Introduction. August 27, 2014

Transcription

1 Introduction August 27, 2014 Atmospheric sciences focuses on understanding the atmosphere of the earth and other planets. The motivations for studying atmospheric sciences are largely: weather forecasting, climate studies, atmospheric chemistry and planetary atmospheres. 1. Composition and Structure The earth s atmosphere plays a central role in: Transferring energy between the sun and the planet s surface. Transferring energy from one region in the globe to another. Influencing the redistribution of atmospheric components such as water vapor, ozone and clouds. Gravity plays a central role because even though the atmosphere has no upper boundary, it is contained by the gravitational field of the planet. It is the reason why the atmospheric mass is concentrated in the lowest 10km. Vertical displacement of air is much smaller than horizontal displacement (vertical and horizontal displacement are only comparable during convective cells and fronts). The earth s atmosphere consists of a mixture of gases: N 2 and O 2 are the dominant constituents of the earth s atmosphere, water vapor accounts for roughly 0.25%, but it is very variable in space from about 10ppm in cold regions to 5% by volume in the tropics. The most important greenhouse gases are water vapor and carbon dioxide - methane and nitrous oxide are also significant contributors to the greenhouse effect. While N 2 O 2 Ar and CO 2 tend 1

2 Figure 1: Top, sun s electromagnetic spectrum and names of each region with approximate percent of energy the sun radiates in various regions. Bottom, absorption of radiation by gases in the atmosphere. The shaded region for each gas represents the percent of radiation each gas can absorb. Source: Ahrens 2

3 Table 1: Fractional concentrations by volume of the major gaseous constituents of the Earth s atmosphere, with respect to dry air Constituent Fractional concentration by volume Nitrogen (N 2 ) 78.08% Oxygen (O 2 ) 20.95% Argon (Ar).93% Water vapor (H 2 O) 0-5% Carbon Dioxide (CO 2 ) 380 ppm% Neon (Ne) 18 ppm% Methane (CH 4 ) 1.75 ppm% Krypton (Kr) 1ppm% Hydrogen (H 2 ) 0.5 ppm% Nitrous oxide (N 2 O) 0.3 ppm% Ozone (O 3 ) 0-0.1ppm% Figure 2: Composition of the Atmosphere 3

4 to be quite uniform and largely independent of height due to mixing by turbulent fluid motions, water vapor tends to be concentrated within the lowest few kilometers because it condenses and precipitates out when air is lifted. Ozone and other highly reactive trace species exhibit heterogeneous distributions because they don t remain in the atmosphere long enough to be well mixed. 1a. Thermal and Dynamical Structure The simplest picture of the earth s pressure, density and thermal structure is given in Figure 3 Figure 3: Source: Salby Figure 1.2 4

5 Troposphere Means turning sphere. Characterized by convection overturning, with a global-mean lapse rate of 6.5 K/km, driven mostly by surface heating. Upper boundary is about 10km marked by a sharp change in lapse rate. Highest elevation in the tropics and lowest in polar regions (about 8km). Accounts for 80% of the mass of the atmosphere. Stratosphere Means layered sphere. Initially temperature is constant, and then it increases (negative lapse rate) due to ozone heating, which results from the absorption of solar UV. There are only weak vertical motions (due to the increase in temperature with height) and is dominated by radiative processes. Residence times of particles are much longer in the stratosphere than in the troposphere. Upper boundary lies at about 50km where temperature reaches its maximum. Temperatures are warmest in the summer pole and decrease steadily towards the winter pole. Mesosphere Ozone heating diminishes. Radiative processes and convection are both important in the mesosphere. The mesopause lies at 85km where a second minimum of temperature is reached. Thermosphere Is not electrically neutral, ionization of molecules by energetic solar radiation produces a plasma of free electrons and ions. There is an increase of temperature with height due to absorption of solar radiation in association with the dissociation of diatomic nitrogen and oxygen molecules and the stripping of electrons from atoms. 2. Trace Constituents Beyond the primary constituents, air contains a variety of trace species: 2a. Carbon Dioxide Well mixed throughout the homosphere with a nearly uniform mixing ratio r CO2 399ppmv. However, unlike the primary constituents of air, CO 2 is tied to human activities. The concentration of atmospheric CO 2 has increased from a preindustrial value of about 280 ppm to 379 ppm in 2005 to its current value. It is produced naturally near the surface.the burning of fossil fuels has steadily increased the amount of carbon dioxide in the atmosphere. This gas is involved in trapping radiant energy near the earth s surface, and its increase has caused concerns over 5

