Atmospheric Composition and Structure
A&OS C110/C227: Review of
thermodynamics and dynamics I
Robert Fovell
UCLA Atmospheric and Oceanic Sciences
rfovell@ucla.edu
1
Notes
•
Everything in this presentation should be familiar
•
Please feel free to ask questions, and remember to refer to
slide numbers if/when possible
•
If you have Facebook, please look for the group
“UCLA_Synoptic”. You need my permission to join. (There are
two “Robert Fovell” pages on FB. One is NOT me, even
though my picture is being used.)
2
Elementary stuff
3
The atmosphere
•
Primordial atmosphere
•
Volcanic activity, rock outgassing
•
H
2
O vapor, CO
2
, N
2
, S… no oxygen
•
Origin of oxygen: dissociation of water vapor by absorption of
UV (minor), and photosynthesis (major)
•
Present composition of
dry air
•
78% N
2
•
21% O
2
•
1% Ar
•
“Minor” constituents of dry air include
•
CO
2
0.039%, CH
4
0.00018%, O
3
< 0.00005%
4
Time series of CO
2
5
Atmosphere: Dry and moist
•
Dry air constituents are well-mixed and vary only slowly over
time and space
•
Roughly constant over lowest 80 km (50 mi)
•
Very convenient for thermodynamic calculations
•
Water vapor (“wv”) 0-4% of total atmospheric mass, but also
concentrated near surface
for these reasons
•
Surface source
•
Efficient return mechanism (precipitation)
•
Absolute humidity is a very strong function of temperature (T)
•
Revealed by Clausius-Clapeyron equation
6
Standard atmosphere
•
Averaged over time
and horizontal space
•
Four layers:
•
Troposphere
•
Stratosphere
•
Mesosphere
•
Thermosphere
•
“Lapse rate” = how T
decreases
with
height
Temperature vs. height for standard atmosphere
7
Standard atmosphere
•
Troposphere
•
“turning sphere”
•
Averages 12 km (7.5
mi) deep
•
Top =
tropopause
•
T range 15˚C @ sfc to -
60˚C at tropopause
•
Average tropospheric
lapse rate: 6.5˚C/km
(19˚F/mi)
Temperature vs. height for standard atmosphere
8
Standard atmosphere
•
Stratosphere
•
“layered”… very stable
•
Extends upward to 50
km
•
Top =
stratopause
•
T increases with
height (lapse rate
negative)
•
UV interception by O
2
and O
3
•
“lid” for troposphere…
in a sense
Temperature vs. height for standard atmosphere
9
Standard atmosphere
•
Mesosphere
•
“middle sphere”
•
T decreases with
height again
•
Top =
mesopause
•
Thermosphere
•
Very hot… and yet no
“heat” (very little
mass)
•
Freeze and fry
simultaneously
Temperature vs. height for standard atmosphere
10
Standard atmosphere
•
Tropospheric T
variation
15˚C at surface
-60˚C at 12 km
elevation
•
If “warm air rises and
cold air sinks”, why
doesn’t the
troposphere turn
over?
Temperature vs. height for standard atmosphere
11
Pressure
•
Pressure = force per unit area
p = N/m
2
= Pascal (Pa)
•
Air pressure largely due to weight of overlying air
•
Largest at the surface, zero at atmosphere top
•
Decreases monotonically with height (z)
•
Pressure linearly proportional to mass
12
Pressure
13
g ~ 9.81 m/s
2
at sea-level
Sea-level pressure (SLP)
mb =
millibar
hPa =
hectopascal
1 mb = 100 Pa
14
For surface p = 1000 mb:
50% of mass below 500 mb
80% of mass below 200 mb
99.9% of mass below 1 mb
Various p and z levels
15
Infer how pressure varies with height
Pressure vs. height
16
P
0
= reference (surface) pressure
H
= scale height
Density =
= mass/volume
17
Infer how density varies with height
p
and
vs. height
18
and
and ln
Warm air rises and cold air
sinks…
•
NOT always true.
