Masayuki Hara

The Non-hydrostatic Icosahedral Atmospheric Model: description and development

This article reviews the development of a global non-hydrostatic model, focusing on the pioneering research of the Non-hydrostatic Icosahedral Atmospheric Model (NICAM). Very high resolution global atmospheric circulation simulations with horizontal mesh spacing of approximately O (km) were conducted using recently developed supercomputers. These types of simulations were conducted with a specifically designed atmospheric global model based on a quasi-uniform grid mesh structure and a non-hydrostatic equation system. This review describes the development of each dynamical and physical component of NICAM, the assimilation strategy and its related models, and provides a scientific overview of NICAM studies conducted to date.

Intermodel variability of future changes in the Baiu rainband estimated by the pseudo global warming downscaling method

[1]xa0Changes in the Baiu rainband owing to global warming are assessed by the pseudo global warming downscaling method (PGW-DS). The PGW-DS is similar to the conventional dynamical downscaling method using a regional climate model (RCM), but the boundary conditions of the RCM are obtained by adding the difference between the future and present climates simulated by coupled general circulation models (CGCMs) into the 6-hourly reanalysis data in a control period. We conducted the multiple PGW-DS runs using the selected Coupled Model Intercomparison Project Phase 3 (CMIP3) multimodel data set, giving better performance around East Asia in June, and the PGW-DS run using the multiselected CGCM model ensemble mean (PGW-MME run). The PGW-MME and PGW-DS runs show an increase in precipitation over the Baiu rainband and the southward shift of the Baiu rainband. The PGW-MME run has good similarity to the average of all PGW-DS runs. This fact indicates that an average of the multiple PGW-DS runs can be replaced by a single PGW-DS run using the multiselected CGCM ensemble mean, reducing the significant computational expense. In comparison with the GCM projections, the PGW-DS runs reduce the intermodel variability in the Baiu rainband caused by the CGCMs themselves.

Why a Large Amount of Rain Falls over the Sea in the Vicinity of Western Sumatra Island during Nighttime

Abstract Precipitation measurements from the Tropical Rainfall Measuring Mission satellite indicate that annual rainfall over the sea in the vicinity of western Sumatra Island is among the highest on the earth, and most of this rainfall occurs during nighttime. Surface meteorological observations at Tabing on the western coast of the island show frequent occurrences of sudden offshore winds accompanied by an abrupt drop in surface temperatures in the late afternoon and evening. Model simulations for a 1-month period during the rainy season of the region successfully simulate the satellite-observed regional distribution and diurnal variation of rainfall. The simulation results show that convection develops across a wide area over the mountainous areas of the island at similar times in the afternoon with the development of thermally induced local circulations. At these times of the day, convection over the sea along the western coast of the island is suppressed by the thermally and topographically induced d...

Impact of extensive irrigation on the formation of cumulus clouds

[1]xa0Irrigated farmland in an arid region creates a prominent heat contrast along its boundary. A statistical analysis of satellite images indicated that the frequency distribution of clouds has a clear contrast between the Hetao Irrigation District in China and the surrounding dry area in the summer. In this paper, we use a mesoscale model to clarify the impact of the extensive irrigation on the cloud formation. The numerical experiment assuming simplified irrigation indicates that the colder surface of the irrigated area induces a surface wind toward the dry area, where the surface temperature is higher. This wind is a part of a land-use-induced circulation. The circulation matures in the early afternoon and generates clouds near the border of the dry area and irrigated area.

Interdecadal variability of the early summer surface heat flux in the Kuroshio region and its impact on the Baiu frontal activity

[1]xa0In the Kuroshio region (KR), the early summer surface heat flux (SHF) exhibits a rapid increase around 1990 as a climate shift, which is rooted in the oceanic interdecadal variability in the North Pacific. The sea surface temperature (SST) in the KR, where SST is largely controlled by subsurface ocean circulations, is positively correlated with the local upward SHF. The positive SHF anomalies in the 1990s reinforced the Baiu frontal activity with an atmospheric large-scale meridional dipole in the western North Pacific. As such, the oceanic interdecadal variability has an impact on the atmospheric circulation through coherent SST and SHF variations even in early summer in the KR.

