Lecture 07 February 10, 2010 Water in the Atmosphere: Part 1

Transcription

1 Lecture 07 February 10, 2010 Water in the Atmosphere: Part 1 About Water on the Earth: The Hydrological Cycle Review 3-states of water, phase change and Latent Heat Indices of Water Vapor Content in the Atmosphere (1) precipitable water, (2) absolute humidity, (3) relatively humidity, (4) vapor pressure of water (5) specific humidity, (6) mixing ratio, (7) dew point temperature, (8) frost point Distributions of Water Vapor in the Atmosphere Measuring Humidity Introduction to Clouds Formation Methods of Achieving Saturation Two Cooling Processes Uplifting Mechanisms Types of Condensation & deposition (dew, frost, fog, cloud)

2 About Water on the Earth Over 70% of the Earth is covered by water a water planet. Water simultaneously exists in three states (solid, liquid, gas). Water can shift between states very easily phase change. Water has large specific heat, latent heat is energy associated with phase change, plays significant role in weather & climate. The hydrologic cycle refers to the regular cycle of water through the Earth-atmosphere system. Water is a variable gas ( 0 4% ), a greenhouse gas. In the temperature range on Earth, adding water vapor to the atmosphere or lowering the air temperature in saturated air can lead to condensation or deposition.

3

4

5 Latent Heat Energy associated with phase change between different states (solid, liquid, gas) of a substance. Atmospheric processes mainly involves latent heat associated with water (ice, liquid water, water vapor). Latent heat of evaporation / vaporization: from liquid water to water vapor, energy is added to liquid water to raise its temperature, molecules gain more kinetic energy to break free into the atmosphere. evaporative cooling: energy used to evaporate liquid water instead to raise body temperature (examples: sweat, wet surface). this energy held by those escaped molecules as latent in the atmosphere, and released when the reverse process condensation (from water vapor to liquid water) occurs e.g., dew, fog, clouds, energy to fuel severe weather events such as hurricanes. for the same net radiation gain by a surface, the presence of liquid water redirect some of this available energy as latent heat and reduce sensible heat (cooler surface temperature).

6 Latent Heat Latent heat of fusion: melting of ice to liquid water Reverse process: freezing, from liquid water to solid ice Latent heat of evaporation = 7.5 times latent heat of fusion Latent heat of sublimation: from solid ice to water vapor Reverse process: deposition, water vapor solid ice In addition to evaporative cooling, wet environments are cooler also because more water vapor in the atmosphere which absorbs more near-infrared insolation and reduces the amount to reach and heat Earth s surface and less heating of the lower atmosphere by the Earth s surface. Globally, 21 units of latent heat are transferred from the Earth s surface to the atmosphere, leaving only 8 units as sensible heat from the surface to the atmosphere.

7 Indices of Water Vapor Content in the Atmosphere Precipitable Water The amount of water in a column of air, measured in depth per unit area. It cannot show vertical water variability, but does vary horizontally. Global average ~ 25 mm Deserts: < 10 mm Tropics > 40 mm

8 Saturation over water surface: when evaporation and condensation reach equilibrium. This occurs at normal temperature on Earth. Saturation over ice surface: equilibrium between sublimation and deposition. Saturation can occur whether dry air exists or not, so the statement air holds water is not accurate. Net evaporation Saturation liquid water liquid water liquid water

9

10 Indices of Water Vapor Content in the Atmosphere Vapor pressure Dalton s law: the total atmospheric pressure p equals the sum of partial pressures of all gases. Partial pressure due to water vapor is called vapor pressure and denoted by e. Both p and e may change with the air temperature. The maximum e is called saturation vapor pressure, denoted by e s. e s is greater in hotter air.

