Atmospheric dynamics

Introduction

In association with the appearance of the digital computer in 1940, science and technology also has accomplished a rapid development. Especially in the field of weather, the computer was introduced from early time and was developed for the importance of the military research computer. Then, the computer usage in the weather forecasting has been coming to play a center role for the outlook service recently.

Moreover, as a base of forecast, the use of computer for the numeric calculation in meteorology (theory, observation and experiment) has developed as one field of the modern physics.

The atmospheric mechanics field is a group that starts approaching the essence of the weather by mainly using this numeric calculation (a part of theory). The numeric calculation is to calculate various mathematical equations by using a concrete numerical value. Especially in meteorology, because the atmosphere is distributed in three dimensions on the earth and the data often becomes huge amount, it impossible to treat it manually. Then, by using the computer in the data processing, many previous problems that can not be understood are being clarified now.  

Atmospheric simulation

Atmospheric numerical simulation is a reproduction of the physical phenomenon (for instance, wind, cloud, rain, snow, etc) that happens in the actual atmosphere by using a computer.

The reproduction does not mean that the atmospheric phenomenon actually occurs in the computer, but the condition of the atmosphere is reproduced virtually by the numerical value. Based on some theories and experiences, by using the control equations, an actual value (for instance, temperature, atmospheric pressure, wind direction, wind velocity, cloud amount, precipitation amount, etc) in which atmospheric phenomenon described is possible to be searched. Practically, because this calculation is too huge, a computer can calculate it better than human being.

But, the computer does not only do that calculation for us. For the atmospheric simulation necessity, the people make programs, and then the computer should read it. This program is called as model (numerical value).

Global warming

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The atmospheric general circulation (mean meridional circulation) changes when the CO2 increases, is studied by using numerical model. The atmospheric general circulation means "The circulation over the global atmosphere (flow)", it is recognized as the tropospheric Hadley circulation, mid-latitudinal circulation, stratospheric Brewer-Dobson circulation, etc. Because the atmospheric general circulation reflects the angular momentum transport and the atmospheric heating, the understanding of the atmospheric general circulation is related to the understanding of the dynamical structure over the atmosphere. Moreover, the transportation of the material, such as ozone, can be examined.

The left figure shows the atmospheric general circulation (unit: 10^10 kg/s) of the present climate in December to February. It is averaged from the east to the west, where the horizontal axis is latitude and the vertical axis is the atmospheric pressure. The atmosphere moves clockwise in the red area along the equal value line (otherwise, anti-clockwise in the blue area). The right figure shows the change in the atmospheric general circulation when the CO2 density increases twice the present value. (CO2 is twice climate - present climate). This figure shows the strengthening of the circulation from the equator towards the North Pole in the stratosphere (upper layer more than 200hPa) and the weakening by latitude of the circulation towards the extratropical cyclone in the troposphere (lower layer less than 200hPa).

Asian Monsoon

The Asian summer monsoon is recognized as one of the main monsoon systems and is separated into two subsystems, the Indian monsoon and the East Asian monsoon. The Asian summer monsoon onset is commonly signified by the strengthening of westerlies that is induced by the land-sea heat thermal between the Eurasian continent and the surrounding ocean. The westerlies enrich water vapor as a result of the cross equatorial flow off the east coast of Africa, and then induces heavy precipitation around South and Southeast of Asia. This mechanism is usually started about in mid May over the South East Asia regions and the end of May over the Indian Ocean, and then intensified during the boreal summer. In addition to the land-sea thermal contrast, the SST also affects the Asian summer monsoon westerlies and the monsoon precipitation.

In order to observe the influence of the SST, against the land-sea thermal contrast, to the onset/strengthening of the Asian summer monsoon in detail, we do some kinds of the SST experiments by using a Global Circulation Model (GCM), Global Spectral Model (GSM) T63L40 at Japan Meteorological Agency (JMA). Currently, our impact studies of the SST experiments are focused to the Indian monsoon and the South China Sea monsoon.

Our previous experiments indicate that both land-sea thermal contrast and SST considerably contributes to the onset of the Asian summer monsoon. The land-sea thermal contrast induces low-level wind surrounding the Eurasian continent, and makes a primary contribution to the formation of the Asian monsoon westerlies. The seasonal march of the SST destabilizes the stratification, induces the ITCZ jump from the southern to the northern hemisphere, strengthens the Hadley circulation, and then enhances the monsoon westerlies through effective transport of the absolute angular momentum.

Our present experiments show that the SCS significantly strengthens/weakens the monsoon westerlies over the subtropical Indian Ocean and also increases/decreases the precipitation over the SCS region and the Bay of Bengal. Some experiments are still done to get the clearer result. Besides that, the observational data analysis is also considered to confirm the model result.

Precipitation over the Tibetian Plateau

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The altitude of the Tibetian Plateau is about 5000m and the surface pressure is about 500 hPa. Recently, from the observational data result, it is recognized that there is precipitation that is due to the development of the low-pressure and front (trough type), and the precipitation that is due to the ground heating and the development of the cumulus convection (Tibetian high type) in the monsoon period.