Altitude dependency of future snow cover changes over Central Japan evaluated by a regional climate model

[1]xa0The Sea of Japan side of Central Japan is one of the heaviest snowfall areas in the world. We investigate near-future snow cover changes on the Sea of Japan side using a regional climate model. We perform the pseudo global warming (PGW) downscaling based on the five global climate models (GCMs). The changes in snow cover strongly depend on the elevation; decrease in the ratios of snow cover is larger in the lower elevations. The decrease ratios of the maximum accumulated snowfall in the short term, such as 1 day, are smaller than those in the long term, such as 1 week. We conduct the PGW experiments focusing on specific periods when a 2 K warming at 850 hPa is projected by the individual GCMs (PGW-2K85). The PGW-2K85 experiments show different changes in precipitation, resulting in snow cover changes in spite of similar warming conditions. Simplified sensitivity experiments that assume homogenous warming of the atmosphere (2 K) and the sea surface show that the altitude dependency of snow cover changes is similar to that in the PGW-2K85 experiments, while the uncertainty of changes in the sea surface temperature influences the snow cover changes both in the lower and higher elevations. The decrease in snowfall is, however, underestimated in the simplified sensitivity experiments as compared with the PGW experiments. Most GCMs project an increase in dry static stability and some GCMs project an anticyclonic anomaly over Central Japan, indicating the inhibition of precipitation, including snowfall, in the PGW experiments.

The Impact of Orographically-Induced Gravity Waves on the Diurnal Cycle of Rainfall over Southeast Kalimantan Island

Abstract Precipitation measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite indicate that the southeastern area of Kalimantan (Borneo) Island receives much less rainfall than elsewhere on the island during the period from July to October. Results from surface meteorological observations show that the diurnal cycle of rainfall differs greatly between the eastern and western coasts of the island. Rainfall on the western coast of the island is frequent in the afternoon and evening, whereas almost all rainfall on the eastern coast occurs in the morning. Meanwhile, the Global Positioning System (GPS)-derived precipitable water (PW) on the eastern coast shows a substantial decrease in moisture in the afternoon and evening. Numerical experiments with a mesoscale model reveal that gravity waves driven by diurnal heating of the elevated land surface of the mountains on Sulawesi Island, which lies approximately 300 kilometers to the east of Kalimantan Island, significantly affect the diurnal cycle of rainfall over southeast Kalimantan Island.

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...

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. n nWe 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.

Impact of the winter North Atlantic Oscillation (NAO) on the Western Pacific (WP) pattern in the following winter through Arctic sea ice and ENSO. Part II: multi-model evaluation of the NAO–ENSO linkage

AbstractnA 1-year-lagged relationship between the North Atlantic Oscillation (NAO) in winter and El Niño/Southern Oscillation (ENSO) in the following year was examined with a multi-model ensemble analysis using Coupled Model Intercomparison Project phase 3 models. A coherency index was developed as a measure of the coherence between the NAO and ENSO in each model and used as a weighting factor in the ensemble model. Weighted multi-model ensemble means of the regressed field on the maximum covariance analysis coefficients between the surface pressure field in the North Atlantic (NAO field) and the sea surface temperature field in the equatorial Pacific (ENSO field) were explored. The results indicated that when the wintertime NAO was in its negative (positive) phase, anomalous atmospheric circulation associated with a large (small) Eurasian snow mass anomaly intensified (weakened) the Asian cold surge and the westerly wind burst (WWB) in the warm pool region. Intensification of the WWB initiates El Niño. A sensitivity experiment using an idealized dry general circulation model revealed that the atmospheric response to anomalous, near-surface cooling associated with an anomalous Eurasian snow mass induced higher surface pressure near the Tibetan plateau and thus intensified the Asian cold surge and WWB. Linear regression analyses applied to the reanalysis data strongly supported the model results. Our results therefore suggest an influence of the NAO on the initiation of El Niño via a process that involves Eurasian snow anomalies associated with the NAO phase.