Water cycle in the environment

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Water cycle in the environment

• Role of water in the atmosphere
• Clouds and precipitation (weather)
• Impact on radiative balance
• Impact on thermodynamic processes and vertical
  stability of the atmosphere through evaporation
  and condensation.
• Cleaning of the atmosphere
• Participation in chemical processes.

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Water cycle in the environment
type volume (km3) share ()
surface water 230 580 0.0171
underground water 8 406 720 0.625
ice 29 190 000 2.15
water vapor in the atmosphere 12 900 0.001
oceans 1 321 890 000 97.2
sum 1 359 700 000 100.0
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Obieg wody w przyrodzie
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Moisture Variables

• There are numerous ways to quantify the amount of
  water vapor in the air.
• 1. Vapor pressure e - partial pressure of water
  vapor. Also a saturation vapor pressure. This is
  the fundamental way to measure the amount of
  vapor.
• 2. Vapor density ?v - defined by equation of
  state for vapor. Also called absolute humidity

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The absolute humidity changes as air rises and
descends
with the same amount of water vapor in the parcel
of air, an increase in volume decreases the
absolute humidity, while an decrease in volume
increases the absolute humidity
parcel size H2O vapor content absolute humidity
2 m3 10 g 5 g/m3
1 m3 10 g 10 g/m3
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• 3. mixing ratio r - mass of water vapor per unit
  of dry air. Usually expressed in g/kg, but most
  correct when unitless (i.e. kg/kg). Typical range
  for globe 0-25 g/kg.
• 4. specific humidity q mass of water vapor per
  unit mass of moist air,

where Rd287.05 Jkg-1K-1 gas constant for
dry air Rv461.51 Jkg-1K-1 gas constant for
water vapor
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The absolute humidity and mixing ration do not
change as air rises and descends
parcel weight H2O vapor weight specific humidity
1 kg 1 g 1 g/kg
1 kg 1 g 1 g/kg
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• The amount of water vapor the atmosphere can
  hold is limited. This limit is given by the
  saturation vapor pressure, which is an
  exponential function of temperature only.
• When SVP is equaled or exceeded, vapor moves to
  the liquid phase and latent heat is released. The
  possibility of supersaturation exists, but is
  only observed in atmosphere to a small degree

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Reaching Saturation

• There are several different processes that a
  parcel of air may undergo in order to reach
  saturation
• These processes define temperatures which can be
  used to indicate the amount of moisture in the air

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What happens at saturation?

• Though not entirely obvious, water begins to
  condense.
• As vapor condenses, latent heat is released
• Lapse rate of parcel changes dramatically.

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phase changes of water

• evaporation / condensation (Lp2462103 J
  kg-1)
• icing / melting (Lt 334103
  J kg-1)
• sublimation / resublimation (Ls2834103 J
  kg-1)

specific heat of water 4103 J kg-1
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Water vapor pressure
The total pressure inside the parcel of air is
equal to the sum of pressures of the individual
gases. The partial pressure of water vapor is
called the actual vapor pressure e. Actual vapor
pressure is measured in hPa, kPa or mb. The
greatest values of actual vapor pressure are
observed near equator, where they can reach 40
hPa. On average at 2 m 20 hPa. Low values of
actual vapor pressure are observed are observed
near the poles in winter. Usually it doeas not
exceed 2 hPa. Extremely low values were noted on
Syberia in Vierchojansk, where during winter the
actual vapor pressure can drop even to 0.03 hPa
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Relative Humidity

• 5 Relative Humidity RH --Ratio of mixing ratio to
  its saturation value. Expressed in percent
• This is what many instruments measure

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Relative Humidity

• most common measure, but not as useful as many
  others
• Function of two variables temperature and the
  amount of water vapor

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Virtual Temperature

• 6 Virtual Temperature TV temperature of dry air
  having the same density as that of a sample of
  moist air at the same pressure

Tv (1 0.61q) T
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Dewpoint

• 7 Dewpoint Td - temperature to which moist air
  must be cooled, holding p and w constant, in
  order to reach saturation wrt water.
• ws at dewpoint w of moist air.