In the present research, these precipitation types are picked up and the numerical simulation experiments are done. The experimental results show that in case of the trough type, water vapor flux comes from the southern part of the sea to the Himalayan mountains valley and it is important to the precipitation over the Tibetian Plateau. (In case of the Tibetian high type, the big problem is still remained, so that it can not be concluded well yet). Then, concerning to the trough type, the next plan is to observe in detail how the water vapor flux passes over the Himalayan mountain valley, and then towards the Tibetian Plateau. Besides that, the period and precipitation of the low-pressure will also be examined.

Typhoon

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A typhoon is a type of cyclone which has synoptic scale low-pressure system, and generally generates in the tropics. A typhoon often causes the disaster to japan because that is accompanied strong wind and heavy rainfall. As a feature structure, eyewall of typhoon and spiral rainband are seen. It is known that favorable environmental conditions for tropical cyclogenesis are warm sea surface temperature, moist atmosphere and weak vertical wind shear condition. A typhoon is formed condensational heating of water vapor which drives rising motion and is maintained. In tropics, the water vapor evaporates actively from warm sea surface, so active deep convection occures. A disturbance of monsoon depression, easterly waves and upper cold low organizes these convection and then it leads to the formation of typhoon.

However, in spite of a thought to appropriate condition for tropical cyclogenesis over tropical region, it is rare cases that tropical disturbances grow into typhoon. So development process of typhoon is yet poorly understood. This is because observational data is poor in tropics and it is difficult to conduct an observation of typhoon so that a formation of typhoon occur over the ocean.

Therefore, numerical simulation is effective to understand formation and development process of typhoon. In our laboratory, we reproduce development of typhoon using Non-Hydrostatic Model (NHM) developed by Japan Meteorological Agency and Meteorological Research Institute to clarify these processes.

Low-level cloud

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The cloud that is developed between the surface to about 2km altitude, is called as low-level cloud. The low-level cloud is few accompanied with the intense precipitation, so that it tends to seem not to be important. However, a large influence is exerted in the heat budget and the earth radiation budget over the land surface, so that a lot of low-level clouds are developed over the sea. Therefore, the low-level cloud is a forecast object that cannot be disregarded for the forecast of the yamase cloud that brings the cold weather and the climate forecast of the earth.

On the other hand, because the size of each cloud is small, the low-level cloud is not possible to examined directly by the the rough mesh (10km or more in horizontal) numerical forecast model. Therefore, the technical parameterization that less than the grid scale is used to examine the low-level cloud. It is essential to understand the mechanism of the low-level cloud to improve the parametarization accuracy. To understand the mechanism of the low-level cloud, a fine mesh numerical model, that is called as cloud resolving model, that has a physical and dynamical process of the cloud is considered to be a strong tool.

The aims are to improve the forecast accuracy of the low-level cloud by simulating the resolving cloud model, and describing the result in the expression of the low-level cloud by using the numerical forecast model. Moreover, the satellite remote sensing of the shipboard observation of the low-level cloud according to yamase that is done by Prof. Shoji Asano's groupd in the "Center for Atmospheric and Oceanic Studies" and the satellite remote sensing of the cloud are compared, verified and then the numerical model is improved in advanced.

Gravity current

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Gravity current (density current) is horizontal flow caused by air density differences due to temperature differences. In the atmosphere, cold front, sea-land breeze front and thunderstorm outflow formed under the bottom of cumulonimbus (downburst) have similar structures and behaviors of gravity current.

The leading edge of these gravity current forms several tens of meter raised head higher than surface, called 'noes', due to the effect of surface friction. Cold air (denser air) intrudes into warm air (lighter air) in the nose, so nose becomes an unstable structure. When disturbances evolve in the nose, the breakdown of nose occurs and which generates complicated structure of lobes and clefts.

Fig shows 3-dimensional fine structure of gravity current reproduced using numerical simulation, which is an isosurface of -1 K deviation of potential temperature. Cold air put on about 3km height falls and it collides with ground then it extends horizontally. An uniform front of gravity current change into lobe-and-cleft structure with time. We reproduce 3-dimensional complicated fine structure formed by gravitational instabillity.

High resolution*1 non hydrostatic*2 atmospheric numerical model*3 development

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The numerical simulation model treats steep geographical features and complex objects, proposes the originality of the new model, and conducts the non-hydrostatic atmospheric numerical model development. In the next work, by the development of the numerical calculation method and computer, we expect that the calculation mesh of horizontal scale can be minimized to be 100m*4.

Currently, the computational model of the mechanism process only, almost has stability, efficiency and accuracy. The below figures show the results of the main model calculation (for example, flow in the city buildings in Tokyo Otemachi) of the direct numerical simulation (DNS; Direct Numerical Simulation) of the flow surrounding the barrier (for example, cube and mountain) and LES (Large Edy Simulation) of atmospheric turbulent flow in complex geographical features.

  • Fig. 1. The DNS result of the flow that covers the cube
  • Fig. 2. Shape and grid calculation of the Tokyo Otemachi city buildings
  • Fig. 3. Surface wind and surface temperature in which the building influence is strongly received

*1 Calculation by using fine mesh
*2 Not approximated methods to improve the computational efficiency. The calculation mesh horizontal length scale for each grid is less than about 10km, is needed.
*3 The model who takes out part of area in the earth is called as an area model. Because the target area is narrow, the calculation with a fine mesh is possible. The other one to calculate the entire earth is called as a global model.
*4 Currently, the Japan Meteorological Agency still applies the 10km horizontal scale mesh to Japan areas.

Last-modified: 2017-03-16 (Thu) 20:29:23 (494d)