2.2 GAME-Radiation Activity

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2.2.1 Introduction

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The redistribution of radiative energy is the main mechanism for the formation of the global climate (Hansen et al., 1981; Shi et al., 1994). This mechanism of energy redistribution is especially complicated in the Asian monsoon region with the monsoon circulation caused by the ocean-land contrast at the continental scale. There are numerical simulations, for example, indicating that upper and lower level clouds play large but different roles in the energy flow on a continental scale. The effect of global warming due to greenhouse gas increase is especially large and sensitive to the radiative budget structure over land areas. Therefore, to understand the climate formation of this region, it is important to study the effect of clouds, water vapor and surface conditions on the radiative budget of this region.

Our knowledge is, however, not accurate enough for the modeling of the above mentioned processes. For example, there is a large uncertainty in the estimation of the globally averaged surface radiation budget, more than 20 Wm-2 even with the state of art modeling of the radiation processes (Stephens and Tsay, 1990; Li et al., 1995). This indicates that the diabatic heating may be poorly modeled in numerical climate models. Therefore, extensive studies of radiation processes, including the solution of the anomalous absorption problem, are indispensable for improving the climate modeling. It is especially important to study the effects of clouds, water vapor and surface conditions upon the radiation budget through extensive monitoring with satellites and ground-based radiometers.

Anthropogenic aerosols are another important factor affecting the radiation budget in the Asian monsoon region. Numerical simulations have shown that an introduction of a large loading of anthropogenic aerosols in this region improves the large scale simulation of temperature, precipitation and energy flux distributions (Mitchell et al., 1995; Santer et al., 1996). The radiative forcing of anthropogenic aerosols has been estimated to be comparable with the radiative forcing due to a greenhouse gas increase. The globally averaged direct radiative forcing of aerosols is estimated as ?0.3 to ?0.9 Wm-2 (Charlson et al, 1992; Taylor and Penner, 1994), and the indirect forcing as about ?1.3 Wm-2 (Jones et al., 1994). There is, however, a large uncertainty in the estimation of the forcing, especially in their indirect forcing evaluation. The sign of the indirect forcing will even become positive in the case of continental convective cloud systems (Kaufman and Nakajima, 1993). This large uncertainty comes fr

The above discussion indicates that it is important to have an extensive study of the radiation budget and of the various factors affecting the radiation field in the Asian monsoon region. We thus set the major science objectives of the GAME Radiation Activity as follows:

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(1) To understand the surface radiation budget distribution over the Asian monsoon region.

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(2) To understand the role of clouds, aerosols, water vapor, and surface conditions in determining the radiation budget regime of the earth-atmosphere system in the region.

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(3) To establish satellite remote sensing techniques for estimating surface radiation budget and optical properties of atmospheres and surfaces.

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Since these activities have to be performed at a continental scale, it is essential to have close collaboration with the ongoing international radiation programs, such as WCRP/International Satellite Cloud Climate Project (ISCCP), Baseline Surface Radiation Network (BSRN) Project, Surface Radiation Budget (SRB) Project, WMO/Global Atmospheric Watch (GAW) System, and others.

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2.2.2 A plan for the GAME-Radiation activity

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As discussed in the preceding section, accurate measurements of solar radiative fluxes and thermal radiative fluxes at the surface in the Asian monsoon region should be performed to understand the large scale climate formation of this region. To attain this objective, the following strategy will be adopted:

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(1) Establish high-accuracy radiation sites to measure the surface radiation budget,

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(2) collect existing radiative flux data,

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(3) calibrate the existing data with data from the high-accuracy radiation sites,

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(4) compare and analyze these flux data with satellite data to obtain the surface radiation budget distribution over the Asian monsoon region,

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(5) and study the effects of clouds, aerosols, water vapor and surface condition upon the obtained radiation budget distribution.

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A. Surface radiation measurements

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Data archive of the following radiative and related quantities in the Asian region is important to accomplish the objectives of the Activity.

