Moist Processes

SOEE1400: Lecture 11

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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:– 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

Sinks:– 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

Cp ≈ 1004 J kg-1 k-1

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.

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:

– Large scale 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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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+M2

T1 TmixT2

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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.

Liftingcondensation

level

Saturation mixing ratioequal to actual water

vapour mixing ratio of parcel

Dew pointat 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.

T

Q1

T

Q2

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The Föhn Effect

0 m

100 m

200 m

300 m

400 m

500 mLifting condensation level

Unsaturated air coolingat -0.98°C per 100m

Saturated air coolingat -0.5°C per 100m

10°C

Unsaturated air warmingat +0.98°C per 100m

9.02°C8.04°C

7.06°C

6.08°C

5.58°C

5.08°C

6.54°C

7.52°C

8.50°C

9.48°C10.46°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.

4.58°C

5.56°C

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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 (long-

wave) 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