MET 112 Global Climate Change - Lecture 6

Clouds and Global Climate

Dr. Craig ClementsSan Jose State University

QuestionsQuestions

What role do clouds play on the Earth’s climate?

What would happen to our climate if clouds were to increase/decrease?

Hydrological Cycle

Water in the atmosphereWater in the atmosphere

Definitions:

– Evaporation:

– Condensation:

– Precipitation:

Process where a liquid changes into a gasProcess where a liquid changes into a gas

Any liquid or solid water that Any liquid or solid water that falls from the atmosphere to falls from the atmosphere to the ground. (i.e. RAIN!)the ground. (i.e. RAIN!)

Process where a gas changes into a liquidProcess where a gas changes into a liquid

Water freely Water freely evaporating and evaporating and condensingcondensing

Since more water Since more water molecules are molecules are evaporating than evaporating than condensing, then condensing, then net evaporation is net evaporation is occurring.occurring.

Lid on:Lid on:

Now, evaporation Now, evaporation and condensation and condensation are equal. The air are equal. The air above water is above water is now called now called ‘saturated’.‘saturated’.

The humidity is The humidity is now 100%now 100%

CondensationCondensation

The process by which water vapor changes to a cloud droplet

Water vapor molecules may ‘stick’ to

condensation nuclei and grow (billions) to eventually form cloud droplet.

Examples of condensation nuclei include:a.a. DustDustb.b. SaltSaltc.c. SmokeSmoke

Condensation occurs primarily as temperature cools:-colder the molecules more likely they are to ‘stick’ to other molecules

Clouds and radiationClouds and radiation

Cloud - Climate InteractionsCloud - Climate Interactions

Albedo effect - COOLINGAlbedo effect - COOLING Clouds reflect incoming solar radiation. Clouds reflect incoming solar radiation.

The cloud droplet size and total water content The cloud droplet size and total water content determine the overall reflectivity. determine the overall reflectivity.

Greenhouse effect - WARMINGGreenhouse effect - WARMING Clouds are good absorbers (and emitters) of long Clouds are good absorbers (and emitters) of long

wave (infrared) radiation.wave (infrared) radiation.

Indirect Effect due to aerosols (I) Certain aerosols may enhance cloud production and

character Recall how clouds form

– Water vapor condenses to liquid water– This processes requires ‘cloud condensation nuclei’– Examples of cloud condensation nuclei

Dust, salt, smoke (all of which are natural aerosols) So, aerosols (with both natural and anthropogenic origin)

– may serve as cloud condensation nuclei.

Indirect Effect due to aerosols (II)

More cloud condensation nuclei– would enhance cloud production

The question then is how would clouds change– Current understanding is that This processes would increase cloud albedoThis processes would increase cloud albedo

This idea of enhanced cloud formation by increases in aerosols is termed – The ‘indirect effect’ of aerosols

Understanding of these processes is currently incomplete.– But model results suggest more low clouds and thus

Total effect on Earth’s energy budget– Cooling– Radiative Forcing: negative– Cirrus cloud (high clouds) production possible, but still uncertain how

important.

Clouds and day to day Clouds and day to day temperaturestemperatures

Imagine that you are going camping in the Sierras Imagine that you are going camping in the Sierras with your friends. On the first day (and evening) it with your friends. On the first day (and evening) it is cloudy, while on the second day (and evening) it is cloudy, while on the second day (and evening) it is clear. Based on this information alone:is clear. Based on this information alone:

Which day would be warmer?Which day would be warmer?

Which evening would be warmer?Which evening would be warmer?

Explain your answers.Explain your answers.

Low and High cloudsLow and High clouds

Consider two types of clouds:Consider two types of clouds:

1.1. Low levels clouds Low levels clouds

2.2. High levels cloudsHigh levels clouds

Q: How is the Earth’s surface energy budget Q: How is the Earth’s surface energy budget different for low clouds compared to high different for low clouds compared to high clouds?clouds?

Clouds and climateClouds and climate

Cloud A: Low level, (dark, thick)Cloud A: Low level, (dark, thick)

Cloud B: High level, light Cloud B: High level, light (sub visible or thin)(sub visible or thin)

Excellent reflector of incoming radiation; good absorber/emitter of infrared radiation

Fair/poor reflector of incoming radiation; good/excellent absorber/emitter of infrared radiation

So, clouds both warm and cool the earth. Overall, though, clouds act to cool the

earth

Cirrus clouds

Cirrocumulus clouds

Altocumulus clouds

Stratocumulus clouds

Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis

stratus clouds

Explain how the earth’s climate would change as a result of aircraft contrails.

