Transcript slides

Moist Processes
SOEE1400: Lecture 11
Water in the Atmosphere
• Almost all the water in the atmosphere is
contained within the troposphere.
• Most is in the form of water vapour, with some
as cloud water or ice.
• Typical vapour mixing ratios are:
~10 g kg-1 (low troposphere) (can be up to ~20 g kg-1)
~1 g kg-1 (mid troposphere)
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METEOSAT Water vapour image : 041019 – 1200 UTC
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METEOSAT visible image : 041019 – 1200 UTC
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Sources and Sinks
Sources:
Sinks:
– Evaporation from
surface: requires
energy to supply latent
heat of evaporation –
sunlight, conduction
from surface (cools
surface).
– Evaporation of
precipitation falling
from above: latent
heat supplied by
cooling of air
– Precipitation: rain,
snow, hail,…
– Condensation at the
surface: dew, frost
• N.B. Most of the water in
the atmosphere above a
specific location is not
from local evaporation,
but is advected from
somewhere else.
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Buoyancy Effects
Water in the atmosphere
has important effects on
dynamics, primarily
convective processes.
– Water vapour is less dense
than dry air
– Latent heat
released/absorbed during
condensation/evaporation.
• molecular weight of water
= 18 g mol-1
• mean molecular weight of
dry air ≈ 29 g mol-1
water vapour = 0.62 air
 A mixture of humid air is
less dense than dry (or
less humid) air at the
same temperature and
pressure
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Latent Heat
Latent heat of evaporation
of water
Lv ≈ 2.5 MJ kg-1
large compared with specific
heat of dry air
Evaporation of 1 gram of
liquid water (=1 cm3) into 1
cubic metre of air:
latent heat used ≈ 2500 J
cools air by ≈ 1.9 K.
Cp ≈ 1004 J kg-1 k-1
Similarly latent heat is
released and air warmed
when liquid water
condenses out – e.g. as
cloud droplets.
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Adiabatic lifting may occur on
many scales:
– Largescale ascent along a warm
or cold front (100s of kilometers)
– The rise of individual convective
plumes to form cumulus clouds
(~100m to ~1km)
– Forced ascent over topographic
features (hills, mountains) to form
orographic cloud (~1km to >10s
km).
– Gravity waves above, and
downwind of mountains (few km).
Radiative cooling
(non-adiabatic process)
• Direct radiative cooling of the air
takes place, but is a very slow
process.
• Once cloud has formed, radiative
cooling of the cloud droplets (and
cooling of surrounding air by
conduction of heat to drops) is
much more efficient.
Radiative cooling  reduced
saturation vapour pressure 
more condensation  higher cloud
water content.
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Addition of water vapour, at
constant temperature,
raising humidity to
saturation point.
– Will occur over any water
surface. Since temperature
decreases with altitude,
evaporation into unsaturated
surface layer can result in
saturation of the air in the upper
boundary layer.
– Cold air moving over warmer
water can sometimes produce
‘steam fog’ : common in the
arctic, and observed over rivers
and streams on cold mornings.
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T1
Tmix
T2
Mixing of two unsaturated air
masses as different temperatures
such that final humidity is above
saturation point
The Temperature and vapour
pressure resulting from mixing is
are averages of the initial values
in proportion to masses of each
being mixed
e.g.
Tmix = T1*M1 + T2*M2
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M1+M2
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Adiabatic Lifting
• As a parcel of air is lifted, the
pressure decreases & the parcel
expands and cools at the dry
adiabatic lapse rate.
• The level at which saturation
occurs is called the lifting
condensation level.
Saturation mixing ratio
equal to actual water
vapour mixing ratio of parcel
Lifting
condensation
level
Dew point
at surface
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• If the parcel continues to rise, it
will cool further; the saturation
mixing ratio decreases, and
more water condenses out.
• Condensation releases latent
heat; this offsets some of the
cooling due to lifting so that the
saturated air parcel cools at a
lower rate than dry air.
