CE 394K.2 Hydrology Atmospheric Water and Precipitation

1. CE 394K.2 HydrologyAtmospheric Water and Precipitation • Literary quote for today: “In Köhln, a town of monks and bones,And pavements fang'd with murderous stonesAnd rags, and hags, and hideous wenches;I counted two and seventy stenches,All well defined, and several stinks!Ye nymphs that reign o'er sewers and sinks,The river Rhine, it is well known,Doth wash your city of Cologne;But tell me, nymphs, what power devineShall henceforth wash the river Rhine?” Samuel Taylor Coleridge, “The City of Cologne”, 1800Contributed by Eric Hersh

2. Questions for today (1)  How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth? (2) What are the factors that govern the patterns of atmospheric circulation over the earth? (3)  What are the key variables that describe atmospheric water vapor and how are they connected? (4)  What causes precipitation to form and what are the factors that govern the rate of precipitation? (5)  How is precipitation measured and described? (Some slides in this presentation were prepared by Venkatesh Merwade)

3. Questions for today (1)  How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth? (2) What are the factors that govern the patterns of atmospheric circulation over the earth? (3)  What are the key variables that describe atmospheric water vapor and how are they connected? (4)  What causes precipitation to form and what are the factors that govern the rate of precipitation? (5)  How is precipitation measured and described? (Some slides in this presentation were prepared by Venkatesh Merwade)

4. Heat energy • Energy • Potential, Kinetic, Internal (Eu) • Internal energy • Sensibleheat – heat content that can be measured and is proportional to temperature • Latent heat – “hidden” heat content that is related to phase changes

5. Energy Units • In SI units, the basic unit of energy is Joule (J), where 1 J = 1 kg x 1 m/s2 • Energy can also be measured in calories where 1 calorie = heat required to raise 1 gm of water by 1°C and 1 kilocalorie (C) = 1000 calories (1 calorie = 4.19 Joules) • We will use the SI system of units

6. Water Volume [L3] (acre-ft, m3) Water flow [L3/T] (cfs or m3/s) Water flux [L/T] (in/day, mm/day) Energy amount [E] (Joules) Energy “flow” in Watts [E/T] (1W = 1 J/s) Energy flux [E/L2T] in Watts/m2 Energy fluxes and flows Energy flow of 1 Joule/sec Area = 1 m2

7. MegaJoules • When working with evaporation, its more convenient to use MegaJoules, MJ (J x 106) • So units are • Energy amount (MJ) • Energy flow (MJ/day, MJ/month) • Energy flux (MJ/m2-day, MJ/m2-month)

8. Internal Energy of Water Water vapor Water Ice Heat Capacity (J/kg-K) Latent Heat (MJ/kg) Ice 2220 0.33 Water 4190 2.5 2.5/0.33 = 7.6 Water may evaporate at any temperature in range 0 – 100°C Latent heat of vaporization consumes 7.6 times the latent heat of fusion (melting)

9. Water flux Evaporation rate, E (mm/day) Energy flux Latent heat flux (W/m2), Hl Latent heat flux r = 1000 kg/m3 lv = 2.5 MJ/kg 28.94 W/m2 = 1 mm/day Area = 1 m2

10. Radiation • Two basic laws • Stefan-Boltzman Law • R = emitted radiation (W/m2) • e = emissivity (0-1) • s = 5.67x10-8W/m2-K4 • T = absolute temperature (K) • Wiens Law • l = wavelength of emitted radiation (m) All bodies emit radiation Hot bodies (sun) emit short wave radiation Cool bodies (earth) emit long wave radiation

11. Net Radiation, Rn Ri Incoming Radiation • Ro =aRi Reflected radiation • = albedo (0 – 1) Re Rn Net Radiation Average value of Rn over the earth and over the year is 105 W/m2

12. Net Radiation, Rn H – Sensible Heat LE – Evaporation G – Ground Heat Flux Rn Net Radiation Average value of Rn over the earth and over the year is 105 W/m2

13. Energy Balance of Earth 70 20 100 6 6 26 4 38 15 19 21 Sensible heat flux 7 Latent heat flux 23 51 http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/radiation_balance.html

14. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 600Z

15. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 900Z

16. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 1200Z

17. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 1500Z

18. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 1800Z

19. Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003 2100Z

20. Latent heat flux, Jan 2003, 1500z

21. Questions for today (1)  How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth? (2) What are the factors that govern the patterns of atmospheric circulation over the earth? (3)  What are the key variables that describe atmospheric water vapor and how are they connected? (4)  What causes precipitation to form and what are the factors that govern the rate of precipitation? (5)  How is precipitation measured and described? (Some slides in this presentation were prepared by Venkatesh Merwade)