•
True statement is:
less dense air rises,
more dense air sinks
•
Note near-surface air,
although warm
, is
also more dense
Temperature vs. height for standard atmosphere
19
Warm air rises and cold air
sinks…
Temperature vs. height for standard atmosphere
20
Basic thermodynamics concepts
21
System and environment
•
System
= what we wish to study
•
View as
control mass
or
control volume
•
Control mass (CM)
•
Define some mass, hold fixed, follow it around
•
Control volume (CV)
•
Define and monitor a physical space
•
Environment
= everything else that may interact with the
system
22
System states
•
Systems may be
open
or
closed
to mass
•
Open systems permit mass exchange across system boundaries
•
Our CVs are usually open
•
Strictly speaking, a CM is closed
•
Closed systems may be
isolated
or
nonisolated
•
Isolated systems do not permit energy transfer with environment
•
Closed, isolated system = environment doesn’t matter
23
Lagrangian vs. Eulerian
•
CM is the Lagrangian viewpoint
•
Powerful, desirable but often impractical
•
Total derivatives
•
Freeway example
•
CV is the Eulerian viewpoint
•
Observe flow through volume
•
Partial derivatives
24
Air parcel
•
Our most frequently used system
•
CM (usually!) – Lagrangian concept
•
Monitor how T, p, and V change as we follow it around
25
Conventions
•
We often use CAPITAL letters for
extensive
quantities, and
lower case for
specific
quantities
•
Specific = per unit mass
•
Example:
•
U is internal energy, in Joules
•
u is specific internal energy, in J/kg
•
Unfortunately, “u” is also zonal wind velocity
•
Aside:
•
Temperature T is essentially specific, but capitalized (and isn’t per
unit mass anyway)
•
Pressure p is fundamentally extensive, but lower case (and isn’t
per unit mass anyway)
26
Energy and the 1
st
law
•
Total energy = KE + PE + IE
•
Conserved in absence of sources and sinks
•
Our main use of 1
st
law: monitor changes in internal energy (IE
or u) owing to sources and sinks
•
How do we change system u? With energy transfer via
•
heat Q or q
•
work W or w
•
Caveat: w is also vertical velocity, and q may also refer to
water vapor specific humidity
27
Work
•
Work = force applied over a distance
•
Force: N, distance: m
•
Work: Nm = J = energy
•
Our principal interest: CM volume compression or expansion
(dV) in presence of external pressure (p)
•
W > 0 if dV > 0
28
Work
29
W > 0 when system expands against
environment
Heat
•
Diabatic
heat
•
Diabatic
: Greek for “passable, to be passed through”
•
Internal energy exchanged between system and environment
•
q > 0 when energy flow is INTO system
•
Adiabatic
= system is
isolated
•
Adiabatic
: Greek for “impassable, not to be passed through”
30
Caution on nomenclature
•
We should use
diabatic
when the energy exchange is between
system and environment
•
But, what if the heat source or sink is
inside
the system?
•
That’s adiabatic, but q ≠ 0
•
Our interior heat source will be water changing phase
•
Dry adiabatic
: q = 0
•
No heat source, outside OR inside
•
“dry” really means no water phase changes
•
Moist adiabatic
: q ≠ 0, but heat source/sink is
inside
system
•
“moist” implies water phase change
•
Synonyms include “saturated adiabatic” and “wet adiabatic”
•
Can also be referred to as “diabatic”!
31
1
st
law and Carnot cycle
32
1
st
law
•
In the absence of ∆KE and ∆PE
•
Other ways of writing this
33
Most of my examples will be per unit mass.
State properties
•
Internal energy u is a
state property
•
Changes in state properties are
not path-dependent
•
Other state properties include m, T, p,
, V, etc.
34
State properties
35
Path-dependence
•
Work and heat are
path-dependent
36
Path-dependence
•
A
cyclic
process
starts and ends with
the same state
property values
•
… but the cyclic
process can have
net
heat exchange
and
do
net work
37
Path-dependence
38
Black path
Path-dependence
39
Red path
Carnot cycle
•
4-step piston cycle on a CM
•
2 steps of volume expansion, 2 of volume compression
•
2 steps are isothermal, 2 are (dry) adiabatic
•
Warm and cold thermal reservoirs external to system
•
Start and end with temperature T
1
and volume V
1
40
Carnot – Step 1
41
Isothermal volume expansion
Add heat Q
A
from warm
reservoir
T
2
= T
1
V
2
> V
1
Carnot – Step 2
42
Adiabatic volume expansion
No heat exchange
T
3
< T
2
V
3
> V
2
Carnot – Step 3
43
Isothermal volume compression
Lose heat Q
B
to cold thermal
reservoir
T
4
= T
3
V
4
< V
3
Carnot – Step 4
44
Adiabatic volume compression
No heat exchange
T
1
> T
4
V
1
< V
4
Returned to original state T
1
, V
1
.
