The characteristics and predictability of climate anomalies over the tropical Indian Ocean (TIO) and South Asian region during the boreal summer (June-July-August) of 2010 are investigated on the basis of atmospheric regional model simulations and five forecasts obtained from Asia-Pacific Economic Cooperation Climate Center coupled models. The robust features of summer 2010 are the basin-wide TIO warming and enhanced (suppressed) rainfall over the north Indian Ocean and maritime continent (head Bay of Bengal and parts of monsoon trough region). Our regional atmospheric model experiments corroborate that rainfall over South Asia was mostly determined by the TIO sea surface temperature (SST) warming during summer 2010. Most of the coupled models and their multi-model ensemble (MME) used in this study successfully predict the robust features over the TIO and/or South Asian region with 01 May 2010 initial condition. The positive rainfall anomalies over the west coast of India, southern Peninsular India, and central Bay of Bengal are qualitatively well predicted by the MME. Suppressed rainfall over the northeast Bay of Bengal associated with the northwestward extension of the northwest Pacific ridge is also reasonably predicted by the MME. On the other hand, the MME has a moderate skill in predicting positive rainfall anomalies over the convective zone of southeast TIO due to weak local SST warming. Further, the coupled models and their MME fail to predict the anomalous positive rainfall in northern Pakistan because of their inability in predicting mid-latitude circulation anomalies. This study reveals that the predictive skill of rainfall and circulation anomalies during summer 2010 over the TIO and South Asia is largely attributable to the Indian Ocean basin-wide warming during the decay phase of El Niño. These results indicate that the accurate simulation of the TIO SST by coupled models is critical in determining the 2010 South Asian summer monsoon rainfall.
This study examined seven indices of the cool summer climate in northern Japan in terms of spatial representativeness and interannual variability using weather observation station data and reanalysis data from the Japanese 25-year Reanalysis/Japan Meteorological Agency Climate Data Assimilation System in June-August for 1979-2010. The indices are constructed using sea level pressure (SLP) and surface air temperature (SAT): area-average SLP over the Okhotsk Sea; north-south SLP difference in northern Japan; east-west SLP difference along the Tsugaru Strait; east-west SLP difference along the Soya Strait; SAT anomaly from climatology at a location along the coasts of the Pacific and the Okhotsk Sea; diurnal variance of SAT at a location along the coasts of the Pacific and the Okhotsk Sea; and time coefficient of the east-west oscillation mode of SAT in northern Japan. The last two are newly proposed. The atmospheric fields represented by the indices commonly show the following features: the developed Okhotsk high at the surface and a mid-troposphere ridge to its northwest; southward extensions of low SAT, high SLP, low specific humidity, and high cloud water content along the Pacific coast of northern Japan and along the Japan Sea coast of the Eurasian continent; and strong easterly/northeasterly winds to the east and west of northern Japan. Furthermore, these indices show consistent interannual variabilities and clearly detect cool summers in northern Japan in the past. Meanwhile, differences between the indices lie in the location of the ridge in the mid-troposphere and the vertical structure of the Okhotsk high, the center locations of low SAT and enhanced easterly/northeasterly surface winds, and the degree of the southward extensions of the cool air along the Pacific coast of northern Japan and along the Japan Sea coast of the Eurasian continent. Based on these characteristics, we can choose a suitable index for the intended use.
