Localized, band-shaped heavy rainfall was observed over the Niigata-Fukushima area on July 13th 2004 and the over Fukui area on July 18th 2004. Both areas are located on the Japan-Sea side of the Japan Islands. These heavy rainfall events were a result of an intensification of convective instability over the Baiu frontal zone, induced by the inflows of low-level humid air and middle-level dry air. The middle-level air was considerably warm, not colder than the surrounding air. This indicates that the formation factors of heavy rainfall should be examined using convective instability, rather than potential instability. Numerical simulations using a cloud-resolving model with a horizontal grid of 1.5 km were carried out in an attempt to reproduce these heavy rainfall events. The Niigata-Fukushima heavy rainfall was reproduced well, while the Fukui heavy rainfall was not. This failure of the Fukui case could be the result of the uncertain analysis of the wind field over the Sea of Japan that determined the movement of low-level humid air. edevelopment of new observation systems over the sea, where upper-air sounding is seldom operated, are required to improve heavy rainfall predictions and to prevent such failures.
A major tornado spawned by a supercell is reproduced by a fine-resolution three-dimensional numerical simulation, and its genesis mechanism and structure are clarified. The tornado, which is associated with a maximum vertical vorticity of 0.85 s-1 and a pressure drop of 27 hPa, originates from one of the small-scale vortices on the gust front that forms between a warm moist environmental air and a rain-cooled air produced by the storm. Only the small-scale vortex that develops into a major tornado is located right under the low-level updraft associated with the low-level mesocyclone; the others that fail to develop are not. Several interesting previously-unexamined characteristics of the threedimensional structure of the simulated tornado vortex are also reported.
In this study, a realtime wildfire smoke prediction model is developed as an application of the PUFF model, which has been developed for the purpose of the realtime volcanic ash prediction. As a case study, the PUFF model is applied to the wildfire occurred in May 2003 near Lake Baykal and near Lake Khanka in Siberia. As a result of the PUFF model simulation, it is found that the fire smoke from Lake Khanka can reach northern Japan on 22 May 2003 if the smoke is released on 18 May 2003. The result explains the observed aerosol increase at around 3000 m height in northern Japan and the red sun observed in Sapporo Japan. There was another huge wildfire near Lake Baykal reported simultaneously by the NOAA satellite. However, it is found that the smoke is unlikely to reach Japan by the upper air wind condition. It is inferred by the PUFF model simulations that the abnormal increase of aerosols around 22 May 2003 in northern Japan was originated from the wildfire near Lake Khanka released around 18 May 2004.
The role of mountain ranges upon the arid climate formation in northwestern China is investigated using regional climate model. Two kinds of numerical experiments are carried out with real topography and with modified topography which does not contain the TianShan Mountains. Simulated rainfall distribution with real topography shows maximum precipitation in upwind slope of the mountain and minimum precipitation in downwind side of it, which coincides with the features of rain-shadow in other regions of the world. However, numerical experiment without the TianShan Mountains also simulates the area of distinct minimum precipitation in the same location. The arid climate, therefore, is formed even though the rain shadow effect is absent. The difference in rainfall amount between these two experiments is very small around downwind arid region. On the other hand, more intense and frequent rainfall events tend to occur in upwind of the mountain range due to forced lifting of environmental flow, which just emphasizes the precipitation contrast between upwind and downwind side of the mountain. The precipitation accompanied with the synoptic scale disturbances is suppressed over the arid region whether the TianShan Mountains exist or not.
The 46-year daily data of NCEP/NCAR Reanalysis are analyzed to investigate the effects of the equatorial quasi-biennial oscillation (QBO) on the large scale dynamics in the extratropical stratosphere and troposphere. Composites of the data in northern winter months with respect to the westerly or easterly phase of the QBO show that the stratospheric polar vortex is colder and stronger in the westerly phase in accord with previous studies. Statistical significance of the composite difference is tested by the large sample method with roughly 2000-day dataset for each phase of the QBO. Independence of serial daily data is taken account of by evaluating an effective time between independent samples. As a result, the most significant composite difference of the temperature is found near the tropopause in high latitudes, although the frequency distributions of the temperature for the two phases of the QBO overlap each other heavily.
Analysis results of data from lidar and AWS observations performed in the northwestern part of the Taklimakan Desert show a sudden increase in the backscattering ratios in the lower troposphere on 13 April 2002, with strong easterly winds. The Terra satellite reveals that the sudden increase is caused by the outbreak of an extensive dust storm. Numerical simulations, using a regional model, reproduce the strong easterly winds prevailing in a wide area of the desert, originating from north of the Tienshan Mountains. These conclude that the strong easterly winds lift a large amount of sand particles off the ground and results in the dust storm. In addition, a nose-shaped profile of the increased backscattering ratios indicates passage of a gravity current.
