Atmospheric Water

1
Atmospheric Water

• Global energy balance
• Atmospheric circulation
• Atmospheric water vapor
• Reading Sections 3.1 and 3.2 for today, 3.3 and
  3.4 for next Tues
• Also, please read article by Morel from Gewex
  News, Vol. 17 No. 4, Nov 2007

2
Atmospheric Water

• Global energy balance
• Atmospheric circulation
• Atmospheric water vapor

3
Radiation

• Two basic laws
• Stefan-Boltzman Law
• R emitted radiation (W/m2)
• e emissivity (0-1)
• s 5.67x10-8W/m2-K4
• T absolute temperature (K)
• Wiens Law
• l wavelength of emitted radiation (m)

All bodies emit radiation
Hot bodies (sun) emit short wave radiation Cool
bodies (earth) emit long wave radiation
4
Net Radiation, Rn
Ri Incoming Radiation

• Ro aRi Reflected radiation
• albedo (0 1)

Re
Rn Net Radiation
Average value of Rn over the earth and over the
year is 105 W/m2
5
Net Radiation, Rn
LE Evaporation
H Sensible Heat
G Ground Heat Flux
Rn Net Radiation
Average value of Rn over the earth and over the
year is 105 W/m2
6
Energy Balance of Earth
70
20
100
6
6
26
4
38
15
19
21
Sensible heat flux 7 Latent heat flux 23
51
http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/energy/radiation_balance.html
7
Diurnal Variation
Diurnal variation of fluxes, July 2003 San
Marcos Basin
Downward shortwave
Upward Longwave
Downward longwave
Upward shortwave
Ground
Fluxes in W/m2
Latent
Sensible
8
Energy Balance in the San Marcos Basin from the
NARR (July 2003)
Note the very large amount of longwave radiation
exchanged between land and atmosphere
Average fluxes over the day
495
61
72
112
3
310
415
Net Shortwave 310 72 238 Net Longwave
415 495 - 80
9
Increasing carbon dioxide in the atmosphere (from
about 300 ppm in preindustrial times)
We are burning fossil carbon (oil, coal) at
100,000 times the rate it was laid down in
geologic time
10
Absorption of energy by CO2
11
Atmospheric Water

• Global energy balance
• Atmospheric circulation
• Atmospheric water vapor

12
Heating of earth surface

• Heating of earth surface is uneven
• Solar radiation strikes perpendicularly near the
  equator (270 W/m2)
• Solar radiation strikes at an oblique angle near
  the poles (90 W/m2)
• Emitted radiation is more uniform than incoming
  radiation

Amount of energy transferred from equator to the
poles is approximately 4 x 109 MW
13
Hadley circulation
Atmosphere (and oceans) serve to transmit heat
energy from the equator to the poles
Warm air rises, cool air descends creating two
huge convective cells.
14
Conservation of Angular Momentum (Coriolis Force)
No external forces on air, so mV1r1 mV2r2
mV1r1
r1 lt r2 so V1 gt V2
mV2r2
Intertropical Convergence Zone
V1
r1
Earth rotation
r2
V2
Earth rotation
Looking down from North Pole, earth is rotating
counterclockwise
Near equator, air starts to fall behind the
earth
15
Atmospheric circulation
Circulation cells
Polar Cell

• Hadley cell
• Ferrel Cell
• Polar cell

Ferrel Cell
Winds

• Tropical Easterlies/Trades
• Westerlies
• Polar easterlies

Latitudes

• Intertropical convergence zone (ITCZ)/Doldrums
• Horse latitudes
• Subpolar low
• Polar high

