Comparison of Long-term Downward Radiation Observations atTateno with JRA-25 and ERA-40
Nozomu Ohkawara1 and Yasuo Hirose1
1Ozone and Radiation Division, Aerological Observatory, Japan Meteorological Agency, Japanohkawara@met.kishou.go.jp, y-hirose@met.kishou.go.jp

INTRODUCTION
Aerological Observatory of Japan Meteorological Agency (Tateno) has been carrying out long-term observationof shortwave and longwave radiation at surface since 1957, International Geophysical Year (IGY). Tateno is oneof the stations of WCRP Baseline Surface Radiation Network (BSRN) (Ohmura et al., 1998) and has been
providing accurate radiation data to researchers from 1996 via the BSRN data archive. In recent years, several
long-term reanalyses were conducted to produce global homogeneous datasets of atmospheric physical parameters
for more than several decades. Among of them, JRA-25 and ERA-40 provide high quality datasets with hightemporal and spatial resolution (Onogi et al., 2007, Uppala et al., 2005). In these reanalyses, many atmosphericparameters such as surface air temperature and precipitation are well represented. In this study, the downward
shortwave and longwave radiation at surface of the reanalyses are compared with observations at Tateno over along period to assess their quality.
DATA1) Tateno observations
Radiation observation started in 1957 at Tateno and downward shortwave and longwave radiation have been
observed since then. For longwave radiation, the observation couldn’t be carried out in daytime or in case of rain
because of technical difficulty in instrument in early times. From 1976, all-day continuous observation has been
carrying out for longwave radiation. For shortwave radiation, continuous observations are available from 1957,
the start of radiation observation at Tateno. In the comparisons, the period from 1979 to 2004 are used for both
shortwave and longwave radiation in accordance with the time coverage of the reanalyses.

Table 1 Latitude and Longitude (°) of Tateno andused grids of ERA-40 and JRA-25

Figure 1 Location of Tateno and used gridsof JRA-25 and ERA-40

Name

Latitudelongitude

Tateno36.050° N

140.133° E

JRA-2535.327° N

139.500° EERA-40

37.500° N

140.000° E2) Reanalyses dataThe nearest land grid points from Tateno of JRA-25 and ERA-40 are used in the comparisons. Figure 1 andTable 1 show locations of Tateno and used grids of JRA-25 and ERA-40. The spatial resolutions are T106, whichis comparable to lat-lon grid with 110km interval, for JRA-25 and 2.5°x2.5° lat-lon grid for ERA-40. Used dataperiods are from 1979 to 2004 for JRA-25 and from 1979 to 2001 for ERA-40, respectively.
METHOD
The reanalyses data are compared with Tateno observations under two different sky conditions, all sky andclear sky condition.
In this study, clear sky is defined as 0 to 0.10 cloud coverage and we use 3-hourly eye observation for clouds atTateno, which is available from 06LT to 21LT, in detecting clear sky. In the shortwave radiation comparisons
under clear sky condition, the days of clear sky throughout the daytime (06, 09, 12, 15 and 18LT) are selected andannual mean daily clear sky value is compared. It is noted that clear sky days are mostly observed during thewinter season at Tateno. On the other hand, annual mean nighttime (21-03LT) value is compared for longwaveradiation. In this case, clear sky days are selected by using eye observation at 21LT. It should be noted that clear
sky radiation data at surface (element names: CSDSFsfc, CSDLFsfc) on the clear sky days are used for JRA-25,
while all sky radiation data at surface (element names: SSRDsfc, STRDsfc) on the clear sky days are used forERA-40. It is considered that ERA-40 data used in comparisons under clear sky condition contain some cloudeffects, so we cannot discuss in detail about the comparison results of ERA-40 under clear sky condition.
In the comparisons under all sky condition, annual mean daily values are compared for both shortwave andlongwave radiation. Used element names of the reanalyses are DSWRFsfc and DLWRFsfc for JRA-25, andSSRDsfc and STRDsfc for ERA-40.
COMPARISONS OF DOWNWARD SHORTWAVE RADIATION1) Conversion of reanalyses data
Before the comparison, the reanalyses data are converted to the value at Tateno according to the each gridcenter latitude, i.e. they are multiplied by the ratio between the daily total clear sky solar flux over Tateno andthose over the center of the grids.
2) Results
The comparison results for downward shortwave radiation are shown in Figure 2. JRA-25 and ERA-40 have thesame long-term trend under all sky and clear sky conditions. It is also shown that the reanalyses denote the sametendency as Tateno observations under the both conditions, in general. Under all sky condition, ERA-40 is wellaccorded with Tateno observations with very slight differences especially from the end of the 1980s under all skycondition. JRA-25 is well accorded with Tateno observations with a little overestimation (12.1W/m2) under clear
sky condition although it has rather a large overestimation (50.0W/m2) under all sky condition. In JRA-25,
subtropical marine stratocumulus off west coast is well represented by adopting a simple but competentstratocumulus parameterization (Kawai et al., 2006), but the result implies that JRA-25 underestimates cloudamount over Tateno.