• The saturated (or wet)
adiabatic lapse rate is NOT
constant, but depends upon
both the temperature and
pressure.
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• The high the air temperature,
the greater the saturation mixing
ratio, and the more water vapour
can be held in a parcel of air.
• Because the gradient of the
saturation vapour pressure with
temperature increases with
temperature, a given decrease
in temperature below the dew
point will result in more water
condensing out at higher
temperatures than at low, and
hence more latent heat is
released.
• Thus the wet adiabatic lapse
rate decreases as the
temperature increases.
Q1
Q2
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T
T
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The Föhn Effect
4.58°C
Saturated air cooling
at -0.5°C per 100m
5.08°C
5.58°C
Lifting condensation level
Unsaturated air cooling
at -0.98°C per 100m
9.02°C
10°C
6.08°C
7.06°C
8.04°C
Unsaturated air warming
at +0.98°C per 100m
5.56°C
500 m
400 m
300 m
200 m
100 m
0m
6.54°C
7.52°C
8.50°C
9.48°C
10.46°C
11.44°C
The different lapse rates of unsaturated and saturated air mean that air flowing
down the lee side of a mountain range is frequently warmer than the air on the
upwind side. In the Alps this warm dry wind is called the Föhn, in American
Rockies it is known as a Chinook. The onset of such winds can result in very
rapid temperature rises (22°C in 5 minutes has been recorded) and is
associated with rapid melting of snow, and avalanche conditions.
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Precipitation
(recap from lecture 3)
• Cloud droplets require a condensation nucleus (CCN)
on which to form; growth then occurs by deposition of
water molecules from vapour. CCN are abundant.
– Growth is limited by local supersaturation of the vapour
– Growth rate decreases as droplet size increases
• Cloud droplets are typically 10 to 30 m in diameter.
Growth/evaporation can occur within a few 10s of
seconds.
• Rain drops are typically 0.5 to 5 mm in diameter, growth
from the vapour would take several hours – longer than
the lifetime of typical convective clouds.
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• In order to grow into rain drops, cloud droplets must
grow by coalescence
– Larger droplets fall faster than small ones, and can collide with
them
– The process by which a sufficient number of large enough
droplets is generated remains a topic of active research
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• Ice nuclei (IN) are not abundant – in many cases ice will
not form spontaneously unless the temperature is down
to about -40°C
– The air may commonly be supersaturated relative to ice
– Commonly there are just a few ice crystals, which then grow very
fast, “stealing” all the available water, and falling out of the cloud
as they grow large.
• Ice crystals provide a more efficient process of forming
rain
– Saturation vapour pressure over ice is less than that over water
 ice crystals grow at expense of water droplets
– If ice crystal touches a cold droplet, the droplet freezes
– Once large enough, ice crystals – or clumps of crystals – fall past
droplets and collect them. Rapid growth of soft hail pellet
(graupel) by riming.
– Graupel falls from cloud, melting before reaching the surface as
rain
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Cloud Radiative Effects
• Clouds play an important controlling role in the
global radiation budget.
– Reflection of incoming solar (short-wave) radiation
– Absorption of both solar and thermal infra red (longwave) radiation (incoming & outgoing)
– Emission of infra red radiation (up and down)
• The altitude, type, and thickness of cloud, along
with that of clouds above & below determines
whether the local net effect is to warm or cool
the air & surface below.
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Vertical Transport
• Deep convective clouds play a major role
in mixing boundary layer air – along with
moisture, aerosol particles, and gases
(both natural and man-made pollutants) –
up into the free-troposphere.
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Chemistry
• Clouds provide an environment within which
aqueous phase chemical reactions can take
place within the atmosphere
• Aerosol particles can be substantially modified
within clouds
– Aqueous phase reactions with gases dissolved in
droplets
– Coalescence of droplets brings multiple aerosol
particles together
• Chemically different aerosol may react
• On evaporation of droplet, a single aerosol particle is formed,
containing material from all contributing droplets
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