Future climate changes over East Asia are studied from ensemble simulations of the coupled climate model ECHO-G, based on the Intergovernmental Panel on Climate Change (IPCC), Special Report on Emissions Scenarios (SRES) A2 and B2 scenarios. Three ensemble experiments are performed: the A2 scenario experiment with greenhouse-gas (GHG) plus sulfate aerosol forcing (referred to as A2), and the A2 and B2 scenario experiments, with GHG forcing only (A2G and B2G respectively). All experiments show that East Asian near surface temperature (T2m) and precipitation (PCP) will increase in the 21st century with larger amplitudes than global means. Seasonally varying changes are found as a larger warming in winter and fall and a stronger PCP in summer. Relative roles of large-scale and convective precipitations (LSP and CP) are analyzed extensively. A mass flux scheme with an adjustment closure is used for cumulus parameterization. In the global mean, LSP dominates total PCP increase whereas CP controls PCP reductions near the equator in December-January-February (DJF) and 30-40°S in June-July-August (JJA). The latter originates from a weakening of the northern winter Hadley circulation and an increased static stability in the Southern Hemisphere, supporting previous results. For the East Asian mean, the CP change explains most of the increase of total PCP in JJA while the LSP change plays a more critical role in DJF. The LSP increase over the North Pacific in DJF is well associated with strengthened [weakened] baroclinicity north [south] of 40°N, i.e., a poleward shift of storm track. Aerosol effects on East Asian climate change (A2 minus A2G patterns) are characterized by cooling and drying with patterns similar to those of the mean changes. This is inconsistent with localized features found in previous works, indicating large uncertainty in regional responses to aerosol forcing. A possible impact of GHG mitigation over the late 21st century (A2G minus B2G patterns) is more pronounced in T2m than in PCP changes, with similar patterns as in aerosol effects. Simulated CP and LSP contributions to PCP changes are insensitive to the aerosol effect as well as that of GHG mitigations.
The numerical simulations were performed to study the intrusion of a shallow cold front along the China coastal mountain range, and the deformation of the front by the Central Mountain Range (CMR) in Taiwan during the IOP-9 (1600 UTC June 14-1700 UTC June 15, 1987) of the Taiwan Area Mesoscale Experiment (TAMEX). The essential features of the observed front, such as the faster movement of the eastern part of the front than the western part, were well reproduced. The control and sensitivity simulations suggested that the frontal deformation in Taiwan was caused by the dynamical interaction between the front and the underlying topography. As the cold air approached northern Taiwan from the north, a relative high pressure with anticyclonic circulation built up on the windward side. More oncoming flow was diverted to the northeast of Taiwan than to the northwest and caused the difference of the frontal speed between the eastern and western parts of the front. The dynamics of the frontal movement along southeast China was similar to the dynamics of coastally trapped mesoscale ridges, or orographical jet. The momentum budget equation showed that the contribution of the nonlinear advection, ageostrophic forcing, and friction were equally important to the local change of the wind just behind the front. This might indicate that the dynamics of the front was more complicated than that described by the density current theory derived from an irrotational fluid. The invicid sensitivity simulation revealed a faster propagating front, and a stronger wind behind its leading edge in comparison with the control simulation. The front bears no dynamical resemblance to either an orographically trapped density current or a Kelvin wave, because the major forces in the momentum budget are the pressure gradient force, Coriolis force and advection terms.
The present report studies features of the polar-air outbreak and the associated air-mass transformation over the east coast of Asia simulated in an AGCM (T106L52: a primitive equation spectral model, which has 52 σ-levels and triangular spectral truncation at wave-number 106) that used climatological SST in comparison with the features described in several observational studies. The large-scale circulations are properly reproduced for January Y07 (the 7th year after the spin up integration). A typical case of polar-air outbreak simulated in January is studied in detail. During the polar-air outbreak, cyclonic northwesterly polar-air streams over the Sea of Japan and the northwestern Pacific, and anticyclonic northeasterly polar-air streams over the East China Sea and the South China Sea are reasonably simulated. During the polar-air outbreak, the sensible and latent heat fluxes simulated over the Sea of Japan reach to ∼150 and ∼250 W m−2, while those over the East China Sea reach to ∼75 and ∼250 W m−2, respectively. These simulated fluxes agree with the fluxes evaluated in observational studies. The multi-layer structure of the transformed air-mass, including the unstable surface layer, the subcloud layer, and the cloud layer capped by the stable layer, simulated over the coastal areas of Asia is consistent with the observations for the cases of typical polar-air outbreak. However, the precipitation over the coastal areas of Japan simulated during the polar-air outbreak is significantly small as compared with the observation. This is due to the insufficient horizontal resolution of the T106L52 to simulate mesoscale circulation systems which induce snowfalls. In addition, the duration of the polar-air outbreak simulated in the AGCM is relatively short as compared with that in the real atmosphere, since the anticyclone over the continent tends to extend eastward in the model.
