
3. Mixing ratio: is the mass of water vapor in grams per
kilogram of air.
4. Dew point temperature: the temperature at which the vapor in
a cooled parcel of air begins to condense.
Water Vapor Pressure versus Temperature
Water Vapor Pressure versus Temperature
Dew Point Temperature
Air parcel 50% relative humidity
Dew point
Mixing Ratio versus Temperature
Note, that warm air can
hold a lot more water
vapor.
What would that mean
for a “green house”
world with warmer
temperatures?
Why does it rain more in
the tropics than at the
poles?
Water in the atmosphere thermodynamic properties of moist air
Phase changes of water.
Phase changes are the transition between different states of a
substance. They are accompanied by the absorption or the
release of heat. In the normal conditions that exist in the climate
system, some substances can be found in only one state (most
atmospheric gases, for example). Water can be found in all 3
states.
Gas
Liquid
Solid
Water in the atmosphere Energy for phase changes
Phase changes of water.
Water in the atmosphere Energy for phase changes
The liquid-vapor phase transition in water takes up (or
gives out) 2.25 to 2.5 x106 Joules/kg (540-600
calories/gm) This heat is known as the latent heat of
vaporization/condensation. At the sea-air boundary,
water coexists as vapor and liquid. Unless the air is
saturated, water evaporates continuously from the
liquid side of the interface. This process draws heat
from the evaporating liquid and cools it. Alternatively,
if vapor condenses (as in clouds), the surrounding air is
warmed.
Latent Heat flux: Ocean cooled by evaporation !
Water in the atmosphere Energy for phase changes
Phase changes of water.
In the cold polar oceans, liquid water and ice are in
equilibrium with each other. The heat required to melt
ice into water is much less than that required to turn
water into vapor. In melting water we need 0.33 x106
Joules/kg (80 calories/gm) (so called the latent heat of
melting). This heat is returned in the process of fusion
(when water freezes).
Water in the atmosphere Energy of phase changes
Phase changes of water.
Water vapor can also be in equilibrium with ice. In this
case, molecules of water can cross the boundary
between the ice surface into the air, just as they do over
a water surface. The transition between the solid phase
and the vapor phase is called sublimation. When ice
turns directly into vapor (sublimation) the heat required
per gram of ice is the sum of the latent heat of melting
and the latent heat of vaporization - a total of 2.5 to 2.8
x106 Joules/kg (620-680 calories/gm).
Atmospheric
Processes
Finally back to
convection.....
Water in the atmosphere thermodynamic properties of moist air
Stability of moist air -moist convection.
The largest differences in behavior between moist and dry
air thermodynamics is in the cooling process
encountered under lifting of air parcels. This is because
when air containing water vapor is lifted up it begins to
cool at the dry adiabatic lapse rate. But when it reaches
its dew point temperature, saturation occurs, and water
droplets begin to condense inside the rising parcel,
forming a cloud. (lifting condensation level)
Water in the atmosphere thermodynamic properties of moist air
Stability of moist air -moist convection.
Adiabatic cooling of rising moist air
Dry air:
Gd = - DT /DZ = 9.8 °K/km
Moist air:
Gm = - DT /DZ = 6.5 °K/km
Why? Latent heat release.
Water in the atmosphere thermodynamic properties of moist air
J
Stability of moist air -moist convection.
Water in the atmosphere thermodynamic properties of moist air
Water in the atmosphere thermodynamic properties of moist air
Clouds
as a
function
of
height
Water in the atmosphere near equatorial convection
Clouds
as a
function
of
height
Water in the atmosphere Convection drives circulation
Clouds
as a
function
of
height
Convection in the atmosphere why localized at the equator?
Light
Water