The representations of extreme weather during the landfall of Typhoon Songda (2004) in regional simulations at 1-km resolution are described and compared for two cloud-resolving models: Non-hydrostatic Model (NHM) and the Weather Research and Forecasting model (WRF). Both models, using the same 5-km-mesh outputs from NHM as their initial and boundary conditions, successfully reproduced the observed typhoon track and intensity. The comparison of surface winds indicated that WRF evaluates more enhanced extremes than NHM; on the other hand, the representations of rainfalls indicated that the extremes of hourly and accumulated rainfalls simulated by the two models are evaluated differently. Slight differences in the model topography between the two models, though produced by the same terrain dataset, were shown to significantly affect the representations of the extremes in each model. It should be recognized that not only the differences in model numerics and physics but also slight changes in the reproduction of topography induce differences in the representations of extreme weather.
To validate the reproducibility of simulated typhoons and related precipitation, perfect boundary condition experiments, forced by 20-km-mesh Japan Meteorological Agency (JMA) operational regional objective analysis (RANAL) data from June to October in 2002-2006, are performed using a 5-km-mesh cloud-system resolving nonhydrostatic regional climate model (NHM-5km). NHM-5km reproduced all 57 typhoons observed in the model domain during the experiment period. Relative to JMA best track data for the typhoons, the average position error of typhoon centers is 130 km at 72 hours after typhoon genesis or when the typhoon first enters the model domain. The simulated minimum central pressures of typhoons are similar to those in RANAL data. The precipitation within (outside of) a 300-km radius from the center of each typhoon is defined as direct (indirect) precipitation. The distributions of the simulated direct and indirect precipitation of typhoons show good agreement with precipitation analyses by JMA. For hourly precipitation amounts exceeding 5 mm, direct precipitation is observed more frequently than is indirect precipitation, especially for amounts exceeding 20 mm. These features are well reproduced by NHM-5km. The appearance frequency of simulated direct precipitation for hourly precipitation amounts also shows a good quantitative agreement with that measured by raingauges.
Regional climate experiments using a cloud-system resolving non-hydrostatic model with a horizontal resolution of 5 km are conducted in order to project changes in precipitation extremes in the vicinity of Japan during the warm season due to global warming. The results of a global 20-km model from May to October for 10-year periods in the present and future climates are adopted as the initial and boundary conditions. The simulated results project an increment in 90th-percentile values of daily precipitation on the Pacific side of the Japanese Islands during July in the future climate. Monthly mean fields during the month in the future climate show that abundant specific humidity of 20 g kg-1, 3 g kg-1 larger than that in the present climate, is supplied from the Pacific Ocean. In the future climate, mean equivalent potential temperature reaches 370 K near the sea surface south of western Japan, while a middle-level (around 5 km) drier region extends from the Asian Continent to the northwestern part of western Japan. As a result, convective instability is intensified in the vicinity of Japan and intense, deep convective systems form on the Pacific side of Japan.
The influence of global climate change due to greenhouse effects on the earth’s environment will require impact assessment, mitigation and adaptation strategies for the future of our society. This study predicts future ocean wave climate in comparison with present wave climate based on the atmospheric general circulation model and global wave model. The annual averaged and extreme sea surface winds and waves are analyzed in detail. There are clear regional dependences of both annual average and also extreme wave height changes from present to future climates. The wave heights of future climate will increase at both middle latitudes and also in the Antarctic Ocean, with a decrease at the equator.
We assessed the reproducibility of regional-mean probable precipitation in Japan estimated based on the results of three runs of a 20km-mesh global climate model (MRI-GCM20) and one run of a 20km-mesh regional climate model (MRI-RCM20). These models inappropriately simulate the annual maximum daily precipitation at individual grid points, especially in relation to the spatial pattern, but estimate the regional mean reasonably well. MRI-GCM20 runs simulated data over annual maximum regional-mean 10-day precipitation better than 1-day. Precipitation and return values estimated by MRI-GCM20 were quite different from those by MRI-RCM20. Among the three MRI-GCM20 runs, one forced by SST without year-to-year variability estimated the standard deviation of the annual maximum precipitation as smaller than those forced by SST with year-to-year variability. The reproducibility of the average and standard deviation of the annual maximum precipitation is correlated with the reproducibility of 30-year return values.