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Water vapor in the atmosphere
Earth surface is a main source of water vapor in
the atmosphere. That is why the greatest amount
of water vapor is observed in the lower
troposphere. Water vapor content near the
surface amounts on average 0.2 near the poles
and 2.5 near the equator. In some cases in can
reach even 4
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Changes of water vapor content with the height
Above boundary layer the water vapor content
decreases with the height ezponentialy (faster
than other components of air). At altitude 13-20
km in temperate region the content od water vapor
is about 10-6 (mass of water vapor per unit of
mass of dry air). Above 20 km water vapor content
increases slightly with the heightnand at 23-50
km the pearl clouds are observed build from ice
crystals. Above 70 km water molecules break up
because of solar radiation at wavelenghts 0.1657
?m and water vapor content decreases to 10-8
.. Trace amount of water vapor are observed up to
90 km.
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Saturation water vapor pressure
For each temperature there is an amount of water
vapor saturated the air E. If water vapor
pressure becomes greater than saturation water
vapor pressure then the condensation occurs. The
condensation nucleus are necessary. So sometimes
it can happen that e gt E.

• Saturation water vapor pressure is
• greater for concave surfaces than for the flat
  ones
• lower for roztworów soli than for pure water

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Saturation water vapor pressure
Clausius-Clapeyron equation describes the impact
of temperature on saturation water vapor pressure
where E06.11 hPa, T0273 oK. Ratio L/Rv is
different for supercooled water (5423oK) and
ice (6139oK) (because of differences in heat of
evaporation and heat of sublimation). that is why
saturation water vapor presssure is greater for
supercooled water than for ice.
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Mixing ratio and specific humidity at saturation
and
Because usually r,q lt 0.04 than r ? q
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Absolute humidity of saturated air
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Other characteristic of humidity
Saturation deficit, d, it is a difference between
the saturation vapor pressure maksymalnym at
given temperature E and actual pressure e d E
e Dew point temperature, ? (or Td), temperature
to which it is necessary to cool the air to
saturate it relative to flat surface. Dew point
deficit, ?, the difference between given
temperature T and the dew point temperature Td
? T Td
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Precipitable water
Precipitable water is the total content of water
in the atmosphere in the of the air. It is equal
to the layer of water if all the water from the
atmosphere condense near the surface. It is
measured in kgm-2 or w mm.
Annual course of precipitable water over Central
Poland
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Precipitable water
styczen
Mean values of precipitable water in the period
1958-2003 in kg/m2 (mm)
lipiec
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Daily humidity course
Daily course of relative humidity is oposite to
the course of temperature with one maximum and
one minimum.
In daily course of vapor pressure and specific
and absolute humidity two maximas and two minimas
can be distinguished.

• morning minimum is caused by temperature

• afternoon minimum is caused by convection. This
  minimum is not observed at the seaside and in the
  mountains.

Dobowy przebieg róznych charakterystyk
wilgotnosci na stacji Lódz-Lublinek wartosci
usrednione z 74 letnich dni z pogoda radiacyjna
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Annual course of humidity
Annual course of water vapor pressure (as well as
absolute humidity, specific humidity and mixing
ratio) is parallel to the annual course of
temperature in summer the content of water vapor
is the greatest and in winter the smallest. Is is
caused by the relation of E (saturation vapor
pressure) on temperature. Annual course of the
relative humidity is oposite to the course of
temperature. But in monsun regions the relative
humidity is much greater during summer than in
winter. It is related to different features of
air mases approaching these regions in winter and
summer.
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Evaporation
Evaporation takes place were the body change
phase from liquid to gasous. It happens in each
temperature. The evaporation from plants is
called transpiration.
Potential evaporation (or evaporative capacity)
is the maximum possible evaporation at given
temperature, not restricted by the amount of
water. Actual evaporation is an amount of water
which really evaporate. The rate of evaporation
Fw is measured in kgm-2s-1 or mmday-1 and
is - proportional to saturation deficit (E-e), -
opposite proportional air pressure p, - relate to
shape of surface of evaporating body (coefficient
A), - related to wind speed (function f(v))
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CLOUDS
A cloud is a visible aggregate of tiny water
droplets or ice crystals suspended in the air.

