Sachiho A. Adachi

Comparison of the Impact of Global Climate Changes and Urbanization on Summertime Future Climate in the Tokyo Metropolitan Area

AbstractIn this study, the impact of global climate change and anticipated urbanization over the next 70 years is estimated with regard to the summertime local climate in the Tokyo metropolitan area (TMA), whose population is already near its peak now. First, five climate projections for the 2070s calculated with the aid of general circulation models (GCMs) are used for dynamical downscaling experiments to evaluate the impact of global climate changes using a regional climate model. Second, the sensitivity of future urbanization until the 2070s is examined assuming a simple developing urban scenario for the TMA. These two sensitivity analyses indicate that the increase in the surface air temperature from the 1990s to the 2070s is about 2.0°C as a result of global climate changes under the A1B scenario in the Intergovernmental Panel on Climate Change’s Special Report on Emissions Scenarios (SRES) and about 0.5°C as a result of urbanization. Considering the current urban heat island intensity (UHII) of 1.0°...

Moderation of Summertime Heat Island Phenomena via Modification of the Urban Form in the Tokyo Metropolitan Area

AbstractThis study investigated the moderation of the urban heat island via changes in the urban form in the Tokyo metropolitan area (TMA). Two urban scenarios with the same population as that of the current urban form were used for sensitivity experiments: the dispersed-city and compact-city scenarios. Numerical experiments using the two urban scenarios as well as an experiment using the current urban form were conducted using a regional climate model coupled with a single-layer urban canopy model. The averaged nighttime surface air temperature in TMA increased by ~0.34°C in the dispersed-city scenario and decreased by ~0.1°C in the compact-city scenario. Therefore, the compact-city scenario had significant potential for moderating the mean areal heat-island effect in the entire TMA. Alternatively, in the central part of the TMA, these two urban-form scenarios produced opposite effects on the surface air temperature; that is, severe thermal conditions worsened further in the compact-city scenario because...

An Oceanic Impact of the Kuroshio on Surface Air Temperature on the Pacific Coast of Japan in Summer: Regional H2O Greenhouse Gas Effect

AbstractThis study used a 4-km resolution regional climate model to examine the sensitivity of surface air temperature on the Pacific coast of Japan to sea surface temperature (SST) south of the Pacific coast of Japan during summer. The authors performed a control simulation (CTL) driven by reanalysis and observational SST datasets. A series of sensitivity experiments using climatological values from the CTL SST datasets over a 31-yr period was conducted. The interannual variation in surface air temperature over the Pacific coast was well simulated in CTL. The interannual variation in SST over the Kuroshio region amplified the interannual variation in surface air temperature over the Pacific coast. It was found that 30% of the total variance of interannual variation in surface air temperature can be controlled by interannual variation in SST. The calculated surface air temperature on the Pacific coast increased by 0.4 K per 1-K SST warming in the Kuroshio region. Note that this sensitivity was considerabl...

Assimilating All-Sky Himawari-8 Satellite Infrared Radiances: A Case of Typhoon Soudelor (2015)

AbstractJapan’s new geostationary satellite Himawari-8, the first of a series of the third-generation geostationary meteorological satellites including GOES-16, has been operational since July 2015. Himawari-8 produces high-resolution observations with 16 frequency bands every 10 min for full disk, and every 2.5 min for local regions. This study aims to assimilate all-sky every-10-min infrared (IR) radiances from Himawari-8 with a regional numerical weather prediction model and to investigate its impact on real-world tropical cyclone (TC) analyses and forecasts for the first time. The results show that the assimilation of Himawari-8 IR radiances improves the analyzed TC structure in both inner-core and outer-rainband regions. The TC intensity forecasts are also improved due to Himawari-8 data because of the improved TC structure analysis.

