GAME-jpのみなさま: 増田(地球フロンティア)です。
Water and Energy Balance Study (WEBS)の一環として、
地表面の放射フラックスの、
地上観測(AAN)と衛星観測によるグローバルデータとの比較をしています。
まだ始めたばかりですが、速報を書いてみました。
それぞれのstationの観測をなさったかたに個別に送って
チェックをお願いするべきところですが、
WEBSの活動紹介を兼ねて、メイリングリストにお送りします。
ただし、バイナリファイルの添付はご迷惑のかたもあると思いますので、
図(3枚、各約20キロバイトのPNG画像またはPS形式)は、
ご希望のかたにだけあらためて送ります。
この場を借りて恐縮ですが、特に各地点の観測をなさったかたには、
ぜひコメントをお願いいたします。
Comparison between station-based and satellite-based observations of
radiative fluxes at surface in monthly mean basis (draft)
20 Feb. 2003
Kooiti Masuda (Frontier Research System for Global Change, Yokohama, Japan)
== Introduction ==
GAME AAN (http://www.suiri.tsukuba.ac.jp/Project/aan/aan.html) provides
continuous monitoring of surface energy fluxes at various locations in Asia.
In order to know large-scale distribution of those fluxes, we have to
utilize either satellite observations or physical models.
Zhang and Rossow's FD dataset (http://isccp.giss.nasa.gov/flux.html) is
a product containing radiative fluxes at the surface (also at the
top-of-atmosphere) based on satellite observations. It is obtained by
calculation of radiative transfer making use of cloud information of ISCCP.
As an activity of GEWEX WEBS, monthly mean values of radiative fluxes at
surface in AAN and FD are being compared.
The spatial resolution of FD data is 2.5 degree latitude/longitude,
and representativeness of AAN stations for this spatial scale is very
questionable. Thus this is just comparison, not verification of either part.
Nevertheless, I try to explain discrepancies found from the comparison
(which may be turn out to be wrong later), in order to find better way
of utilization of surface and satellite-based observations.
Major field experiments of GAME consists of four regional experiments:
Siberia, HUBEX, Tibet and Tropics.
Here I introduce one cases each of Siberia, Tibet and Thailand.
AAN includes a station in HUBEX (Shouxian), but its data so far published
contain only few months where monthly averages can be discussed.)
== 1. Siberia ==
The station is Spasskaya Pad (62.25 deg. N, 129.62 deg. E) near Yakutsk.
Radiative fluxes are observed at 32 m above ground,
above the canopy of larch (deciduous needle-leaf) forest.
The FD grid box is 60 - 62.5 deg. N, 127.5 - 130 deg. E.
Comparison was possible from March 1998 to August 1999.
[There are some problems in the AAN data.
- Upward longwave flux suddenly shifted to the larger values at 06 UTC,
29 April 1999 and came back down at 10 UTC, 28 May 1999. The values
between these events are discarded in this comparison.
- There is no record of upward shortwave radiation since July 1998.
- Files for July to December 1998 were available in two formats,
and values of some components were different.
I prefer the files stored in a format other than AAN format
because they contain more valid values.]
* Agreement of downward shortwave radiation is good.
* The seasonal cycle of upward shortwave radiation is somewhat different.
FD shows sharp decrease of albedo at the time of snowmelt.
AAN shows similar values of upward shortwave radiation in spring and
summer, so relatively small changes in albedo.
The discrepancy is probably due to inhomogeneity of the land cover.
* Agreement of both downward and upward longwave radiation is good
except in winter.
In winter, both fluxes are larger in FD than in AAN.
I suspect that the temperature profile used for production of FD
did not capture strong temperature inversion in eastern Siberia.
== 2. Tibet ==
The station is Amdo (32 deg. 14 min. N, 91 deg. 38 min. E).
The landscape is flat grassland.
The FD grid box is 30 - 32.5 deg. N, 90 - 92.5 deg. E.
Comparison was possible from May to December 1998.
* Agreement of downward shortwave radiation is generally good
(with difference about 20 W/m^2).
* For upward shortwave radiation, FD shows smaller value than AAN
in July-December 1998, but the difference is within the range
of interannual variation of FD data. (FD values of 1996 are nearer
to AAN values of 1998 than FD values of 1998.)
* Agreement of both downward and upward longwave radiation is good
in winter, but not in summer.
In June to August, both fluxes are smaller in FD than AAN.
I suspect that the temperature profile used for production of FD
did not capture daytime warmth of the ground on the plateau.
== 3. Tropics ==
The station is Kog-Ma (18 deg. 45 min. N, 98 deg. 54 min. E, 1290 m
above sea level), near Chiang Mai, northern Thailand.
Radiative fluxes are observed at 50.5 m (43.4 m for upward shortwave)
above ground, above the canopy of evergreen forest.
The FD grid box is 17.5 - 20 deg. N, 97.5 - 100 deg. E.
Comparison was possible for the whole years of 1998 and 1999.
Also, downward shortwave radiation observed at agrometeorological stations
at Mae Cho near Chiang Mai (18 deg. 55 min. N, 99 deg. E) and
Lampang (18 deg. 18 min. N, 99 deg. 17 min. E), as part of GAME-Tropics
activity, are included in the comparison.
* For downward shortwave radiation, AAN shows smaller value than FD,
often by 50 W/m^2.
Agromet stations show values closer to FD, but they also show
smaller values than FD in late dry (hot) seasons of March-April.
I suspect that the cause is regional inhomogeneity of cloudiness.
* Agreement of albedo seems to be good.
* Both downward and upward longwave radiation between AAN is FD
are similar in general, but month-to-month time series look different.
FD data have occasional "troughs" in curves of both fluxes in August
1999 and July 1998, which do not appear in AAN.
If the large variation of FD is representative of the grid box,
the AAN station may be a place exceptionally insensitive to large-scale
weather condition.
== List of acronyms ==
AAN: GAME Asian Automatic Weather Station Network
GAME: GEWEX Asian Monsoon Experiment
GEWEX: Global Energy and Water Cycle Experiment
HUBEX: Huaihe river basin Experiment
ISCCP: International Satellite Cloud Climatology Project
WEBS: Water and Energy Balance Study
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増田 耕一 (MASUDA Kooiti)
地球フロンティア研究システム 水循環領域
236-0001 横浜市 金沢区 昭和町 3173-25
e-mail masuda.kooiti@nasda.go.jp, masuda@jamstec.go.jp
電話 045-778-5538 (直通), 045-778-5700 (代表)
Fax 045-778-5706 (3階共用)
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