Long-term variations of atmospheric transmittance from ... - GEWEX

Long-term variations of atmospheric
transmittance from pyrheliometer
measurements
Nozomu Ohkawara
Atmospheric Environment Division,
Global Environment and Marine Department,
Japan Meteorological Agency (JMA)
12 th BSRN Science and Review and Workshop, 1 Aug. 2012, Potsdam, Germany

Background
• The long-term variation of the transmittance was reported for North
America and for Europe
• The long-term change in the transmittance measured over Japan is
presented.
• The weight of aerosol in determining the variation in transmittance is
also analyzed.
How was it
change?
Martin Wild, Hans Gilgen, Andreas Roesch, Atsumu Ohmura, Charles N. Long,
Ellsworth G. Dutton, Bruce Forgan, Ain Kallis, Viivi Russak, Anatoly Tsvetkov (2005)

Direct solar radiation measurements in Japan
• JMA has been conducting direct solar radiation measurements by
pyrheliometers since International Polar Year (I.P.Y.) in 1932.
• Although the number of observation stations differs by age, the
measurements were conducted at 14 stations, which covered all
climatic zones in Japan, for a long time.
• The direct solar radiation was measured 3 times a day at 09, 12 and 15
Local Apparent Time (LAT) when there was no cloud in the direction of
the sun.
BSRN station
A map of 14 stations of direct solar radiation measurement in Japan used in this study.

Station list used in this study (14 stations)
Station
WMO
identifier
Start of
observation
Sapporo 47412 Aug.1932 -
Nemuro 47420 Sep.1953 -
Akita 47582 Aug.1932 -
Miyako 47585 Jun.1952 -
Wajima 47600 May 1953 -
Matsumoto 47618 Jan. 1936 -
Tateno 47646 Dec.1957 -
Yonago 47744 Feb.1940 -
Shionomisaki 47778 Aug.1932 -
Fukuoka 47807 Aug.1932 -
Kagoshima 47827 Apr.1940 -
Shimizu 47898 Aug.1932 -
Ishigakijima 47918 Jan.1941 -
Naha 47936 Sep.1932 -
BSRN station

Data rescue
Recently, the results of
direct solar radiation
measurements by
pyrheliomaters were
digitized, providing the
basis for the study of longterm
variations of
atmospheric transmittance
for more than 70 years.
An original record of pyrheliometer measurements
(at Fukuoka station in Aug. 1932)

Traceability to the international solar radiation scale
Before 1970
• Several Ångström pyrheliometers calibrated in Stockholm, Sweden
were imported and used as national standard pyrheliometers.
• Ångström scale 1905 was transfered to operational standard siver
disk pyrheliometers, and then scale was converted to Smithsonian
scale.
Smithsonian scale (JMA) = Ångström scale 1905 x 1.035
“The operational standard silver disk pyrheliometer used for
calibration of operational pyrheliometers in early times was
extremely stable and its sensitivity remained virtually unchanged
for more than 35 years since the start of the observation in 1932”
(Japan Meteorological Agency, 1975).

transfer the
pyrheliometric scale
Ångström pyrheliometer
(national standard )
Silver disk pyrheliometer
(operational standard & operational pyrheliometer)

1970-
JMA has been participating IPC to calibrate national(regional)
standard pyrheliometers against international scale since IPC-III in
1970.
IPC-III
1970
IPC-XI
2010

Used pyrheliometer & scale of solar radiation measuremnents
Period
Type of
pyrheliometer
Scale of solar radiation
measurements
Conversion factor to
WRR
1932- Silver disk
pyrheliometer
1957-1
970
Silver disk
pyrheliometer
Smithsonian scale (JMA)
(Ångström scale 1905 x1.035)
1.026 / 1.035
=0.9913
IPS-1956 (JMA)
1.026 / (1.035x0.98)
(Ångström scale 1905 x1.035x0.98) =1.012
1971-1
977
1978-1
980
Silver disk
pyrheliometer
Thermoelectric
pyrheliometer
1981- Thermoelectric
pyrheliometer
IPS-1956
IPS-1956
WRR
1.022
1.022
1.000
All of the measurements were converted in WRR, before calculating
the transmittance.
As for the solar constant, a fixed value of 1367 W/m 2 recommended
by World Meteorological Organization (WMO) was used.

