In order to study temporal and spatial variations of atmospheric CH4 quantitatively, we originally improved a measurement system for carbon and hydrogen isotopic ratios (δ13C and δD) of CH4 to attain high-precision measurements. By analyzing 100 mL aliquots of an ambient air sample, the precision of our system is 0.080‰ for δ13C and 2.20‰ for δD(1σ), which are one of the highest precisions reported so far. The system consists mainly of aCH4 preconcentration device and a continuous-flow gas chromatograph isotope ratio mass spectrometer equipped with a combustion furnace and a pyrolysis furnace for measurements of δ13C and δD. The preconcentration trap temperature was maintained at -130 ± 1°C during collection of CH4 from the air sample by passing it through the trap, then at -83 ± 1°C while remaining air components such as N2 and O2 except for CH4 escaped, and finally at 100 ± 1°C for CH4 elusion. The isotopic values are measured on a mass spectrometer, relative to respective reference gases. For this study, the δ13C and δD values of the reference gases were calibrated against our primary standards provided by the IAEA: our δ13C primary standard is NBS18, whereas our δD primary standards are V-SMOW and SLAP. To ensure the long-term stability and reproducibility of our measurement system, a calibrated whole air stored in a high-pressure cylinder, which was called “test gas,” was measured at least twice on each day when sample measurements were made. To measure small air samples, such as those extracted from ice cores, we also examined the relation between the sample size and the measured value of δ13C and δD: gradual enrichment of the δ13C occurred with decreasing CH4 content less than 8 nmol whereas no such effect could be seen for the δD. Furthermore, preliminary results of latitudinal distributions of δ13C and δdD were discussed along with CH4 concentrations obtained by our shipboard air-sampling program.
This paper describes and evaluates the performance of a regional climate model developed at the State Key Laboratory of Numerical Modeling for Atmospherics Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP). The model has been developed based on the numerical forecast model, Advanced Regional Eta-coordinate Model (AREM), of LASG/IAP. An advanced radiation package and a common land surface scheme have been included into the AREM. The new model is regarded as the Climate version of AREM and named CREM. To evaluate its performance in reproducing the summer climate over eastern China, the CREM with 37 km horizontal resolution was integrated from 1 May to 1 September of 1995-2004. The lateral boundary forcing data is derived from the NCEP-DOE (National Centers for Environmental Prediction-Department of Energy) reanalysis data at 6-h intervals. Evaluations on the model performance indicate that the CREM can reasonably reproduce the spatial distributions of climatological monthly (June, July, and August) mean precipitation and circulation over eastern China. The simulated monthly mean precipitation has high spatial correlation with the observation over central-eastern China, except with larger variability. Compared with the precipitation, both the spatial distribution and the spatial variability of geopotential height at 500-hPa and the zonal wind at 200-hPa are better simulated. The interannual variation of precipitation anomalies over central-eastern China is well captured by the model. The variation of the Meiyu front is realistically depicted except that the simulated rainbelt shifts northward in July. A case study of the 1998 flooding year suggests that the model can reproduce not only the seasonal mean precipitation and circulation, but also the temporal evolution of the rainfall and monsoon system over eastern China. In particular, the two northward-propagating intra-seasonal oscillation events are successfully captured in the simulation. The main deficiency of CREM is the overestimation of rainfall amount and the northward shift of the rainbelt, which probably results from the high frequency of rainstorms and exaggerated thermal contrast between central-eastern China and southern China.
A series of numerical experiments are conducted to examine the sensitivity of the numerical simulation of Hurricane Emily’s (2005) early rapid intensification to the cumulus parameterization schemes in the advanced research version of Weather Research and Forecasting (WRF) model at different horizontal resolutions. Results indicate that the numerical simulations are very sensitive to the choices of cumulus schemes at 9 km grid spacings. Specifically, with different cumulus schemes, the simulated minimum central sea level pressure (SLP) varies by 41 hPa during the 54 h forecast period. In contrast, only about 10 hPa difference is produced in minimum central SLP by varying planetary boundary layer (PBL) parameterization schemes in the same simulation period. Physical and dynamic mechanisms associated with this sensitivity are investigated. It is found that the intensity of the simulated storm depends highly on the magnitude and structure of surface latent heat flux and convective heating rate over the storm eyewall. The use of cumulus schemes is helpful for the model to reproduce those favorable conditions that cause the storm deepening. However, at 3 km resolution, the cumulus schemes do not result in any notable difference in the storm intensity and track forecasts. Only a slight difference is found in the simulated storm precipitation structure. Compared with cumulus schemes, the PBL schemes have significant impacts on Emily’s intensity forecast at 3 km resolution; the minimum central SLP varies by 37 hPa with the use of a different PBL scheme in the WRF model.