The tourism sector is sensitive to the effects of climate change. This is the first study that examines the relationship between tourism and climate change over Japan using data from projections of future climate. We apply a statistical downscaling method to climate model data and estimate how tourism on a city-scale over Japan may be affected by the expected near-future global warming. We used the tourism climate index (TCI) to evaluate the effect of meteorological factors on tourism. We estimated TCI using data from observatories of the Japan Meteorological Agency (JMA), and compared it with monthly changes in tourist number at Morioka city, and annual variations in tourists at 38 areas in Japan. In general, TCI shows a positive correlation with tourists numbers, although the correlation depends on location. In mountainous regions such as Osorezan and Tsurugizan, TCI is clearly correlated with the number of tourists. As expected, TCI under present climate conditions identifies summer as the most comfortable season for tourism. We also estimate TCI under future climate conditions (around the year 2040) using data from the Model for Interdisciplinary Research on Climate (MIROC) and five climate models from the Coupled Model Intercomparison Project Phase 3 (CMIP3). For future climate over large areas of Japan, TCI generally increases in spring/autumn and decreases in summer because effective temperature move into the comfortable and uncomfortable range, respectively. This indicates that the comfortable season for tourists will change in the future from summer to spring and autumn. TCI in the winter season showed large variance between models owing to differences in predicted temperatures in the models.
Seasonal variations of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), and nitrous oxide (N2O), in the mid-troposphere over the western North Pacific, are investigated using air samples collected onboard a C-130H aircraft. These samples were obtained between Atsugi Base (35.45°N, 139.45°E) and Minamitorishima (MNM; 24.28°N, 153.98°E), once a month, from September 2010 to September 2012. Increasing trends of CO2 and N2O and large variability of CH4 and CO (at approximately 6 km) have been found. During summer, concentrations of CH4 and CO were found to increase with height over MNM. High concentrations of CH4 were persistently observed in the mid-troposphere throughout the observation period. The average enhancement ratios of CH4 to CO above background levels (ΔCH4/ΔCO) were 0.47 and 1.2 ppb/ppb for winter-spring and summer-fall, respectively. The results suggested that the high CH4 concentrations originated primarily from fossil fuel combustions in winter-spring, while there could be an additional contribution from increased biogenic sources during summer-fall. Because a surface station in MNM rarely observed the summer-fall high CH4 concentration values in the mid-troposphere, the aircraft measurements could provide a powerful constraint on the CH4 emission estimates for Asia, in addition to that provided by the surface measurements. This aircraft measurement program is regularly conducted for the long-term monitoring of the greenhouse gases in the mid-troposphere, and it has a significant role for filling the data gap of the existing measurement network.
During a cold-air outbreak between 25 and 27 January 2009, a broad cloud band formed along the coastal region from San-in to Hokuriku on the Sea of Japan side of the Japanese Islands. Along the mountain flank, precipitation in the cloud band was intensified. Intensification processes of the broad cloud band and microphysical characteristics of the intensified precipitation region were examined. During the lifetime of the cloud band, two low-pressure systems successively developed in the central Sea of Japan. Between the San-in and Hokuriku coastal regions located south of the low-pressure systems, westerly winds were predominant. It can be theoretically explained that the winds are blocked at least below 900 m by a high mountain region in Hokuriku. When the predominant westerly winds flowed into a high-pressure-gradient region produced by the blocking, unbalanced flow with ageostrophic winds formed. The relatively high pressure forced the westerly winds toward the left, resulting in southwesterly winds. The southwesterlies made a convergence with the predominant westerlies, the area of which corresponded to the intensified precipitation region. The correlation coefficient between vertical and horizontal polarized return signals (ρhv) averaged for 2 days indicates that melting particles were present below a height of several hundred meters in some periods and/or regions with surface temperature higher than 0°C. Above the melting level, the radar reflectivity (Zh) maximum in the intensified precipitation region was more than or equal to 35 dBZ during one-third of the 2-day lifetime of the cloud band. For portions of Zh ≥ 30 dBZ, the mode of specific differential phase (KDP) had negative values, which indicates the predominance of prolate graupel in the intensified precipitation region.