16
Effect of land mass distribution
Uneven distribution of land and ocean, coupled
with different thermal properties creates spatial
variation in atmospheric circulation
A) Idealized winds generated by pressure gradient
and Coriolis Force. B) Actual wind patterns
owing to land mass distribution
17
Shifting in Intertropical Convergence Zone (ITCZ)
Owing to the tilt of the Earth's axis in orbit,
the ITCZ shifts north and south. 
Southward shift in January
Creates wet Summers (Monsoons) and dry winters,
especially in India and SE Asia
Northward shift in July
18
ITCZ movement
http//iri.ldeo.columbia.edu/7Ebgordon/ITCZ.html
19
Atmospheric Water

• Global energy balance
• Atmospheric circulation
• Atmospheric water vapor

20
Structure of atmosphere
21
Atmospheric water

• Atmospheric water exists
• Mostly as gas or water vapor
• Liquid in rainfall and water droplets in clouds
• Solid in snowfall and in hail storms
• Accounts for less than 1/100,000 part of total
  water, but plays a major role in the hydrologic
  cycle

22
Water vapor
Suppose we have an elementary volume of
atmosphere dV and we want quantify how much
water vapor it contains
Water vapor density
dV
ma mass of moist air mv mass of water vapor
Air density
Atmospheric gases Nitrogen 78.1 Oxygen
20.9 Other gases 1
http//www.bambooweb.com/articles/e/a/Earth's_atmo
sphere.html
23
Specific Humidity, qv

• Specific humidity measures the mass of water
  vapor per unit mass of moist air
• It is dimensionless

24
Vapor pressure, e

• Vapor pressure, e, is the pressure that water
  vapor exerts on a surface
• Air pressure, p, is the total pressure that air
  makes on a surface
• Ideal gas law relates pressure to absolute
  temperature T, Rv is the gas constant for water
  vapor
• 0.622 is ratio of mol. wt. of water vapor to avg
  mol. wt. of dry air

25
Daltons Law of Partial Pressures
John Dalton studied the effect of gases in a
mixture. He observed that the Total Pressure of a
gas mixture was the sum of the Partial Pressure
of each gas. P total P1 P2 P3 .......Pn
The Partial Pressure is defined as the pressure
of a single gas in the mixture as if that gas
alone occupied the container. In other words,
Dalton maintained that since there was an
enormous amount of space between the gas
molecules within the mixture that the gas
molecules did not have any influence on the
motion of other gas molecules, therefore the
pressure of a gas sample would be the same
whether it was the only gas in the container or
if it were among other gases.
http//members.aol.com/profchm/dalton.html
26
Avogadros law
Equal volumes of gases at the same temperature
and pressure contain the same number of molecules
regardless of their chemical nature and physical
properties. This number (Avogadro's number) is
6.023 X 1023 in 22.41 L for all gases.
Dry air ( z xy molecules)
Moist air (x dry and y water vapor)
Dry air
Water vapor
rd (xy) Md/Volume
rm (x Md yMv)/Volume
rm lt rd, which means moist air is lighter than
dry air!
27
Saturation vapor pressure, es
Saturation vapor pressure occurs when air is
holding all the water vapor that it can at a
given air temperature
Vapor pressure is measured in Pascals (Pa), where
1 Pa 1 N/m2
1 kPa 1000 Pa
28
Relative humidity, Rh
es
e
Relative humidity measures the percent of the
saturation water content of the air that it
currently holds (0 100)
29
Dewpoint Temperature, Td
e
Td
T
Dewpoint temperature is the air temperature at
which the air would be saturated with its current
vapor content
30
Water vapor in an air column

• We have three equations describing column
• Hydrostatic air pressure, dp/dz -rag
• Lapse rate of temperature, dT/dz - a
• Ideal gas law, p raRaT
• Combine them and integrate over column to get
  pressure variation elevation

2
Column
Element, dz
1
31
Precipitable Water

• In an element dz, the mass of water vapor is dmp
• Integrate over the whole atmospheric column to
  get precipitable water,mp
• mp/A gives precipitable water per unit area in
  kg/m2

2
Column
Element, dz
Area A
1
32
Precipitable Water, Jan 2003
33
Precipitable Water, July 2003
34
January
July