The Baiu (Mei-yu) front over East Asia in the global warming climate as well as that in the present one, is studied using outputs of a non-hydrostatic regional model with a horizontal grid size of 5 km (NHM). The NHM was run in June and July for ten years, applying a spectral boundary coupling method to reduce the horizontal phase differences of large-scale disturbances using the outputs of a global climate model with a grid size of 20 km. In the global warming climate, the Baiu front is likely to stay over the southern Japan Islands around the latitudes of 30 N-32 N and will not move northward. Therefore, the activity of the Baiu front maintains longer than that in the present climate, and the precipitation increases. On the other hand, the precipitation decreases over the northern Japan Islands and the northern Korean Peninsula. Years with no end of the Baiu season are often seen, and the frequency of occurrence of heavy rainfall greater than 30 mm h-1 increases over the Japan Islands.
In this study, to elucidate the inner structure of Typhoon Rusa (2002), a numerical simulation was conducted using a two-way triple-nested cloud-resolving nonhydrostatic model with a 2 km horizontal grid size on the finest nested mesh. The model successfully reproduced the features of polygonal eyewall structure observed in Typhoon Rusa. The simulated asymmetric structure in the inner-core region was dominated by a number of mesovortices within or near the eyewall in the lower and middle troposphere. Meso-lows on the horizontal scale of 20-30 km were found at the kinks of polygonal eyewall, and between them meso-highs existed. The modification of radial flow by the mesovortices affected not only the location of eyewall, but also the convective activities in the eyewall by causing the interaction between the eye and the eyewall. The horizontal distribution of the potential vorticity (PV) showed the wave-like pattern related to the mesovortices near the eyewall, which was quite similar to the ideal numerical experiments of Schubert et al. (1999) and Kossin and Schubert (2001).
We propose a retrieval method of Asian dust (Yellow sand or Kosa aerosol) columnar amount around source regions using a near ultraviolet radiometry observation from space. The method simultaneously retrieves an optical thickness and mode radius of Kosa aerosol, and then derives its columnar amount. The method was applied to ADEOS-II/GLI data in the spring of 2003 around Taklimakan desert source region, inland China. The retrieved optical thickness and mode radius were about 0.34 and 1.75 µm, respectively, at a validation site. They are comparable to the in situ observations conducted within the framework of ADEC project. The estimated columnar amount around a validation site is about 2.77 g m-2, which seems reasonable under a relatively calm situation. The method should be further validated with a regional model simulation study, and then it is useful to monitor Asian dust around source regions from space in the future.
On-board direct eddy flux measurement is essential in the quantitative air-sea flux evaluation as sea truth flux. Present authors have developed a new real-time automated eddy flux system and it is operating as a routine measurement on R/V MIRAI, JAMSTEC. As compared to our previous system, ship motion correction scheme is simplified and real-time eddy flux data processing system is introduced as the automated system. As a continuously operating real time eddy flux ship in the world, this is a unique system. The quality control of the flux datasets is essential as the data is collected continuously including during unfavorable conditions. Preliminary quality control was applied and the results show the reasonable values as compared to the bulk parameters. Based on the detailed quality control, the bulk transfer coefficients can be parameterized in various conditions.
Long-term changes of precipitation intensity were analyzed using a dataset which was recently compiled by the Japan Meteorological Agency (JMA). After some quality check, data of four-hourly, daily, and hourly precipitation at 46, 61, and 8 stations, respectively, were used for the period 1898-2003 on the condition that data for at least 80 years were usable in each month. As the measure of precipitation intensity, ten categories were defined so as to equate the total precipitation amount in each month at each station. The result is characterized by increase of precipitation in high categories, namely intense precipitation, and decrease in low categories. The linear trend for the highest and lowest categories is ±20-30% per century. This feature is found invariably for four-hourly, daily, and hourly precipitation, and qualitatively for all the seasons and regions.
Two sets of forecast-analysis cycle experiments were performed with and without direct assimilation of precipitation amounts using JMA mesoscale four-dimensional variational data assimilation system (Meso4D-Var). With a devised cost function of precipitation observation, which is derived from the exponential distribution, Meso 4D-Var successfully assimilated precipitation data in various weather situations throughout a one-month experiment period. The result of experiments shows that the precipitation assimilation by Meso 4D-Var improves the model forecasts of both weak and moderate precipitation and it ameliorates a spin-up problem of the model precipitation.
This study investigated diurnal cycles in convection and precipitation over the complex mountain-valley terrain of the southern Tibetan Plateau (TP) during the mature phase of the summer monsoon. Cloud-cover frequency (CCF) for high cloud increased after 13 LST (07 UTC) over the mountain ranges along 28.5°N and 30.2°N, reaching a maximum near 18 LST (12 UTC). Areas of high CCF subsequently moved towards the valley area along 29.3°N; relatively high CCF persisted there until early morning. Tropical Rainfall Measuring Mission (TRMM) PR data show a nearly identical variation in rainfall frequency. Formation and development of convective-type clouds and phase differences in the diurnal cycle were strongly affected by TP topography. Possible mechanisms for convective enhancement over the southern TP are also discussed.
