5. NUMERICAL MODELLING

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5.1 Scientific Objectives

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As is explained in the Introduction, the main thrust of the GAME is as follows;

(1) to understand the role of the Asian Monsoon in the global energy and water cycle, and

(2) to investigate the predictability of the Asian Monsoon and to improve the water management over the Asian Monsoon region by developing the seasonal prediction capability of the Asian Monsoon.

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There is no doubt that the monsoon phenomena is caused by the heat contrast between land and ocean. In order to achieve these goals, various research topics from large-scale phenomena to turbulence scale phenomena relating to the land-atmosphere interaction should be defined and investigated. However, as understanding of land-atmosphere interaction is less understood than the air-sea interaction, research on the interaction between land and atmosphere is emphasized in the GAME. Especially, it should be noted that heterogenous and small-scale features are dominant in the land surface, which should be integrated into the large-scale model project. As GAME is a climate-related project, where their contribution to the time-averaged fields is emphasized in the GAME. For this purpose, those characteristics are integrated into the large-scale atmospheric circulations. In other words, the multi scale interaction should be intensively investigated.

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5.2 Modelling of Large-scale Phenomena

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5.2.1 Predictability of the Asian monsoon

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Whether the Interannual variability of the Asian Monsoon is predictable, or not is an interesting scientific and important practical issue, because if it is predictable, a great improvement can be expected in the water resource management. There are many researches which have been done for this subject. Based on the numerical experiments, Palmer and Anderson(1994) reviewed the possibility of the seasonal forecast, where they confirmed that while Sahel rainfall seems to be extremely predictable,Indian monsoon rainfall is much less so. They attributed the reason to the intra-seasonal fluctuation of the Asian Monsoon(so called, active-break phase, and/or 30-60 day fluctuation). The reason why the Asian Monsoon rainfall has a chaotic behavior should be investigated further. Especially, Webster (1983) proposed the land-atmosphere interaction is the mechanism for the active-break cycle of the monsoon. This mechanism has not yet been fully investigated and is worth being done. During the GAME, intensive flux measurements by using AAN monitoring networks will be planned. These unprecedented dataset will provide a new base for these topics.

At the same time, another possibility exits for the intra-seasonal fluctuation; 30? 60 day fluctuation can be considered to be a collective motion of convective system. Usually, it is generated over the Arabian Sea and propagates eastward. Sometimes, it tends to weaken over the maritime continent and redevelop over the Western Pacific ocean. At the same time, synopic disturbances are generated in accordance with these fluctuations. The organizing mechanism is not yet clearly understood although intensive efforts have been conducted (for example, TOGA-COARE) weakening and reintensification mechanism is unclear. These issues are also interesting topic investigated during the GAME.

In short, the following topics should be investigated;

(1) Mechanism for the inter-seasonal fluctuation with special emphasis on the land-atmosphere interaction,

(2) Life cycle of the depression in the Asian Monsoon region,

(3) Weakening mechanism over the maritime continent and reintensification mechanism.

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5.2.2 Interaction between ENSO , the Asian monsoon and the Eurasian snowfall

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Correlation between the intensity of the Asian Monsoon and the Eurasian snowfall has been pointed for a long time (for example, Hahn and Shukla(1976), Yasunari(1987) and others). At the same time, the linkage between the Asian Monsoon and the ENSO has been suggested by many authers (Meehl, 1987; Yasunari,1990, and others). However, the interaction between these three components in the climate system has not yet been fully understood. The main reason is poor observational data over the land surface areas, such as ground wetness, albedo and snow amount. However, tremendous data will become available in the GAME, and many new intensive research can be possible.

First, the AMIP type numerical experiments should be conducted to investigate the sensitivity of the land-surface condition to the atmospheric general circulations. During the AMIP project, only SST was specified. If we can get reliable land surface observations, we can specify both the land surface condition and SST. In other words, we can specify all of the boundary condition. Then, we can simulate what kind of atmospheric circulation might be occurred due to this boundary condition. This clarifies various weakness of the present state-of-arts model.

Secondly, numerical experiments with respect to the relation between Eurasian snow fall and monsoon activity should be done. Shen et al. (1996) found the distinct relationship between Eurasian snowfall and the intensity of the Asian Monsoon in the CCSR AMIP simulation . In order to confirm this conclusion further studies area necessary. Especially, further improvements in the land surface parameterization is necessary. During the GAME project the great improvement is expected in the land-surface parameterization. CCSR have a plan to conduct similar experiments by using longer SST data and other land surface parameterization.

Thirdly, further simulation should be conducted by using a coupled atmosphere-ocean model. It is famous that there exits a strong coupling between atmosphere and ocean over the Arabian Sea region. Whether this coupling have a serious impact on the Asian Monsoon is also an interesting issue.

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5.2.3 Establishment of one-month prediction of the Asian monsoon

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A long-range forecast is critical issue in the tropical countries. As is explained in 5.2.1, there is a possibility of a chaotic nature in the seasonal forecast. In order to overcome this difficulty, an ensemble forecast system should be established. For the purpose, present skill of the 1 month forecast should be estimated. Its sensitivity to the initial state should be intensively investigated. At the same time, efficient and economical way of the ensemble forecast should be established. Especially, as the parameterization of convective process plays a key role in the Asian Monsoon, its sensitivity to it should be emphasized.

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5.2.4 Spring predictability barrier

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It is now widely recognized that there exists a predictability barrier of the ENSO forecast in the spring (Webster and Yang,1992). They suggested that it is related to the onset of the Asian Monsoon . Whether this is a universal result or a model-dependent result should be carefully investigated. If the onset mechanism is very important, it is necessary to understand the mechanism of the warming up process of land surface. This may be related to the melting process of snowfall and development of convection. Orographic effect to these processes should be paid much attention. These studies should be conducted in the GAME.

