lecture04 atmosphere

8/2/2019 Lecture04 Atmosphere

1/29

Lecture 4

The Atmosphere


2/29

Atmospheric Pressure


3/29

If warm air rises and cold air sinks why doesnt the Troposphere turnover?!


4/29

The saying that warm air risesand cold air sinks is only true

when looking at a constant

pressure level. This is

because density varies

drastically with height. Inreality, less dense air rises and

more dense air sinks. !

For the sake of this class, one can think of aconstant pressure level as a level of constant

height. !


5/29

The global atmospheric circulation

and its seasonal variability is drivenby the uneven solar heating of the

earths surface.!


6/29


7/29

Because earths rotation axis is tilted relative to theplane of its orbit around the sun, there is seasonal

variability in the geographical distribution of sunshine.!December!

June!

September!

March!


8/29

Warm colors: net gain in energy!Cool colors: net loss!

Source: NCEP/NCAR Reanalysis Project, 1959-1997Climatologies. Animation: Department of Geography, Universityof Oregon, March 2000 !


9/29

tmp2m_web.gif

The geographical distribution oftemperature and its seasonal variability

closely follows the geographicaldistribution of sunshine. !


10/29


11/29

Temperature plays a direct role indetermining the climate of every region.Temperature differences are also key in

driving the global atmospheric

circulation. Warm air tends to risebecause it is less dense, while cold airtends to sink because it is more dense.As we will see, this sets the atmosphere

in motion.!


12/29

As air becomes cooler, its capacityto hold water in vapor formdecreases significantly. This isknown as the Clausius-Clapeyronrelationship. This means that if aircontaining water vapor cools down,

it will eventually becomesaturated

with water vapor. As the air coolsfurther, enough water vaporcondenses into droplets to maintainthe air at its saturation point.!


13/29

Measurements of watervapor in the atmosphere

at Black Rock Forest

during the early fall of1995. The saturationcurve of water vapor is

clearly evident.

Graphic courtesy of John Knox


14/29

The Clausius-Clapeyron relationship leadsus to the concept of relative humidity.

Relative humidityis defined as the ratio of

the amount of water vapor in the air to the

amount of water vapor that air would hold ifit were saturated. So completely saturatedair has a relative humidity of 100%. If the air

has half as much water vapor as it can hold,

the relative humidity is 50%.!


15/29

Air temperature just above the Earths surface!


16/29

Total amount of water vapor in the atmospheric column!


17/29

In the lowest 10 km of the atmosphere (the troposphere),temperature decreases with height. So when warm air

rises, it cools. Because of the Clausius-Clapeyronrelationship, this often means that the air is quickly broughtto saturation. Condensation begins, and clouds form. !

If the air is especiallybuoyant, condensation

continues, causing the waterdroplets to increase in size.Eventually the water droplets

are so large they begin tocoalesce and fall as

precipitation. For thisreason, rising motion is oftenassociated with precipitation.(Note that the troposphere

contains 90% of the

atmospheres mass.)!


18/29


19/29

Temperature differences are key indriving the global atmosphericcirculation. Warm air tends to rise

because it is less dense, while cold air

tends to sink because it is more dense.This sets the atmosphere in motion,and ultimately leads to stark contrastsin hydrology. The tropical circulation is

a good example of this.!


20/29

Where the earths surface isheated the most by the sun, rising

motion occurs (A). This rising

motion leads to cooling,

condensation (B), and precipitation

(C). Then to complete the

circulation cell, the air movesnorthward and southward away

from the zone of rising motion.

Eventually it also sinks back to the

surface (D). Since the air hasbeen depleted of moisture, and

warms as it descends, its relativehumidity becomes extremely low.

This leads to extremely dryconditions. This tropical circulation

cell is known as the Hadley Cell.!

The Hadley Cell!


21/29


22/29

(1)!

(2)!

The Hadley Cell has a profound influence on hydrology and climate!


23/29

Evidence of the Hadley cell is clearly seen in satellitesnapshots of the tropics from space. The band of cloudiness

associated with the rising branch of the Hadley cell is known as

the Intertropical Convergence Zone, or ITCZ. !


24/29

Average Precipitation in December!


25/29

Persistent features in the global precipitation climatology

Inter-tropical convergence zone (ITCZ). The east-west-orientedband of intense convection located over the warmest regions ofthe tropics.!Sub-tropics. The large areas of sinking, typically located at about20 latitude, that compensate for the rising motion of the ITCZ.

Little precipitation falls in these regions.!Monsoon. A tropical seasonal phenomenon driven by contrasts inland-sea temperature. When the land is warm relative to theocean, air rises over the land, drawing in moist air from the ocean.Intense precipitation typically follows on land.!Mid-latitude jet stream. An intense air current that moves to theeast in both hemispheres. The jet stream is turbulent, particularlyin wintertime. The eddies it generates are wintertime storms, andare associated with high precipitation.!


26/29

Keep in mind that in our examination of theseasonal variation of surface air

temperature, precipitable water, rising

motion, and precipitation weve been

looking at long-term averages, orCLIMATOLOGIES. The day-to-dayvariations in these quantities can look

quite different from the climatology. !


27/29

Accumulated Precipitation from Sep 10-16 2009from the TRMM (Tropical Rainfall Measurement

Mission) satellite


28/29

Recent Precipitation, 9 April 2012

rainfall estimate courtesy of Japan Aerospace Exploration Agency (JAXA)!


29/29

Weather vs ClimateWeatheris the short-time-scale (< a few days)evolution of the of the atmosphere.!Climateis the statistics of weather.Weather prediction. The evolution of the state variablesof the atmosphere is governed by nonlinear dynamics with

chaotic evolution, and is therefore inherently unpredictablebeyond a certain period of time (~ 2 weeks).!Climate prediction. Predicting the statisticsof weather isstill possible at much longer time scales. Predictions ofclimate come in two versions. 1) Predicting the the time

evolution of natural climate variability, such as El Nio, wherethe slow evolution of the ocean permits prediction severalmonths in advance. 2) Predicting the climate response to anexternally specified change, such as to the anthropogenicemission of greenhouse gases. The external effect is referredto as a forcing or driver of climate change.

lecture04 atmosphere

Documents