Large amounts of reactive gases and aerosols are emitted from urban areas. Megacities, including the Tokyo Metropolitan Area (TMA), are very large, concentrated sources of these species affecting local, regional, and global ozone (O3) and aerosol levels. Emissions strongly influence air quality and climate on these scales. In 2003-2004, we made intensive measurements of O3 and chemical composition of aerosol particles with diameters less than 1 µm (PM1 aerosol) together with their precursors for the first time in Tokyo, Japan, as a part of the series of Integrated Measurement Program for Aerosol and Oxidant Chemistry in Tokyo (IMPACT) campaigns. Using these data, we investigated the formation and transport processes of O3 and PM1 aerosols through the analysis of their temporal variations near the urban center of Tokyo and regions downwind. Key findings obtained in these studies are reviewed in this paper.
During a cold-air outbreak, a broad cloud band is occasionally observed over the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) that forms over the Sea of Japan from the base of the Korean Peninsula to the Japanese Islands. On 14 January 2001, a broad cloud band associated with the JPCZ (JPCZ cloud band) extended in a southeastward direction from the base of the Korean Peninsula to Wakasa Bay, and it stagnated for half a day. The JPCZ cloud band consisted of two cloud regions: one was a long cloud band extending along its southwestern edge (a developed convective cloud band), and the other was the region consisting of cloud bands normal to a wind direction of winter monsoon (transversal cloud bands). The structure and formation mechanism of the transversal cloud bands were examined on the basis of observations (e.g., satellite images, in situ measurement and cloud-pro.ling radar data from an instrumented aircraft and upper-air soundings from observation vessels) and simulation results of a cloud-resolving model with a horizontal resolution of 1 km. The transversal cloud bands had the following characteristic structures; they extended along a northeast-southwest direction, which was parallel to the direction pointed by the vertical shear vector of horizontal wind in the mixed layer, they mainly consisted of convective clouds, which slanted with height toward the down-shear side, and they widened and deepened toward southwest, as the depth of the mixed layer increased. An examination of simulation results presented that the transversal cloud bands were accompanied by roll circulations. The axes of rolls were oriented nearly parallel to the direction of the vertical shear vector in the mixed layer. An analysis of the eddy kinetic energy budget indicated that the roll circulations derived most of its energy from the mean vertical shear and the buoyancy.
An afternoon heavy rainfall event in northern Taiwan, observed on June 20, 2000 during the monsoon break, is investigated using surface observation and Doppler radar data and a nonhydrostatic model with a horizontal grid spacing of 1.33 km. Heavy rainfall was brought majorly by two precipitation systems, namely A and B. System A was initiated and developed in Taipei Basin, associated with a local wind convergence line. System B was formed on the western slopes south of Taipei Basin, extended northward of the Taipei Basin, and lasted for 4 h. The formation and maintenance mechanisms of the two systems are examined using sensitivity experiments in which the terrain of Central Mountain Range (CMR) or Yangmin Mountain (YM) located north of the Taipei Basin is removed or reduced. In addition, the effect of cold outflow from System A on System B is examined. The wind convergence line responsible for forming and maintaining System A is brought from the interaction between the prevailing southwesterly winds and the easterly/northeasterly flow caused by the terrain effect of CMR. Furthermore, the northeasterly onshore flow and the low-level northwesterly/northerly flow resulting from the prevailing southwesterly winds turning southward over southern YM, enhance System A inside Taipei Basin. System B is produced over relatively high sloped areas (elevations exceeding 200 m) in southwestern Taipei Basin by the prevailing southwesterly winds. Over relatively low lands (elevations less than 200 m), the cold outflow from System A inside Taipei Basin converges with the prevailing southwesterly winds and helps the maintenance of System B.
The kinematic and thermodynamical structures of two longitudinal-mode (termed “L-mode”) snow bands over the Sea of Japan occurring on February 8, 1991 and January 21, 1993 are analyzed mainly based on dual-Doppler radar data. The L-mode snow bands with multicellular structure in 1991 and 1993 formed, respectively, at the early onset of and toward the end of cold-air outbreaks, where the magnitude of the band-transverse vertical shear was roughly 2 × 10-3 s-1 approximately in the lower-half of the mixed layer. This magnitude was larger than that associated with L-mode snow bands characterized by axi-symmetric circulation, which will be described in Part II. Thermodynamical structures and the spatial distributions of water substances in the two snow bands were inferred from variational-based retrieval. A pronounced feature of the airflow structures in both snow bands was upshear-tilting updrafts in the band-transverse vertical cross-section. At least two factors could account for their formation: the existence of a certain depth of the vertical layer of the environmental band-transverse system-relative wind components directing to the upshear, and the lower terminal fall velocities of snow and graupels. The retrieval results showed that both snow bands had a subsaturated cold pool beneath the band around the surface, whose collision with the unstable ambient air could have been responsible for overall upward motion in the bands. With regard to the energetics of the band circulation, energy production by buoyancy and the band-transverse shear was dominant. The repeated formation of new cells was observed in the two snow bands in the downshear side, which may have been triggered by the low-level collision of the outflow from active cells or a cold pool with the unstable ambient air. As the new cell developed enough, the older cell significantly decayed. Consequently, the successive formation of cells did not change the overall echo pattern of the L-mode snow bands without producing elongated echoes branching o. into the downshear direction at large angles to the orientation of the L-mode snow bands.
