let us “together” understand the “atmospheric …...composition of the earth’s atmosphere...

Let us “together” understand

the “Atmospheric Thermodynamics”

An Interactive Session

D. BALA SUBRAHAMANYAM

NUMERICAL ATMOSPHERE MODELLING

Space Physics Laboratory, Vikram Sarabhai Space Centre

Thiruvananthapuram - 695 022

E-mail: [email protected];

Home Page: http://subrahamanyam.webs.com/

D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 1 / 25

Composition of the Earth’s Atmosphere

Different Gases of the Earth’s Atmosphere

So-called “Permanent” Gases:

N2, O2, Ar and traces of other inert gases

Water (H2O):

in all three of its phases (i.e., ice, liquid & vapour)

Variable gasesous constituent other than water:

CO2, O3, N2O, CH4

Aerosols:

solid & liquid particles suspended in air


Life is too Simple; Isn’t it ???

Vertical Layers of the Earth’s Atmosphere

[Figure Courtesy: http://web.atmos.ucla.edu/]


I wish... “Life was simpler !!!”

Vertical Layers of the Earth’s Atmosphere

[Figure Courtesy: http://www.vtaide.com/png/atmosphere.htm]


Let us enjoy the “Complexity of Life”

Incompressible Fluids and Compressible Fluids

Earth

Earth

Compressible atmosphere - No de!nite upper boundary

Incompressible ocean - Well-de!ned upper boundary

[Figure Courtesy: “A First Course in Atmospheric Thermodynamics”, by: Grant W. Petty]


Earth’s Atmosphere as a “Fluid”

Incompressible Fluids and Compressible Fluids

INCOMPRESSIBLE FLUIDS:

hypothetical type of fluids (introduced for the convenience of calculations)

it does not change the volume of the fluid due to external force

COMPRESSIBLE FLUIDS:

every fluid that we encounter in our lives

compressibility of a fluid is the reduction of volume in presence of external force


Thermodynamic Systems and Environment

THERMODYNAMIC SYSTEM:

... which we are specifically interested in

ENVIRONMENT (UNIVERSE):

... everything else other than the system

Different Types of Thermodynamic Systems


Air Parcels as Thermodynamic Systems

An “AIR PARCEL” is simply an imaginary sample of air, often taken to

be representative of a particular location in the free atmosphere.

Four Basic Assumptions of an “Air Parcel”

(1) The parcel is sealed from outside air. Thus, there is no mixing of parcel

and environmental air once the parcel is created.

(2) Parcel size itself is irrelevant.

(3) The parcel is insulated from its surrounding environment, so there is no

heat transfer by conduction, or radiation across a parcel boundary.

(4) The parcel sides are flexible, which means if outside pressure changes,

inside pressure adjusts to match it.


Air Parcels as Thermodynamic Systems

Some Interesting Facts about “Air Parcels”

Meteorological balloons are not a good

proxy for an air parcel, as the outside and

inside pressures are clearly different (i.e.,

violation of our fourth assumption:

uniformity in pressure.

For an air parce, its pressure is always the

same as environmental pressure. Hence, if

the air parcel is warmer than the

environment, the parcel will tend to rise.

Similarly, if we find a parcel to be cooler

than its surroundings, it will be more

dense, and therefore it will tend to sink.


Formulation of Thermodynamic Relations

Intensive and Extensive Variables

Intensive Variables:

These variables do not depend on the amount of matter in the system.

e.g., temperature or pressure

Extensive Variables:

These variables depend on the size of the systems.

e.g., internal energy

In principle, every extensive variable can be converted to its corresponding

intensive form by normalizing it by the amount of matter it describes.

e.g., specific volume, v = V/M;

where V = volume of the whole system, and M = amount of mass.


Exchange of Heat between a System & Environment

Zeroth Law of Thermodynamics


The Concept of “Temperature”

Temperature and Molecular Kinetic Energy

Temperature is ultimately a measure

of the kinetic energy associated with

the chaotic motions of the molecules.

A substance in which the molecules

are flying around madly has a higher

temperature than the same substance

when the molecules are more or less

at rest.

Absolute Temperature: For an ideal gas, we may define the absolute

temperature as proportional to the mean translational kinetic energy of the

constituent molecules. It implies that a temperature of absolute zero is reached

when all molecular motion has decreased to its minimum possible level.


Temperature Scales

Inter-Conversion between different Temperature Scales

Fahrenheit, Celsius and Kelvin


First Law of Thermodynamics

Energy is conserved, its form can be converted


“Water Vapour” in Atmospheric Thermodynamics

The Hydrologic Cycle (A Simplified Schematic)

[Figure Courtesy: Meteorology Today by C. Donald Ahrens]


Three Phases of Water

Water Vapour (Gaseous), Water (Liquid) and Ice (Solid)



Measurement Units of Water Vapour

Different Moisture Variables



Measurement Units of Water Vapour ...

Moisture Variables

Absolute Humidity:

= Mass of water vapour/Volume of air

Specific Humidity:

= Mass of water vapour/Total mass of air

Mixing Ratio:

= Mass of water vapour/Mass of dry air

Vapour Pressure (e):

= Amount of pressure exerted by the actual water vapour present in atmosphere

Saturation Vapour Pressure (esat):

= Amount of pressure exerted by water vapour if air is made saturated at a given T

Relative Humidity:

= water vapour content/water vapour capacity = e/esat x 100%



Saturation Vapour Pressure Vs. Temperature



Dew Point Temperature

The dew point temperature is the temperature at which the air can no longer hold all of

the water vapour which is mixed with it, and some of the water vapour must condense

into liquid water. The dew point is always lower than (or equal to) the air temperature.

an approximation: Td = T - ((100 - RH)/5.)


Geopotential (Φ)

Physical Relevance of the “Geopotential”

The geopotential Φ at any point in the Earth’s atmosphere is defined as the

work that must be done against the Earth’s gravitational field to raise a mass

of 1 kg from sea level to that point. In other words, Φ is the gravitational

potential per unit mass.The units of geopotential are J kg−1 or m2 s−2.


Enthalpy (a thermodynamic potential)

Enthalpy (H) is a defined thermodynamic potential that consists of the

internal energy of the system (U) plus the product of pressure (p) and

volume (V) of the system.

H = U + pV

The U term can be interpreted as the energy required to create the system, and the pV

term as the energy that would be required to make room for the system if the pressure of

the environment remained constant.


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