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5.2.5 Validation for satellite data and its use for the large-scale modelling

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During the GAME period, unprecedented data will become available including measurement of surface fluxes and radiation fluxes. However, these data is spatially limited and the horizontal extension of these data should be considered. There exits two methods to extend these point measurements to the continental scale; (1) using a numerical model and (2) using a remote sensing data by satellite. The first method is realized in the 4DDA technique, which will be discussed later. The second method is related to the validation of satellite data which is discussed in the different section, At the same time, a combination of numerical technique and remote sensing technique is critical for further advance. For this purpose, ground-truth observation is essential. For example, there exist a possibility to obtain the vertical profile of convection by using the TRMM measurements. These data is also very effective in validating the model performance.

Water budget is another important issue to discuss the Asian Monsoon. However, there are a lot of problems to estimate the moisture fields over the ocean. There exits a possibility of improvement to estimate moisture field by using TMI measurement. These coupling between numerical modelling and remote sensing data is a hot topic, and should be intensively persuaded in the GAME.

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5.3 Modelling of Mesoscale Atmospheric Processes

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Numerical studies on the mesoscale atmospheric processes are carried out in order to clarify follows:

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(1) Heat and water exchange processes between the atmosphere and land surface, especially detailed heat and water budget in the Monsoon region.

(2) Large-scale heat/moisture transport processes in the Asian monsoon system, between land and ocean areas, particularly from the Indian and Pacific Oceans.

(3) Role of diurnal cyles and/or local circulation in the energy and water exchanges over complex terrain of monsoon Asia.

(4) Role of mesoscale convection and topogaraphically induced processes in the monsoon climate system.

(5) Predictability of regional rainfall and circulations in the Asian monsoon region.

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The core members of mesoscal modelling group are providing the following three models to study above items, assisting other research groups who intend to study a part of GAME research plan with these mesoscale models.

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(1) Simple models: Simple two- or three-dimensional models with radiation and turbulent parametarization including soil surface process, developed by University of Tsukuba (Kimura, 199x). This model will mainly apply to process studies of the land surface and atmosphere interaction.

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(2) JSM(Japan Spectrum Model) developed by JMA (Japan Meteorological Agency), which is a hydrostatic comprehensive model. This model will mainly apply to the meso-alpha phenomena and long term simulation. Science team of GAME Japan obtained the permission of JSM from JMA. The permitted code is a version of 1988, which is being revised in University of Tokyo.

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(3) RAMS( Regional Atmospheric Modelling System) developed by Colorado State University, which is a non-hydrostatic model with full parametarization. This model will mainly apply to the meso-beta and gamma phenomena. Three licenses of RAMS were purchased by the GAME project. The CCSR of Univ. of Tsukuba, Kyoto Univ., and Univ. of Tsukuba are the official users. Since NIES ( National Institute for Environmental Science) has bee already an official user, four institutions related with GAME have the licenses. All the national/international researchers involved in GAME can use this model as part of cooperative research programs with one of these groups. Some parameterization schemes, radiation, surface process, initialization etc., will be modified to fit the GAME area.

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Numerical studies using other models or their own models are also expected to carried out by some research groups. The core members will also provide fundamental data such as topography and basic meteorological data. The core members are having frequently hold meetings with GAME mesoscale modellers to discuss their results and future reserach plan and to help each other to achieve the object.

For meso-scale modeling, precipitable water observed by GPS receiverare usefull for initiariztion or evaluation of the model. GPS network data by IGS(International GPS service for Geodynamics) will be available.

Cloud-scale modelling and Cloud-scale 4DDA will be also carried outusing radar data obtained in IOP.

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Modelling studies of four regions are follows:

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(1) GAME-Toropics

Core member is Dr.Satomura, Kyoto Univ. Studies for the diurnal variation of mesoscale disturbance and 4DDA for the cloud-cluster scale will be carried out using RAMS.

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(2) GAME-Subtoropices (HUBEX)

Dr.Tsuboki, IHAS of Nagoya Univ. is a core member. 4DDA for Regional scale with JSM and for cloud-cluster scale with ARPS (non-hydrostatic model by Oklahoma Univ.) will be carried out. Cooperative studies on numerical modelling are planned between Japan and China.

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(3) GAME-Tibet

Core members are Dr. Kimura, Univ. Tsukuba and Dr.Takayabu, Meteorological Research Institute. The former applies The simple models and RAMS to both plateau scale and basin scale. The latter mainly apply the newest version of JSM and studies surface parameterizarion. Science groups of Japan and China will promote cooperative studies on modelling studies. The both intend to approve US modelling group to join their modelling studies.

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(4) GAME-Siberia

Dr.Yamazaki,Tohoku Univ. is a core member. Surface parameterization for Siberian region including permafrost will be investigated using the simple model and others.

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5.4 Modeling of Basin-scale Hydrological Processes

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Macro-hydrologaical model to be joined with atmospheric models mentioned in 5.3 will be developed to investigate following items:

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(1) Heat and water exchange process between the atmosphere and

land surface, especially effects of land-use, vegetation and snow cover.

(2) Estimation or correction method for the effects of inhomogeneous surface

and complex topography on the atmospheric heat budget.

(3) To develop estimation method of land surface parameters from satellite

data (for example, NDVI and surface radiation temperature and so on).

(4) Impact of initial soil moisture on long term forecast by the models.

(5) Interaction between meteorological phenomena with different scales.

(6) Long term water circulation in basin-scale regions.

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Core menbers are Drs. Lu and Koike, Univ. Nagaoka and Dr. Kimura, Univ.Tsukuba.

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