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Moist Processes
ENVI1400: Lecture 7
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ENVI 1400 : Meteorology and Forecasting 2
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 : 0410191200 UTC
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METEOSAT visible image : 0410191200 UTC
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Typical cloud water contents are:
cumulus (early stage) : 0.20.5 g m-3
cumulus (later stage) : 0.51.0 g m-3
cumulonimbus : 3 g m-3(>5 g m-3observed invery strong updrafts)
alto-cumulus : 0.20.5 g m-3
stratocumulus / stratus : 0.10.5 g m-3
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Sources and Sinks
Sources: Evaporation from
surface: requiresenergy to supply latent
heat of evaporationsunlight, conductionfrom surface (coolssurface).
Evaporation ofprecipitationfallingfrom above: latentheat supplied bycooling of air
Sinks: Precipitation: rain,
snow, hail,
Condensation at thesurface: dew, frost
N.B. Most of the water inthe atmosphere above a
specific location is notfrom local evaporation,but is advected fromsomewhere 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 duringcondensation/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 (orless 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 specificheat of dry air
Cp 1004 J kg-1k-1
Evaporation of 1 gram of
liquid water (=1 cm3) into 1
cubic metre of air:
latent heat used 2500 Jcools air by 1.9 K.
Similarly latent heat is
released and air warmedwhen liquid water
condenses oute.g. as
cloud droplets.
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Condensation Conditions
Temperature is reduced to
below dew point.
Two most common mechanisms
for cooling are:
Contact cooling : loss of heat toa surface colder than the
overlying air, e.g. following
advection over a cooler surface,
or due to radiative cooling of the
surface at night.
Dynamic cooling : adiabatic
lifting results in very efficient
cooling of the air. (see below)
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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 reducedsaturation 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 temperaturedecreases with altitude,
evaporation into unsaturated
surface layer can result in
saturation of the air in the upper
boundary layer.
Cold air moving over warmerwater can sometimes produce
steam fog : common in the
arctic, and observed over rivers
and streams on cold mornings.
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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
M1+M2T1 Tmix
T2
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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.
As the parcel cools, the
saturation mixing ratio
decreases; when it equals the
actual water vapour mixing ratio
the parcel becomes saturated
and condensation can occur.
The level at which saturation
occurs is called the lifting
condensation level.
Lifting
condensationlevel
Saturation mixing ratio
equal to actual watervapour mixing ratio of parcel
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 rateis 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 highertemperatures than at low, and
hence more latent heat is
released.
Thus the wet adiabatic lapse
rate decreases as the
temperature increases.
T
Q1
T
Q2
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The Fhn Effect
0 m
100 m
200 m
300 m
400 m
500 m
Lifting condensation level
Unsaturated air cooling
at -0.98C per 100m
Saturated air cooling
at -0.5C per 100m
10C
Unsaturated air warming
at +0.98C per 100m
9.02C8.04C
7.06C
6.08C
5.58C
5.08C
6.54C
7.52C
8.50C
9.48C
10.46C11.44C
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 Fhn, in American
Rockies it is known as a Chinook. The onset of such winds can result in very
rapid temperature rises (22C in 5 minutes has been recorded) and is
associated with rapid melting of snow, and avalanche conditions.
4.58C
5.56C