Atmospheric Properties II Martin Visbeck DEES, Lamont-Doherty Earth Observatory [email protected]

Atmospheric Properties II

Martin VisbeckDEES, 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 (, the mass of a unit volume, in Kg/m3).

Ideal Gas Law

p = R T or p / (R T)

Ideal Gas Law


Ideal Gas Law


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


I do know the units of all the other properties....

R = p / (T)

p: [Pa or N/m2]

T: [K]

: [kg/m3]

Ideal Gas Law

p: [Pa]

T: [K]

: [kg/m3]

=> equation

R = p / (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 K-1 m3 kg-1

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 K-1 m3 kg-1



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 K-1 m3 kg-1



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

E = - 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.

p = - g z where g is the acceleration of gravity = 9.8 m/s2.




How do the units work out here?




p ~ Pressure [ Pa]

Density [kg m-3]

z ~ Length [m]

=> [Pa] = [kg m-3] [m/s2] [m] = [ kg / (m s2)]




p ~ Pressure [ Pa]

Density [kg m-3]

z ~ Length [m]

=> [Pa] = [kg m-3] [m/s2] [m] = [ kg / (m s2)]

=> [N m-2] = [ kg / (m s2)] *m-2

=> [N] = [ kg m /s2]


The drop of pressure with height

Exponential Function !


Pressure

Density

Adiabatic cooling of rising air

d = - T /Z = 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.



Ok this was dry enough.....

WaterLets talk about.....

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).


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 importantly, reflects short wave radiation into space.

Water cycles through the atmosphere-ocean-land system

Water cycles through the atmosphere-ocean-land system

Reservoirs:

Atmosphere: 0.001%Land: 2.43%Ocean: 97.57 %

Water cycles through the atmosphere-

ocean-land system

Fluxes in 1012 m3/year

Total reservoir is1383 1015 m3

Cycle time is~1000 years

99

62

37324

361

A

L

O


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

Dew point

Air parcel 50% relative humidity

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?


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



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 !



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


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).


Finally back to convection.....


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)



Adiabatic cooling of rising moist air

Dry air:

d = - T /Z = 9.8 °K/km

Moist air:

m = - T /Z = 6.5 °K/km

Why? Latent heat release.





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?

Water

Light

Documents

Atmospheric Properties II Martin Visbeck DEES, Lamont-Doherty Earth Observatory [email protected]