In this study, we analyze the local structure of the wave activity flux associated with maintenances of the Arctic Oscillation (AO) index by separating it in quasi-stationary and transient components. For maintenances to the positive phase of the AO, it is found that the wave activity of the transient component plays an important role, supplementing that of the quasi-stationary component in the mid latitude upper troposphere. The local patterns of wave activity indicate a southeastward flux over North America from the transient waves and the Atlantic from the transient and quasi-stationary waves, strengthening the polar jet. Similarly, for maintenances to the negative phase, it is found that the transient component makes a more important contribution than the quasi-stationary component. It is shown that the local patterns of wave activity indicate a northeastward flux over the North Atlantic from the transient waves and the North Pacific from the quasi-stationary waves, enhancing the weaker polar jet. It is concluded that the positive feedback between wave activity flux and zonal mean flow is greatest over the two major storm track regions.
Five global climate models are used to estimate the local enhancement of Arctic warming attributable to sea ice retreat in B2-scenario greenhouse simulations. The models show a wide range of ice retreat, resulting in a corresponding range in the enhancement of warming. The enhancement is highly seasonal, varying locally from essentially zero in the summer to several degrees (°C) in the late autumn and early winter. Its magnitude increases with the threshold decline in ice concentration used to define retreat because higher thresholds better isolate the warming enhancement signal over ice retreat areas. A threshold of 20% ensures that all models in this study have enough ice retreat area to sample the enhancement because all start with ice concentrations at least that high over substantial northern hemisphere areas. All estimates are lower bounds because they do not account for advective effects.
A global warming response, interannual variability, and their relationship have been examined on mid-winter storm-tracks, using a high-resolution atmospheric general circulation model. In the western Pacific, global warming makes the storm-track stronger and the westerly jet weaker, closely related to the leading mode of the storm-track variability. Much projection onto the leading mode is also realized for the global warming response in the Western Hemisphere; the storm-track is suppressed along with weaker zonal wind there at least in this model. It is therefore found that there are two types of interannual variability of storm-tracks (one for the western Pacific and the other for the Western Hemisphere), both mostly explain the global warming response.
In this study, energy spectrum and energy flow of the large-scale atmospheric motions are examined in reference to the Arctic Oscillation index. Attention is concentrated to the barotropic component of the atmosphere in the framework of the 3D normal mode decomposition. According to the result of the observational analysis in the phase speed domain, the Arctic Oscillation (AO) is characterized as the energy increase at the meridional index l=3 with simultaneous decrease at l=5 of the zonal field. The result is consistent with the theory of the AO by the singular eigenmode of the global atmosphere. The accumulation of energy at the eigenmode of the AO is explained by the enhanced energy flux FZi associated with the zonal-wave interaction. It is found in this study that the energy flux FZi comes from eddies at the spherical Rhines speed cR where the planetary-scale Rossby wave becomes stationary. The result suggests that the low-frequency variability associated with the AO is maintained by energy flux from cR, which is compensated by the up-scale energy cascade from synoptic eddies in addition to the forced stationary planetary waves by topography.
We performed numerical experiments on layered structures in the tropical mid-troposphere, which are often observed by radiosonde and airborne observations as anticorrelation between humidity and ozone, to investigate their three-dimensional structures and transport processes. We reproduced a thin layered structure of high humidity, which has a horizontal scale of about 1000 km and a vertical extent of about 1 km. Particle transport experiments around the layered structure show that the wet part of the layered structure is advected from the intertropical convergence zone by northwesterly winds, while dry parts above and below it come from the southeast direction. Streamline analysis shows that the thin layered structure is produced by vertical shear of a horizontal wind component. This shear is associated with longitudinal displacement of a stagnation point, which is located between two synoptic-scale vortices, with height.
In order to improve the forecast of convection, the MM5 3D-Var (3-Dimensional Variational Data Assimilation) system is applied to initialize the mesoscale nonhydrostatic model, MM5, for a convective case on 12 to 13 June 2002. The BES (Background Error Statistics) is the key component used in a 3D-Var system, which helps to extract the useful information from the observations and to make the final analysis balanced. In this study, an approach is proposed to construct a case-dependent BES file for a convective case with a relatively inexpensive computing cost. By using MM5 3D-Var system and MM5 forecast model, a series of the 3D-Var experiments with the default BES and the newly constructed BES are carried out. The results showed that the assimilation of the conventional and GPS (Global Positioning System) PW (Precipitable Water) data with the new BES improved the forecast of convection. The real-time GPS PW itself also has a positive impact on the forecast of convection.
In this study, intensities and trends of Hadley, Walker, and monsoon circulations are compared for the IPCC 20th Century simulations and for 21st Century simulations, using the upper tropospheric velocity potential data. As a result, we showed significantly weaker biases in Walker and monsoon circulations for the JJA climate in the IPCC 20th Century simulations. The dispersal in the scatter diagram of the model biases is considerably large. The same analyses are applied for the IPCC 21st Century simulations to investigate the trends of these tropical circulations in response to the projected global warming. As a result, it is anticipated that Hadley, Walker, and monsoon circulations are weakened by 9, 8, and 14%, respectively, by the late 21st Century, according to the ensemble mean of the IPCC model simulations. Considering the large model biases demonstrated for the IPCC 20th Century simulations, further studies are needed to quantify those trends.
