Water in the Tropical Atmosphere

The importance of the hydrological cycle in our planet

Evaporation of water: transport heat from the surface to upper levels within the

updrafts (ascending currents)

Condensation releases Condensation releases Latent Heat to the Latent Heat to the atmosphere: main atmosphere: main driving forces for driving forces for tropical disturbancestropical disturbances

Clouds reflect solar radiation back to Clouds reflect solar radiation back to space resulting in low solar incoming space resulting in low solar incoming radiation available to heat the surfaceradiation available to heat the surface

Rainfall returns to Rainfall returns to the ground, is the ground, is transported by transported by rivers, used by the rivers, used by the biosphere and can biosphere and can evaporate and re-evaporate and re-enter the enter the atmosphereatmosphere

Evapotranspiration

• Atmospheric moisture originates from the earth’s surface, where water is in its liquid or solid phase and is transformed into water vapor

• Water vapor can be transported vertically (ascending currents) or horizontally (winds)

Water vapor originates from two processes: evaporation and evapotranspiration

EvaporationEvaporation takes place from water, ice surfaces (even in tropical regions!) , soil and over land, rocks or vegetation that are wet after rainfall

TranspirationTranspiration: is the same process as evaporation but carried out by organisms, mainly plants: is the same process as evaporation but carried out by organisms, mainly plantsThe combination of The combination of Transpiration + Evaporation =evapotranspirationTranspiration + Evaporation =evapotranspirationEvapotranspiration Evapotranspiration indicates the total flow of water vapor into the atmosphere.indicates the total flow of water vapor into the atmosphere.

Symbols: E= evaporation, ET=evapotranspiration ; T= transpirationSymbols: E= evaporation, ET=evapotranspiration ; T= transpiration

E

ET

ETETTT

E

E

transpirationThe roots of plants remove water from the soilwater evaporates from leaf surface

Rainforests like the Amazon represent a large source of transpiration and evaporation (evapotranspiration) to the atmosphere

What controls evapotranspiration

• 1) The capacity of the air to take up water vapor

• 2) the amount of energy available for use in the evaporation and transpiration processes

• 3) degree of turbulence at the lower layers of the atmosphere necessary to transfer the water vapor near surface to higher levels in the atmosphere.

Evaporation CondensRate Rate

Suppose that circles represent water vapor molecules

Saturation for a given Temperature T is reached when evaporation rate equals the condensation rate. If one molecule evaporates another will condensate

1) The air capacity to take up water vapor

1) The air capacity to take up water vapor

This figure shows the relationships between the maximum quantity of water vapor that can be observed at a given temperature T (oC). For each temperature, there is a maximum vapor pressure that can be observed with respect to a surface of waterThis figure indicates that the capacity of air to retain moisture increases rapidly with temperatureWarm tropical masses can take up more water vapor than cold ones5oC5oC

Saturation water vapor pressure and temperature relationships

Warm climatesCold climates

LargerLargerdifferencedifference

1) The air capacity to take up water vapor

• The actual amount that can be absorbed into the atmosphere also depends on the humidity of the air. The lower it is, the more favorable are the conditions for further evaporation.

• The dry tropics have very high rates of evaporation (importance of the semi-permanent anticyclones over the oceans)

2) The energy for evapotranspiration is provided by solar radiation: available in large quantities in the tropics

World Daily Solar Insolation Map ( from : http://www.oksolar.com/abctech/solar-radiation.htm)This map shows the amount of solar energy in hours, received each day on an optimally tilted surface (as determined by engineers when solar panels are installed) during the worst month of the year (note how cloudiness are important in tropical regions!).

3) Turbulence in the atmosphere

• Turbulence depends on horizontal winds and also on convection (vertical currents)

• Winds can be weak too close to the equator but convection can be strong in this regions.

• Trade winds can be very strong in some seasons and some years and increase evaporation.

Annual distribution of evaporation

Note how evaporation is larger over tropical oceans (dominated by anti-cyclones) when compared with forested regions such as the Amazon (large amount of rainfall)Deserts are regions with low clouds but low available moisture to evaporateRemember also that high annual evaporation depends also on the availability of moisture on the ground! Dry deserts have all ingredients for high evaporation (solar radiation, winds, turbulence, low humidity) but have no water to evaporate (200mm/year)!

Comparison: precipitation and evaporation rates (distinct unities)

HH

Highest values of evaporation (~ 2000mm/yr) occur over the subtropical oceans where the subtropical anticyclones dominate in both hemispheres. That is, evaporation exceeds precipitation: ocean desertsEvaporation rates are high where warm ocean currents exist (Gulf Stream – SE North America, Kuroshiro – SE Japan) (~ 2000mm year)Where cold currents dominate (off the west coasts of Africa and South America) evaporation rates are less than 1000mm/year

cm/year

Why low evaporation rates occur in oceanic equatorial regions?

Answer: in these regions precipitation dominate, cloud cover is high, air masses are often at or near saturation, wind velocities are low and in some regions upwelling of cold water is also observed. All these factors operate to reduce evaporation rates

Meridional distribution of evaporation over the oceans

Little seasonal variation at the EquatorLittle seasonal variation at the EquatorTrades are stronger in the winter Hemisphere: rate of evaporation increases

In contrast, land evaporation rates are highest for the equatorial regions, why?