Impact of high‐resolution sea surface temperature and urban data on estimations of surface air temperature in a regional climate

We investigated the impact of using high-resolution sea surface temperature (SST) data and a sophisticated urban model on the simulation of surface air temperature (SAT) in the Nagoya metropolitan area using a regional climate model. The spatially detailed structure of SST, expressed in high-resolution SST data, had relatively little impact on SAT. On the other hand, the difference in areal mean value of SST strongly affected SAT across a wide range of land surfaces. When a spatially inhomogeneous distribution was used for the urban fraction and anthropogenic heat, and appropriate physical properties for building materials were given according to the specific urban categories, we achieved significant improvements in both the diurnal range of SAT and its daily mean. Based on a comparison with an additional sensitivity experiment for building albedo, the sophistication of urban fraction and thermal parameters related to building materials had a comparable impact on SAT as presumable building albedo in the daytime, while they indicated a larger impact on the nighttime SAT. We conclude that: (1) the areal mean SST is critical rather than its resolution for the climatological average of SAT over the land; (2) the simultaneous refinement of the urban fraction and building material parameters, as well as an appropriate building albedo setting, greatly improves the representation of SAT; and (3) the refinement of areal mean SST and the urban data have the same degree of importance for a better representation of the SAT.

CONeP: A cost-effective online nesting procedure for regional atmospheric models

Abstract We propose a cost-effective online nesting procedure (CONeP) for regional atmospheric models to improve computational efficiency. The conventional procedure of online nesting is ineffective because computations are executed sequentially for each domain, and it does not enable users freely to determine the number of computational nodes. However, CONeP can completely avoid this limitation through three actions: 1) splitting the processes into multiple subgroups; 2) making each subgroup manage just one domain; and 3) executing the computations for each domain simultaneously. Since users can assign an optimal number of nodes to each domain, the model with CONeP is computationally efficient. We demonstrate the computational advantage of CONeP over the conventional procedure, comparing the elapsed times with both procedures on a supercomputer. The elapsed time with CONeP is markedly shorter than that observed with the conventional procedure using the same number of computational nodes. This advantage becomes more significant as the number of nesting domains increases.

Contributions of changes in climatology and perturbation and the resulting nonlinearity to regional climate change

Future changes in large-scale climatology and perturbation may have different impacts on regional climate change. It is important to understand the impacts of climatology and perturbation in terms of both thermodynamic and dynamic changes. Although many studies have investigated the influence of climatology changes on regional climate, the significance of perturbation changes is still debated. The nonlinear effect of these two changes is also unknown. We propose a systematic procedure that extracts the influences of three factors: changes in climatology, changes in perturbation and the resulting nonlinear effect. We then demonstrate the usefulness of the procedure, applying it to future changes in precipitation. All three factors have the same degree of influence, especially for extreme rainfall events. Thus, regional climate assessments should consider not only the climatology change but also the perturbation change and their nonlinearity. This procedure can advance interpretations of future regional climates.Changes in climatology and perturbation will lead to different impacts on regional climate change, but their effect remains a subject of debate. Here the authors develop a new downscaling procedure that reveals the importance of both changes on the regional climate and examines their nonlinear effect.

Decomposition of the large-scale atmospheric state driving downscaling: a perspective on dynamical downscaling for regional climate study

AbstractIn this study, we provide a perspective on dynamical downscaling that includes a comprehensive view of multiple downscaling methods and a strategy for achieving better assessment of future regional climates. A regional climate simulation is generally driven by a large-scale atmospheric state obtained by a global climate simulation. We conceptualize the large-scale state based on reconstruction by combining decomposed components of the states, such as climatology and perturbation, in different global simulations. The conceptualization provides a comprehensive view of the downscaling methods of previous studies. We propose a strategy for downscaling regional climate studies based on the concept of covering a wider range of possibilities of large-scale states to account for the uncertainty in global future predictions due to model errors. Furthermore, it also extracts the individual influences of the decomposed components on regional climate change, resulting in better understanding of the cause of the change. We demonstrate a downscaling experiment to highlight the importance of the simultaneous consideration of the individual influences of climatology and perturbation.