Changes in cloudsChanges in clouds

Increases in low level clouds will:Increases in low level clouds will:– cool the surface (cooling outweighs cool the surface (cooling outweighs

warming)warming)

Increases in high level clouds will:Increases in high level clouds will:– warm the surface (warming outweighs warm the surface (warming outweighs

cooling)cooling)

Controls on ClimateControls on Climate

Seasonal temperature and precipitation patterns Seasonal temperature and precipitation patterns are generally attributable to: are generally attributable to:

LatitudeLatitude Mountains and highlandsMountains and highlands Land and water locationLand and water location Prevailing windsPrevailing winds Pressure and wind systems Pressure and wind systems Ocean currentsOcean currents

QuestionsQuestions

Indicate the warmest and coldest areas of the Indicate the warmest and coldest areas of the Earth.Earth.

Consider the temperature at 60N latitude. Indicate Consider the temperature at 60N latitude. Indicate on the map the coldest and warmest places at 60N. on the map the coldest and warmest places at 60N.

What is the temperature difference between these What is the temperature difference between these locationslocations

What factors might explain this temperature What factors might explain this temperature difference?difference?

Why is there not a similar difference seen at 60S?Why is there not a similar difference seen at 60S?

Annual Surface TemperatureAnnual Surface Temperature

Climate controls: Climate controls: Latitude/MountainsLatitude/Mountains

LatitudeLatitude– Higher latitude climates are generally Higher latitude climates are generally

(cooler/warmer)(cooler/warmer)– Lower latitudes climates are generally Lower latitudes climates are generally

(cooler/warmer)(cooler/warmer) MountainsMountains

– Higher altitudes climate are generally Higher altitudes climate are generally (cooler/warmer): cooler temperatures(cooler/warmer): cooler temperatures

– Windward side of mountains are generally Windward side of mountains are generally (cooler/warmer) and (wetter/drier), than leeward side(cooler/warmer) and (wetter/drier), than leeward side

Climate controls: Climate controls: Latitude/MountainsLatitude/Mountains

LatitudeLatitude– Higher latitude climates are generally Higher latitude climates are generally

(cooler/warmer)(cooler/warmer)– Lower latitudes climates are generally Lower latitudes climates are generally

(cooler/warmer)(cooler/warmer) MountainsMountains

– Higher altitudes climate are generally Higher altitudes climate are generally (cooler/warmer): cooler temperatures(cooler/warmer): cooler temperatures

– Windward side of mountains are generally Windward side of mountains are generally (cooler/warmer) and (wetter/drier), than leeward side(cooler/warmer) and (wetter/drier), than leeward side

Controls on ClimateControls on Climate

Seasonal temperature and precipitation patters Seasonal temperature and precipitation patters are generally attributable to: are generally attributable to:

LatitudeLatitude Mountains and highlandsMountains and highlands Land and water locationLand and water location Prevailing windsPrevailing winds Pressure and wind systems Pressure and wind systems Ocean currentsOcean currents

Controls on Climate: OceansControls on Climate: Oceans

Ocean TemperaturesOcean Temperatures– Coasts of continents are affected by ocean Coasts of continents are affected by ocean

temperatures: Generally less temperature temperatures: Generally less temperature extremes compared to interior of continentsextremes compared to interior of continents

– Cold oceans: Cold oceans:

– Warm oceans:Warm oceans:

generally produce cooler/drier conditionsgenerally produce cooler/drier conditions

generally produce more warm/humid conditionsgenerally produce more warm/humid conditions

Cold ocean Warm ocean

Dry Humid

Controls on Climate: Controls on Climate: Pressure systemsPressure systems

Rising and sinking motion associated with low Rising and sinking motion associated with low and high pressure affects climateand high pressure affects climate

Areas where pressure seasonally low, Areas where pressure seasonally low, –

Areas where pressure seasonally high, Areas where pressure seasonally high, –

Tropics: rainyTropics: rainy

Subtropical high (30N/30S): warm and drySubtropical high (30N/30S): warm and dry

January Average sea-level Pressure and surface wind patternJanuary Average sea-level Pressure and surface wind pattern

July Average sea-level Pressure and surface wind patternJuly Average sea-level Pressure and surface wind pattern