Yasutaka Wakazuki

Performance of Long-Term Integrations of the Japan Meteorological Agency Nonhydrostatic Model Using the Spectral Boundary Coupling Method

The spectral boundary coupling (SBC) method, which is an approach used to couple a limited-area model with a large-scale model, was introduced into a nonhydrostatic model. To investigate whether the SBC method works well in a long-term integration of a high-resolution nonhydrostatic model, two numerical experiments were conducted with a model having a horizontal grid interval of 5 km. In one experiment, the SBC method was employed, while it was not in the other experiment. The time integration in both experiments was over a 40-day period. The nonhydrostatic model was nested into objectively analyzed fields, instead of the forecasts from an extended-area model. Predicted patterns of sea level pressure and precipitation were compared with objective analyses, and data provided by the Global Precipitation Climatology Project (GPCP), respectively. The predicted rainfall amounts and surface temperature over the Japanese islands were statistically evaluated, making use of the analyzed rainfall and surface data observed by the Japan Meteorological Agency (JMA). All results examined in the present study exhibited better performances with use of the SBC method than those without the SBC method. It was found that the SBC method was highly useful in long-term simulations by a high-resolution nonhydrostatic model.

Incremental dynamical downscaling for probabilistic analysis based on multiple GCM projections

Abstract A dynamical downscaling method for probabilistic regional‐scale climate change projections was developed to cover the inherent uncertainty associated with multiple general circulation model (GCM) climate simulations. The climatological increments estimated by GCM results were statistically analyzed using the singular vector decomposition. Both positive and negative perturbations from the ensemble mean with the magnitudes of their standard deviations were extracted and added to the ensemble mean of the climatological increments. The analyzed multiple modal increments were utilized to create multiple modal lateral boundary conditions for the future climate regional climate model (RCM) simulations by adding them to reanalysis data. The incremental handling of GCM simulations realized approximated probabilistic climate change projections with the smaller number of RCM simulations. For the probabilistic analysis, three values of a climatological variable simulated by RCMs for a mode were analyzed under an assumption of linear response to the multiple modal perturbations.

Current and future carbon budget at Takayama site, Japan, evaluated by a regional climate model and a process-based terrestrial ecosystem model

Accurate projection of carbon budget in forest ecosystems under future climate and atmospheric carbon dioxide (CO2) concentration is important to evaluate the function of terrestrial ecosystems, which serve as a major sink of atmospheric CO2. In this study, we examined the effects of spatial resolution of meteorological data on the accuracies of ecosystem model simulation for canopy phenology and carbon budget such as gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) of a deciduous forest in Japan. Then, we simulated the future (around 2085) changes in canopy phenology and carbon budget of the forest by incorporating high-resolution meteorological data downscaled by a regional climate model. The ecosystem model overestimated GPP and ER when we inputted low-resolution data, which have warming biases over mountainous landscape. But, it reproduced canopy phenology and carbon budget well, when we inputted high-resolution data. Under the future climate, earlier leaf expansion and delayed leaf fall by about 10 days compared with the present state was simulated, and also, GPP, ER and NEP were estimated to increase by 25.2%, 23.7% and 35.4%, respectively. Sensitivity analysis showed that the increase of NEP in June and October would be mainly caused by rising temperature, whereas that in July and August would be largely attributable to CO2 fertilization. This study suggests that the downscaling of future climate data enable us to project more reliable carbon budget of forest ecosystem in mountainous landscape than the low-resolution simulation due to the better predictions of leaf expansion and shedding.

Physical Responses of Convective Heavy Rainfall to Future Warming Condition: Case Study of the Hiroshima Event

An extreme precipitation event happened at Hiroshima in 2014. Over 200 mm of total rainfall was observed on the night of August 19th, which caused floods and many landslides. The rainfall event was estimated to be a rare event happening once in approximately 30 years. The physical response of this event to the change of the future atmospheric condition, which includes a temperature increase on average and convective stability change, is investigated in the present study using a 27-member ensemble experiment and pseudo global warming downscaling method. The experiment is integrated using the Japan Meteorological Research Institute non-hydrostatic regional climate model. A very high-resolution horizontal grid, 500 m, is used to reproduce dense cumulonimbus cloud formation causing heavy rainfall in the model. The future climate condition determined by a higher greenhouse gas concentration is prescribed to the model, in which the surface air temperature globally averaged is 4 K warmer than that in the preindustrial era. The total amounts of precipitation around the Hiroshima area in the future experiments are closer to or slightly lower than in the current experiments in spite of the increase in water vapor due to theatmospheric warming. The effect of the water vapor increase on extreme precipitation is found to be canceled out by the suppression of convection due to the thermal stability enhancement. The fact that future extreme precipitation like the Hiroshima event is not intensified is in contrast to the well-known result that extreme rainfall tends to be intensified in the future. The results in the present study imply that the response of extreme precipitation to global warming differs for each rainfall phenomenon.

Simulations of Forecast and Climate Modes Using Non-Hydrostatic Regional Models

Two applications with a cloud-resolving model are shown utilizing the Earth Simulator. The first application is a case in the winter cold-air outbreak situation observed over the Sea of Japan as a forecast mode. Detailed structures of the convergence zone (JPCZ) and formation of mechanism of transverse convective clouds (T-modes) are discussed. A wide domain in the horizontal (2000 × 2000) was used with a horizontal resolution of 1 km, and could reproduce detailed structures of the JPCZ as well as the cloud streets in the right positions. It is also found that the cloud streets of T-modes are parallel to the vertical wind shears and, thus, similar to the ordinary formation mechanism as longitudinal convective ones. The second application is changes in the Baiu frontal activity in the future warming climate from the present one as a climate mode. At the future warming climate, the Baiu front is more active over southern Japan, and the precipitation amounts increase there. On the other hand, the frequency of occurrence of heavy rainfall greater than 30 mm h-1 increases over the Japan Islands.