To demonstrate the importance of regional climate models over Japan, performance of the Meteorological Research Institute Regional Climate Model (MRI-RCM) was compared to the results of a super-high-resolution atmosphere general circulation model whose grid interval was 20 km (GCM20). The grid intervals (20 km) of the models are almost the same. Although both models reproduced the observed annual fluctuation of temperature adequately, the reproduced temperature of the GCM20 was somewhat lower than the observation, especially in winter. The temperature of the GCM20 has a negative bias of about -0.7°C, while the MRI-RCM has a negative bias of -0.1°C. Both models adequately reproduced the precipitation distribution during the winter monsoon. However, the precipitation of the GCM20 is less than the observation in early summer to autumn, while the MRI-RCM adequately reproduced precipitation during the same season. Therefore, for surface temperature and precipitation, the reproducibility of the MRI-RCM is generally better than that of the GCM20.
A coupled ocean-atmosphere climate model is used to depict changes in precipitation characteristics around Japan in the 21st century. A comparison between high (T106 atmosphere) and medium (T42) resolution versions for the present-day climate shows that the higher resolution version better represents not only the mean but also the frequency distribution of precipitation. The climate projection for the 21st century by the high resolution version shows that mean precipitation increases more than 10% in 100 years from the present, especially in warm seasons. Increases in frequencies of non-precipitating and heavy (≥30 mm day-1) rainfall days and decrease in relatively weak (1-20 mm day-1) rainfall days are significant.
A physical process of basin-wide warming in the equatorial Indian Ocean (IO) is examined in view of the seasonally different coupling processes between the monsoon circulation and the modulated Walker circulation associated with major El Niño. In the composite analysis of six warm episodes, the reversed Walker circulation along the equator appears over the tropical IO through the Pacific Ocean, resulting in anomalous surface easterlies over the equatorial IO prior to the mature phase of the El Niño. From boreal fall to winter, the divergent easterly wind anomalies prevent the development of the climatological equatorial westerly. Using a reduced gravity ocean model, it is shown that the modulated surface flow over the equatorial IO has a potential to contribute to the basin-wide warming through the heat exchange of the ocean surface. These results imply that the strong seasonality involved in monsoon is important to understand the seasonal phase-locked feature of interannual SST anomalies.
Simulations under present (end of the 20th Century) and future conditions (end of the 21st Century with SRES A1B scenario) by using a 20 km-mesh atmospheric general circulation model (AGCM) over 10 years are conducted and the changes in snow due to global warming are investigated. The seasonal march of the snow cover in the present simulation is comparable to that of satellite-based observational data. Distributions of the simulated snow cover and snow water equivalent (SWE) reflect the detailed geographical features. Due to global warming, the beginning of the snow-accumulating season (the end of the snow-melting season) will occur later (earlier) in most snow regions, and the snow cover will decrease except for very few exceptions. SWE will also decrease in wide areas, but over the cold regions (Siberia and the northern parts of North America), SWE will increase due to increases of snowfall in the coldest season. In both the change and the percentage change of the SWE, we can find that the detailed geographical features effect on them. In Japan, the SWE will decrease over the heavy snow areas. However, the percentage changes are relatively smaller over the colder areas.
The Meteorological Research Institute (MRI) and the Japan Meteorological Agency (JMA) projected climate change over Japan due to global warming using a high-resolution Regional Climate Model of 20 km mesh size (RCM20) developed in MRI. Projection was made for 2081 to 2100 following a SRES-A2 scenario. Precipitation projected by RCM20 indicated that increased daily precipitation will be seen during the warm season from June to September. Except for this period, the precipitation amount will not change much or will slightly decrease around Japan. The increase during the warm season will be seen only in the western part of Japan. A possible cause of the increase is an El Niño-like SST pattern in the future. Due to the future increased summer SST in the eastern equatorial Pacific, anti-cyclonic circulation to the south of Japan will intensify and will induce a strong water vapor flux along the rim of the anti-cyclonic anomaly. The intensified flux will converge over the western part of Japan and may increase precipitation. Surface air temperature is projected to increase more than 2°C around Japan in January. In summer, the temperature increase will be lower by about 1°C than in winter.
We developed a high-resolution, Coupled Atmosphere-Ocean Regional Climate Model to project climate change over Japan with more sophisticated atmosphere-ocean interaction. The model represented sea surface temperature (SST) distribution well. An SST bias over the Japan Sea was improved by 10 to 20% compared to a non-coupled ocean model driven by atmospheric forcing in a GCM. These results indicate that realistic, high-resolution atmospheric conditions positively impact on the reproduction and projection of oceanic conditions. SST in the future projection simulation was 1 to 6°C warmer than in the present climate run. A particularly remarkable warming signal was projected to the sea east of Hokkaido in both summer and winter. The model also simulated cooling SST off Sanriku (143E/39N) in winter, which has never obtained by non-coupled ocean model. These projected SST results are likely to be related to future changes in surface wind.
