Let us “together” understand the “Atmospheric …...Composition of the Earth’s Atmosphere...
Transcript of 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
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Life is too Simple; Isn’t it ???
Vertical Layers of the Earth’s Atmosphere
[Figure Courtesy: http://web.atmos.ucla.edu/]
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I wish... “Life was simpler !!!”
Vertical Layers of the Earth’s Atmosphere
[Figure Courtesy: http://www.vtaide.com/png/atmosphere.htm]
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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]
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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
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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
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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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 8 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 8 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 8 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 8 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 8 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 9 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 10 / 25
Exchange of Heat between a System & Environment
Zeroth Law of Thermodynamics
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 11 / 25
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.
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Temperature Scales
Inter-Conversion between different Temperature Scales
Fahrenheit, Celsius and Kelvin
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First Law of Thermodynamics
Energy is conserved, its form can be converted
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“Water Vapour” in Atmospheric Thermodynamics
The Hydrologic Cycle (A Simplified Schematic)
[Figure Courtesy: Meteorology Today by C. Donald Ahrens]
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Three Phases of Water
Water Vapour (Gaseous), Water (Liquid) and Ice (Solid)
[Figure Courtesy: Meteorology Today by C. Donald Ahrens]
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 16 / 25
Measurement Units of Water Vapour
Different Moisture Variables
[Figure Courtesy: Meteorology Today by C. Donald Ahrens]
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 17 / 25
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%
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 18 / 25
Measurement Units of Water Vapour ...
Saturation Vapour Pressure Vs. Temperature
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 19 / 25
Measurement Units of Water Vapour ...
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.)
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 20 / 25
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.
D. Bala Subrahamanyam (SPL, VSSC) SPL JRF Orientation Course October 16, 2014 21 / 25
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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