It is important to evaluate precisely whether observational data that include screen-level air temperatures could be affected by the environment around meteorological surface observation stations. It is well known that atmospheric radiation (downward long-wave radiation) from the atmosphere and clouds affects the temperature of the ground as well as observational air temperature data, but there are few stations that observe atmospheric radiation. Therefore, various formulas have been proposed and developed to estimate the atmospheric radiation under clear sky conditions that use air temperature and water vapor pressure; these are used in earth surface models to estimate average hourly thermal energy budgets in the planetary boundary layer. It is necessary to verify whether the formulas are applicable for recent data in Japan, because these formulas were developed with data collected at local observation stations during specific periods.
In this study, the accuracy of the familiar formulas used for estimation of diurnal atmospheric radiation under clear sky conditions was evaluated. Results from the formulas were compared with observational data from five stations, namely Sapporo, Tateno (Tsukuba), Fukuoka, Ishigaki Island, and Marcus Island, at which renovated solar and infrared radiation observations commenced on 31 March 2010. It was found that there were noticeable differences between observations and calculations as well as their seasonal variations. Therefore, the coefficients of Brutsaert (1975), which are comparatively theoretical, were adjusted to fit the regional meteorological conditions. The new Brutsaert-type formulas caused the differences and seasonal variations to disappear. Furthermore, in order to be applicable to various meteorological conditions including cloudy skies, the new formula for clear sky conditions was corrected by using sunshine duration and optical air mass. With these corrections, the average of differences between observations and calculations became close to zero.
A new calibration system of methane (CH4) standard gases by using a wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) analyzer was developed at the Japan Meteorological Agency (JMA) in collaboration with the Meteorological Research Institute. We used two sets of CH4 primary standard gases with mole fractions assigned based on the World Meteorological Organization (WMO) CH4 mole fraction scale maintained by the National Oceanic and Atmospheric Administration to test the performance of the new WS-CRDS calibration system. Our results showed high repeatability (0.06 nmol mol−1) and reproducibility (0.07 nmol mol−1) of measurements and good linearity against the WMO CH4 mole fraction scale. The CH4 calibration results for the new system agree well with those of the previous JMA calibration system, which employed a gas chromatograph with a flame ionization detector (GC/FID). These tests indicate that the new WS-CRDS CH4 calibration system at JMA will provide results that are consistent with those of the previous GC/FID system but with precision that is one order of magnitude higher. We also evaluated the stability and consistency of the JMA calibrations over the past 10 years by examining data from the World Calibration Centre (WCC) Round Robin comparison experiments in Asia and the regions in the southwest Pacific. The results of our study clearly demonstrate that the new calibration system will provide more precise CH4 measurements and improved traceability to the WMO scale of atmospheric CH4 measurements for the JMA/WCC comparisons.