To evaluate the impact of climate change on snowfall in Japan, a hydrological simulation was made in the Agano River basin by using a regional climate model’s output. A hindcast experiment was carried out for the two decades from 1980 to 1999. The average correlation coefficient of 0.79 for the monthly mean discharge in the winter season showed that the interannual variation of the river discharge could be reproduced and that the method can be used for climate change study. The future hydrological response to global warming in the 2070s was investigated using a pseudo-global-warming method. In comparison to data from the 1990s, the monthly mean discharge for the 2070s was projected to increase by approximately 43% in January and 55% in February, but to decrease by approximately 38% in April and 32% in May. The flood peak in the hydrograph was moved forward by approximately one month, changing from April in the 1990s to March in the 2070s. Furthermore, the projection for the 10-year average snowfall amount was projected to be approximately 49.5% lower in the 2070s than in the 1990s.
This study assessed the future change of the 30-year return level of daily precipitation due to global warming for Kagoshima Prefecture, (except for the Amami Islands) in southern Kyushu, Japan, using regional frequency analysis. The 20km-mesh regional climate model (MRI-RCM20) was used for this analysis. The present climate data was reproduced for the years 1981 through 2000, and the future climate data was projected for the years 2081 through 2100 under the greenhouse gas emission scenario SRES A2. Over Kagoshima Prefecture, the future change of the regional average of annual maximum daily precipitation was projected to increase by only 3.3%. However, the quantile that corresponds to the non-exceedance probability of the 30-year return level was projected to increase by 14.5%. As a result, in Kagoshima Prefecture, the 30-year return level of daily precipitation was projected as likely to increase by 18.3% in about one hundred years.
The controlled simulation output of a super-high resolution atmospheric model (AGCM20) was evaluated from a hydrologic point of view, using a distributed hydrologic model and observed river discharge data. The AGCM20 output for the current climate condition should be able to provide a current river flow pattern when it is converted into river discharge information using a reasonably well-prepared hydrologic model. For this evaluation, a distributed hydrologic model was composed for various basin scales ranging from 60 km2 to 8,772 km2 in the Tone River Basin, Japan, and calibrated in advance, using observed data of each sub-basin. Most sub-basins in the study area provide more than 25 years of observed discharge data, and the evaluations were conducted by comparing seasonal patterns of simulated and observed discharges. The result shows that the GCM output provides excellent discharge output when the basin scale is more than 5,000 km2, while its utilization into a smaller basin remains limited. The proposed hydrologic evaluation method provides direct and comprehensive understanding to hydrologists in addition to the conventional evaluation method.
The strong wind field caused by Typhoon Songda, which passed through Kyushu in Japan in 2004, was simulated numerically. A non-hydrostatic mesoscale model was used for the numerical simulation. The simulated wind field was compared with the observed wind field in terms of wind speeds and directions, which were measured by the wind observation network, NeWMeK. It was found that the temporal variation of the calculated wind speeds at 1 km hori-zontal mesh grids correspond to the average of the observed wind speeds over a ten to fifteen minute time period. The temporal variation of the calculated wind directions showed good agreement with the observations. The maximum values of the calculated wind speeds were highly correlated to the maximum values of the observed wind speeds. The applicability of the mesoscale model for the wind hazard assessment of buildings was examined, and it was found that the maximum values of the calculated wind speeds at higher altitudes showed higher correlations with the rates of damaged houses caused by Typhoon Songda.