11 Non-linear increase of saturation vapor pressure (e s ) with increasing temperature, same temperature increase leads greater increase in e s at higher temperature. T 1 = T 2 = 10 o C But e s, 1 < e s, 2 e s, 2 T 2 e s, 1 T 1

12 Indices of Water Vapor Content in the Atmosphere Absolute humidity = It is simply the density of water vapor. mass of water vapor (kg or g) volume of air (m 3 ) It depends on temperature because at constant pressure, air expands or contracts when heated or cooled change air volume. It depends on pressure because at constant temperature, air volume increases with increasing altitude and decreasing pressure. Other problems: How do you weigh the water vapor in the air? How do you determine the volume of an air parcel? Not widely used in free atmosphere: weather & climate

13 Indices of Water Vapor Content in the Atmosphere m v q = = m r = m v m d m v m v + m d Specific humidity q (g kg -1 ) Mixing ratio r (g kg -1 ) m v = mass of water vapor m d = mass of dry air m = mass of atmosphere Numerical values of q and r are very close because m v << m d Read examples in textbook! p128 Mainly scientific application. Both q and r independent of temperature and pressure because either water vapor mass nor dry air mass changes with volume of air when air expands or contracts thus, they are very useful in comparing water vapor in the air at locations of different temperatures and pressures Saturation values are denoted by q s and r s Only problem: unfamiliarity to the public.

14 Indices of Water Vapor Content in the Atmosphere Relative humidity (RH) is the percentage of actual water vapor content in the air relative to that (maximum water vapor content) in saturated air. Although RH is defined in many classical and modern texts using the ratio e / e s, International Meteorological Organization in 1947 adopted the ratio r / r s. However, the difference is very slight, thus: e RH = x 100% e s q = x 100% q s r = x 100% r s RH is a poor choice for comparing actual water content in the air at different places with different temperature, or same place at different time of the day or of the year as temperature changes with time.

15 RH depends on air temperature. RH changes even though actual water vapor content does not. This is because when temperature changes, the maximum water vapor content at saturation (e s, q s and r s ) all change as indicated by the change in the size of the open circles.

16

17 Daily RH changes mainly due to temperature change

18

19 Indices of Water Vapor Content in the Atmosphere Dew point temperature or dew point = the temperature to which an air parcel needs to be cooled in order to reach saturation (100% RH). Dew point (temperature) = or < air temperature. When air temperature < dew point, condensation occur. Frost point = below 0 o C temperature to which an air parcel needs to be cooled in order to reach saturation (100% RH). When air temperature < frost point, deposition occur. Simple to use, easy to interpret. Mainly depends on actual water vapor content in the air. Higher dew point, higher actual water vapor content in the air

20 Dew point temperature

21

22 The brown and green parcels have the same RH (75%), but different dew points (~14 C) vs (~26 C) Dew point temperature is directly related to the amount of water vapor in the air and is widely used (local weather report and daily weather maps)

23 January, dew point July, dew point Distribution of Water Vapor in the Atmosphere Water vapor in the atmosphere either from local evaporation or from horizontal transport by advection of moisture from other regions (warm moist ocean). Warm July yields higher water vapor content in the atmosphere than cold January The blue arrows indicate the direction of decreasing dew point or water vapor content in the atmosphere from source regions (warm Gulf of Mexico or Pacific).

24 Measuring Humidity Hair Hygrometer: hair length changes with the relative humidity (human or horse hair) Sling Psychrometer Wet Bulb Dry Bulb Hygrothermograph: hair hygrometer & bimetallic strip are combined

25

26

27 Introduction to Clouds Formation Water vapor condenses on condensation nuclei suspended in the air (fog & cloud) in saturation without condensation nuclei, super-saturation with RH > 100% is required to produce fog & clouds. Clouds are visible aggregates of minute droplets of water or tiny crystals of ice. Clouds are instrumental to Earth s energy (radiation) and moisture balances. Most clouds form as air parcels are lifted & cooled to saturation.

28 Saturation Specific Humidity Methods of Achieving Saturation Examples: (1) condensation on bathroom mirror during shower, (2) precipitation fog Adding water vapor Cooling Example: clouds Temperature

29 Methods of Achieving Saturation Mixing cold air with warm moist air Examples: contrails behind jetliners, Steam fog

30 Two Different Cooling Processes Diabatic process: Heat is added or removed from an air parcel. Air parcel cooled by surface, radiation fog. More important for fog formation than for clouds. Adiabatic processes: No heat added or removed from an air parcel. Cooling by expansion or warming by compressing. Most important for clouds formation.