Effects of atmosphere-ocean coupling have been investigated using the CCSR/NIES/FRCGC coupled and atmospheric general circulation models (CGCM and AGCM). The latter is integrated with monthly sea surface temperatures (SSTs) taken from the former. The given SSTs being independent of the atmospheric fluctuations, the AGCM supplies more water vapor to the atmosphere to adjust larger air-sea temperature difference. In our AGCM, the summertime land temperatures are higher due to greater greenhouse effect, because the change of the cloud amount is too small to affect the radiative fluxes. More evaporation induces stronger rainfall in some regions, and circulation and moisture distribution control the horizontal distribution of rainfall. Hence the coupling effect in rainfall distribution could change if the climate condition changed. As an example in our model, the decoupling does not affect Japanese summer rainfall under the preindustrial condition but strengthens it under the global warming condition.
Rainfall diurnal variation over the Indonesian Maritime Continent simulated by a super high resolution atmospheric general circulation model (GCM-TL959) is examined. GCM-TL959 is successful in representing spatio-temporal characteristics of observed rainfall diurnal cycle over the region. The model also seems to simulate local circulations such as land-sea breeze. The speed of rainfall migration over land and coastal sea in GCM-TL959 is consistent with that of a gravity current.
Daily rainfall and δ18O were observed from August to November 2001 at three stations in Thailand. Water origins were estimated using a tagged-water transport model. The temporal and spatial distributions of water origins showed a clear transition from the Indian Ocean to the Pacific Ocean as the Asian monsoon retreated from Indochina. A new definition of the withdrawal date of the Asian monsoon over Indochina is proposed that considers the transition of the origin of water vapor. The daily variability of precipitation isotopes at three stations in Thailand was reproduced at statistically significant levels using a Rayleigh-type isotope circulation model. Therefore, the atmospheric water balance in the model can be validated reliably with these precipitation isotopes. Nevertheless, the simulated δ18O values were obviously underestimated, perhaps because of uncertainties in the evaporating isotope ratios and the lack of isotope physics at small scales.
Mesoscale convective systems (MCSs) that bring rainfall in the vicinity of Kyushu Island, Japan during the late Baiu season in the present and global warming climates are examined by a non-hydrostatic regional climate model (NHM) with the horizontal grid of 5 km. In the global warming climate, two types of MCSs appear in the vicinity of Kyushu Island. One travels from the Chinese Continent and the other from the southern part of the East China Sea to Kyushu Island. These two MCSs often merge over the sea southwest of Kyushu Island, and they rapidly develop to bring heavy precipitation to the vicinity of Kyushu Island. Among the latter, MCSs with low cloud-tops below 4 km MSL (Mean Sea Level) are found. In the comparison with the present climate, the averaged cloud and rain water mixing ratios in the vicinity of Kyushu Island become much larger, and the peak altitude of the mixing ratios are about 0.5-1.0 km higher in the global warming climate. The cloud water mixing ratio between 2-4 km MSL increases in the global warming climate, corresponding to MCSs with low cloud-tops. These results suggest one of the processes to produce heavy rainfall in the vicinity of Kyushu Island in July in the global warming climate.
Dust layers in the free troposphere were observed with the lidars in Suwon, Gosan, and Tsukuba in March 7-9, 2005. The observed dust distributions were compared with the results of the regional and global dust transport models (CFORS, NRL NAAPS, and SPRINTARS). The results with the global models reproduced the dust layer qualitatively, but the regional model did not. This suggests the source of the dust layers is located outside of the modeled region of the regional model that includes Taklimakan Desert and Gobi Desert. The global models showed the plumes were from the Sahara Desert, and the both models showed there was no major dust emission in Taklimakan and Gobi Deserts during the observation period. The trajectory analysis using NOAA HYSPLIT showed that the dust originated in the Sahara Desert 5-10 days before.
Influence of El Niño/Southern Oscillation (ENSO) on the stratosphere-troposphere dynamical linkage during stratospheric sudden warming (SSW) events is investigated based upon the composite analysis using 44 winter record of NCEP-NCAR reanalysis data. Stratospheric waves with zonal wavenumber 1 (2), which are linked with tropospheric teleconnections over the Pacific-North America-Atlantic sector (the Eurasian sector), are important for the onset of SSWs during the warm (cold) phase of ENSO. During the cold phase, easterly anomalies associated with SSWs penetrate into the troposphere, and annular-mode-like height anomalies appear subsequently in the troposphere. On the other hand, during the warm phase, easterly anomalies are restricted in the stratosphere, and zonally asymmetric height anomalies are observed in the troposphere about 10 days after the warming peak in the stratosphere. Modulation of storm track activities due to lower stratospheric anomalies is found to be important for the downward penetration of zonal wind anomalies into the troposphere.
Numerical experiments are performed using a non-hydrostatic regional climate model with a horizontal resolution of 5 km to study changes in the characteristic features of disturbances appearing over the Baiu frontal zones due to global warming. In this study, disturbances are defined as those with precipitation greater than 20 mm/6 hr within a radius of 100 km. An increase in the number of disturbances is found in the Baiu frontal zone over western Japan in the warming climate. The increase is caused by the lengthening of the Baiu duration. In addition, the disturbances are likely to be much more detected by the intensification of precipitation. Among such disturbances, those with intense precipitation and eastward-tilting vertical structures in high-pass-filtered fields are more frequently seen in the warming climate. They are considered to be meso-α-scale baroclinic instability modified by diabatic heating due to large precipitation. Our results suggest that the changes in the numbers of disturbances are induced by an increase in the supply of water vapor to the Baiu frontal zone and convectively unstable stratifications in the lower atmosphere.