(a)

(b)
250

200
150
100
50

Tateno Observations ( mean diff. )( 50.0 W/m2)
JRA-25
ERA-40( 8.4 W/m2)

0

Shortwave Radiation (W/m2)

Shortwave Radiation (W/m2)

250

200
150
10050

Tateno Observations ( mean diff. )( 12.1 W/m2)
JRA-25(-35.4 W/m2)
ERA-4001975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

Figure 2 Time series of annual mean downward shortwave radiation, in W/m2: (a) under all skycondition and (b) under clear sky condition. Mean differences (Reanalyses – Tateno Observations)for the whole comparison periods are also denoted.

COMPARISONS OF DOWNWARD LONGWAVE RADIATION
1) Bias estimation and correction of Tateno observations
The earlier types of instrument for longwave radiation have less quality and observations by these instruments
contain biases. Before the comparisons, Tateno observations in nighttime (hourly mean data at 12UTC) under clear
sky condition are compared with radiative transfer calculations and systematic biases due to instrument changes are
estimated and corrected. As radiative transfer model, MODTRAN (Kneizys et al. 1996) is adopted. For temperature,pressure and water vapor profile, significant level radiosonde data at Tateno at 12UTC are used in the radiativetransfer calculations. It is noted that the radiosonde data are used after applying bias correction according to results ofsimultaneous radiosonde measurements (World Meteorological Organization, 1996, The Aerological Division of JMA,1983). To take long-term variability of carbon dioxide into consideration, monthly mean carbon dioxide total column
amount in the mid latitude of the northern hemisphere (60° N-30° N), which are supplied by Atmospheric
Environment Division of JMA, are used in the calculations. We use default profiles in the radiative transfer model forother greenhouse gases. For aerosol, only background stratospheric aerosol is taken into account in the calculations.
Figure 3 shows differences of longwave radiation between Tateno observations and MODTRAN calculations. It isknown that the most recent type of instrument has good quality with uncertainty less than about 3W/m2 (Gröbner et al.,2007), so bias for each type of instrument is defined here as deviation from the most recent type of instrument(horizontal thick line in figure 3). To evaluate the bias correction, Tateno observations are compared with black bodyradiation emitted by a black body at surface air temperature (Figure 4). It is shown that Tateno observations are wellaccorded with blackbody radiation after the bias correction especially around 1980. From this result, it is confirmedthat the bias correction for longwave radiation works well.

Difference (Tateno - MODTRAN)(W/m2)

30
20
100-10Each data-20

Mean Diff. with Stdev. for each instrument type-30
1975

1980

1985

1990

1995

2000

2005

Longwave Radiation (W/m2)

Figure 3 Differences of longwave radiation between Tateno observations and MODTRAN
calculations, in W/m2. The vertical lines represent the transition of instrument, and the square andthe vertical bars represent the mean and the standard deviation for each instrument type,respectively. The horizontal thick line represents the mean difference of the most recent type ofinstrument.

400
Upward (Before Corr.)

380

Upward (After Corr.)

360

Blackbody Radation(Surf. Air Temp.)
Downward (Before Corr.)

340

Downward (After Corr.)

320
300
1975

1980

1985

1990

1995

2000

2005

Figure 4 Time series of 5-year running mean upward, downward longwave radiation and blackbody
radiation emitted by a blackbody with surface air temperature at Tateno, in W/m2. Thick lines
represent upward and longward radiation after applying the bias correction and thin lines represent
those without the correction, respectively.
2) Results
The comparison results for downward longwave radiation are shown in Figure 5. The two reanalyses have thesame long-term trend and tendency as Tateno observations for the whole period. The results also show that theboth reanalyses have underestimations against Tateno observations as seen in ERA-15, which is the previous
reanalysis at ECMWF, and other GCMs (Morcrette, 2001). JRA-25 seems to have a larger underestimation(-22.5W/m2) than that of ERA-40 (-9.0W/m2). To verify the assumption of the cloud amount underestimation inJRA-25 from the shortwave comparison results, we compare the differences between JRA-25 data and Tateno
observations under the two conditions (Figure 6). It is shown that the difference under all sky condition(-22.5W/m2) is greater than that under clear sky condition (-19.0W/m2). This result corresponds with theassumption of the cloud amount underestimation in JRA-25.

(b)
350
300
250
200150Tateno Observations ( mean diff. )JRA-25(-22.5 W/m2)( -9.0 W/m2)
ERA-40100

Longwave Radiation (W/m2)

Longwave Radiation (W/m2)

(a)

350
300
250
200
150

Tateno Observations ( mean diff. )JRA-25(-19.0 W/m2)
ERA-40( 4.6 W/m2)

100
1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

Difference (Tateno - JRA) (W/m2)

Figure 5 Time series of annual mean downward longwave radiation, in W/m2: (a) under all sky
condition and (b) under clear sky condition. Mean differences (Reanalyses – Tateno Observations)for the whole comparison periods are also denoted.