6 global warming. GCM integrations have estimated significant increases in global temperatures due to CO 2 forcing. On short timescales, large exchanges of carbon occur through photosynthesis which removes carbon from the atmosphere and stores it in organic molecules in phytoplankton and leafy plants. Photosynthesis involves absorption of light near.43 and.66 µm. CO 2 + H 2 O + energy CH 2 O + O 2 (1) And through respiration and decay: CH 2 O + O 2 CO 2 + H 2 O + energy (2) Which oxidizes organic matter and returns the CO 2 to the atmosphere. Respiration releases an equivalent amount of energy in the form of heat. Emissions of CO 2 from fossil fuel use and from the effects of land use change on plant and soil carbon are the primary sources of increased atmospheric CO 2. Since 1750, it is estimated that about 2/3rds of anthropogenic CO 2 emissions have come from fossil fuel burning and about 1/3rd from land use change. About 45% of this CO 2 has remained in the atmosphere, while about 30% has been taken up by the oceans and the remainder has been taken up by the terrestrial biosphere. About half of a CO 2 pulse to the atmosphere is removed over a time scale of 30 years; a further 30% is removed within a few centuries; and the remaining 20% will typically stay in the atmosphere for many thousands of years. 2b. Water Vapor Water vapor is continuously produced in some regions and leaves the atmosphere through advection or precipitation in others. It is the single most important trace species in the atmosphere because of its role in radiation, cloud formation and exchanges of energy with the ocean. Confined almost exclusively to the troposphere, its mixing ratio decreases steadily with altitude from about 20g/kg in the tropics to a few ppm in the tropopause. There is also a sharp decrease with latitude. Its characteristic lifetime is on the order of 10 days. You calculate it by dividing the mass of water in the atmosphere ( 30 kg m 2 ) by the mean rainfall rate (0.3 cm day 1)Most of the water vapor is produced in the tropical oceans and carried vertically by convection and horizontaly by large-scale eddies and the general circulation. 6

7 Figure 4: Source: IPCC AR4 7

8 Figure 5: Source: Dingman 8

9 i. Residence Time A fundamental physical principle governing the behavior of an element in the atmosphere is conservation of mass. In any imaginary volume of air the following balance must hold: Rate of species flowing in (F in ) - rate of species flowing out (F out ) + rate of introduction of species (P ) - rate of removal of species (R) = rate of accumulation of species in imaginary volume ( dq dt ) dq dt = (F in F out ) + (P R) (3) If the amount of the substance Q is not changing in time, dq = 0, all the dt sources are balanced by the sinks and steady-state conditions are said to hold (F in F out ) = (P R) (4) If the volume is the entire atmosphere F in = F out = 0. We define the average residence time of lifetime τ as: τ = Q P +F in Q R + F out (5) And under steady state: τ = In the entire atmosphere is taken as the reservoir then under steady state: τ = Q R = Q P (6) 2c. Ozone By intercepting UV radiation, ozone allows life as we know it to exist. Early life forms lived in the ocean, liberated oxygen that photodissociated by UV radiation O 2 + hv 2O (7) Atomic oxygen then recombines with O 2 to form ozone in the reaction O 2 + O + M O 3 + M (8) where M is a third body needed to carry off excess energy liberated by the combination of O and O 2. The cycle then closes by dissociation by UV radiation. 9

10 Figure 6: Source: Wallace and Hobbs 10

11 O 3 + hv O 2 + O (9) and the combination of atomic oxygen and O 3 to form O 2. O + O 3 2O 2 (10) This closed cycle involves no net loss of components, and since the only result is the absorption of solar energy, this cycle can process UV radiation very efficiently. Ozone is concentrated in the stratosphere, reaching a maximum at a about 10ppmv near 30km, there is largest concentration in the tropics due to large UV radiation. In the lower stratosphere ozone has a lifetime of several weeks. It is quickly destroyed in the troposphere because of water solubility and can be oxidized in the surface. Even though most stratospheric ozone is produced in the tropics, the greatest column abundances are found at middle and high latitudes. The distribution over the Southern Hemisphere has column abundances of less than 200 DU that delineate the Antarctic Ozone Hole due to increasing levels of atmospheric chlorine. 2d. Methane Produced primarily by bacterial and surface processes that occur naturally, but anthropogenic sources such as mining and industrial activities may constitute as much as 20% of CH 4. r CH4 1.7ppmv and is well mixed in the troposphere. It oxidizes in the stratosphere. Methane concentrations are also increasing steadily and can lead to increased temperatures. Methane has an 9-year residence time in the atmosphere and is removed by the oxidation reaction: 2e. Chlorofluorocarbons CH 4 + 2O 2 CO 2 + 2H 2 O (11) Industrial halocarbons like CCl 4 (CF C 10) and CF Cl 3 (CF C 11) and CF 2 Cl 2 (CF C 12) are used widely as aerosol propellants, in refrigeration, and in a variety of manufacturing processes. Their release has increased steadily since the WWII. They are not soluble in water and are well mixed in the troposphere. CFCs are eventually transported into the stratosphere, where UV radiation photodissociates them. Free chlorine liberated via dissociation can destroy ozone and this is responsible for the ozone hole, very high clouds that rarely form except 11