Diurnal and seasonal variation of transmittance
Results
annual variations of monthly mean transmittance averaged among selected 7 stations.
Monthly mean transmittance values at each observation time (09, 12 and 15LAT) and
diurnal mean values averaged over the whole data period from 1932 to 2005 are
plotted.

• Diurnal variations between 09, 12 and 15LAT for each month are
smaller than the range of the annual variations of monthly mean
values.
• The difference in the transmittance among each observation time
within the same month is less than 0.02, while the difference between
the largest and smallest months is much larger and reaches 0.13.
• If a prolonged missing observations existed for several months, the
seasonal variations could influence the annual means. This tendency
must be taken into account for calculating the annual means.

Long-term variations of transmittance
Methods
To analyze the long-term variations of atmospheric transmittance,
the seasonal atmospheric transmittance was first calculated from
monthly mean for each year, so that each season has the same
weight in the annual mean value.
• spring: March-May
• summer: June-August
• fall: September-November
• winter: January and November-December
If there was any missing seasonal transmittance in a year, annual
mean would not calculated.

Results
• From 1933 to late 1940s, the transmittance remained stable at around
0.74 to 0.75.
• Decreasing phase followed to the mid-1980s when it reached 0.69.
• It then turned into be increasing phase till the early 2000s marking the
level of 0.71.
Long-term variations of annual mean transmittance at 14 stations in Japan.
A black thick line shows average among 14 stations.

Cause of long-term variations of transmittance
• Besides the large volcanic eruptions, the variation is caused
mostly by the aerosol and the precipitable water vapor (PWV) in
the atmosphere.
• Japan has a dense coverage of the rawinsonde observation
starting in 1955.
• The impact on the long-term variation of transmittance by that of
water vapor content in the atmosphere was estimated using
precipitable water vapor (PWV) from rawinsonde measurements.

Bias correction for rawinsonde measurements
During the period from 1957 to 2005,
three types of rawinsonde were used for
upper air observation.
The biases among them were reported as
the intercomparison results, beforehand
(Japan Meteorological Agency, 1983;
World Meteorological Organization,
1996).
pressure
humidity
temperature

Monthly mean precipitable water vapor from rawinsonde data
Monthly mean precipitable water vapor (PWV) was calculated from
rawinsonde measurements at 00 and 12 UTC.
It was confirmed bias correction for rawinsonde data worked well.
Annual mean precipitable water vapor calculated from rawinsonde
measurements at Tateno station.

Calculation of transmittance due to water vapor
Transmittance due to water vapor content in the atmosphere was
calculated from monthly mean PWV at 09, 12 and 15LAT on 15th day of
each month by Gueymard’s method (1998).
Then monthly mean and annual mean transmittance were calculated.

Results
• The variability of annual mean PWV is about 5 mm around the longterm
mean of 22.13mm on average among the stations during the
period from 1957 to 2005.
• The variation in PWV would cause the transmittance change of
only about 0.01.
Long-term variations of annual mean precipitable water vapor (PWV), transmittance
due to PWV (Tw) and total zenith transmittance (T) averaged among 7 stations in
Japan. PWV and Tw are plotted for the period from 1957 to 2005 while T is plotted
for the whole analyzed period.

Results (cont.)
• Main cause of the variation of transmittance is due to the direct
effect by aerosols in the atmosphere.
• Among all aerosols in the atmosphere, the contribution of natural
sources to the variation of the total aerosol concentration is very
small (Streets et al., 2009) leaving the possiblity to anthropogenic
aerosols.

Summary
• From 1933 to late 1940s, total transmittance remained stable
at around 0.74 to 0.75, followed by the decreasing phase to
the mid-1980s when it reached 0.69. It then turned into be
increasing phase till the early 2000s marking the level of
0.71.
• Both mean transmittance and its variable range are smaller
than those in Europe.
• The majority of these variations is caused by aerosol, and
water vapor changes making only a minor contribution.
• The aerosol changes are considered to be due to changes in
anthropogenic sources.

Danke!!