Cycle is complete.
45
Apply 1
st
law
46
Carnot on T-V diagram
47
Carnot on T-V diagram
48
Carnot on T-V diagram
49
Carnot on T-V diagram
50
Carnot on T-V diagram
51
Carnot on T-V diagram
52
Carnot on T-V diagram
53
No net ∆V
But did net W
Conceptual summary
54
Heat flow diverted
to do work
Question for thought #1
55
The isothermal expansion (Q
A
) occurred at a
higher temperature than the
Isothermal compression (Q
B
).
What does this imply for the work?
What does this imply for the pressure?
Q
B
is waste heat.
What does this imply for the
efficiency of this heat engine?
Is there a limit to efficiency?
Is the limit found in the 1
st
law?
Question for thought #2
56
Can you design a cyclic process that does
no net work?
What would it look like on a T-V diagram?
Useful forms of the 1
st
law
57
• for ideal gases only (where
h
= enthalpy)
• these can be used to create these useful forms (
= 1/
= specific volume)
• we can also write this in terms of potential temperature
• for dry air, c
p
= 1004 J/(kg K), and c
v
= 717 J/(kg K)
[end]
58
The presentation covers fundamental concepts related to the Earth's atmosphere, including its composition, origin of oxygen, dry and moist layers, standard atmosphere layers, and temperature variations. Key topics discussed include the primordial atmosphere, atmospheric constituents, water vapor distribution, and tropospheric and stratospheric characteristics. The content provides insights into thermodynamic calculations and atmospheric dynamics, offering a comprehensive overview for better understanding atmospheric science.
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Presentation Transcript
A&OS C110/C227: Review of thermodynamics and dynamics I Robert Fovell UCLA Atmospheric and Oceanic Sciences rfovell@ucla.edu 1
Notes Everything in this presentation should be familiar Please feel free to ask questions, and remember to refer to slide numbers if/when possible If you have Facebook, please look for the group UCLA_Synoptic . You need my permission to join. (There are two Robert Fovell pages on FB. One is NOT me, even though my picture is being used.) 2
The atmosphere Primordial atmosphere Volcanic activity, rock outgassing H2O vapor, CO2, N2, S no oxygen Origin of oxygen: dissociation of water vapor by absorption of UV (minor), and photosynthesis (major) Present composition of dry air 78% N2 21% O2 1% Ar Minor constituents of dry air include CO2 0.039%, CH4 0.00018%, O3 < 0.00005% 4
Atmosphere: Dry and moist Dry air constituents are well-mixed and vary only slowly over time and space Roughly constant over lowest 80 km (50 mi) Very convenient for thermodynamic calculations Water vapor ( wv ) 0-4% of total atmospheric mass, but also concentrated near surface for these reasons Surface source Efficient return mechanism (precipitation) Absolute humidity is a very strong function of temperature (T) Revealed by Clausius-Clapeyron equation 6
Standard atmosphere Averaged over time and horizontal space Four layers: Troposphere Stratosphere Mesosphere Thermosphere Lapse rate = how T decreases with height 7 Temperature vs. height for standard atmosphere
Standard atmosphere Troposphere turning sphere Averages 12 km (7.5 mi) deep Top = tropopause T range 15 C @ sfc to - 60 C at tropopause Average tropospheric lapse rate: 6.5 C/km (19 F/mi) 8 Temperature vs. height for standard atmosphere
Standard atmosphere Stratosphere layered very stable Extends upward to 50 km Top = stratopause T increases with height (lapse rate negative) UV interception by O2 and O3 lid for troposphere in a sense 9 Temperature vs. height for standard atmosphere
Standard atmosphere Mesosphere middle sphere T decreases with height again Top = mesopause Thermosphere Very hot and yet no heat (very little mass) Freeze and fry simultaneously 10 Temperature vs. height for standard atmosphere
Standard atmosphere Tropospheric T variation 15 C at surface -60 C at 12 km elevation If warm air rises and cold air sinks , why doesn t the troposphere turn over? 11 Temperature vs. height for standard atmosphere
Pressure Pressure = force per unit area p = N/m2 = Pascal (Pa) Air pressure largely due to weight of overlying air Largest at the surface, zero at atmosphere top Decreases monotonically with height (z) Pressure linearly proportional to mass 12
Pressure g ~ 9.81 m/s2 at sea-level 13
Sea-level pressure (SLP) For surface p = 1000 mb: 50% of mass below 500 mb 80% of mass below 200 mb 99.9% of mass below 1 mb mb = millibar hPa = hectopascal 1 mb = 100 Pa 14
