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.

[1] Concentrations of homologous (C 2 -C 11 ) dicarboxylic acids were measured in the atmosphere over Chichi-jima Island (27°04 0 N, 142°13 0 E), western North Pacific, from April 1990 to November 1993. The monthly averaged concentrations of straight-chain C 2 -C 7 dicarboxylic acids, and some branched, keto-, and unsaturated dicarboxylic acids showed a maximum in winter to spring and a minimum in the summer season. In contrast, straight-chain C 8 -C 11 dicarboxylic acids showed an opposite trend with a maximum in spring to summer. Principal component analysis revealed that there is a source characterized by a strong contribution of C 2 -C 7 dicarboxylic acids, phthalic acid, some keto-dicarboxylic acids, and unsaturated-dicarboxylic acids, whereas there seems to be another source(s) for the C 8 -C 11 dicarboxylic acids. A combined model of backward trajectories and emission inventories was used to better understand the sources and source regions of the dicarboxylic acids. The results show that concentrations of various C 2 -C 7 dicarboxylic acids in the western North Pacific atmosphere are largely controlled by their emissions and/or their precursor emissions from anthropogenic sources in the Asian continent, followed by long-range atmospheric transport. On the other hand, longer-chain dicarboxylic acids cannot be explained by long-range atmospheric transport from a continental region, which implies emissions and/or formation from natural sources, possibly from the ocean.

The structures of temperature and velocity fields in the tropical tropopause layer (TTL) in boreal winter are investigated using an atmospheric general circulation model (AGCM). The model reveals strong upward motions in the lower part of the TTL over the maritime continent and the western tropical Pacific, corresponding to the “stratospheric fountain” region, and downward motions in the upper part of the TTL over Indonesia, representing the stratospheric drain. In the TTL, strong easterlies prevail, and the cold ascent region tilts eastward. A down‐slope flow over the upward‐bulging isentropic surface produces the downward p velocity over Indonesia. In addition, reduction of longwave heating over deep convection suppresses the upward motion. The model simulates the observed stratospheric drain signature well, without convective overshootings. A trajectory analysis using the AGCM‐simulated three‐dimensional wind and temperature is performed to clarify the entry process of air parcels from the tropical troposphere to the stratosphere and to investigate the dehydration process during passage through the TTL. Tropospheric air parcels are advected upward to the bottom of the TTL mainly from the stratospheric fountain region. A pair of anticyclonic circulations in the tropical western Pacific entrains air parcels, which then pass through the equatorial cold region several times during the slow ascent in the TTL. This slow spirally ascending motion brings about low humidity in the stratosphere, despite the local downward motion over Indonesia. In addition, transient disturbances, particularly low‐frequency disturbances, produce intermittent upward motions over the fountain region, resulting in effective dehydration of the air. The spiral ascent and transient mechanisms are key factors in the dehydration process in the TTL. The interannual variation in the water vapor mixing ratio into the tropical lower stratosphere with the El Niño/Southern Oscillation cycle is also estimated, and it is found that in La Niña years, air is more dehydrated.

SUMMARYThe progress and status of a new atmospheric re-analysis, JRA-25 the Japanese 25-year Re-analysis which covers the 26 years from 1979 to 2004, are introduced. Observational data include some newly produced for JRA-25 and advantages and drawbacks of performance are briefly described. JRA-25 has many advantages such as its handling of precipitation amounts, tropical cyclone analysis, and the extent of low-level cloud along western continents that are among the best compared to other re-analyses. The snow analysis is also good and stable. JRA-25 outputs analysis products every 6 hours.

A new long-term reanalysis project, JRA-25, has set as one of its main goals the realistic representation of tropical cyclones (TCs) in the reanalysis. To supplement in situ observations near TCs, wind profile data (TCR data) are retrieved based on best track information and assimilated. This paper addresses the benefits of using TCR data for the analysis of TCs.TC representation in the JRA-25 reanalysis is compared with other reanalysis data, and an experimental reanalysis (Control) without TCR data. The general result is that JRA-25 successfully represents the location and intensity of each TC. Among TC basins in the Northern Hemisphere, the TC representation in the JRA-25 data in the eastern North Pacific, and in the tropical Atlantic, where upper observations are sparse, is superior compared to other datasets that did not assimilate TCR data. In the western North Pacific, especially around Japan and the East China Sea, TCR data give little improvement in TC representation, since conventional upper observations are dense there and upstream of the active TC regions.TC detection rates in the Northern basins and over the entire globe, are computed using an objective procedure from the reanalysis datasets. The rate in JRA-25 is the highest in all basins among the datasets, and consistent through the period. The high qualified TC representation in intensity and location has a positive effect on a flow field, and hydrologic cycle around TCs. Case studies of TC track forecasts for a recurving TC, in which reanalysis data are used as initial conditions, suggest the JRA-25 data are more realistic than the Control. For these reasons, TCR data is effective in representing TCs and surrounding atmospheric conditions in JRA-25.

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The possible impact of the winter monsoon intensity and sea surface temperature (SST) gradient on the activity of explosively developing extratropical cyclones around Japan is investigated using the Weather Research Forecasting model. Two independent long-term integrations over 18 winters from 1993/94 to 2010/2011 are conducted using prescribed observed SST data (OS run) and spatially smoothed data (SS run). The OS run is successful in reproducing the spatial distributions of the explosive cyclone activity in the vicinity of Japan under both strong and weak winter monsoon conditions. Under strong monsoon conditions, the Kuroshio, the Kuroshio Extension, and the Japan Sea subpolar fronts give rise to enhanced near-surface baroclinicity through the increase in heat and moisture fluxes from the ocean surface, resulting in frequent occurrence of the explosive cyclone activity along those fronts.

Abstract.Interannual