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(1) Sunshine duration

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This is the most well measured quantity in the Asian region. We will use the existing archived data for our study.

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(2) Global, direct and diffuse solar radiative fluxes

These quantities can be measured by pyranometer and/or pyrheliometer. We will adopt the BSRN method to make accurate measurements of the global solar radiative flux, i.e., adding direct flux from pyrheliometer and diffuse flux from pyranometer with a shadow disk to block the direct radiation. It is also important to collect the existing data of the global solar radiative flux, since this quantity has been measured in many sites in the region without systematic archiving.

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(3) Longwave radiative flux

It is important to measure the longwave radiative flux for estimating the effect of cloud and water vapor on the radiation budget. The establishment of longwave flux sites is strongly encouraged, since this quantity has been measured at very few sites in the Asian region.

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(4) Atmospheric turbidity

This quantity can be an index of aerosol loading, and be used to correct retrievals of global solar radiative flux from sunshine duration data. Atmospheric turbidity can be measured using a sunphotometer or a sky radiometer. We will make an effort to establish a sky radiometer network, since this instrument can measure the atmospheric turbidity and related aerosol microphysical quantities with better accuracy than sunphotometry for long term measurements (Nakajima et al., 1996).

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(6) Ancillary data

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Vertical profiles of temperature and water vapor will be obtained from the nearest sonde station. Column water vapor in clear sky conditions can be obtained by the sky radiometer. Ozone amount will be obtained from satellite measurements. Those quantities are needed to calculate radiative fluxes to compare with measured values.

There are two BSRN sites in the Asian region operated at Tsukuba, Japan and at Wudaoliang, China. Adding two to three BSRN type sites will improve the coverage of high accuracy radiation measurement network in the region. The GAME-Radiation Activity will establish GAME high-accuracy radiation sites for the accurate measurement of radiative fluxes and related ancillary quantities with the following equipment (Fig. 2.2-1):

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? Upward and down looking pyranometers

? Upward and down looking pyrgeometers

? A Pyrheliometer with an automatic solar tracking system

? A sky radiometer

? A Mie lidar

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We will make 1 to 5 minute samplings of instantaneous, integrated and dispersion of the corresponding quantities. The sites should be near sonde stations. To achieve the accuracy shown in Table 2.2-1, the pyranometers and pyrgeometers should be ventilated; upward looking pyranometer and pyrgeometer should be blocked from the direct solar radiation by a shadowing mechanism.

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Table 2.2-1 Anticipated accuracy of radiation measurements.

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Quantities Accuracy (Wm-2)

Downward shortwave flux 10

Upward shortwave flux 10

Downward longwave flux 15

Upward longwave flux 15

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A lidar system will be introduced to monitor the aerosol profile, cloud base height, and cirrus cloud structure. The latter two monitorings are important to connect the surface data to satellite-received longwave radiation data. A sky radiometer (PREDE/SKR-01L) will be introduced to obtain aerosol optical properties, column water vapor and ozone amounts. A similar radiometer network has been developed by NASA/AERONET project (Holben et al., 1996).

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B. Satellite remote sensing

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Retrievals of the surface radiation budget and optical properties of the atmosphere and at the surface have not been studied fully over land surface area. The GAME-Radiation Activity is, therefore, concerned with comprehensive satellite data analyses to produce the large scale radiative budget field. The following parameters are planned to be retrieved for the Activity:

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(1) Surface shortwave and longwave radiative fluxes: The quantities will be obtained from GMS radiances for every 1‘ί1 degree box over the GAME area. Retrieved values will be compared and validated with ground-based measurements.

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(2) Cloud microphysical parameters: Optical thickness and effective particle radius will be retrieved from AVHRR over the GAME area (Nakajima and Nakajima, 1995). It is important to investigate the radiative effects of a reduction of the effective cloud particle radius over continental areas that is observed by Han et al. (1992). Partitioning of shortwave radiation absorption into visible and near-infrared spectral regions is important for studying the cause of anomalous absorption (Hayasaka et al., 1995). Further studies of clouds, water vapor and precipitation will be needed from visible-infrared sensors and microwave sensors, such as AVHRR, SSM/I and SSM/T2. A detailed validation and process studies will be performed with combined satellite and ground-based data sets over the GAME area.