The influences of the SST and land surface temperature on the onset of the Asian summer monsoon are studied by using an atmospheric GCM, Global Spectral Model (GSM) of Japan Meteorological Agency (JMA). After confirming from five-year control run with the climatological SST that the model performs the Asian summer monsoon well, we make the impacts of two kinds of main experiments, i.e., SST fixed run and solar fixed run, focusing on the Asian summer monsoon onset. In the SST fixed run, the model is run from Apri1 lst until June 30th under the SST, which is fixed at the value of April 1st. To see impacts systematically, an ensemble average over five cases is compared with that of the control run. The fixed SST considerably reduces the Somali jet, the cross equatorial water vapor transport, the northern-hemispheric evaporation, and then suppresses the ITCZ jump from the southern hemisphere to the northern hemisphere. It also reduces the low-level westerlies from Southeast Asia, which brings the monsoon onset. In the solar fixed run, the model is run from April 1st until June 30th under the solar condition, which is fixed at that of April 1st. The fixed solar condition reduces the land-sea thermal contrast, suppresses the heat low over the continent, and reduces low level westerlies in a geostrophic sense. The above experiments indicate that both land-sea thermal contrast and SST considerably contribute to the onset of the Asian summer monsoon. The land-sea thermal contrast induces low-level wind surrounding the Eurasian continent, and makes a primary contribution to the formation of the Asian monsoon westerlies. The seasonal march of the SST destabilizes the stratification, induces the ITCZ jump from the southern to the northern hemisphere, strengthens the Hadley circulation, and then enhances the monsoon westerlies through effective transport of the absolute angular momentum.
Aerosol optical characteristics of Asian dust are studied by combining Global Ozone Monitoring Experiment (GOME) data, with Geostationary Meteorological Satellite (GMS-5) visible data, collected during a Yellow Sand event occurred on 7 April 2000. Retrieved results were compared with those from solar aureole measurements at Anmyon-Do, Korea. It was shown that the single scattering albedo of Asian dust can be as low as 0.76, much smaller than the generally known values of 0.9 in the Asian dust source region, such as Dunhuang or 0.93 for Saharan dust. This finding suggests that Asian dust can be much absorbing aerosols. The overall atmospheric forcing efficiency (radiation fluxes per unit aerosol optical thickness at 0.5 μm) of Asian dust observed on 7 April 2000, is about 102 Wm−2 in the atmospheric layer, and −116.9 Wm−2 at the surface. These results strongly indicate that the regional surface and atmospheric radiation energy budget, can be significantly altered by the presence of Asian dust.
An intensive radiosonde observation with time intervals of 3 h was performed in June 2002 at Syowa Station (39.6°E, 69°S) in the Antarctic. A wavelike disturbance with a wave period of 12-15 h, having a nearly barotropic structure was observed above a height of 22 km in the time period of 27-28 June 2002. A result of the hodograph analysis suggests that the short-period disturbance is not due to an inertiagravity wave. A similar short-period disturbance is observed in the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis data. Results of detailed analysis using the ECMWF data show that the short-period disturbance has a horizontal wavelength of about 2000 km, and propagates along a potential vorticity minimum region with a horizontal phase velocity of about 40 m s−1. This phase velocity is equal to the background horizontal wind velocity at the potential vorticity minimum. A necessary condition for the barotropic instability is locally satisfied at the potential vorticity minimum. However, it is rather appropriate that the short-period disturbance is interpreted as a neutral wave than as an unstable wave in the barotropically unstable background flow. This result implies any unknown mechanism of the suppression of barotropic instability in the locally unstable background flow associated with a disturbed polar vortex.