This paper has investigated the mesoscale structure and evolution of a Meiyu/Baiu front and precipitation along the front observed in the downstream region of the Yangtze River on 21 June 2002 by using data from intensive observations of upper-air, surface, and five Doppler radars, as well as GMS IR and GANAL re-analysis data. It is found that the front collocated with a large-scale wind shear line. The frontal zone was characterized by a subsynoptic-scale low-level jet (LLJ) to the south and a thermally direct circulation in the middle troposphere south of the surface front. The front evolved from an inactive front with little convection along it to an intensive one triggering a strong meso-α-scale rainband. The front initially intensified mainly in association with the divergence related to the evaporative cooling of precipitation systems north of the front and further developed when strong convection evolved along the front. The meso-α-scale rainband triggered by the front was composed of several meso-β-scale convective systems. Meso-β-scale convective systems were narrow and consolidated in the western part, but wide and weak in the eastern part of the downstream region of the Yangtze River. Three-dimensional kinematic and reflectivity structures of two meso-β-scale convective systems, where one was in the western part and the other was in the eastern part, have been examined comprehensively. In the western part, the convective system evolved in a quasi-steady state and was characterized by a deep and strong convective cell just north of the front and limited stratiform precipitation further north of the front. The orientation of the LLJ to the front was at a sharp angle. The primary updraft triggered by the front sloped largely northward in the lower troposphere and became nearly upright and strong from the middle troposphere. In the eastern part, on the other hand, the convective system changed remarkably with time and was featured by multiple shallow and weak convective cells across the front and extended stratiform precipitation both south and north of convective cells. The LLJ that oriented nearly normal to the front overran the front at lower levels and penetrated far to the north of the front. It appears that the variable structure and evolution of the LLJ would have a great impact on the development of distinct convective systems in the downstream region of the Yangtze River. The mesoscale along-frontal variability of the Meiyu/Baiu frontal zone appears to be responsible for distinct modes of convective organization along the front in a limited distance.
In the 1998 monsoon season, precipitation from midnight through early morning was frequently observed in the central Tibetan Plateau (TP). It occurred on several successive days and tended to be accompanied by low-level easterly winds. Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis data also showed easterly component winds as a part of anti-cyclonic circulations in the northeastern plateau systematically occurring with a prevailing synoptic scale trough at around 100°Einthe mid-latitudes. A numerical simulation by the Weather Research and Forecasting (WRF) model with a resolution of 15 km reproduced the development of a convergence flow over the central TP, and corresponding zonal cloud formation was confirmed by METEOSAT-IR images. The simulation results indicated that the precipitation system was developed through with dissipating nighttime low-level convective instability by low-level convergence flows, and sonde observations confirmed the existence of an instability layer. Sensitivity experiments revealed that the convergence pattern over the central TP was produced by the dynamical stagnation effect of the plateau topography to the northwesterly general flows from the mid-latitudes. Many former studies have explained nocturnal precipitation in monsoon Asia as a part of an orographically induced diurnal variation. However, this study characterized the nighttime precipitation over the central TP due to synoptic-scale convergence. Two reasons are given to explain the substantial precipitation enhancement in the night. One is the daytime thermal convections that depend strongly on the topography, which dissipate synoptic-scale convergences zone, and the other is the low-level moistening after the maturing of the monsoon season to increase nighttime convective instability.
Reproducibility of land-surface air temperatures and land precipitation in the twentieth century by an atmospheric general circulation model (AGCM) MJ98 was investigated focusing on long term trends and year-to-year variability. The MJ98 model was jointly developed by the Meteorological Research Institute (MRI) and the Japan Meteorological Agency (JMA), and has a 270-km horizontal grid spacing (T42) with 30 vertical levels. Forcing the MJ98 model with observed historical sea surface temperatures (SST) and observed historical CO2 concentrations, six-member ensemble integrations were conducted for 130 years from 1872 to 2001. Simulated land-surface air temperature and land precipitation were validated against observational data of the Climate Research Unit (CRU) from 1872 to 2001 and from 1951 to 1997, respectively. The model reproduces the observed positive trend of annual mean global average land-surface air temperature as well as decadal variability and year-to-year variability. The model simulates the observed positive trend of global average land-surface air temperature for all four seasons and the annual mean, though the magnitude is underestimated. The seasonality of the simulated trend is weak compared with that of the observation. At each grid point, the model generally reproduces positive trends of annual mean temperature over land. However, the simulated trends are underestimated especially over the middle and higher latitudes of the Northern Hemisphere, which can be partly attributed to the inability of model to simulate the increasing boreal wintertime trend of the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) over the last few decades. The model's ability to reproduce the year-to-year variability of the annual mean temperature is relatively higher in coastal regions than in inland regions. In the case of annual mean land precipitation, the model simulates the observed negative trend for the global average, its negative trend for the Northern Hemisphere, and its positive trend for the Southern Hemisphere, although these observed trends are not statistically significant. The model fails to reproduce year-to-year variability. The model generally reproduces the distribution of trend of global annual mean land precipitation, but large discrepancies between observation and simulation are found over Asia, Australia and southern Africa.