Using a high-horizontal-resolution atmospheric general circulation model (AGCM), impacts of SST warming and CO2 increase on the tropical cyclone (TC) climatology are investigated. The SST effect is examined from numerical experiments in which SST is uniformly higher/lower by 2 K, without changing the atmospheric CO2 concentration. The CO2 effect is shown from doubled and quadrupled CO2 experiments with a fixed SST condition. The results demonstrate that the increases in CO2 have large impacts to reduce TC frequency globally, while the SST changes have relatively small influences on the TC frequency. The SST warming causes significant increase in climatological precipitation, and this indicates intensification of convective heating and should have some influences to activate the atmospheric circulation in terms of vertical mass flux in the tropics. In the high-SST experiment, however, larger warming in the upper troposphere causes higher dry static stability, which should have some impacts to weaken the atmospheric circulation. It seems that these two conflicting factors, in terms of TC frequency, may cancel out to a large extent. As the effect of CO2 enhancement, precipitation decreases significantly in the tropics, which may lead to the reduction in TC frequency.
Recent climate trends in the East Asia during the Baiu season (June) of 1979-2003 are investigated. The precipitation is significantly decreasing around Philippines and the northeastern part of China, while increasing along the Baiu frontal zone (the southeastern part of China and Japan) and the western part of Siberia. The pressure trend that corresponds to the precipitation has barotropic structure in the midlatitudes and baroclinic structure in the subtropics. For the barotropic structure to be maintained, the Rossby wave propagation from the western part of Siberia and Tibetan Plateau seems to play an important role. On the other hand, the trend of diabatic heating around Philippines and Japan is important for the baroclinic structure. The pressure trend is investigated by a linearized primitive model. The results support the above idea that the effect of the Rossby wave propatation is important for the barotropic pressure trend in the midlatitudes and that the trend of apparent diabatic heating is an important factor for the baroclinic pressure trend in the subtropics. Interannual variability is also investigated. It is suggested that the trend of surface temperature around Mongolia and Philippines is related to the pressure trend.
The Baiu (Mei-yu) front over East Asia in the global warming climate as well as that in the present climate is simulated using a non-hydrostatic regional climate model (NHM) with a horizontal resolution of 5 km, which explicitly calculates the microphysical processes without a cumulus parameterization. The simulation results of the present and global warming climates were compared to clarify the change in the characteristics of the distributions of cloud and precipitation particles. The vertical profile of the mixing ratio of each hydrometeor category slightly shifts about 1 km higher in the global warming climate. The mixing ratios are generally larger in the global warming climate by a factor of about 1.3 at the middle of the Baiu frontal zone. No significant difference is found in the mass ratios of solid precipitating water to total water and non-precipitating water to total water between the climates. This is a reflection of that the shape of each hydrometeor profile is similar between the climates. The approximate formulas of the mass ratios among water substances were obtained.
We use a relatively high resolution (T106) atmospheric general circulation model to simulate precipitation along the tracks of tropical cyclones (TCs) within the western North Pacific basin under present-day and doubled CO2 climates. The simulated mean precipitation associated with TCs is in agreement with observational data. The simulation predicts that a doubling of atmospheric CO2 will result in an increase in TC precipitation. We attribute the predicted increase in precipitation to increased atmospheric moisture content, which is partly offset by the effects of decreased TC intensity. Mean TC precipitation and the frequency of extreme precipitation (≥150 mm/day) over Japan are predicted to increase with CO2 doubling, despite an accompanying decrease in the frequency and intensity of TCs.
A coupled general circulation model, MIROC3.2, was used to investigate the impacts of global warming on the El Niño-Southern Oscillation (ENSO) variability. The model has relatively fine resolution which captures the atmosphere and ocean dynamics well. The model shows an overall warm trend in the tropical Pacific under the greenhouse warming, particularly in the central equatorial Pacific. Superimposed on such trend is the increase in amplitude of interannual ENSO variability. A sensitivity analysis shows that ocean, rather than the atmosphere, is responsible for the strengthening. The greenhouse warming tends to increase the surface waters more, leading to strengthened subsurface vertical stratification in the equatorial ocean. The strengthening of the subsurface stratification is greater in the western half of the equatorial Pacific where the unit amount of surface wind stress change tends to accompany in-situ subsurface temperature anomalies greater under the warmed climate. This as well as the change of the mean meridional temperature gradient appears to be responsible for the increase in ENSO amplitude.
Changes in indices of extremes between the present-day climate and a future warmer climate are projected over Japan using a global 20-km-mesh atmospheric model. Comparisons with observed data show that the indices on temperature extremes are represented well in the model, while less intense precipitation biases are found. In the future climate simulation around 2090, the number of frost days decreases by 20-45 days with larger decrease along the Sea of Japan than the other area. Growing season length increases about a month. Changes in the temperature extremes are not uniform over Japan, showing usefulness of projections using a high-resolution model. Although changes in precipitation extremes are small and not significant over a large part of Japan, statistically significant increase in indices of heavy precipitation is found in western part of Japan and Hokkaido.
