Atmospheric Water Global energy balance Atmospheric circulation Atmospheric water vapor Reading: Sections 3.3 and 3.4 for next Tues

Atmospheric Water Global energy balance Atmospheric circulation Atmospheric water vapor

Radiation Basic laws –Stefan-Boltzman Law R = emitted radiation (W/m 2 ) T = absolute temperature (K), and  = 5.67x10 -8 W/m 2 -K 4 with  = emissivity (0-1) –Water, Ice, Snow ( ) –Sand (0.76) “Gray bodies emit a proportion of the radiation of a black body Valid for a Black body or “pure radiator”

Net Radiation, R n R i Incoming Radiation R o =  R i Reflected radiation  albedo (0 – 1) R n Net Radiation ReRe Average value of R n over the earth and over the year is 105 W/m 2

Net Radiation, R n R n Net Radiation Average value of R n over the earth and over the year is 105 W/m 2 G – Ground Heat Flux LE – EvaporationH – Sensible Heat

Energy Balance of Earth Sensible heat flux 7 Latent heat flux 23 19

Diurnal Variation Diurnal variation of fluxes, July 2003 San Marcos Basin Fluxes in W/m 2 Downward shortwave Upward Longwave Downward longwave Upward shortwave Ground Latent Sensible

Energy Balance in the San Marcos Basin from the NARR (July 2003) Average fluxes over the day Net Shortwave = 310 – 72 = 238; Net Longwave = 415 – 495 = - 80 Note the very large amount of longwave radiation exchanged between land and atmosphere

Increasing carbon dioxide in the atmosphere (from about 300 ppm in preindustrial times) We are burning fossil carbon (oil, coal) at 100,000 times the rate it was laid down in geologic time

Absorption of energy by CO 2

Atmospheric Water Global energy balance Atmospheric circulation Atmospheric water vapor

Heating of earth surface Heating of earth surface is uneven –Solar radiation strikes perpendicularly near the equator (270 W/m 2 ) –Solar radiation strikes at an oblique angle near the poles (90 W/m 2 ) Emitted radiation is more uniform than incoming radiation Amount of energy transferred from equator to the poles is approximately 4 x 10 9 MW

Hadley circulation Warm air rises, cool air descends creating two huge convective cells. Atmosphere (and oceans) serve to transmit heat energy from the equator to the poles

V1V1 Conservation of Angular Momentum (Coriolis Force) Intertropical Convergence Zone mV 1 r 1 mV 2 r 2 r1r1 r2r2 V2V2 Looking down from North Pole, earth is rotating counterclockwise Earth rotation No external forces on air, so mV 1 r 1 = mV 2 r 2 r 1 V 2 Earth rotation Near equator, air starts to “fall behind” the earth

Atmospheric circulation 1.Tropical Easterlies/Trades 2.Westerlies 3.Polar easterlies 1.Intertropical convergence zone (ITCZ)/Doldrums 2.Horse latitudes 3.Subpolar low 4.Polar high Ferrel Cell Polar Cell 1.Hadley cell 2.Ferrel Cell 3.Polar cell Latitudes Winds Circulation cells

Effect of land mass distribution A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution Uneven distribution of land and ocean, coupled with different thermal properties creates spatial variation in atmospheric circulation

Shifting in Intertropical Convergence Zone (ITCZ) Owing to the tilt of the Earth's axis in orbit, the ITCZ shifts north and south. Southward shift in January Northward shift in July Creates wet Summers (Monsoons) and dry winters, especially in India and SE Asia

ITCZ movement

Atmospheric Water Global energy balance Atmospheric circulation Atmospheric water vapor

Structure of atmosphere

Atmospheric water Atmospheric water exists –Mostly as gas or water vapor –Liquid in rainfall and water droplets in clouds –Solid in snowfall and in hail storms Accounts for less than 1/100,000 part of total water, but plays a major role in the hydrologic cycle

Water vapor Suppose we have an elementary volume of atmosphere dV and we want quantify how much water vapor it contains Atmospheric gases: Nitrogen – 78.1% Oxygen – 20.9% Other gases ~ 1% dV m a = mass of moist air m v = mass of water vapor Water vapor density Air density

Specific Humidity, q v Specific humidity measures the mass of water vapor per unit mass of moist air It is dimensionless

Vapor pressure, e Vapor pressure, e, is the pressure that water vapor exerts on a surface Air pressure, p, is the total pressure that air makes on a surface Ideal gas law relates pressure to absolute temperature T, R v is the gas constant for water vapor is ratio of mol. wt. of water vapor to avg mol. wt. of dry air (=18/28.9)

Dalton’s Law of Partial Pressures John Dalton studied the effect of gases in a mixture. He observed that the Total Pressure of a gas mixture was the sum of the Partial Pressure of each gas. P total = P1 + P2 + P Pn The Partial Pressure is defined as the pressure of a single gas in the mixture as if that gas alone occupied the container. In other words, Dalton maintained that since there was an enormous amount of space between the gas molecules within the mixture that the gas molecules did not have any influence on the motion of other gas molecules, therefore the pressure of a gas sample would be the same whether it was the only gas in the container or if it were among other gases.

M d ~ 0.22* *28 ~ 28.9 Avogadro’s law Equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties. This number (Avogadro's number) is X in L for all gases. Dry air (21% O 2, 78% N 2, 1% other ) Water vapor (H 2 O) Dry air ( z = x+y molecules)Moist air (x dry and y water vapor)  d = (x+y) * M d /Volume  m = (x* M d + y*M v )/Volume  m <  d, thus moist air is less dense than dry air M v = 2* = 18 Moist air is lighter than dry air

Saturation vapor pressure, e s Saturation vapor pressure occurs when air is holding all the water vapor that it can at a given air temperature Vapor pressure is measured in Pascals (Pa), where 1 Pa = 1 N/m 2 1 kPa = 1000 Pa

Relative humidity, R h eses e Relative humidity measures the percent of the saturation water content of the air that it currently holds (0 – 100%)

Dewpoint Temperature, T d e Dewpoint temperature is the air temperature at which the air would be saturated with its current vapor content T TdTd

Water vapor in an air column We have three equations describing column: –Hydrostatic air pressure, dp/dz = -  a g –Lapse rate of temperature, dT/dz = -  –Ideal gas law, p =  a R a T Combine them and integrate over column to get pressure variation elevation Column Element, dz 1 2

Precipitable Water In an element dz, the mass of water vapor is dm p Integrate over the whole atmospheric column to get precipitable water,m p m p /A gives precipitable water per unit area in kg/m 2 Column Element, dz 1 2 Area = A

Precipitable Water, Jan 2003

Precipitable Water, July 2003

January July