Five misocyclones occurring along a narrow cold frontal rainband (NCFR) were detected by single, X-band Doppler radar at Yokosuka, Japan on April 20, 2006. Each of the misocyclones, which formed in succession in the core and gap regions of the meandering NCFR over a period of 30 minutes, had a short lifetime. Three of the five misocyclones generated near the surface and reached altitudes of up to 4 km ASL. The diameters of the lower-level misocyclones increased with altitude and vorticity in the order of 10-2 s-1 was observed near the surface. Two of the five misocyclones had similar diameters and vorticity, and at more than 6 km ASL. One of the observed misocyclones was related to the tornado in Fujisawa and as a non-supercell type tornado within the NCFR. This tornado-related misocyclone had the largest vorticity (5 × 10-2 s-1) near the surface (300 m ASL) and was characterized as having the reduced diameter below the cloud base, which is considered typical of a tornado.
In normal mode analysis, the unstable mode grows as an exponential function of time, and the stable mode oscillates as a trigonometric function of time. At a critical point of the relevant parameter space, the mode becomes marginally unstable. The marginally unstable mode is a degenerate mode of growing and decaying modes, as shown by Xu (2007) in the symmetric instability. In this note, we examine marginally unstable modes of gravitational, barotropic and baroclinic instabilities. As in the case of symmetric instability, the marginally unstable modes grow as a linear function of time. In the case of gravitational instability, the linear growth of the marginally unstable mode is rather trivial. On the other hand, in the cases of barotropic and baroclinic instabilities, the linear growth is not trivial, and can be qualitatively explained in terms of the PV thinking.
Future changes in daily and monthly surface air temperature variability are associated with temperature extremes and thus will have a large impact on our life and various sectors. In this study, we analyze variability of surface air temperature by 14 CMIP3 coupled ocean-atmosphere general circulation model results. We first assess the 20th century climate in coupled models (20C3M) experiment for the period 1981-2000 with three reanalysis datasets and then show the changes in future climate for the period 2081-2100 under the SRES A1B scenario. The interannual variability of simulated monthly mean surface air temperature agrees with the observations based on three reanalysis datasets. Although there are large model-to-model scatters in the future changes of interannual variability, the ensemble mean of the available model results shows a large decrease in variability over the Northern Hemisphere high latitude region in winter, and an increase over the Northern Hemisphere mid-latitudes and over the tropical land region in summer. For the daily temperature variability, the model ensemble mean underestimates the reanalysis data, probably related with less developed synoptic disturbances in the models than in reality. In future, the daily temperature variability is projected to increase over land in the Northern Hemisphere summer and in the tropics, and to decrease over the ocean throughout the year, consistent with the projected weakening of cyclonic disturbances. Inter-model variability is smaller than in the case for the interannual variability of monthly mean temperature.
This study examines the global warming impact on tropical cyclone (TC) genesis frequency over the western North Pacific basin (0°-40°N, 100°E-180°; WNP) projected by five atmosphere-ocean coupled general circulation models that participate in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3), and exhibit high performances in simulating horizontal distribution of annual-mean frequency under the current climate condition. TC-like disturbances are detected and counted in simulations for the 20th-century climate experiment and global warming experiments. It is revealed that all of the five models project an increasing trend of the frequency in the eastern part of the analysis domain, especially over the central North Pacific (5°-20°N, 150°E-180°; CNP), and a decreasing trend in the western part, with a maximum decrease over the South China Sea (10°-25°N, 110°-120°E; SCS). The former increasing trend can be interpreted by analogy with interannual variability related to El Niño and Southern Oscillation (ENSO). This is because projected changes of sea surface temperature and large-scale circulation field exhibit an El Niño-like pattern, and on the other hand, more TCs are observed in the CNP during the El Niño phases. Relative vorticity in the lower troposphere and vertical wind shear would become more favorable for TC genesis, as in El Niño situation. The authors conclude that these two dynamic factors are major contributors to the projected increase of the frequency in the CNP. Over the SCS, projected environmental conditions are diagnosed as more favorable for TC genesis than the current ones, in spite of the decrease projection of the frequency. The authors discuss that the projected decrease may be associated with a projected weakening of the activity of tropical depression-type disturbance that can later be developed into TC.