High-resolution simulations on the vertical transport of dust in a fair-weather condition over desert regions are performed with a cloud-resolving modeling approach. Basic processes relevant to the dust transport are examined by focusing on the role of boundary-layer and cloud convection. Although the model settings are simplified, the simulation well reproduces the diurnal variability of fair-weather convective boundary layer and cloud development observed over a Chinese desert. Dry convective motion within the boundary layer and moist convection above the boundary layer play a significant role in regulating the vertical mixing and transport of dust in the atmosphere, and in enhancing the surface winds and thus the dust emission by the downward transport of high momentum in upper levels. The present modeling framework is useful in understanding the small-scale effects on the vertical dust transport associated with boundary-layer convection and cloud convection in a fair-weather environment.
A classification of snow clouds, called the “snowfall mode,” is proposed based on Doppler radar observations at 10-minute intervals at Nagaoka in 1999/2000 winter season. Using 795 hours of data at an altitude of 1.6 km, six snowfall modes were defined: longitudinal line (L-mode), transversal line (T-mode), spreading precipitation (S-mode), meso-β scale vortex (V-mode), mountain-slope precipitation (M-mode), and local-frontal (discontinuity) band (D-mode). In migrating snow clouds, a sub-class, referred to as snowfall with coastal intensification (xI-mode, where x is L, T, S and V) was defined. A sample snapshot and the mean Ze are shown for each snowfall mode. The frequency of occurrence of the snowfall modes indicated that both of the longitudinal cloud streets and the meso-α scale disturbances occupied about 1/3 of the analysis period. About 18% of the precipitation in the analysis period was considered to be under orographic effects. The prevailing wind direction differed between the snowfall modes although a west-northwesterly wind dominated.
Characteristics of systematic differences between TRMM Microwave Imager (TMI) and Precipitation Radar (PR) rain estimates derived from TRMM-3G68 version 5 datasets are analyzed and the possible factors contributing to these differences are discussed. This study utilizes conditional rain estimates which represent the average rain rate of rain pixels over a domain instead of unconditional rain (which is the domain averaged rain) used in most of the previous studies. It is found that TMI rainfall estimates are larger than PR estimates over most of the equatorial oceans. The distribution of rainfall differences with respect to their convective percentages brings out their characteristics under different convective/stratiform regimes. Different life stages of tropical convection are identified utilizing TMI and PR convective percentages, rain cover, Visible Infrared Scanner (VIRS) brightness temperature and the difference between TMI and PR rain estimates. Our result suggests that TMI and PR rainfall differences are associated with these life stages of organized convection. At low TMI convective percentages and high PR convective percentages, representing the formative stage of convection, PR rain estimates are found to be larger than TMI. But, significantly large differences in rain estimates, with overestimation of rain by TMI, occur when PR convective percentage is < 60% and TMI convective percentage is > 50% which is identified as mature to decaying stages of convection. During these stages, due to the presence of overshooting tall clouds, the time lag between the occurrence of maximum rain and the highest cloud top height, the increased back radiation from ice-scattering near the cloud top level, and the melting layer effect result in large overestimation of rain by TMI.
CO2 exchange was measured using eddy covariance technique during 2001, 2002, and 2003 in a humid temperate C3/C4 co-occurring grassland in Japan. The interannual variability in cloudiness of Asian monsoon will be an important forcing in CO2 exchange. Early summer and autumn is a period with continuous rainy and cloudy days peculiar to the Asian monsoon region. The early summer rainy season (baiu) in 2001 ended earlier than 2002. In 2003, the baiu season was protracted, and the summer had cooler conditions than 2001 and 2002. Annual net ecosystem exchange was 9 gC m-2 in 2001 and 17 gC m-2 in 2003, and a weak CO2 (-78 gC m-2) sink in 2002. In 2002, the maximum annual gross primary production was 2426 gC m-2, and contributed to negative net ecosystem CO2 exchange. The annual variations of gross primary production and net ecosystem exchange were sensitive to that of photosynthetic photon flux density.
Following the previous finding that drought occurred in summer monsoon over India synchronizing with the Equatorial Indian Ocean Oscillation (EQUINOO) negative phase and El Niño, a full analysis of moisture transport is carried out to explore the mechanism of the drought. It is suggested that the key component of relating the drought with the EQUINOO negative phase and El Niño tends to be a suppressed moisture convergence over the Arabian Sea, which may yield a low moisture transport across the western coast of India and the consequent drought. Among the recent drought years, the drought in 2002 is considered to be special with respect to its month-long duration in July. The moisture transport to India was comparable with those in the other drought years, while the Hadley cell completely reversed and expanded a downward belt over India. Both EQUINOO negative phase and El Niño were at the extreme states in this period and are considered to be responsible for the anomalous Hadley cell and the 2002 drought.