31 Adiabatic Cooling & Warming DALR = dry adiabatic lapse rate = 10 o C km -1

32 Adiabatic Cooling Rising air cools at a consistent rate, called the dry adiabatic lapse rate (DALR): Unsaturated air 10 o C/km (1000m) If the cooling decreases the air temperature to the dew point and below, the lapse rate changes to the wet adiabatic lapse rate (WALR) saturated air varies:.4-9 o C/km These lapse rates apply no matter what lifting mechanism is at work

33 Note: Average environmental lapse rate (ELR) is 6.5 C km -1 (page 17 of textbook) Condensation release latent heat warm atmosphere to offset adiabatic cooling. e a = e s WALR = Wet adiabatic lapse rate is always lower than the dry adiabatic lapse rate and ranges between 4 and 9 C km -1.

34 Adiabatic Cooling Why is the wet adiabatic lapse rate different? Because when condensation occurs after the dew point is reached, latent heat is released. This heat is added to the atmosphere, so the rate at which air cools is offset by the heat added to the atmosphere. The wet adiabatic lapse rate is always lower than the dry adiabatic lapse rate. 4 o C/km 9 o C/km more condensation warmer, moist air ex: Tropics less condensation cooler, dry air ex: Poles

35 SALR = saturated adiabatic lapse rate = the lapse rate when saturated air parcel rises SALR is not constant, more latent heat is released by warmer air to offset adiabatic cooling thus, lower SALR rate, than cooler air

36 Uplifting Mechanisms

37 Uplifting Mechanisms (a) Orographic lifting: over mountains and hills On the windward side of the barrier, air is displaced toward higher altitudes, undergoes adiabatic cooling, possibly to saturation, even rain On the leeward side, descending air warms adiabatically through compression leading to a rain-shadow Windward Side Leeward Side

38 Uplifting Mechanisms (b) Frontal Wedging When boundaries (fronts) between airmasses of unlike temperatures migrate, warmer (less dense or lighter) air is pushed aloft This results in adiabatic cooling and cloud formation (c) Convergence Air mass is non-uniformly distributed over Earth Air advects from areas of more mass to areas of less mass Air moving into these low pressure regions converges Stimulates rising motions and adiabatic cooling (d) Convection Localized surface heating leads to local free convection Vertical motions are stimulated from the surface upward resulting in towering clouds and a chance for intense precipitation over small spatial scales

39 Forms of Condensation & Deposition Dew Liquid condensation on surface objects Diabatic cooling of surface air typically takes place through terrestrial radiation loss on calm, cool, clear nights Surface air becomes saturated and condensation forms on objects acting as condensation nuclei Frost Similar to dew except that it forms when surface temperatures are below freezing Deposition occurs instead of condensation May be referred to as white or hoar frost

40 Dew Frost Frozen Dew Occurs when normal dew formation processes occur followed by a drop in temperature to below freezing Causes a tight bond between ice and the surface Forms dangerous black ice on roadways

41 Fog Simply a surface cloud when air either cools to the dew point, or moisture added, or when cooler air is mixed with warmer moister air Radiation Fog Occurs when near surface air chills diabatically to saturation through terrestrial radiation loss on clear cool nights Require a slight breeze to vertically mix air through a shallow column

42 Advection Fog Occurs when warm moist air moves across a cooler surface Air is chilled diabatically to saturation Common on the U.S. west coast as warm, moist air from the central Pacific advects over the cold California ocean current Frequently develop near boundaries of opposing ocean temperatures, e.g.: off northeast US coast Upslope Fog The only fog developed through adiabatic cooling Occur when air is advected over land surfaces which increase in elevation A common occurrence in the Great Plains of the U.S. where warm, moist air advects from the Miss. River Valley towards the Rocky Mountains