• criterions of classification
• composition
• way of developing
• appearance

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Composition
Liquid clouds are built from water droplets only.
They develop in above 0C temperatures as well as
at temperatures slightly lower than 0C .
Ice clouds are composed from ice crystals only.
They exist in temperatures below -40C .
Mixed clouds are composed from both water
droplets and ice crystals. They exist in
temperatures below -0C but above -40C .
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Appearance
According to International Cloud Classification
ten principal cloud froms are divided into four
groups. Each group is identified by the height
of the cloud's base above the surface high
clouds, middle clouds and low clouds . The fourth
group contains clouds showing more vertical than
horizontal development. This system was
introduced by Abercromby and Hildebrandsson who
expanded the original Howard classification.
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Cloud types
Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus
(Cu) Altostratus (As) Altocumulus (Ac)
Nimbostratus (Ns) Stratocumulus (Sc) Stratus
(St) Cumulus (Cu) Cumulonimbus (Cb)
high clouds
middle clouds
low clouds
clouds with vertical development
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Approximate height of cloud base
polar temperate tropical
high clouds 3 - 8 5 - 13 6 - 18
middle clouds 2 - 4 2 - 7 2 - 8
low clouds lt 2 lt 2 lt 2
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Cloud types
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CIRRUS
thin wispy cloud blown by high winds into long
streamers called mares' tailes. They are build
from ice crystals only. They do not give any
precipitation.
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CIRROCUMULUS
Appear as small, founded, white puffs that may
occur individually or in long rows. When in rows
they have a rippling appearance that
distinguishes them from the silky look of cirrus
and sheetlike look of cirrostratus.
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CIRROSTRATUS
The thin, sheetlike high clouds that often cover
the entire sky. They are so thin that the moon or
sum can be seen through them. They are composed
from ice crystals, so refract the light passing
through them producing a halo.
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ALTOSTRATUS
Middle level cloud. It is a grey or blue-grey
(never white) cloud that covers oten the entire
sky. In thinner parts the sun (or moon) can be
dimly visible (translucidus). If the cloud is
thick (opacus) than the sun light could not be
seen through the cloud.
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ALTOCUMULUS
Middle clouds that appear as gray, puffy masses
sometimes rolled out in parallel waves or bands.
The sky can be seen between individual particles
of the cloud.
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NIMBOSTRATUS
Dark gray "wet " looking cloud layer associated
with more or less continuous precipitation. The
intensity of precipitation is usually low to
moderate. The base of the cloud is usually
difficult to define
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STRATUS
A uniform grayish cloud that often covers the
entire sky. Its base is very low over the groud
sometimes resembling the fog. It gives no
precipitation or drizzle
nebulosus (St neb foggy, murky) foggy,
uniform curtain, without any details fractus
(St fra frayed) cloud with unregular shape and
frayed appearance
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STRATOCUMULUS
A low lumpy cloud layer. It appears in rows , in
patches or as rounded masses with blue sky
visible betwen individual cloud elements. The
color of stratocumulus ranges from light to dark
gray.
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• the processes of saturation
• evaporation,
• cooling
• mixing

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Clouds appear when air becames supersaturated Usua
lly it happens when air mass ascends and cool.s
Ascending air mass expands and its temperature
drops. When temperature is decreasing, its
relative humidity is increasing up to the moment
the air becames saturated (relative humidity
100) When rising of air parcel continues part
of water vapor condences or resublimates. Water
vapor condences on small particles called
aerosol. If the aerosol is hydroscopic than
condensation or resublimation c an happen at
saturation lower than 100
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Convective clouds
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Orographic clouds
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Orographic clouds
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Frontal clouds
Ci
Cs
As
Ns
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Convective clouds made by convergence
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Fogs
radiative
advective
from evaporation
orographic
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Many Deserts Are Located Where a High Percentage
of the Precipitation Moisture Originates Over
Land These Areas are Not Supplied With Much
Moisture
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Climate feedbacks

• Water vapor feedback
• Ice/snow albedo feedback
• Cloud feedback

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Water vapor feedback
Surface temperature
Atmospheric H2O
()
Greenhouse effect
? Positive feedback loop
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Snow/ice albedo feedback
Surface temperature
Snow and ice cover
()
Planetary albedo
? Another positive feedback loop
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What about clouds?
Some reflection
10 km
Cirrus clouds
(Thin)
More reflection
Altitude
Cumulus/stratus clouds
(Thicker)
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What about clouds?
?Tc4
Cirrus clouds High and cold
10 km
?Tc4
Altitude
Cumulus/stratus clouds
?Tw4
Low and warm
?Tw4
?Ts4
Ts
Tw
Tc
Temperature
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What about clouds?

• Cumulus and stratus clouds
• Low and warm
• Small greenhouse effect
• Big effect on albedo
• These clouds cool the climate
• Cirrus clouds
• High and cold
• Large greenhouse effect
• Smaller effect on albedo
• ? These clouds warm the climate

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Cloud feedback

• Most models predict that cloudiness should
  increase as the climate warms
• If low clouds increase the most, then the
  feedback will be negative
• If high clouds increase the most, then the
  feedback will be positive
• The balance of evidence suggests that cloud
  feedback is negative. However, this is highly
  uncertain, as clouds are sub-grid-scale in size
  and are therefore difficult to model.

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gpcp/maps/frontmap.gif
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