Sensitivity analysis on lake temperature under the A1B SRES climate change scenario has been evaluated by coupling MRI-GCM with 20 km outputs and the Biwa-3D integrated assessment model. The non-hydrostatic 3D hydrodynamic model featuring Very Large Eddy Simulation (VLES) has been verified and compared with field observations in 2002 (the present-day year, PDY). A significant temperature increase in surface water, recording more than 34 degrees in the East Coast of the North Basin of the Lake Biwa, has been projected for the simulation for the very hot year (VHY), determined from comparison with MRI-GCM output for the year 2099, which may induce a catastrophic impact on lake water quality during the period. Weak stratification is predicted to start from March in VHY, compared with observations of stratification that doesn’t commence until after April in PDY. The thickness of the epilimnion which is around 15–20 m in PDY increases to 25–30 m in August in VHY due to a much higher atmospheric temperature. According to the model outputs, the vertical mixing may not always decrease due to the accumulated heat in the hypolimnion which has a higher temperature than the temperature during the winter. Accumulated heat in the hypolimnion may induce catastrophic degradation in the lake ecosystem.
This study projects the river discharge in the Magdalena River basin, Colombia, considering projected climate conditions for the 21st century, by using a 20-km-mesh atmospheric global climate model and a 0.5°-mesh global river routing model under a greenhouse gas emission scenario. The climatological annual mean river discharges along the main stream of the Magdalena River do not change significantly, however precipitation, evaporation, and total runoff into the river show statistically significantly changes over most of the Magdalena River basin. By the end of the 21st century, the projected climatological monthly mean river discharge at Puerto Berrio decreases statistically significantly in April, October, and November (P < 0.05), compared to current values, whereas it shows a distinct increase for June through August, thereby reducing the present bimodality of its seasonal variation. Minimum climatological monthly mean river discharge in February could be lower at the end of the 21st century than in the current condition. These results should help increase the awareness of the changing river discharge in the Magdalena River basin, and prepare adaptation strategies to face these challenges.
Production and transportation of suspended sediment was investigated by sampling surface soils in 18 domains over an entire river basin. Particle size distributions alone suggested that all domains were a source of suspended sediment. Chemical composition analysis however showed that the transport rate of suspended sediment was more closely related to the type of surface soil than to land use. A method was developed which enables estimation of the relative suspended sediment transport rates from each domain of a river basin to its downstream end. The model’s validity was verified through good agreement with field sampling results.
We constructed historical (1900–) high-resolution (0.05° × 0.05°) daily precipitation data over the Japanese land area as part of the product of the “Asian Precipitation – Highly-Resolved Observational Data Integration Towards Evaluation of the Water Resources” (APHRODITE) project. This product APHRO_JP is derived from rain gauge observations and is intended to accurately represent both mean and extreme values. Due to new interpolation techniques developed in APHRODITE, estimation accuracy for orographic precipitation is improved, and bias for long-term amount is reduced, even for the early 20th century in which the observation network was sparse in space. Moreover, the product can be used for statistical analysis of heavy precipitation up to about 150 mm/day, over a long term period (≥ 100 years). APHRO_JP enables diverse research, including validation of meso-scale models and analysis of the longterm extreme precipitation trend in Japan.
This study presents a stochastic typhoon model (STM) for estimating the characteristics of typhoons in the present and future climate conditions. Differences between statistical characteristics of present and future typhoons were estimated from projections by an Atmospheric General Circulation Model (AGCM) under a climate change scenario and are taken into account in the stochastic modeling of future typhoons as a climate change signal. From the STM results which utilize the Monte Carlo simulation, it was found that the frequency of typhoon landfall in Japan, especially in three major bay areas, will decrease and the mean value of typhoon central atmospheric pressure will not change significantly. An important point is that the arrival probability of stronger typhoons will increase in the future climate scenario.
This study aims to reveal the factors most useful for evaluating the influence of wind on entrainment from the lower to upper layer in a two-layer system. Lake Abashiri, which is a typical brackish lake, is chosen as a study area because a distinct two-layer system exists due to salt-wedge intrusion from the ocean. To evaluate entrainment velocity between two layers, a 3D numerical computation is applied, showing good agreement with field observations. Computations suggest that the entrainment velocity estimated using the Richardson number is smaller than that from field observations. Thus, other factors are computed suggesting that use of the Lake number is more effective in estimating entrainment velocity due to internal wave breaking and interfacial fluctuation than use of the Effective Wedderburn number.
