The Japan Meteorological Agency (JMA) conducted the second Japanese global atmospheric reanalysis, called the Japanese 55-year Reanalysis or JRA-55. It covers the period from 1958, when regular radiosonde observations began on a global basis. JRA-55 is the first comprehensive reanalysis that has covered the last half-century since the European Centre for Medium-Range Weather Forecasts 45-year Reanalysis (ERA-40), and is the first one to apply four-dimensional variational analysis to this period. The main objectives of JRA-55 were to address issues found in previous reanalyses and to produce a comprehensive atmospheric dataset suitable for studying multidecadal variability and climate change. This paper describes the observations, data assimilation system, and forecast model used to produce JRA-55 as well as the basic characteristics of the JRA-55 product. JRA-55 has been produced with the TL319 version of JMA’s operational data assimilation system as of December 2009, which was extensively improved since the Japanese 25-year Reanalysis (JRA-25). It also uses several newly available and improved past observations. The resulting reanalysis products are considerably better than the JRA-25 product. Two major problems of JRA-25 were a cold bias in the lower stratosphere, which has been diminished, and a dry bias in the Amazon basin, which has been mitigated. The temporal consistency of temperature analysis has also been considerably improved compared to previous reanalysis products. Our initial quality evaluation revealed problems such as a warm bias in the upper troposphere, large upward imbalance in the global mean net energy fluxes at the top of the atmosphere and at the surface, excessive precipitation over the tropics, and unrealistic trends in analyzed tropical cyclone strength. This paper also assesses the impacts of model biases and changes in the observing system, and mentions efforts to further investigate the representation of low-frequency variability and trends in JRA-55.
We investigated the horizontal resolution dependence of atmospheric radionuclide (Cs-137) simulations of the Fukushima nuclear accident on March 15, 2011. We used Eulerian and Lagrangian transport models with low- (15-km), medium- (3-km), and high- (500-m) resolutions; both models were driven by the same meteorological analysis that was prepared by our data assimilation system (NHM-LETKF) for each horizontal resolution. This preparation was necessary for the resolution-dependent investigation, excluding any interpolation or averaging of meteorological fields. In the results, the 15-km grid analysis could not reproduce Fukushima’s mountainous topography in detail, and consequently failed to depict a complex wind structure over mountains and valleys. In reality, the Cs-137 plume emitted from the Fukushima Daiichi Nuclear Power Plant (FDNPP) was mostly blocked by Mt. Azuma and other mountains along the Naka-dori valley after crossing over Abukuma Mountains on March 15, 2011. However, the 15-km grid simulations could not represent the blockage of the Cs-137 plume, which unnaturally spread through the Naka-dori valley. In contrast, the 3-km and 500-m grid simulations produced very similar Cs-137 concentrations and depositions, and successfully produced the plume blockage and deposition along the Naka-dori valley. In conclusion, low-resolution (15-km grid or greater) atmospheric models should be avoided for assessing the Fukushima nuclear accident when a regional analysis is needed. Meanwhile, it is reasonable to use 3-km grid models instead of 500-m grid models due to their similarities and the high computational burden of 500-m grid model simulations.
The calibration biases of reflectivity (ZH) and differential reflectivity (ZDR) from an operational Mt. Bisl S-band dual polarization radar are derived to improve the accuracy of rainfall estimation. The effect of radar calibration in rain estimation is examined by using data from the dense rain gauge network. The calibration biases of ZH are calculated by using the self-consistency constraint between ZH and specific differential phase shift (KDP). This procedure is performed every 2.5 min. The biases are varied from -3.3 dB to 0.8 dB during the period between July 2010 to October 2011. The ZDR calibration biases are obtained by two methods: 1) vertically pointing measurements, and 2) comparison of observed data with the average ZH-ZDR relationship derived from disdrometric data. The ZDR biases are varied from 0.25 dB to 0.7 dB and both methods show similar results. This ZH-ZDR technique can be applied for a volume scan and does not require a special scan. The rainfall relationships, R(ZH), R(ZH, ZDR) and R(ZH, ξDR), where ξDR = 100.1ZDR, are derived from measured disdrometer data and then adjusted with gauge data. The verification of rainfall estimation is performed by applying 1) average ZH and ZDR calibration biases for the entire period and 2) adaptive calibration biases that vary each rain event. The application of adaptive calibration biases is more effective for R(ZH, ZDR) and R(ZH, ξDR) than that for R(ZH), thus indicating the necessity of frequent calibration of ZH and ZDR.
The modulation of tropical cyclone (TC) genesis over the western North Pacific (WNP) by the intraseasonal variability (ISV) is investigated in this study. The two leading ISV modes, i.e., the 40-day Madden-Julian oscillation (MJO) and the 16-day quasi-biweekly oscillation, are found to exert significant impacts on TC genesis over the WNP. A majority of TC geneses over the WNP is found to occur during the period when both the modes are active, suggesting a joint influence of the two modes on TC genesis over the WNP. The modulation of TC genesis over the WNP by the two leading ISV modes can be well depicted by the genesis potential index (GPI). Contributions of the four terms to the total GPI anomalies are further analyzed to determine the key factors involved in modulations of TC genesis by both of the ISV modes. Results indicate that while, in general, the low-level absolute vorticity and the mid-level relative humidity are the two most important factors affecting WNP TC genesis, relative roles of the four GPI factors also tend to be dependent on the ISV phases. This study provides further understanding of the ISV modulation of WNP TC genesis, which could benefit the intraseasonal prediction of the TC activity.
