07 Moist Processes

8/12/2019 07 Moist Processes

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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.


7/16ENVI 1400 : Meteorology and Forecasting 7

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



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.



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)



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.



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.



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

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07 Moist Processes