Lecture 6: The Hydrologic CycleLecture 6: The Hydrologic Cycle

EarthsClimate_Web_Chapter.pdf, p. 10, 16-17, 21, 31-32, 34

Components of the Hydrologic CycleComponents of the Hydrologic Cycle

Evaporation, Condensation, Transport, Precipitation, Transpiration, Runoff,

Groundwater Flow

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/smry.rxml

Water and the PlanetsWater and the Planets

Earth: the only planet where water can exist in three forms.

Water is essential to life.

Where Is Water On Earth?Where Is Water On Earth?

Water is a vital component of the climate cycle.

Atmosphere: 0.001%

Groundwater 0.58%

Surface Water 0.02%

Oceans 97.39%

Ice/Snow 2.0%

1. Most Rapid MovementIn Vapor Phase, in the Atmosphere

2. Most Water and Energy StorageIn Liquid Phase, in the Oceans

3. Most of Water useful to Humanity in Rivers, Lakes, Subterranean Water, and Ice and Snow, as Fresh Water

Movement of Water Between Stores

Hydrologic CycleHydrologic Cycle

Water VaporWater Vapor1. Highly variable spatially

2. Importance to climate and climate change

* Important part of the water cycle; ocean-to-land atmospheric vapor transport balances land-to-ocean runoff.

• Near 0% over deserts•3–4% over tropical oceans/jungles• Decreases rapidly with altitude; most is within a few km of the surface• Decreases rapidly with latitude; at the equator is 10 times that at the poles

* The most important greenhouse gas: water vapor-temperature feedback.* Water vapor condenses to form clouds, thus clouds–radiation feedback. Clouds release rainfall, reflect solar radiation, and reduce the infrared radiation emitted by Earth.

Cloud Development Causes of a rising air mass

Rising air expands, cools, and Rising air expands, cools, and condenses to form cloudcondenses to form cloud

What causes the air to rise?What causes the air to rise?

1. Surface heating and convection1. Surface heating and convection2. Widespread ascent due to convergence 2. Widespread ascent due to convergence

of surface airof surface air

3. Orographic uplift3. Orographic uplift 4. Uplift along weather fronts4. Uplift along weather fronts

Cold FrontCold Front Warm FrontWarm Front

Steps in Making Precipitation

6. Drops/crystals falls as rain/snow when they are too heavy to be suspended by upward motion

1. Water vapor in air1. Water vapor in air

2. Air with vapor rises, expands, and cools2. Air with vapor rises, expands, and cools

3. Vapor condensates around nuclei to form droplets (clouds)

4. Droplets suspended by atmospheric 4. Droplets suspended by atmospheric upward motion and turbulenceupward motion and turbulence

5. Droplets collide and coalesce into drops 5. Droplets collide and coalesce into drops in warm clouds and droplets diffuse to in warm clouds and droplets diffuse to ice crystals in cold cloudsice crystals in cold clouds

Orographic PrecipitationOrographic Precipitation

Winds blowing moist air toward a mountain will experience Winds blowing moist air toward a mountain will experience orographic uplift to an elevation where orographic uplift to an elevation where dew pointdew point is reached and is reached and clouds are formed.clouds are formed.

When the condensed moisture falls as rainfall, the When the condensed moisture falls as rainfall, the leeward sideleeward side of of the mountain is kept in a the mountain is kept in a rain shadowrain shadow..

Topographic Controls on PrecipitationTopographic Controls on Precipitation

Westerly winds blowing moist air from the Pacific Ocean encounter Westerly winds blowing moist air from the Pacific Ocean encounter several mountain ranges that create patterns of rising air and several mountain ranges that create patterns of rising air and precipitation followed by sinking air and warm dry rain shadows.precipitation followed by sinking air and warm dry rain shadows.

Global Annual Mean PrecipitationGlobal Annual Mean Precipitation

http://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.htmlhttp://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.html

Geographic Distribution of Annual P-E (mm)

• Evaporation excess nearly ubiquitous over sub-tropical oceans, with a sharp contrast at coastal regions.• Equatorial ocean evaporation minimum.• Tropical land areas show richest excess in precipitation.• Major desert regions, tundra, and mountainous regions all indicate deficit to marginally-balanced conditions.• Mid-latitude and boreal coastal/maritime environments exhibit adequate precipitation supply over evaporation.

From Paul Houser

Zonal Mean PrecipitationZonal Mean Precipitation

http://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.htmlhttp://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.html

PrecipitationPrecipitation

PrecipitationPrecipitation

Various Satellites Are Monitoring Earth’s Water Cycle Various Satellites Are Monitoring Earth’s Water Cycle

Summary:

• 1. What are three most important elements of the hydrological cycle?– Evaporation/evapotranspiration, transport, precipitation

• How is water distributed in the earth’s climate system?– 97% in ocean, 2% in ice, ~0.6% in ground water, 0.02% river/lake,

0.001% in atmosphere.

• How does each form of water (e.g., vapor, liquid and ice) influence climate?– Vapor: strongest greenhouse gas, liquid/ice forms clouds and

precipitation, river and ice/snow, which influence albedo of the earth, sensible and latent heat fluxes.

• How are clouds and rainfall formed?– Clouds/precipitation are formed by condensation/freeze of the

water vapor in rising motion of the atmosphere either due to unstable thermodynamic stratification or due to mechanical lifting by topographic or lower tropospheric wind convergence.