The Metouia Oasis (South Tunisia) is increasingly being impacted by high soil salinity, leading to yield losses and ultimately to farmland abandon. In this area shallow groundwater rise is considered the major factor responsible for salt accumulation in soils. However, limited data exist on groundwater fluctuations in Metouia Oasis, despite the criticality of such information for accurate soil salinity assessment. In this study, groundwater behavior was monitored at a high measurement frequency. An observation well was installed and equipped with a water level sensor (Diver DI240, Van Essen Instrument) to record the groundwater fluctuation at 30 minute intervals. The results showed that at seasonal basis, groundwater was characterized by a declining phase during spring and summer seasons followed by a rising phase starting from mid-October. Further, the hydrograph was characterized by several rapid water table rises following recharge events followed by extended periods of water table decline. The hourly observations at daily basis displayed strong evidence of diurnal effects on groundwater patterns. The results from this study suggest that consideration should be given to monitoring water table behavior in Metouia Oasis more frequently in order to capture daily and seasonal patterns.
To clarify plot-scale runoff characteristics in bamboo forests, soil properties and surface runoff were measured in a preliminary study of a forested hillslope of moso-bamboo (Phyllostachys pubescens). Infiltration capacities and saturated hydraulic conductivities at 10, 30 and 50 cm depths were similar to or greater in the moso-bamboo forest than in an adjacent broad-leaved forest. This suggests that surface runoff rarely occurs in the moso-bamboo forest. However, surface runoff was observed in seven of 14 storm events. The surface runoff responses to rainfalls were relatively rapid and the amount of surface runoff per storm event depended largely on rainfall. The proportion of the total amount of surface runoff to the total rainfall during the observation period ranged from 19 to 33%, depending on the observation system. This indicates that the greater portion of rainfall infiltrated into the soil. A high density of roots in the surface soil suggests that part of the rainfall was impeded and passed laterally through the surface soil as preferential flow around the root mats and/or the rhizomes, thereby contributing to surface runoff.
Iron is the limiting nutrient of phytoplankton in the Sea of Okhotsk, and the majority of iron in this system is fed by the Amur River. The recent conversion of wetlands, the main source of iron in the Amur River basin, to agricultural lands will likely impact dissolved iron productivity, which may also influence primary production in the Sea of Okhotsk. Therefore this study was conducted to construct a macroscopic index for use in assessing dissolved iron productivity in the basin. Correlation analysis between climate and topographic parameters and the observed dissolved iron concentration in forests and wetlands revealed that the topographic wetness index (TWI) had a significant correlation with dissolved iron concentration. An exponential curve was found to be the best curve to express this correlation. We assumed that dissolved iron concentration for grasslands and agricultural lands, the other two dominant land cover types, could also be expressed by TWI. Based on this assumption, dissolved iron concentration curves for grasslands and agricultural lands were inversely identified by systematic modification of the curve for forests and wetlands. The results suggest that TWI can describe the average dissolved iron concentration of major land cover types in the basin.
This study quantified the uncertainty range of future change in the 30-year return level of daily precipitation due to global warming for Kagoshima Prefecture (except for the Amami Islands), Japan, based on regional frequency analysis, following the results of Ishihara (2010b). The uncertainty due to resampling variability was quantified with a Monte Carlo simulation based on the regional quantile function, using projection results from MRI-RCM20. The 5%–95% uncertainty ranges of the regional 30-year quantile for the present and future climates were 1.941 ± 0.117 and 2.217 ± 0.160, respectively. Moreover, the 5%–95% range of the simulated future change ratio of the regional 30-year quantile was 1.142 ± 0.107. Based on the previous result that the annual maximum daily precipitation averaged over the region is projected to increase by 3.3%, the 5%–95% uncertainty range of the future change ratio of the regional 30-year return level of daily precipitation in the region was projected to be 1.180 ± 0.111. This result indicates that the regional 30-year return level of daily precipitation in the region is likely to increase by 6.9%–29.1% in about 100 years.