Various p and z levels 15 Infer how pressure varies with height
Pressure vs. height 16 P0 = reference (surface) pressure H = scale height
Density = = mass/volume 17 Infer how density varies with height
p and vs. height and and ln 18
Warm air rises and cold air sinks NOT always true. True statement is: less dense air rises, more dense air sinks Note near-surface air, although warm, is also more dense 19 Temperature vs. height for standard atmosphere
Warm air rises and cold air sinks 20 Temperature vs. height for standard atmosphere
System and environment System = what we wish to study View as control mass or control volume Control mass (CM) Define some mass, hold fixed, follow it around Control volume (CV) Define and monitor a physical space Environment = everything else that may interact with the system 22
System states Systems may be open or closed to mass Open systems permit mass exchange across system boundaries Our CVs are usually open Strictly speaking, a CM is closed Closed systems may be isolated or nonisolated Isolated systems do not permit energy transfer with environment Closed, isolated system = environment doesn t matter 23
Lagrangian vs. Eulerian CM is the Lagrangian viewpoint Powerful, desirable but often impractical Total derivatives Freeway example CV is the Eulerian viewpoint Observe flow through volume Partial derivatives 24
Air parcel Our most frequently used system CM (usually!) Lagrangian concept Monitor how T, p, and V change as we follow it around 25
Conventions We often use CAPITAL letters for extensive quantities, and lower case for specific quantities Specific = per unit mass Example: U is internal energy, in Joules u is specific internal energy, in J/kg Unfortunately, u is also zonal wind velocity Aside: Temperature T is essentially specific, but capitalized (and isn t per unit mass anyway) Pressure p is fundamentally extensive, but lower case (and isn t per unit mass anyway) 26
Energy and the 1st law Total energy = KE + PE + IE Conserved in absence of sources and sinks Our main use of 1st law: monitor changes in internal energy (IE or u) owing to sources and sinks How do we change system u? With energy transfer via heat Q or q work W or w Caveat: w is also vertical velocity, and q may also refer to water vapor specific humidity 27
Work Work = force applied over a distance Force: N, distance: m Work: Nm = J = energy Our principal interest: CM volume compression or expansion (dV) in presence of external pressure (p) W > 0 if dV > 0 28
Work W > 0 when system expands against environment 29
Heat Diabatic heat Diabatic: Greek for passable, to be passed through Internal energy exchanged between system and environment q > 0 when energy flow is INTO system Adiabatic= system is isolated Adiabatic: Greek for impassable, not to be passed through 30
Caution on nomenclature We should use diabatic when the energy exchange is between system and environment But, what if the heat source or sink is inside the system? That s adiabatic, but q 0 Our interior heat source will be water changing phase Dry adiabatic: q = 0 No heat source, outside OR inside dry really means no water phase changes Moist adiabatic: q 0, but heat source/sink is inside system moist implies water phase change Synonyms include saturated adiabatic and wet adiabatic Can also be referred to as diabatic ! 31
1st law In the absence of KE and PE Other ways of writing this 33 Most of my examples will be per unit mass.
State properties Internal energy u is a state property Changes in state properties are not path-dependent Other state properties include m, T, p, , V, etc. 34
Path-dependence Work and heat are path-dependent 36
Path-dependence A cyclic process starts and ends with the same state property values but the cyclic process can have net heat exchange and do net work 37
Path-dependence Black path 38
Path-dependence Red path 39
Carnot cycle 4-step piston cycle on a CM 2 steps of volume expansion, 2 of volume compression 2 steps are isothermal, 2 are (dry) adiabatic Warm and cold thermal reservoirs external to system Start and end with temperature T1 and volume V1 40
Carnot Step 1 Isothermal volume expansion Add heat QA from warm reservoir T2 = T1 V2 > V1 41
Carnot Step 2 Adiabatic volume expansion No heat exchange T3 < T2 V3 > V2 42
Carnot Step 3 Isothermal volume compression Lose heat QB to cold thermal reservoir T4 = T3 V4 < V3 43
Carnot Step 4 Adiabatic volume compression No heat exchange T1 > T4 V1 < V4 44 Returned to original state T1, V1. Cycle is complete.
