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2.3 Asian AWS Network

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2.3.1 Objectives

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The Eurasian continent, the largest continent on the earth, plays a predominant role on the seasonal cycle of the planetary-scale surface energy exchange and transport in the climate system. The diverse land surfaces and vegetations, however, characterize the extremely large seasonal and spatial variation of surface sensible and latent energy fluxes over the continent, which in turn may produce the regionality and asymmetries in the seasonal cycle over the continent.

The surface net radiation flux is a fundamental forcing of the sensible and latent energy fluxes. The estimation of these fluxes over the global surface was formerly carried out by Budyko (1956). His pioneering work should be updated by utilizing the satellite as well surface observations with higher quality and resolution over the whole of the continent, which is one of the major tasks of the GEWEX. The surface energy budgets are fundamental forcing of the seasonal march of the climate system. These elements are particularly important over the eastern half of the Eurasian continent, to unravel the role of the land/ocean heating contrast on the Asian winter and summer monsoon systems.

There has been, so far, a considerably dense routine observation network of surface meteorological station in the eastern half of this continent, maintained by the operational meteorological agencies of each country. However, these station data are providing only indirect information for estimating the surface radiation and energy budgets over the broad area of the continent, based mainly upon the bulk method. In addition, the diverse and heterogeneous land surfaces of the continent make it more difficult to estimate the appropriate bulk transfer coefficients for the momentum, heat and moisture fluxes of each station.

The satellite-based SRB (Surface Radiation Budget), combined with the surface-based BSRN (Baseline Surface Radiation Network), has been continuously providing us with data sets of surface radiation elements with continental-scale coverage. The GEBA (Global Energy Balance Archive) conducted by ETH group has been archiving surface energy balance data from various parts of the world. The long-term monitoring of directly-measured energy fluxes, however, has not been undertaken except at a very few micro-meteorological stations (e.g., refer to Bulletin of ERC, Univ. of Tsukuba, 1994). The continental-scale monitoring system of surface energy fluxes and surface conditions (albedo, soil moisture, vegetation etc.), combined with the radiation network mentioned above, will provide us with key information for unraveling the physical processes of the recent climate change (e.g., rapid warming over Siberia and Mongolia) of the continental-scale. This network would also contribute greatly to the validation of surface energy conditions

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derived from satellites, and to the advanced 4DDA as part of the essential data input from the surface.

To remedy the current lack of a surface measurement network, it is important to construct a network of surface stations. Such network should have a major objective of the observation and the detection of seasonal and annual variations of surface fluxes of momentum, heat, and radiation as well as those of soil moisture on the continental scale as part of GAME scientific activities. At the same time, it's objective should include a support for the regional studies, particularly their intensive field experiments. The latter requires the accurate determination of the above variable on a time scale of hour.

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2.3.2 Strategy of AAN

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Because the two major aims require different and somewhat contradictory requirements for the specification and configuration of a surface station, it has been decided to organize AAN activities in two phases.

Phase I for the year 1996 ? 2000 will be the period for the construction, tests, and deployment of the AAN, and also for data acquisition during the intensive observations planned in 1998. For the Phase I, measurements will be made of (i) regular meteorological variables such as temperature, humidity, wind speed, etc., (ii) regular hydrologic variables such as precipitation, (iii) surface turbulent fluxes of momentum, heat and water vapor, (iv) surface radiative fluxes, and (v) soil moisture.

The subsequent Phase II (year 2000 ? 2004) will be devoted for the long term monitoring period, and somewhat reduced number of variables will be monitored during this period.

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2.3.3 Development of AWS

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An important function of the AWS system is to measure the radiation and energy flux components, with the surface parameters (soil moisture, snow cover etc.). Currently, AWS systems to fully satisfy this purpose are not available. Among the candidates as an AWS to be used during Phase I, a Portable Automated Mesonet (PAM) III station being developed at National Center for Atmospheric Research (NCAR) in the U.S., is currently a prime candidate, and scientists in AAN project are working closely with NCAR scientists and engineers to modify and improve the PAM III station to meet needs and requirements for AAN.

PAM III station uses a 3D sonic anemometer and a hygrothermometer to determine surface turbulent fluxes of momentum, heat and water vapor by applying an eddy correlation technique and bandpass covariance method. GAME-AAN is

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considering to add the second hygrothermometer and an infrared radiation thermometer, to allow flux evaluation by means of a Bowen ratio method, a profile method, and of a bulk method in order to increase the reliability of flux determination over a prolonged period in a remote area. In addition, sensors needed to apply time domain reflectometry (TDR) technique will be added to PAM III, in order to add capability to monitor the soil moisture. Furthermore, direct measurements of radiation budget at the surface are are considered where other routine measurements are not available. Novel feature of the PAM III station is its capability to transmit the data through a geostationary satellite. This will allow real time monitoring of the status of the station as well as real time data acquisition.

In order to assess possible problems in cold climate, a prototype station was subjected to a test in a cold temperature down to ?40 within a controlled cold room, and it was verified that the station worked without a problem. However, it was desided to add more protection agains cold temperature by empolying teflon cables wherever needed. Also, the development of another AWS system specifically designed for extreme cold environment is being considered and currently a prototype station is being tested.

For the long term monitoring during phase II, further consideration will be given to the improvements of the station with emphasis on the long term measurements based on the experience obtainable during Phase I.

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2.3.4 Data requirements for AAN monitoring

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To achieve the scientific objectives mentioned in 2.3.1, long-homogeneous and quality-controlled data should be obtained through the AWS network. The national GAME workshop on surface observation and AWS monitoring was held in July, 1996 at Environmental Research Center, University of Tsukuba, and the agreement was met on the guideline for the data required under GAME surface flux monitoring and regional process studies to be implemented during the phase I. The details of meteorological and hydrological elements and their time-space resolutions are list in Table 2.3-1.