12 over Antarctica also play a key role, as does the stratospheric circulation during the spring breakdown of the ozone hole. 2f. Atmospheric Aerosol Suspensions of liquid and solid particles are relevant to radiative as well as chemical processes. They range from thousandths of a micron to several hundred microns, and they are vital because they promote cloud formation as CCNs, they also scatter solar radiation at visible wavelengths and absorb IR radiation. Dust, sea salt, volcanic debris produce natural aerosols. Combustion and industrial processes also produce aerosols. Concentrations are high in urban areas and industrial complexes. Sporadic increases of aerosols produced by major volcanic eruptions have been linked to changes in thermal, optical and chemical properties of the atmosphere. 3. Units In general we will use the metric system (mks for meters, kilograms and seconds). Always always check your units. Perform the calculations in mks units and then convert the answers to whatever other unit you wish. Length: meter (m) Mass: kilogram (kg) Time: seconds (s) Temperature: Kelvin (K) Frequency: Hertz (Hz) [s 1 ] Force: Newton (N) [kg m s 2 ] Pressure: Pascal (Pa) [N m 2 ] Energy: Joule (J) [N m] Power: Watt (W) [J s 1] 12

13 Sometimes, instead of seconds, we will use minutes, hours or days. Most meteorologists use millibar (mb), which is equal to 100 Pa. The atmosphere is a fluid that supports motions ranging from turbulent eddies to circulations having dimensions of the earth itself. It is governed by the laws of mechanics and thermodynamics governing a discrete fluid body (air parcel) by generalizing those laws to a continuum of such systems. There are two frameworks to describe atmospheric behavior: Lagrangian description represents atmospheric behavior in terms of the properties of individual air parcels. It focuses on transformations of properties within an air parcel and on the interactions between that system and its environment. Mathematically we use the total time derivative d/dt to refer to the rate of change following an air parcel as it moves along its threedimensional trajectory. In this course we will focus primarily on the Lagrangian perspective. Eulerian description represents atmospheric behavior in terms of field properties, like instantaneous distributions of temperature, motion and constituents. Governed by partial differential equations, it is convenient for numerical purposes. Mathematically we will use the local derivative / t to refer to the rate of change at a fixed point in a rotating (x, y, z) space. note The vector (denoted by bold letters) x(t) = [x(t), y(t), z(t)] (12) denotes the three-dimensional position of a parcel trajectory, with initial position ζ = (x 0, y 0, z 0 ) and has a motion field v(x, t). dx = (dx, dy, dz) describes the incremental displacement of the material during the time interval dt. Velocity: of this parcel is then v = dx, with components u = dx/dt, v = dt dy/dt and w = dz/dt. While this corresponds to the Lagrangian time rate of change as we have described it, this velocity is equal to the field value v(x, t) at the material elements s position x(t) at time t. In mks units: (m s 1 ). Streamlines are defined by the velocity field, and are everywhere tangential to v(x, t). 13

14 Acceleration: time rate of change of velocity. dv dt. units of [m s 2 ]. Momentum: mass of an object times the object s velocity (m v) [kg m s 1 ]. Mass Density: mass per unit volume ρ [kg m 3 ] Flux: vector defined as a density times a velocity F [kg m 2 s 1 ] Flux Divergence: F = F x + F y + F z [kg m 1 s 1 ] note The Eulerian and Lagrangian rates of change are related by the chain rule d dt = t + u x + v y + w z (13) The terms involving velocities are referred to as advection terms. For a hypothetical conservative tracer, the Lagrangian rate of change is identically equal to zero and the Eulerian rate of change is: t = u x v y w z (14) 14