The structures of deep convections and their environment were examined during the latter half of the active phase of the Madden-Julian oscillation (MJO), using fine temporal resolution of research vessel observation data. During the intensive observation period, the MJO associated with deep convections passed through the observation area. A strong westerly wind also appeared during the active phase of the MJO. Vertical structures associated with convective activity were detected using lag correlations of atmospheric parameters. Convective activity included deep convections, with a nocturnal maximum and another peak 12 to 18 hours earlier. Active convections associated with the first peak developed in the afternoon and decayed within a few hours. Those active convections transported water vapor to the lower to middle troposphere, making the environment favorable for the more intensive deep convections that developed during the night. Westerly winds were intensified in the lower layers during the deep convections.
A band-shaped precipitation system, associated with a cold front, caused heavy rainfall over northern Kyushu, Japan, on 29 June 1999. This precipitation system had a hierarchical structure with different horizontal scales; the precipitation system consisted of several mesoscale convective systems (MCSs), and each MCS consisted of a few convective cells that formed successively on its upstream side. Each of them had different spatial and time scales, as well as different traveling speeds and directions. Low-level humid air from the southwest and middle-level dry air from the west continuously flowed into the precipitation system. This dry air was not colder than the surrounding atmosphere. The inflow of low-level humid air initiated the MCSs, and that of middle-level dry air enhanced, and maintained, the convective instability over the cold front. The heavy rainfall was brought under the maintenance condition of convective instability. The cloud top heights of convective cells were found differently between the western and central parts of the precipitation system. The factors to determine them were examined using the successfu1 simulation results of a nonhydrostatic cloud-resolving model, with the horizontal resolution of 2 km. The intrusion of middle-level dry air into convective cells was considerably larger in the western part than in the central part. This caused the difference in the cloud top heights of the convective cells; the convective cells were less than 7 km high in the western part, whereas, in the central part, most were between 5 and 7 km high and at the height of the tropopause. In the western part, most of the convective cells completely lost their buoyancy because of the significant evaporative cooling of hydrometeors. These results show the two different effects of middle-level dry air; a huge amount inflow suppresses the development of convective cells, while it strongly enhances the convective instability.
To clarify the actual conditions necessary for the neutralization of acid rain by tree leaves, an examination of the neutralization effects of artificially-made acid droplets on several kinds of tree leaves, was conducted using a newly-developed pH-imaging microscope. Through these examinations, it was clarified that broad-leaved tree species such as beech, horse chestnut, Japanese zelkova and oak trees could dilute and neutralize acid droplets more effectively than coniferous trees, such as Japanese cedar, Japanese black pine and Japanese red pine. This result is in accordance with our previous observations conducted in the same site that broad-leaved trees were more effective in diluting and neutralizing throughfall than coniferous trees (Kikuchi et al. 2000, 2004a). Furthermore, the pH values of throughfall in September were higher than those in June for all tree species examined. In the broad-leaved trees, the relationship between the lapse of time, and pH values, could be expressed by a logarithmic approximation. The temporal variations in pH values of beech tree leaves, collected from the D-site (a forest near the Shirakami-Sanchi Natural Heritage Area), and E-site (rural, but relatively heavy traflic area) showed very similar trends. Therefore, the neutralization of acidity in these two areas was basically similar in nature, and did not depend on the amounts and constituents of aerosol substances on their leaf surfaces. The capacity to neutralize acid droplets on the abaxial surface was greater than that on the adaxial surface of beech leaves.
A global atmospheric general circulation model, with the horizontal grid size of about 20 km, has been developed, making use of the Earth Simulator, the fastest computer available at present for meteorological applications. We examine the model’s performance of simulating the present-day climate from small scale through global scale by time integrations of over 10 years, using a climatological sea surface temperature. Global distributions of the seasonal mean precipitation, surface air temperature, geopotential height, zonal-mean wind and zonal-mean temperature agree well with the observations, except for an excessive amount of global precipitation, and warm bias in the tropical upper troposphere. This model improves the representation of regional-scale phenomena and local climate, by increasing horizontal resolution due to better representation of topographical effects and physical processes, with keeping the quality of representation of global climate. The model thus enables us to study global characteristics ofsmall-scale phenomena and extreme events in unprecedented detail.