~ 1200mm1200mm

Answer: high temperature (more important) and high precipitation rates (source of moisture)

In conclusion: What drives evapotranspiration:

• High energy (high temperatures)• Humidity in the atmosphere• Turbulence (convection and winds)• Moisture availability

Over water surfaces, where supply is plentiful, evaporation is not restricted and the values reached are called “potential evaporation”

Over land areas, the actual evaporation is less than the potential except where rainforests exist (such as in the Amazon)

IR 11/19/2009 0000Z IR 11/19/2009 0000Z

Global Average over land: 480 mm/year; Over oceans: 1176mm/year

Humidity

• The outcome of the evaporation process is the vertical transfer of moisture away from the earth’s surface to the atmosphere

• This result in the increase in the atmospheric water vapor level

• This process transfers mass and energy mass and energy (evaporation takes heat from the environment (evaporation takes heat from the environment and transport to upper levels) => LATENT HEAT and transport to upper levels) => LATENT HEAT TRANSFERTRANSFER

The simplest and most widely used instrument for measuring humidity is the sling psychrometer, which

has two thermometers called the wet bulb and dry bulb.

The difference between the two temperatures,the wet bulb depression, depends on themoisture content of the air and can be used

to determine dew point and relative humidity.

Humidity can be expressed in a variety of waysHumidity can be expressed in a variety of ways

Absolute humidity is the density of water vapor (‘red circles’),expressed as the number of grams of water vapor

contained in a cubic meter of air (blue circles)= (g/m3)

The mixing ratio is a measure of the mass of water vapor (red) relative to the mass of the other gases of the atmosphere (dry air in blue) (g/kg)

The maximum possible mixing ratio is called the saturation mixing ratio.

(g/kg)Saturation specific humidity is the maximum specific humidity that cancan exist

at a given temperature and is directly analogous to the saturation vapor pressure.

Relative humidity, RH, relates the ACTUALACTUAL amount of water vapor

in the air to the maximum possible at the current temperature.

RH = (specific humidity/saturation RH = (specific humidity/saturation specific humidity) X 100%specific humidity) X 100%

Saturation for cold air

Saturation for warm air

If the air temperature increases, more water vapor

can exist, and the ratio of the amount of water vapor

in the air relative to saturation decreases.

More water vapor can exist in warm air than in cold air,

so relative humidity depends on both the actual moisture content and the air

temperature.

More on moisture in the atmosphere:

http://www.icess.ucsb.edu/clivac/classes/geog110.htmclass on atmospheric moisture

Mean annual specific humidity (g/kg) ( g water/ Kg dry air)

Increase as latitude decreases Large values where tropical rain forests are presentExtratopical continental regions have less specific humidity than oceans for the same latitudeArctic and Antarctica are very dry regions (drier than the Sahara desert)

January minus July specific humidity g/kg

1) Question for Debate: Explain the reasons for relatively larger differences of specific humidity between January and over eastern USA, West Africa, Northern Australia and Eastern Asia.

2) Why seasonal differences in moisture are not as large over low latitudes in South America?

Winter NH (January) Summer NH (July)

You can justify your answers based on the seasonal variation of moisture

(Differences are for January – July)

Winter NH (January) Summer NH (July)

Moisture is transported from the anticyclone regions over the oceans toward the continents during summer of the respective hemisphere over Africa, Asia and south eastern USA Near the equator over South America The South Atlantic Antycyclone provides moisture during the winter decreasing the seasonal differences

Mean annual meridional transfer of water vapor in the atmosphere (in 1015 kg)

SE trades

NE trades

SE trades transport a great mass of water vapor compared to the NE trades. Note that the peak is ~ 10S and 10-15N

3) Question for debate: explain the reasons (mechanisms) for the meridional transport of moisture to midlatitudes of both hemispheres.

Precipitation versus evaporation: negative values of divergence of water vapor indicates precipitation > evaporation and positive values evaporation >

precipitation December to February June to August

P+ P+P+

P+P+P+

E+E+ E+E+ E+E+ E+E+P+

E+E+E+E+ P+ P+ P+

P+ P+

E+E+ E+E+ E+E+ E+E+ E+E+

Clouds in tropical regions:Clouds in tropical regions:

Mid level: Altostratus As

Low level: Stratus StLow level: Stratocumulus Sc Low level: Nimbostratus Ns

High level Cirrus Ci

Trade wind cumulus Cu Cumulonimbus Cb

Meridional profile of cloud amountsWhat are the most important clouds (regarding frequency and

cover) for tropical oceans?July -AugDec-Feb

2) Low level St (satellite picture)

1)Mid level: Altostratus As3) cumulus Cu 4) Cirrus (Ci)

Question: Is the Northern Hemisphere to the right or left with respect to the equator in the horizontal axes of these figures and why?

Meridional profile of cloud amounts: continents

Dec-Feb July -Aug

1)Mid level: Altostratus As3) cumulus Cu 2) Cirrus (Ci)

2) Low level St (satellite picture)

What are the main conclusions regarding:

1. The importance of water vapor2. Controls of evapotranspiration3. Annual distribution of evaporation4. Importance of anti-cyclones over the oceans5. Trade winds and evaporation6. Meridional transport of water vapor7. Relationship precipitation/evaporation8. Cloud distribution in tropical regions