Scales of Motion and Atmospheric Composition

Transcription

1 Scales of Motion and Atmospheric Composition Atmos 3200/Geog 3280 Mountain Weather and Climate Sebastian Hoch & C. David Whiteman Drusberg and Glaernisch, Switzerland, Sebastian Hoch

2 Atmospheric Scales of Motion ~200 km Synoptic Scale ~2 km Mesoscale Microscale Whiteman (2000)

3 Atmospheric composition Permanent gases % by volume N 2 (nitrogen) 78 O 2 (oxygen) 21 Ar (argon) 1 Variable gases H 2 O (water vapor) CO 2 (carbon dioxide) others % by volume trace

4 Aerosols Aerosols - small solid and liquid particles in atmosphere Can be natural or man-made (anthropogenic) Natural: hydrometeors, dust, pollen, sea spray, terpenes, volcanic ash Man-made: black carbon, air pollutants, dust from mechanical disturbance of soils Diameters: µm Large particles settle out quickly Small particles may have long residence times Aerosols affect transmission of light (thus, visibility) Absorption / Scattering / Reflection / Diffraction / Refraction Ice nuclei (IN), cloud condensation nuclei (CCN) Sites for chemical transformations Respirable aerosols (d < 2.5 µm)

5 Water droplets & settling velocities CLOUD RAIN Table A-1. Settling velocities (cm/s) of different diameter (mm and µm) water droplets in still air at various altitudes (km). 1m = 100 cm diameter diameter droplet type Altitude (mm) (micrometers) z=0 km z=2 km z=4 km z=6 km typical cloud droplet large cloud droplet drizzle small raindrop typical raindrop large raindrop Higher density particles will have larger settling velocities raindrop fall speed ~ 4-8 m/s (that s ~ 9-18 miles/hour) largest raindrop speed ~ 9 m/s

6 Water is the primary variable gas - How measured? Relative Humidity: Ratio of the actual water vapor pressure to the vapor pressure that would occur if the air were saturated at the same temperature. Whiteman (2000)

7 Saturation Vapor Pressure Saturation water vapor pressure: the vapor pressure of water at a given temperature, wherein the water vapor is in equilibrium with either a plane surface of liquid water or ice. Saturation vapor pressure over liquid water as a function of air temperature. Saturation vapor pressure over ice is lower than over water. Whiteman (2000)

8 Dalton s Law (John Dalton, ) Total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the various gases P a = P N2 + P O2 + P Ar + P H20 + P H20 = e is the partial pressure of water vapor (mb) P H20 is an absolute measure of water content in the atmosphere At any given temperature, there is a maximum partial pressure of water vapor that cannot be exceeded. This maximum or saturation vapor pressure, e s, is the partial pressure of water vapor when the air reaches saturation. e s is a function of temperature only.

9 Relative humidity in percent [%] is given by: where e is the actual water vapor pressure and es is the saturation water vapor pressure (the water vapor pressure that the air would have if it were at saturation). The saturation water vapor pressure es is a function of temperature and is calculated by the Clausius-Clapeyron Equation: where e0=6.11 mbar is the saturation vapor pressure at T0=273 K, Rv=461 J K -1 is the gas constant for water, and T is absolute temperature in Kelvin...

10 Absolute Temperature [K] = Temperature [ ] and L is the latent heat (heat per unit mass released or absorbed during a phase change of water between solid, liquid, and gaseous phase). Latent heat of vaporization Lv = 2.5 MJ kg -1 Latent heat of sublimation Ld = 2.83 MJ kg -1 The saturation vapor pressure with respect to an ice surface is lower than for a surface of liquid water. Snow crystals can grow at the expense of water droplets in a cloud containing both liquid and solid phases of water!

11 Dew-point temperature Td [ ] is the temperature to which air must be cooled to become saturated at constant pressure. Dew-point temperature is always less than or equal to the air temperature T. Saturation is reached when T= Td. Dew-point depression, given by T -Td [ ] is a relative measure of the dryness of the air. Dew-point temperature is measured by a dew-point hygrometer, a small mirror that is cooled until dew first forms on it. Once dew forms, the mirror temperature equals the dew-point temperature.

12 Mixing ratio r [kg/kg] is the ratio of the mass of water vapor Mw to the mass of dry air Md in an air volume and can be calculated from: where ε= is the ratio of the gas constants for water vapor and dry air, p is the pressure. Mixing ratios are typically between and kg/kg, although they may reach kg/kg in warm saturated tropical atmospheres. Specific Humidity q [kg/kg] is the ratio of the mass of water vapor Mw to the mass of moist air in an air volume and can be calculated from:

13 Impacts of Vapor Pressure Higher vapor pressure reduces transmission of infrared radiation (IR) Refraction & absorption of solar radiation Flux of water vapor proportional to (e s e); so, for given temperature, lower vapor pressure increases evaporation Compared to free air, vapor pressure is usually higher near mountains Reduces transmission of IR, which may increase temperature Lowers condensation level Lowers drying power, i.e., ability to transfer water from clothing/plants into atmosphere

14 Phases of Water Whiteman (2000) Lysimeters, Greenland Summit, 2002 Transformations between the solid, liquid, and gaseous phases of water result in the release or storage of large quantities of heat. Heat must be supplied when the transformation is from the less dispersed to the more dispersed phase. The quantity of heat associated with individual phase changes is given in Joules per unit mass of water. L v = 2.5 MJ/kg is the latent heat of vaporization, L f = MJ/kg is the latent heat of freezing, and L d = 2.83 MJ/kg is the latent heat of deposition.

15 Phase diagram

16 Importance of water phase changes Heat released or gained in one location can be regained or re-lost at another location and time. Thus, latent heat can be transferred separately from sensible heat. Water is energetically important because of the large values of latent heat that are released or stored during phase changes.