A series of numerical experiments were conducted to investigate the sensitivity of the WRF Model to the advection scheme. The differences in the simulated results between third- and fifth-order schemes are not large, however the vertical velocity field of the former is somewhat smoother. On the other hand, fourth- and sixth-order schemes easily produce grid-scale computational noise, although the time step is set to be smaller than the recommended one. Artificial second-order diffusion damps the noise, however it negates the advantage of the high-order advection scheme. Explicit fourth-order diffusion damps the noise, however it smoothes the field more than the inherent, implicit diffusion in the third-order scheme. Contrary to this, explicit sixth-order diffusion damps noise, but maintains detailed structure and high energy spectral density of the vertical velocity. We recommend the fifth-order upwind scheme for use in the WRF Model.
We use a recently-developed efficient probabilistic estimation technique to estimate the sensitivity of the Earth’s temperature to a doubling of atmospheric carbon dioxide. The method is based on the ensemble Kalman filter which we apply to the CCSR/NIES/FRCGC AGCM (the atmospheric component of MIROC3.2) at T21L20 resolution coupled to a slab ocean. The method combines prior beliefs about the model, with observational data, to simultaneously estimate 25 model parameters in an efficient and objective manner. We perform a sensitivity analysis to investigate the effect of different assumptions regarding model error, since this is a necessarily subjective input which has not yet been well characterised. We attempt to validate the resulting ensembles against out-of-sample data by comparing their hindcasts of the Last Glacial Maximum (LGM) to paleoclimate proxy data, and demonstrate through this that our ensembles of simulations are probably biased towards too high a sensitivity. Within the framework of our single-model ensemble experiment, we show that climate sensitivity of much greater than 6°C is hard to reconcile with the paleoclimate record, and that of greater than 8°C seems virtually impossible. Our estimate for the most likely climate sensitivity is in the region of 4.5°C. Although these results are reasonably consistent with the most widely accepted estimates of climate sensitivity, they disagree with some recent research which has suggested a significant probability of sensitivities well in excess of these values. These results suggest that paleoclimatic evidence could provide a useful, albeit imprecise, constraint on ensemble forecasts of future climate change.
Structure and variability of the tropical tropopause are presented using radio occultation measurements by CHAMP/GPS (CHAllenging Mini satellite Payload/Global Positioning System) from May 2001 to December 2004 (with a total of 175,149 occultations). The tropopause heights defined by both lapse rate and cold point generally show large-scale, off-equatorial maxima (tropopause increase at 20°N or S than at equator), and sometimes even a high tropopause for about 0.3 to 0.65 km (on an average) at 20°N and S simultaneously than at the equator along a particular meridian, in contrast to our previous knowledge. Although this feature has already been reported partially during the summer monsoon season, the present study shows the seasonal and geographical distributions of the tropical tropopause comprehensively using a new promising observational technique. In addition, the vertical shape of the tropopause is found to be sharp in the equatorial region and broad in the subtropics especially in northern winter. Possible mechanisms are discussed in light of dynamical and radiative processes.
Impacts of global warming on gravity wave (GW) momentum flux (MF) in the lower stratosphere are estimated by analyzing results of simulations we performed using a high-resolution coupled climate model. In the “doubled CO2 climate” during the June-August period, a nearly uniform and isotropic increase in MF of 20-40% is obtained at 70 hPa as a result of an upward displacement of the tropopause. The geographical pattern of the largest enhancement of MF is generally coincident with changes in the tropospheric GW source. Eastward and westward components of MF both increase in relation to the enhanced source activity as well as to change in the tropopause height. However, the eastward component is preferentially filtered by the increased background westerly winds in the lower stratosphere. As a result, the net westward MF largely increases in the tropics and the southern mid-latitudes.
Using new gauge-based gridded daily precipitation climatology over monsoon Asia (5-60°N, 65-155°E) with a grid resolution of 0.05°, we validate the precipitation climatology simulated by a global 20-km resolution atmospheric model of the Meteorological Research Institute of the Japan Meteorological Agency. The new gauge-based precipitation climatology explicitly expresses orographic precipitation over the East Asia. The model has the highest resolution of all atmospheric general circulation models currently in use to study global warming. It successfully simulates orographically enhanced precipitation patterns presented in the East Asia climatology (hereafter, EA clim). The model overestimates precipitation averaged over land areas of monsoon Asia, and bias is larger over India and central China. Difference in annual precipitation between the model and EA clim exceeds those between other well-known grid precipitation climatological datasets. EA clim can be used to validate seasonal changes in monsoon precipitation over the domain, including mountainous regions. The 20-km resolution model reproduces seasonal cycles in precipitation over northern China and the Himalayas. However, large biases and seasonal cycle differences occur over India and central and southern China. As the model resolution improves, gridded daily precipitation datasets based on dense rain-gauge networks should be prepared to validate the model results.
A comparison of WRF and MM5 models was conducted to investigate the differences in the performance for a simulation of a heavy rainfall event along the Baiu front. The simulated precipitation pattern from WRF and MM5 roughly agrees with the observations. However, details are different among WRF, MM5, and observations. The position of the heaviest rainfall is found onshore in WRF and the observations, whereas it is found offshore in MM5. The difference in the position is more clearly found in the vertical velocity field. Additionally, the vertical velocity field of WRF is more detailed than that of MM5. Power spectral density of the vertical velocity clearly shows the difference. Sensitivity experiments indicate that this is due to the differences in the numerical scheme for the model dynamics, not in the cloud microphysics.
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