Recent observational studies have described the non-closure of the energy balance when the eddy co-variance (EC) method is used for the measurements. We investigated this problem using a numerical simulation of a heterogeneous surface region. A typical daytime boundary layer was simulated, using the large eddy simulation (LES) method in which horizontal heterogeneity was imposed on the ground surface heating as a one-dimensional sinusoidal variation. This horizontal heterogeneity is expected to produce a mesoscale circulation. We decomposed the total vertical heat flux into the EC turbulent flux, the heat flux due to a mesoscale circulation (hereafter, mesoscale flux), and the “residual flux”. The sum of the mesoscale flux, and residual flux accounts for the energy imbalance if we estimate the total flux only from the EC method. The numerical results demonstrated that larger amplitude of surface heating caused larger mesoscale flux, but smaller residual flux. As a result, the energy imbalance became minima at some weak amplitude of surface heating. The residual flux was caused by the turbulent organized structure (hereafter, TOS), which is a cluster of thermals moving, with a larger time scale than that of individual plumes. The larger surface heating amplitude weakened the TOS due to the following two mechanisms; (1) the TOS is organized in roll due to the strong horizontal pressure gradient, (2) the higher horizontal wind speed, parallel to the mesoscale circulation, advects the TOS faster then the ergodicity works better. The other cases with gepstrophic winds, resulted in the decrease of the energy imbalance with increasing wind velocity.
Changes in the Baiu frontal activity in the future climate are examined, making use of super-high-resolution global and cloud-resolving regional climate models (20-km-mesh AGCM and 5-km-mesh NHM). In the present study, the focus is on the lengthened duration of the Baiu, and the characteristics of the precipitation during the Baiu season in the future climate. First, 10-year global-scale simulations of the present, and future climates are conducted by the 20-km-mesh AGCM. The present climate simulation accurately reproduces the northward shift of the Baiu front with time, and the end of the Baiu season around Japan. In the future climate, the Pacific anti-cyclone remains at the south of the Japan islands even late in July, resulting in the obscure migration of the Baiu front to the north and the lengthened Baiu season. Second, regional climate simulations are conducted by the 5-km-mesh NHM covering East Asia, in order to investigate the small-scale response to large-scale conditions, simulated by the 20-km-mesh AGCM. While the rainfall does not vary in June between the present and future climates, there is more rainfall in July in the future climate. Moreover, the frequency of the precipitation greatly increases with the intensity of the precipitation in July in the future climate simulation. In order to investigate the typical size of the precipitation systems that bring rainfall during the Baiu season, precipitation systems are classified according to the area coverage of the systems. Precipitation systems with an area larger than 90,000 km2 are more frequently seen in July in the future climate, than in the present climate, which corresponds to more rainfall. The increase of the large system in July is most remarkable in the vicinity of Kyushu Island, and the baroclinicity in that area is stronger in the future climate.
The relationships between the natural variability and CO2-induced response over the Pacific region are investigated in terms of the spatial anomaly pattern of SST, sea level pressure and precipitation by a multi-model intercomparison analysis, based on the 18-model results contributing to the IPCC Fourth Assessment Report. The analysis indicates that the CO2-induced response pattern is related with the model natural variability modes, ENSO and AO. In the tropical Pacific, an ENSO-like global warming pattern is simulated by the majority of the models, with mostly El Niño-like change. In the Arctic region, an AO-like global warming pattern is simulated by many models, with the positive definite AO-phase change, if AO-like. It is suggested that the increase in meridional temperature gradient in the upper troposphere, and the lower stratosphere, provides a preferable condition for the positive AO-like change in the high latitudes by intensifying the subtropical jet, while the increase in the static stability provides a preferable condition for the El Niño-like change in the low latitudes, by reducing the large-scale ambient circulations. However, the sign of the mass (SLP) anomaly is incompatible over the North Pacific, between the positive AO-like change and the El Niño-like change. As a result, the present models cannot fu11y determine the relative importance between the mechanisms inducing the positive AO-like change and inducing the ENSO-like change, leading to scattering in global warming patterns in regional scales over the North Pacific.