Japan experienced unusually heavy snowfall and low temperatures in December 2005 owing to the cold air advected from Siberia. As a result of this strong and sustained cold surge, record-breaking snowfalls occurred repeatedly along the coast of the Sea of Japan. To determine the cause and to examine the accuracy of numerical forecasts of such unusual weather as well as to investigate the impact of initial and boundary conditions on the forecasts, we conducted one-month ensemble forecasts for December 2005 by changing lower boundary conditions, such as sea-surface temperature (SST) and sea-ice coverage (SIC). Forecasts were initiated every day between November 1 and December 1 to investigate their dependency on initial atmospheric conditions. Our results showed that SST and SIC appeared to have little impact on the forecasts of a cold December in Japan. Instead, high sensitivity to initial atmospheric conditions was found. Almost all forecasts initiated before November 16, 2005 failed to reproduce the cold December; however, those initiated after that date successfully predicted it. By comparing the differences between successful and unsuccessful members for predicting the cold December, we identified the processes producing the cold December of 2005. In the successful forecasts, we found that blocking developed over the North Atlantic in mid-November 2005. When it decayed, the emitted Rossby waves propagated along the subpolar jet across Siberia, inducing a cold surge over Japan. Moreover, the Rossby waves propagating over the Indian sector along the subtropical jet gave rise to intense convection over the Bay of Bengal and the South China Sea, which led to the deepening of a low-pressure trough over Japan. The unusual weather conditions in December have been due to the simultaneous occurrence of these two processes.
The synoptic-scale condition related to the intense rainfalls on August 17, 1968, over the Kiso?Hida and Nagara River Basin in the central part of Japan is studied by using European Center for Medium-Range Weather Forecasts 40-year reanalysis data and upper, synoptic-surface, local rain-gauge observation data and satellite cloud images. The intense rainfalls occurred within a long cloud belt formed with a low-level moist belt (LMB), which had formed along the northwestern rim of the North Pacific subtropical anticyclone (NPSA). The LMB was sustained by large-scale moisture transport along the northern rim of the NPSA and mesoscale northward moisture transport along a small anticyclone embedded in the NPSA. In the lower and middle troposphere, dry air spread over the Japan Sea after the passage of a severe tropical storm (STS) over the Japan Sea owing to the prevailing westerlies to the southwest of the STS. The northern edge of the LMB was bounded by the dry air. The northern boundary of the LMB was signified as the dry front because of the strong moisture gradient and very weak thermal gradient. Intense rainfalls occurred in the cloud belt, at a distance of ∼1200 km south of the STS, over the river basin where the large-scale and the mesoscale moist flows converged in association with the orographic convergence. Intense convective rainfalls were accompanied by the increase in the convective instability due to the southeastward intrusion of the middle tropospheric dry air over the LMB. Significant low-level jet stream and mesoscale depression were not found around the intense rainfall area.
A numerical model in which the effects of cumulus convection are incorporated as the subgrid-scale and mesoscale organized convection is resolved by the grid (mesoscale-convection-resolving model, MCRM) was developed in the 1980s with an intention of improving the parameterization schemes for moist convection, which had been used since the 1960s. As in many numerical models with parameterization in the 1980s, hydrostatic equilibrium was assumed. The present paper describes a nonhydrostatic version of the MCRM, with some modifications of the subgrid-scale effect formulation used in the hydrostatic MCRM. Numerical experiments are performed to get some evaluation of the performance of the nonhydrostatic version of the MCRM through the comparison of the results in two cases with and without the effects of the subgrid-scale cumulus convection. Although the most efficient horizontal grid size of the MCRM ranges from about 20 km to 5 km, only the results of a 15-km grid case are presented in this paper. The initial condition used in the numerical experiments is idealized (simplified). However, such numerical experiments can be considered useful to understand the behavior of spiral rainbands and eyewall convection in well-developed tropical cyclones and to evaluate the model validity. The subgrid-scale effects in the nonhydrostatic MCRM are not so important as those in the hydrostatic MCRM. However, it is shown that the eyewall and spiral rainbands are not simulated well unless the subgrid-scale effects are incorporated.