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Atmospheric and Geophysical Fluid Dynamics

  • Toshihiko Hirooka, Professor
  • Yasunobu Miyoshi, Associate Professor
  • Huixin Liu, Associate Professor
  • Kensuke Nakajima, Assistant Professor
Our group investigates dynamics of the atmosphere up to about 1000 km height, as well as geophysical fluid of other planets such as Jupitar, Venus, and so on. The atmosphere of that height range consists of the troposphere (~10 km), the stratosphere (10~50 km) including the ozone layer, the mesosphere (50~90km), and the thermosphere (90 km~) including the ionosphere. There exist various spectacular phenomena associated with different physical properties in each atmospheric region. These phenomena are also to be intimately connected to vertical coupling processes through various kinds of waves, and to clarify their details in terms of theoretical and observational bases is an ongoing important subject. The framework of atmospheric dynamics is further applied to much different phenomena observed in the geophysical fluid of other planets.

“Weather Forecasts” in the Stratosphere

Figure 1
Figure 1. Time changes of observed (red lines) and numerically predicted (black dotted lines) temperatures at 80 N around 30 km height during the boreal winter in 2001/2002. The forecasts are initialized on 5 December 2001 in the top panel and 12 December 2001 in the bottom panel. For more details, see the text.

In the stratosphere during the Northern Hemisphere winter, a sudden stratospheric warming sometimes occurs, in which polar temperatures suddenly warm by more than 40–50 K in a week. Figure 1 shows a time change of observed polar temperatures at 80 N (red lines) and 30 km for the period from 1 December 2001 to 31 January 2002, along with the predicted ones (black dotted lines) by the ensemble forecast system starting from 5 December in the top panel and 12 December in the bottom panel. The numerical prediction is performed now by using the ensemble forecast system in which multiple forecasts starts from perturbed initial values denoting initial errors.

We can see that a sudden warming occurred with its warming peak on 28 December. It is found that most of the ensemble forecast members starting from 5 December fail to predict the occurrence of the warming (top), while all the ensemble members starting from 12 December successfully predict the warming (bottom). Therefore, this warming is predictable at least from 16 days in advance; the predictable period of 16 days is significantly long, compared with that in the usual weather forecast, i.e., 7 days. Plausible mechanisms bringing about this good predictability in the stratosphere is now being investigated.

Simulated Circulations in the Lower Thermosphere

Figure 2
Figure 2. Simulated temperatures (contour plots) and the horizontal winds (vectors) in the stratosphere (40 km height; bottom) and the lower thermosphere (100 km height; top) for September on the basis of our whole atmosphere general circulation model.

Our group developed a whole atmosphere general circulation model, which includes the regions from the ground surface to the upper thermosphere/ionosphere up to 500 km. Using this model, we investigate the general circulation in the whole atmosphere and vertical coupling processes between the lower and upper atmospheres.

Figure 2 shows the global distributions of the temperatures (contour plots) and the horizontal winds (vectors) in the upper stratosphere (40 km height) and the lower thermosphere (100 km height) obtained by the whole atmosphere model. The strong eastward wind and planetary scale wave dominate in middle latitudes in the stratosphere, where tidal waves and small scale gravity waves prevail in the lower thermosphere. Thus, the global structure of the general circulation changes significantly with increasing height. It is important and interesting to investigate mechanisms for the global structure of the general circulation in the whole atmosphere.

Earth's Upper Atmosphere

The upper atmosphere refers to regions between 100–1000 km altitudes. It is a unique region containing both neutral and ionized particles (plasma), while the regions above/below contains only ionized/neutral particles. The neutral and ionized part are respectively called the thermosphere and ionosphere. Knowledge of the upper atmosphere has great practical use in our modern high-tech society as it critically affects satellites' life time and orbit control, and wireless communication systems. Therefore, a good understanding of the upper atmosphere is in high demand.

Figure 3
Figure 3. Schematic figures elucidating (top) the space weather and climate, and (bottom) the thermosphere–ionosphere coupling.