Based on the World Climate Research Programme's (WCRP's) Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset, evaluation for the summer monsoon over the Asian and western North Pacific (WNP) sectors is made in terms of reproducibility of the seasonal mean structure. Also investigated is a stepwise eastward progress of convection center from the Indian Ocean toward the WNP in the course of the maturing process of the continental and oceanic monsoons. Most models roughly reproduce seasonal mean broad-scale features on the Asian summer monsoon (ASM), but lower-tropospheric circulation over East Asia (EA) through the WNP and the location and intensity of the North Pacific subtropical high exhibit large inter-model variability. Some of the models fail to reproduce a reversal of the upper-tropospheric meridional temperature gradient over the South Asia and the North Indian Ocean sector. Metrics on the reproducibility of lower-tropospheric circulation of the ASM are also presented, in order to evaluate the reproducibility of the ASM circulation quantitatively. The large inter-model variability over the EA-WNP domains could be attributed to insufficient reproducibility of the oceanic monsoon. In most of the models, the stepwise eastward progress of convection over the South China Sea and WNP commences in May almost concurrently with large-scale circulation, whereas the eastward progress of convection is faster in most of the models than in the observation over the WNP. It is suggested that a teleconnection pattern associated with an intensification of convective activity over the WNP in mid-July is one of the key phenomena in both the observation and the coupled models, given the withdrawal of the Baiu rainy season around Japan. The analysis based on metrics concerning the stepwise eastward progress of convection over the WNP and its vicinity suggests that these models still have some difficulties in reproducing the stepwise eastward progress of convection accurately.
The Silk Road pattern, a wave-like anomaly pattern observed along the summertime Asian jet, is one of the major teleconnection patterns that can influence the East Asian summertime climate. Our analysis based on a reanalysis (JRA-25) dataset confirms the conventional notion that the pattern has a characteristic of a free stationary Rossby wave train, with its horizontal wavenumber close to the stationary Rossby wavenumber determined by the mean intensity of the jet. However, our analysis reveals its more essential characteristic as a dynamical mode whose extraction of available potential energy from the baroclinic Asian jet is highly efficient for its self-maintenance. Our analysis also reveals high sensitivity of its barotropic energy conversion to subtle zonal asymmetries of the Asian jet, which can be regarded as a critical factor to anchor the strongest vorticity anomaly around the western jet core and thereby determine the preferred longitudinal phase alignment of the wave train as observed. In fact, singular value decomposition of a global baroclinic model linearized about the observed mean state for boreal summer leads to identification of a perturbation similar to the Silk Road pattern with respect to its structure and energetics. It is thus indicated that the configuration of the mean flow determines the dominant phase, as well as the meridional location and the wavenumber, of the Silk Road pattern. The aforementioned dynamical characteristics of the Silk Road pattern are found useful for assessing and interpreting the reproducibility of the pattern in the present-day climate simulated in climate models that participated in the phase 3 of the Coupled Model Intercomparison Project (CMIP3). The pattern tends to be identified as the dominant mode of upper-tropospheric meridional wind variability as observed in such models that can reproduce the mean Asian jet realistically, including its zonal structure, which confirms the dynamics of the Silk Road pattern revealed in our observational analysis. On the basis of our analysis, a metric is proposed for assessing the models' reproducibility of the pattern.
A reanalysis dataset is used to establish the relationship between the year-to-year fluctuations in the midwinter storm-track activity over the Far East measured by poleward heat flux associated with subweekly disturbances and the occurrence of the first spring storm with strong southerly winds over Japan (Haru-Ichiban). Our analysis reveals that its early (delayed) occurrence tends to follow the enhanced (suppressed) winter storm-track activity with less (more) apparent minimum in midwinter in the course of the seasonal march. A metric is defined on the basis of the eddy heat flux to measure the reproducibility of the particular seasonal march of the Far East storm-track activity simulated in each of the Coupled Model Intercomparison Project Phase 3 climate models under the present climate. Under a particular global warming scenario, ensemble projection based only on the several models that show the highest reproducibility of the storm-track activity measured with the particular metric indicates that the future enhancement is likely in the midwinter storm-track activity associated with the weakening of the East Asian winter monsoon, implying that Haru-Ichiban is likely to occur earlier in the late 21st century than in the 20th century.