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2.3.5 Coordination for AAN

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AAN project will be carried out through coordination of scientists in GAME regional studies and those in GAME/AAN. The latter forms the AAN working group (WG) which will be directly responsible for the development of an AS, data handling and initial analysis. The former will take care of the maintenance of AS installed in each regional study area. Table 2.3-2 summarizes organization and name(s) of person(s) in charge.

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Table 2.3-1 Requirements for the surface measurement within GAME-AAN

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Measurement site Representative of the area of 10 ? 50 km2

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Measurements and Products

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Fluxes : Sensible and latent heat, soil heat flux and momentum

Radiation : Four components of radiation balance and surface temperature

General meteorology: air temperature, humidity, air pressure, wind speed and

direction, and soil temperature (2 ? 3 depths)

Hydrology: soil moisture, precipitation, (snow depth)

Vegetation height, (Leaf Area Index), (stomatal resistance)

Soil: Soil profiles, hydraulic conductivity, soil moisture

curve, heat conductance, etc.

Others: ? Manual observations of rainfall, etc. should also be

collected whenever they are available at nearby

meteorological/hydrologic station

? Field log of observations/maintenance should be kept,

preferably with photos.

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Observation interval and averaging period

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Fluxes, radiation, and Meteorology: Continuous 30 ? 60 minutes averages

Vegetation: Infrequent intervals should be acceptable as long as

seasonal variations can be obtained.

Soil properties: One time observation at each station, except for soil

moisture measurements

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Methods and Accuracy

Fluxes:

The absolute value of 20 W/m2 can be considered as

desired accuracy.

Any method of deriving fluxes can be accepted as long

as the methods are fully documented.

Meteorology and hydrology :

The same standard method employed in a country

where the station is located should be used

whenever possible.

Soil and vegetation:

More emphasis should be placed on the detection of time

variation of the soil moisture at single site.

Variability of soil moisture in an area should be studies

in a separate, independent experiments in an intensive

observation. For the determination of

physical/physiological properties of

soil/vegetation, measurements by a single rover team

visiting every sites are preferable.

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Measurement period

From 1997 through 1999/2000.

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The network of AWS for measuring the surface radiation, energy fluxes and surface conditions will cover the whole of monsoon Asia, or the eastern half of the Eurasian continent. The AWSs will be arrayed fundamentally along the meridional and zonal section lines crossing over this area, which represent large gradients of climatic conditions and vegetation. A tentative idea of the arrangement of AWS is shown in Fig. 2.3-1. One major meridional section crosses over the Tundra, Taiga of central Siberia, Mongolia, Gobi desert, Tibetan plateau, the Himalayan highland and the Indian sub-continent, representing the temperature gradient from the polar region to the tropical monsoon region. Another zonally-oriented section lies over the humid subtropics in central China to the arid zone in the interior of the continent, representing basically the gradient of moisture and continentality. Several sub-networks with relatively high density of stations will also be considered, related to the intensive regional experiments. The total number of AWS over the whole of this area during Phase I is expected be around 10.

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2.3.6 Data collection and control

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To reliably acquire the measured variables, two data streams are under consideration. One flow of data will take place on site. All the variables measured and processed in a computer on an AWS will be recorded on a memory card. The card will be replaced and recovered at regular intervals by a scientist/engineer of a local counterpart organization responsible for the maintenance of the AWS system. These data will be sent to AAN/WG for further processing of the data.

Another data flow will be through the Japanese geostationary satellite (GMS). The AWSs will have capability to transmit data at regular intervals to the GMS, which in turn will downlink the data to the Japan Meteorological Agency (JMA). These data will then be distributed to the general scientific community through GTS (Global Telecommunications System ) by JMA and through the Internet by the AAN Data and Analysis Center.

GAME/AAN scientists will check the raw data and apply necessary analysis to make raw data more reliable and usable to a general GAME scientists. These are particularly needed for the flux and soil moisture data which require careful treatment of data. These secondary data will also be archived in GAIN and will become available to GAME scientist initially, and then to general scientific community.

The types of data which will be transferred through the GMS and which will become available real time, and those which will be stored on site for later recovery, will be determined by considering available channels of GMS for GAME, requirements of scientific community and needs of data handling.

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Table 2.3-2 : Organization of GAME-AAN

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1. Organization of GAME AAN project in Japan

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I. GAME AAN WG

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Chair Tetsuzo Yasunari (Univ.Tsukuba)

vice chair Teruyuki Nakajima (Univ.Tokyo)

secretariats Michiaki Sugita (Univ.Tsukuba)

Ken'ichi Ueno (Univ. Shiga Pref.)

Data transfer Rikie Suzuki (Univ.Tsukuba)

Sensor group

Radiation Teruyuki Nakajima (Univ.Tokyo)

Michiaki Sugita (Univ.Tsukuba)

Flux Osamu Tsukamoto (Okayama Univ.)

Soil Moisture Ichirow Kaihotsu (Hiroshima Univ.)

IRT Michiaki Sugita (Univ.Tsukuba)

Long term Planning

Ohata (Univ .Shiga Pref.)

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II. The AAN Data and Analysis Center (to be established) and secretariat of GAME

AAN WG (tentative)

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The Envrironmental Research Center, University of Tsukuba

2. Regional Counterpart (tentative)

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Russia: Russian Academy of Sciences

Mongolia: Institute of Meteorology and Hydrology (IMH),

Ministry of Nature and Environment

China: Chinese Academy of Sciences

China Meteorological Administration

Thailand: Thai Meteorological Department

Nepal: Department of Hydrology & Meteorology