3. Atmospheric dynamics

Vertical Structure of the Atmosphere

Figure 3 shows the mean vertical profile of atmospheric temperature. Atmospheric scientists partition the atmosphere vertically on the basis of this thermal structure. The lowest layer, called the troposphere, is characterized by a gradual decrease of temperature with height due to solar heating of the surface. It typically extends to 16–18 km in the tropics and to 8–12 km at higher latitudes. It accounts for 85% of total atmospheric mass. Heating of the surface allows buoyant motions, called convection, to transport heat and chemicals to high altitude. During this rise the water cools and condenses, leading to the formation of clouds. The process of condensation releases heat, providing additional buoyancy to the rising air parcels that can result in thunderstorms extending to the top of the troposphere. The mean decrease of temperature with altitude (called the lapse rate) in the troposphere is 6.5 K km–1, reflecting the combined influences of radiation, convection, and the latent heat release from water condensation.

Figure 3 Mean vertical profile of air temperature and definition of atmospheric layers.

The top of the troposphere, defined by a temperature minimum (190–230 K), is called the tropopause. The layer above, called the stratosphere, is characterized by increasing temperatures with height to reach a maximum of about 270 K at the stratopause located at 50 km altitude. This warming is due to the absorption of solar UV radiation by ozone. A situation in which the temperature increases with altitude is called an inversion. Because heavier air is overlain by lighter air, vertical motions are strongly suppressed. The stratosphere is therefore very stable against vertical motions. Exchange of air with the troposphere is restricted, and vertical transport within the stratosphere is very slow. The residence time of tropospheric air against transport to the stratosphere is 5–10 years, and the residence time of air in the stratosphere ranges from a year to a decade. In summer, the zonal (longitudinal) mean temperature distribution in the stratosphere is determined primarily by radiative processes (solar heating by ozone absorption and terrestrial cooling by CO2, water vapor, and ozone emission to space). In winter, radiation is weaker and the radiative equilibrium is perturbed by the propagation of planetary waves. This generates a large-scale meridional (latitudinal) circulation, called the Brewer–Dobson circulation, transporting air from low to higher latitudes.

The mesosphere extends from 50 km to the mesopause located at approximately 90–100 km altitude, where the mean temperature is about 160 K (120 K at the summer pole, which is the lowest temperature in the atmosphere). In this layer, where little ozone is available to absorb solar radiation, but where radiative cooling by CO2 is still effective, the temperature decreases again with height. Turbulence is frequent and often results from the dissipation of vertically propagating gravity waves, when the amplitude of these waves becomes so large that the atmosphere becomes thermally unstable.

The thermosphere above 100 km is characterized by a dramatic increase in temperature with height resulting primarily from the absorption of strong UV radiation by molecular oxygen O2, molecular nitrogen N2, and atomic oxygen O. Collisions become rare so that a stable population of ions can be sustained, producing a plasma (ionized gas). The temperature above 200 km reaches asymptotic values of typically 500 to 2000 K, depending on the level of solar activity (Figure 4). This asymptotic behavior reflects the small heat content and the high heat conductivity of this low air density region. The corresponding altitude is called the thermopause and varies from 250 to 500 km altitude. Atmospheric pressure is sufficiently low above 100 km that vertical transport of atmospheric species occurs primarily by molecular diffusion. This process tends to separate with height the different chemical species according to their respective mass. As a result, the relative abundance of light species like atomic oxygen, helium, and hydrogen increases with height relative to species like molecular nitrogen and oxygen. Molecular nitrogen dominates up to 180 km, while the prevailing constituent between 180 and about 700 km is atomic oxygen. Helium is the most abundant constituent between 700 km and 1700 km, and atomic hydrogen at higher altitudes. Above the thermopause, atoms follow ballistic trajectories because of the rarity of collisions. In this region of the atmosphere, light atoms (hydrogen) can overcome the forces of gravity and escape to space if their velocity is larger than a threshold value (escape velocity). At that point the atmosphere effectively merges with outer space.

Figure 4 Vertical distribution of the mean temperature for two levels of solar activity with emphasis on the upper atmosphere layers.

Air motions below 100 km are dominated by gravity and pressure forces, following the laws of hydrodynamics. Above 100 km, where ionization produces a plasma, the flow is affected by electromagnetic forces, and more complex equations from magneto-hydrodynamics must be applied. Aeronomy is the branch of science that describes the behavior of upper atmospheric phenomena (with emphasis on ionization and dissociation processes), while meteorology refers to the study of the lower levels of the atmosphere (with emphasis on dynamical and physical processes). The aeronomy literature has its own classification of atmospheric layers.

For example, it refers to the troposphere as the lower atmosphere, to the stratosphere and mesosphere as the middle atmosphere, and to the thermosphere as the upper atmosphere. It defines the homosphere below 100 km as the region where vertical mixing is sufficiently intense to maintain constant the relative abundance of inert gases, and the heterosphere above 100 km as the region where gravitational settling becomes sufficiently important for the relative concentration of heavy gases to decrease more rapidly than that of lighter ones. The atmospheric region above 1700 km is often called the geocorona. It produces an intense glow resulting from the fluorescence of hydrogen excited by the solar Lyman-α radiation at 122 nm. In another nomenclature, one distinguishes between the barosphere, where air molecules are bound to the Earth by gravitational forces, and the exosphere in which the air density is so small that collisions can be neglected. The lower boundary of the exosphere, called the exobase, is located at 400–1000 km.

Aeronomers refer to the ionosphere as the atmospheric region where ionization of molecules and atoms by extreme UV radiation (less than 100 nm) and energetic particle precipitation is a dominant process. Different ionospheric layers are distinguished: the D-region below 90 km altitude, the E-region between 90 and 170 km, the F-region between 170 and 1000 km, and the plasmasphere above 1000 km altitude. The region in which the magnetic field of the Earth controls the motions of charged particles is called the magnetosphere. Its shape is determined by the extent of the Earth’s internal magnetic field, the solar wind plasma, and the interplanetary magnetic field.

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