Japanese 55-year Reanalysis Research Articles

A STUDY ON SYNOPTIC SCALE ATMOSPHERIC STRUCTURE OF SUMMER HEAVY RAINFALL BY JAPANESE 25-YEAR RE-ANALYSIS DATA

A summer heavy rainfall event in Japan is a part of large-scale circulation field. But, heavy rainfall research has not been done from large-scale viewpoint because it is difficult to get high-integrity long-term data. In 2006, Japanese 25-year Re-analysis (JRA-25) data was begun to be published.In this study, by using JRA-25 data, trajectory analysis was carried out and anomalous field of atmospheric conditions about the 35 events in which heavy rainfall occurred in Japan in Baiu season was investigated. Some features of synoptic scale atmospheric structure of summer heavy rainfall was found out. ; intention of Pacific high, westward spread of Pacific high, intension of cyclone near Japan, intension of MA-3, increase of water vapor of moist tongue, pressure variation of Okhotsk Sea. When dry year events were analyzed, they show opposite tendency to the features.

The JRA-25 Reanalysis

A long-term global atmospheric reanalysis, named “Japanese 25-year Reanalysis (JRA-25)” was completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system. The analysis covers the period from 1979 to 2004. This is the first long-term reanalysis undertaken in Asia. JMA's latest numerical assimilation system, and specially collected observational data, were used to generate a consistent and high-quality reanalysis dataset designed for climate research and operational monitoring and forecasts. One of the many purposes of JRA-25 is to enhance the analysis to a high quality in the Asian region.Six-hourly data assimilation cycles were performed, producing 6-hourly atmospheric analysis and forecast fields of various physical variables. The global model used in JRA-25 has a spectral resolution of T106 (equivalent to a horizontal grid size of around 120 km) and 40 vertical layers with the top level at 0.4 hPa. In addition to conventional surface and upper air observations, atmospheric motion vector (AMV) wind retrieved from geostationary satellites, brightness temperature from TIROS Operational Vertical Sounder (TOVS), precipitable water retrieved from orbital satellite microwave radiometer radiance and other satellite data are assimilated with three-dimensional variational method (3D-Var). JMA produced daily sea surface temperature (SST), sea ice and three-dimensional ozone profiles for JRA-25. A new quality control method for TOVS data was developed and applied in advance.Many advantages have been found in the JRA-25 reanalysis. Predicted 6-hour global total precipitation distribution and amount are well reproduced both in space and time. The performance of the long time series of the global precipitation is the best among the other reanalyses, with few unrealistic variations from degraded satellite data contaminated by volcanic eruptions. Secondly, JRA-25 is the first reanalysis to assimilate wind profiles around tropical cyclones reconstructed from historical best track information; tropical cyclones were analyzed properly in all the global regions. Additionally, low-level cloud along the subtropical western coast of continents is well simulated and snow depth analysis is also of a good quality. The article also covers material which requires attention when using JRA-25.

Satellite Assessment of Divergent Water Vapor Transport from NCEP, ERA40, and JRA25 Reanalyses over the Asian Summer Monsoon Region

The divergent component of water vapor transports was constructed using evaporation, precipitation, and total precipitable water estimated from the Special Sensor Microwave Imager (SSM/I). The SSM/I moisture budget parameters were then compared with those from the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40), and the Japanese 25-year Reanalysis Project (JRA25) data over the Asian monsoon region for the May to September (MJJAS) period from 1988 to 2000.The climatology of SSM/I water vapor transports clearly indicates three major water vapor sources for the Asian monsoon, i.e., the subtropical Indian Ocean and Pacific Ocean in the Southern Hemisphere, and the North Pacific Ocean. In contrast, sinks are located in the Asian summer monsoon trough, the equatorial convective zones over the Indian and western Pacific Oceans, and over the East Asian monsoon region from the northern tip of Philippine Sea to the Kuroshio extension region. These sources and sinks are linked to well-known large-scale rotational circulation features, i.e., the cross-equatorial flow associated with monsoon circulation over the Indian Ocean, the anticyclonic circulation along the western periphery of the western North Pacific High, and the cross-equatorial flow north of New Guinea. In conjunction with the fluctuation of these sources and sinks, the northward propagating climatological intraseasonal oscillations in water vapor flux convergence are evident in the South and East Asian monsoon regions in the SSM/I data.From the comparison of water budget parameters of NCEP, ERA40, and JRA25 reanalysis with SSM/I-derived features, we found that the general features of all three reanalyses are in good agreement with those from SSM/I; however, the magnitudes of water vapor transports are comparatively weaker in all three reanalyses than what SSM/I measurements suggested. In addition, much weaker water vapor transports in three reanalyses are found in the intraseasonal oscillation signals with less distinct patterns, compared to what are inferred from the SSM/I measurements.