This study investigates observed interannual changes in the Northern winter stratosphere with El Niño/Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data for 56 years. We focus on changes in occurrence of major stratospheric sudden warmings (MSSWs) as well as in seasonal mean states. Our results reveal complex changes in the MSSW probability with both ENSO and QBO as in the seasonal mean states. However, statistically significant changes at the 90 % confidence level are obtained only for some combinations of ENSO and QBO conditions reflecting the limitation of the data period. When the QBO is in a westerly phase, the MSSW probability increases with the ENSO sea-surface temperature condition in the eastern equatorial Pacific, i.e., from ENSO cold (La Niña), through neutral, to warm (El Niño) years. When the QBO is in an easterly phase, on the other hand, the probability significantly increases for La Niña years than for neutral years, whereas the probability is not significantly different between neutral and El Niño years. A characteristic feature is the high MSSW probability for the La Niña and QBO easterly winters, which is consistent with strengthened stationary wave with zonal wavenumber 1 compared to the climatology. These results suggest the importance of taking into account both ENSO and QBO factors, when one examines the frequency of MSSWs in the Northern winter stratosphere.
The Advanced Microwave Scanning Radiometer 2 (AMSR2) onboard the Global Change Observation Mission 1-Water (GCOM-W1) was launched by the Japan Aerospace Exploration Agency (JAXA) in May 2012. The AMSR2 is the follow-on model of the AMSR-Earth Observing System (AMSR-E) onboard the Aqua satellite. An assessment of the reliability of the soil moisture estimations from the newly launched passive sensor, the AMSR2, was carried out in this study, by using in situ soil moisture data from nine locations on the Korean peninsula during the period from July to October, 2012. The temporal patterns of the AMSR2 had a rough association with the in situ soil moisture measurements. However, there was intermittent striking of the AMSR2 data, in comparison to the in situ time series. For a clearer comparison between the variables, normalizing and filtering methods were applied to the AMSR2 soil moisture data with less systematic differences. The error estimation was based on triple collocation, and the AMSR2 data showed a larger error than the in situ and Global Land Data Assimilation System (GLDAS) soil moisture values. The spatial distributions of the monthly AMSR2 soil moisture were analyzed from the perspective of the corresponding reaction of the soil moisture to the spatial distributions of precipitation. The results provided an overview of the AMSR2 soil moisture product that is useful, despite being somewhat limited over the regions in northeast Asia. This study offers an insight into the applicability of the soil moisture products derived from the AMSR2 sensor. However, further studies are required for better understanding of the AMSR2 products for other areas of the validation task.
Diurnal variations of surface wind speeds during fair weather in the summer were revealed in central Japan, including data at Automated Meteorological Data Acquisition System (AMeDAS) and mountain station data above 2000 m above the mean sea level (a.s.l.) archived by an inter-university cooperative project, in relation to the altitude and concave-convex conditions around each station. AMeDAS stations belonging to Japan Meteorological Agency (JMA) are located below 1500 m, and most of them were categorized as being in concave topography with stronger daytime wind speed anomalies than in nighttime. At stations above 2000 m a.s.l. operated by each university, wind speed anomalies at night were stronger than those during the day except at the station without convex topography within a 1-5 km scale. Nocturnal enhancement of wind speeds at representative mountaintop stations appeared with prevailing Pacific Highs in synoptic pressure patterns, but it did not always appear in the same day and the absolute nocturnal wind speed varied day by day. The degree of concavity was not clearly related to the wind speed anomaly, and the degree of convexity was linearly related to the wind speed anomaly at a scale of approximately 10 km.
This study investigates the frequency of the occurrence of ice crystal habits in midlatitude cirrus clouds that were primarily associated with warm or stationary fronts within synoptic-scale lows. The measurements were performed with a balloonborne hydrometeor videosonde (HYVIS). The predominant types were single bullets at temperatures ranging from -60° to -20°C. Plate-type crystals were dominant at temperatures warmer than -20°C, whereas column or bullet rosette crystals became dominant at temperatures colder than -60°C. The distributions of the ice crystal habits derived from the HYVIS observations were consistent with the results of recent laboratory and field experiments, although the dependency of the ice crystal habits on ice supersaturation was not characterized in this study because only limited accurate humidity data were available under low temperature conditions. The size dependency of the distributions of the axis ratios of column and bullet crystals tended to decrease with increasing crystal dimension. We found no clear temperature dependency of the axis ratios of columnar crystal shapes. The area ratios of the six classified crystal habits were found to be comparable with previously reported ratios. The fact that the area ratio decreased with increasing dimension was apparent for all the crystal types. Polynomial curves fit to plots of area ratio versus maximum crystal dimension for each crystal type evidenced patterns different from those reported in previous studies. When the conventional power-law relationships between cross-sectional area and dimension reported in a previous study were applied to the HYVIS data, we found that the power-law relationships could overestimate the measured cross-sectional area integrated over the 250 m height interval between the cloud base and the cloud top by 10 %-80 %. The degree of overestimation was highly variable between individual cases.