22. Heating of earth surface is uneven Solar radiation strikes perpendicularly near the equator (270 W/m2) Solar radiation strikes at an oblique angle near the poles (90 W/m2) Emitted radiation is more uniform than incoming radiation Heating of earth surface Amount of energy transferred from equator to the poles is approximately 4 x 109 MW

23. Hadley circulation Warm air rises, cool air descends creating two huge convective cells.

24. Coriolis Force Cone is moving southward towards the pole Camera fixed on to the globe (looking southward, cone appears deflecting to the right) Camera fixed in the outer space (cone appears moving straight) the force that deflects the path of the wind on account of earth rotation is called Coriolis force. The path of the wind is deflected to the right in the Northern Hemisphere and the to left in the Southern Hemisphere.

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

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

27. 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 Creates wet Summers (Monsoons) and dry winters, especially in India and SE Asia Northward shift in July

28. ITCZ movement http://iri.ldeo.columbia.edu/%7Ebgordon/ITCZ.html

29. Questions for today (1)  How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth? (2) What are the factors that govern the patterns of atmospheric circulation over the earth? (3)  What are the key variables that describe atmospheric water vapor and how are they connected? (4)  What causes precipitation to form and what are the factors that govern the rate of precipitation? (5)  How is precipitation measured and described? (Some slides in this presentation were prepared by Venkatesh Merwade)

30. Structure of atmosphere

31. 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

32. Water vapor Suppose we have an elementary volume of atmosphere dV and we want quantify how much water vapor it contains Water vapor density dV ma = mass of moist air mv = mass of water vapor Air density Atmospheric gases: Nitrogen – 78.1% Oxygen – 20.9% Other gases ~ 1% http://www.bambooweb.com/articles/e/a/Earth's_atmosphere.html

33. Specific Humidity, qv • Specific humidity measures the mass of water vapor per unit mass of moist air • It is dimensionless

34. 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, Rv is the gas constant for water vapor • 0.622 is ratio of mol. wt. of water vapor to avg mol. wt. of dry air

35. 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 + P3 + .......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. http://members.aol.com/profchm/dalton.html

36. 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 6.023 X 1023 in 22.41 L for all gases. Dry air ( z = x+y molecules) Moist air (x dry and y water vapor) Dry air Water vapor rd = (x+y) * Md/Volume rm = (x* Md + y*Mv)/Volume rm < rd, which means moist air is lighter than dry air!

37. Saturation vapor pressure, es 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/m2 1 kPa = 1000 Pa

38. Relative humidity, Rh es e Relative humidity measures the percent of the saturation water content of the air that it currently holds (0 – 100%)

39. Dewpoint Temperature, Td e Td T Dewpoint temperature is the air temperature at which the air would be saturated with its current vapor content

40. Water vapor in an air column • We have three equations describing column: • Hydrostatic air pressure, dp/dz = -rag • Lapse rate of temperature, dT/dz = - a • Ideal gas law, p = raRaT • Combine them and integrate over column to get pressure variation elevation 2 Column Element, dz 1

41. Precipitable Water • In an element dz, the mass of water vapor is dmp • Integrate over the whole atmospheric column to get precipitable water,mp • mp/A gives precipitable water per unit area in kg/m2 2 Column Element, dz Area = A 1

42. Precipitable Water, Jan 2003

43. Precipitable Water, July 2003

44. January July

45. Questions for today (1)  How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth? (2) What are the factors that govern the patterns of atmospheric circulation over the earth? (3)  What are the key variables that describe atmospheric water vapor and how are they connected? (4)  What causes precipitation to form and what are the factors that govern the rate of precipitation? (5)  How is precipitation measured and described? (Some slides in this presentation were prepared by Venkatesh Merwade)

46. Precipitation • Precipitation: water falling from the atmosphere to the earth. • Rainfall • Snowfall • Hail, sleet • Requires lifting of air mass so that it cools and condenses.

47. Mechanisms for air lifting • Frontal lifting • Orographic lifting • Convective lifting

48. Definitions • Air mass : A large body of air with similar temperature and moisture characteristics over its horizontal extent. • Front: Boundary between contrasting air masses. • Cold front: Leading edge of the cold air when it is advancing towards warm air. • Warm front: leading edge of the warm air when advancing towards cold air.

49. Frontal Lifting • Boundary between air masses with different properties is called a front • Cold front occurs when cold air advances towards warm air • Warm front occurs when warm air overrides cold air Cold front (produces cumulus cloud) Cold front (produces stratus cloud)

50. Orographic lifting Orographic uplift occurs when air is forced to rise because of the physical presence of elevated land.