0-5-10( mean diff. )All Weather (-22.5 W/m2)
Clear Sky (-19.0 W/m2)-15-20-25-30
1975 1980 1985 1990 1995 2000 2005

Figure 6 Comparison between 5-year running mean difference (JRA-25 - Tateno) under all skycondition and that under clear sky condition, in W/m2. Mean differences (Reanalyses – Tateno
Observations) for the whole comparison periods are also denoted.
SUMMARY
ERA-40 and JRA-25 surface radiation data are compared with Tateno observations over a long period. The results
show that the reanalyses have the same long-term trend as Tateno observations in both shortwave and longwave
radiation and well represent realistic long-term trend of shortwave and longwave radiation at surface, in general. For
shortwave radiation, ERA-40 is well accorded with Tateno observations with little differences except before middle of
1980s under all sky condition. Comparisons of shortwave radiation under all sky and clear sky imply that JRA-25 mayunderestimate cloud amount. This assumption is consistent with the results of longwave radiation comparisons underall sky and clear sky conditions for JRA-25. There exist differences between Tateno observations and the reanalyses.
An overestimation for shortwave radiation under all sky condition in JRA-25 is mainly due to an underestimation ofcloud amount over Tateno. For longwave radiation, the both reanalyses have underestimations against observations as
seen in other many reanalyses and GCMs. The cause may radiation process including cloud parameterizations in themodel, and also atmospheric constituents which are not taken into account in the model such as aerosol.
ACKNOWLEDGEMENT
JRA-25 datasets used for this study are provided from the cooperative research project of the JRA-25 long-term
reanalysis by the Japan Meteorological Agency (JMA) and the Central Research Institute of Electric Power Industry(CRIEPI). We wish to thank ECMWF for providing ERA-40 datasets. Thanks to Atmospheric Environment Divisionof JMA for providing carbon dioxide data.
REFERENCES
Gröbner, J. and A. Los 2007: Laboratory calibration of pyrgeometers with known spectral responsivities, Appl.
Opt. 46, 7419-7425
Kawai, H. and T. Inoue 2006: A simple parameterization scheme for subtropical marine stratocumulus, SOLA, 2,17-20.
Kneizys, F. X., and Coauthors 1996: The MODTRAN 2/3 Report and LOWTRAN 7 Model. Phillips Laboratory,Geophysics Directorate, Hanscombe AFB, 261 pp.
Morcrette, J-J. 2001: The surface downward longwave radiation in the ECMWF forecast system. ECMWF
Technical Memorandum No.339, 34pp
Ohmura, A., E.G. Dutton, B. Forgan, C. Frohlich, H. Gilgen, H. Hegner, A. Heimo, G. Konig-Langlo, B. McArthur,G. Muller, R. Philipona, R. Pinker, C.H. Whitlock, K. Dehne, and M. Wild 1998: Baseline Surface Radiation
Network (BSRN/WCRP), a new precision radiometry for climate research. Bull. Amer. Meteor. Soc., 79, 2115 2136.
Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N. Yamazaki, H.
Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose, N. Mannoji and R. Taira 2007: The
JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, 369-432.
The Aerological Division of the Observation Department 1983: Comparisons of the RS 2-80 Type Rawinsondewith the RS II-56 Type One. Sokko - Jiho Japan Meteorological Agency, 50, 373-384 (in Japanese).
Uppala, S. M., P. W. K_allberg, A. J. Simmons, U. Andrae, V. daCosta Bechtold, M. Fiorino, J. K. Gibson, J.
Haseler, A.Hernandez, G. A. Kelly, X. Li, K. Onogi, S. Saarinen, N.Sokka, R. P. Allan, E. Andersson, K. Arpe, M.
A.Balmaseda, A. C. M. Beljaars, L. van de Berg, J. Bidlot, N.Bormann, S. Caires, F. Chevallier, A. Dethof, M.
Dragosavac, M. Fisher, M. Fuentes, S. Hagemann, E. H_olm,B. J. Hoskins, L. Isaksen, P. A. E. M. Janssen, R. Jenne,A. P.McNally, J.-F. Mahfouf, J-J. Morcrette, N. A. Rayner, R. W.Saunders, P. Simon, A. Sterl, K. E. Trenberth, A.
Untch, D.Vasiljevic, P. Viterbo and J. Woollen 2005: The ERA-40 reanalysis. Quart. J. Roy. Meteor. Soc., 131,2961-3012.
World Meteorological Organization 1996: WMO International Radiosonde Comparison — Phase IV: Final Report (S.
Yagi, A. Mita and N. Inoue). Instruments and Observing Methods Report No. 59, WMO/TD-No. 742, 130pp.