Vertical diffusion in a stable boundary layer near the surface is not clearly understood. In the present study, the vertical diffusivity of radon and the height below which the concentration of radon is high are estimated from observations made at two levels (1.5 and 100 m) on the meteorological tower in the campus of the Meteorological Research Institute, Tsukuba, Japan, in November 2006. Seven 12-hour episodes, in which radon concentration near the surface increased, were averaged to make one sequential dataset. The averaged time sequence was divided into two periods: the radon concentration near the surface increased in the first one (Period I) and was stationary in the second (Period II). The estimated vertical diffusivity was less than 0.05 m2 s-1 for Period I and was at most 0.13 m2 s-1 for Period II. The estimated thickness of the radon-rich layer was less than 50 m for the first three hours in the first period; however, inversion height was approximately 100 m. The height under which radon accumulated was somewhat lower than that at the temperature inversion in period I, which suggests that the turbulent transfer was not dominant in the process to generate the temperature inversion layer in this area. Although the vertical diffusion of radon was different between the two periods, it was difficult to distinguish them with surface observation, employed for conventional Pasquill stability categories classification.
Low stratiform clouds (LSCs) are of three different types: stratocumulus (Sc), stratus (St), and sky-obscuring fog (FOG). Ship-based cloud observations (September 1957-August 2002) and air-temperature and sea-level pressure data obtained from the ERA-40 reanalysis are used to investigate the seasonal relationships between the amounts of these LSC types and the estimated inversion strength (EIS) over the global ocean. Although it is known that a single linear relationship applies to the variations in the LSC amount as the sum of those of the LSC types and EIS, two relationships with different sensitivities are found between each LSC-type amount and EIS. The boundary lies at a sea surface temperature (SST) of approximately 16°C. The Sc amount is strongly correlated with EIS in the warm SST regime, whereas no correlation can be observed between them in the cold SST regime. In contrast, although FOG rarely occurs in the warm SST regime, its amount increases with EIS in the cold SST regime. The St amount increases with EIS in both regimes, with higher sensitivity in the cold SST regime. Examination of vertical layers contributing to EIS reveals that an increase in the inferred inversion strength between 850- and 925-hPa levels corresponds to that in the Sc amount in the warm SST regime. In the cold SST regime, as EIS increases, relatively high values of inferred inversion strength between 700- and 850-hPa levels change to a rapid increase in that between 925-hPa level and the surface, which coincides with the transition from Sc to FOG. Temperature advection implied by the air-sea temperature difference provides favorable conditions to the different variations in the two regimes: general occurrence of cold advection in the warm SST regime and cold-to-warm transition of advection in the cold SST regime.
We simulated mesoscale distributions of summertime human heat stress around the Seto Inland Sea in western Japan, using a mesoscale numerical model: WRF-ARW at 2 km horizontal resolution. In this study, the Heat Index, which indicates the real human-felt temperature, was adopted as a heat-hazard index that can represent mesoscale spatial distributions of human heat stress. Our simulations specified regions of undesirably high daytime Heat Index around the Seto Inland Sea for the year 2007, an extremely hot summer. Although the daytime high surface air temperature in the Osaka Plain was the most prominent of the calculation domain (at 1400 JST, August air temperature means of 33-35 °C), high Heat Index regions were broadly found in other land areas around the Seto Inland Sea in addition to the Osaka Plain: the Tokushima, Okayama, Sanuki, and Nakatsu Plains (at 1400 JST, August Heat Index means of 37-39 °C). Daytime differences between air temperature and Heat Index were large in the four previously mentioned plains (excluding the Osaka Plain), showing a difference of 5-7 °C in our simulation. The large differences were caused by the difference of relative humidity in the region. Hence, residents in these plains should carefully avoid high heat stress, because they may be feel hotter than the actual air temperature. To discuss mechanisms increasing the mesoscale Heat Index, sensitivity experiments for mountains and sea surface temperature were conducted. They revealed that those factors worked selectively in the above regions. The existence of mountains induced thermal effects due to a valley-like terrain, and higher sea surface temperature produced warm and moist air transports from the sea. Daytime local circulations, which were topographically and thermally driven, were important for both factors. These mountain and high sea surface temperature influences produced increases of Heat Index of 1-2 °C in the aforementioned four regions.