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(3) Aerosol optical properties: It is found that the ͺ©ngst«Σm exponent derived from a two channel method of AVHRR significantly increases over a large areas of several hundred kilometers near large cities (Higurashi and Nakajima, 1996). This observation suggests that aerosols generated from gases emitted from a large city are significant even on global scale. It is important to estimate land aerosols by combined analyses of AVHRR, OCTS and TOMS data.

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2.2.3 Implementation plan

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A. Schedule

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Two to three GAME high accuracy radiation sites will be established in the GAME period between 1997 and 2001. For 1997, we have selected the following sites:

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? Thailand: Si Samrong agrometeorological station (16.9N, 99.8E). This site is close to the GAME-T measurement site and inside the GAME-T sonde network. The radiative effect of large biomass burning in dry season is one of the important phenomena to study. We will locate a Mie lidar at this site.

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? China: Shou-Xian meteorological observatory (32.6N, 116.8E). This site is one of the HUBEX measurement sites and is located in a horizontally flat area with a relatively small city effect. The instruments are the same as those of the Si Samrong site without a lidar system. There are lidars in the Anhui Institute of Optics and Fine Mechanics, which is located about 100km south-east of Shou-Xian site. The site is inside the HUBEX sonde network.

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We will deploy our instruments, other than the lidar system, around May 1997 after testing the system at Tohoku University. It should be noted that the existing radio sonde sites are not suitable for GAME radiation sites, since those sonde sites are located in relatively large cities and are heavily affected by city air pollution. This problem should be more adequately addressed to make a good radio sonde network for climate studies.

It will be important to exchange information with the Chinese BSRN site in Wudaoliang and the Chinese GAW site in Waliguan.

The GAME-Radiation Activity will make an effort to collect existing radiation data over China for studying the continental scale radiation budget over China. It is also important to collect related data useful for the radiation budget analyses such as meteorological data and cloud base height data. Sites and period of data collection will be decided by the China GEWEX National sub-Committee for GAME-Radiation taking into consideration that data from the first class radiation sites from April of 1997 to March of 2001 are the first priority. The Chinese radiation data sets to be exchanged will be subject to approval of the appropriate Chinese authorities.

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Measurements will also be performed in close collaboration with HUBEX and SCSMEX.

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The schedule for the GAME Radiation Activity is summarized as follows:

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JFY Description of milestones

1995 ? Implementation plan development

? Radiation site system design

? GCM radiation code development

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1996 ? Radiometer system evaluation

? Selection of measurement sites (2 sites)

? Development of satellite retrieval algorithms for surface radiation

budget, clouds, aerosols, water vapor, and precipitation

1997 ? Establishment of GAME radiation sites

? Start data accumulations (ground data, satellite data)

? Start calibration activities

? Start application of satellite retrieval algorithms

? Start large scale radiation budget modeling with GCM

1998 ? Start data set construction and comprehensive analyses

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1999 ? Continue efforts of all the components

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2000 ? Summarize the results

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B. Organizations

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B.1 Participating Organizations

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The GAME-Radiation activity is performed by the following organizations:

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China:

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? National Climate Center

? Institute of Atmospheric Physics

? Anhui Institute of Optics and Fine Mechanics

? Lanzhou Institute of Plateau Atmospheric Physics

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Japan:

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? Center for Climate System Research, University of Tokyo

? Center for Atmospheric and Oceanic Studies, Tohoku University

? Center for Environmental Remote Sensing, Chiba University

? National Space Development Agency of Japan

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Thailand:

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National Research Council of Thailand

Thai Meteorological Department

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B.2 Cooperative Organizations

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The following organizations have expressed their interest in supporting and/or cooperating with the GAME-Radiation activity:

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? Japan Meteorological Agency

? Asia Pacific Network

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