Lecture 6: The Hydrologic Cycle EarthsClimate_Web_Chapter.pdfEarthsClimate_Web_Chapter.pdf, p. 10, 16-17, 21, 31-32, 34

Components of the Hydrologic Cycle Evaporation, Condensation, Transport, Precipitation, Transpiration, Runoff, Groundwater Flow

Water and the Planets Earth: the only planet where water can exist in three forms. Water is essential to life.

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 Movement In Vapor Phase, in the Atmosphere 2. Most Water and Energy Storage In 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 Cycle

Water Vapor 1. 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 condenses to form cloud What causes the air to rise? 1. Surface heating and convection 2. Widespread ascent due to convergence of surface air 3. Orographic uplift 4. Uplift along weather fronts Cold Front Warm 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 air 2. Air with vapor rises, expands, and cools 3. Vapor condensates around nuclei to form droplets (clouds) 4. Droplets suspended by atmospheric upward motion and turbulence 5. Droplets collide and coalesce into drops in warm clouds and droplets diffuse to ice crystals in cold clouds

Orographic Precipitation Winds blowing moist air toward a mountain will experience orographic uplift to an elevation where dew point is reached and clouds are formed. When the condensed moisture falls as rainfall, the leeward side of the mountain is kept in a rain shadow.

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

Global Annual Mean Precipitation

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 Precipitation

Precipitation

Precipitation

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.