Due to its low density, the upper atmosphere is easily subjected to external forcing from both above (the Sun) and below (the lower atmosphere). At the same time, the thermosphere and ionosphere constantly interact with each other. All these coupling processes lead to variations and perturbations of the upper atmosphere, forming the so-called space weather and climate (see the top of Fig. 3).

Co-existing in the same altitude region, the thermosphere and ionosphere constantly interact with each other via collisions. The force felt during such collisions is called drag force, with the one felt by the neutrals being called ion drag, and the one felt by the plasma (mainly ions) neutral drag. Due to ion drag, the neutral thermosphere, which would otherwise feels no electromagnetic forces, shows interesting features. For instance, both the neutral density and wind organize themselves along the geomagnetic equator instead of the geographic equator (see the bottom of Fig. 3), demonstrating the magnetic control of the thermosphere. The thermosphere and ionosphere also respond strongly to solar flares and magnetic storms.

Figure 4
Figure 4. Upper atmosphere coupling to the lower atmosphere.

Though more than 100 km above our head, the upper atmosphere is surprisingly closely linked to the troposphere we live in. For instance, both the thermosphere and ionospheric density show a wave-4 zonal structure (Fig.4), which resembles the global distribution of the cloud and also the land–sea distribution.

Although upward propagating atmospheric waves have been suggested to be the connecting agent, the underlying physical mechanism remains largely unknown. Identifying both dynamical and chemical changes on the vertical chain linking the troposphere and upper atmosphere is the critical frontier topic, which can lead our understanding of the vertical coupling in the atmosphere to a new horizon.

Geophysical Fluid Dynamics

You feel and see wonders of fluids everyday – relax in the bath, feel the gentle wind in jogging, pour drops of milk in a cup of coffee. However, fluid behaves a bit differently in the far greater scales of the Earth or planets, which are the focus of Geophysical Fluid Dynamics.

Flow on Rotating Spherical Earth

Figure 5
Figure 5. Flow in the atmosphere of “aquaplanet”, which is an idealized Earth entirely covered with an ocean.

Rotation and Sphericity of the Earth affects the motion of the air or water on it in a number of aspects. Figure5 illustrates the flow driven by a warm water pool of Amazon size. We see a belt of strong wind extends eastward along the equator. This result is obtained in an idealized earth, all covered with ocean, on which we see the character of flow on rotating spherical planet more clearly than on the Earth which has many complex features like continent and mountains.

Formation of cloud and rain drops

Figure 6
Figure 6. Jupiter's thunderstorm (yellow arrow in the left panel) observed by Galileo Orbiter. Lightnings are observed at night (right panels).

When flowing up or down in a large distance, due to the change of temperature or pressure, the phase change of fluid occurs, resulting in the formation of cloud and rain. Everybody knows those on the Earth, but clouds and rain develop also in the atmospheres of planets and its satellites. Figure 6 shows lightning in the thunderstorms of Jupiter's atmosphere. Cumulonimbus clouds of methane develop in the atmosphere of Titan, the largest satellite of Saturn. Of course, there are many poorly known issues on these extra terrestrial clouds.

From Geophysical Fluid Dynamics to Pan-Planetary Fluid Dynamics

Figure 7
Figure 7. A large scale vortex emerging in a numerical model constructed based on the equations for the Earth's ocean, which is similar to Jupiter's Great Red Spot (bottom panel).

Geophysical Fluid Dynamics originates from the consideration on the dynamics of the Earth's atmosphere and oceans. Later, it extends its field to include the fluids on the other planets. For example, the behavior of the Great Red Spot on Jupiter can be understood based on the mathematical framework used for the Earth's oceans (Fig. 7). Now, thousands of “exoplanets” are found around stars other than our own sun. So the development of “Pan-Planetary Fluid Dynamics”, with which possible great variety of the atmospheres and oceans can be explored and understood, is one of our dreams. Such framework should be useful also in considering the environment of our own Earth in the distant past and future.