Yoshiyuki Kajikawa - Academia.edu (original) (raw)
Papers by Yoshiyuki Kajikawa
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 2020
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 2019
ABSTRACT: A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations i... more ABSTRACT: A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations is defined by using 850 hPa zonal wind averaged over the area (5°S-15°S, 110°E-130°E). This circulation index reflects monsoonal rainfall variability over Northern Australia and maritime continent. The index can be used to depict the seasonal cycle (for instance the onset) and measure the intraseasonal, interannual, and interdecadal variations of the Australian monsoon. The interannual variation of the Australian monsoon onset determined by the AUSMI agrees well with that derived from the rainfall and winds at Darwin in the previous studies. We found a significant anti-correlation between the monsoon onset date and the seasonal (DJF) mean AUSMI anomalies; namely an early onset is accompanied by a strong Australian summer monsoon and vice versa. These interannual variations are also strongly associated with El Niño-Southern Oscillation (ENSO). In contrast, the retreat dates are not signifi...
Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences... more Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, School of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, China, Indian Institute of Tropical Meteorology, Pune, India, Center for Climate Physics, Institute for Basic Science, and Department of Atmospheric Sciences, Pusan National University, Busan, South Korea, RIKEN Center for Computational Science, and Research Center for Urban Safety and Security, Kobe University, Kobe, Japan, Met Office Hadley Centre, Exeter, United Kingdom
In this study, the effect of sub-mesoscale topography (i.e., topographical features smaller than ... more In this study, the effect of sub-mesoscale topography (i.e., topographical features smaller than a few kilometers in size) on precipitation associated with thermally driven local circulations over a mountainous region is examined in the absence of synoptic-scale precipitation systems through a 100-m-mesh large-eddy simulation experiment. The observed effect of topography on precipitation is different to that identified in previous studies; sub-mesoscale topography is observed to induce a weakening effect on precipitation in this study, while previous studies have suggested that sub-mesoscale topography enhances precipitation. This discrepancy between studies is owing to differences in the scale of the topography and the precipitation-inducing system under consideration. Previous studies have focused on precipitation associated with synoptic-scale systems, where mechanical orographic forcing is dominant. The mechanism of the topographic effect where thermal orographic forcing is domi...
Deep moist convection plays a crucial role in transporting energy and momentum in the Earth’s atm... more Deep moist convection plays a crucial role in transporting energy and momentum in the Earth’s atmosphere. Examining the convection is fundamentally important for better understanding of various atmospheric phenomena, e.g., typhoons, as well as the convection itself. We developed a simple methodology for detecting the lifecycle of convection from discrete spatiotemporal data such as output of model simulations or reanalyses, which are often used in the Atmospheric Science research. The method includes only a few thresholds and hence it can be applied to data of global atmosphere in which environmental conditions for the convection are largely different in space and time. We tested and verified the method on a global simulation with high spatiotemporal resolution.
Geophysical Research Letters
Atmospheric deep moist convection can organize into cloud systems, which impact the Earth's clima... more Atmospheric deep moist convection can organize into cloud systems, which impact the Earth's climate significantly. High-resolution simulations that correctly reproduce organized cloud systems are necessary to understand the role of deep convection in the Earth's climate system. However, there remain issues regarding convergence with respect to grid spacing. To investigate the resolution necessary for a reasonable simulation of deep convection, we conducted grid-refinement experiments using state-of-the-art atmospheric models. We found that the structure of an updraft ensemble in an organized cloud system converges at progressively smaller scales as the grid spacing is reduced. The gap between two adjacent updrafts converges to a particular distance when the grid spacing becomes as small as 1/20-1/40 of the updraft radius. We also found that the converged inter-updraft distance value is not significantly different between Reynolds-averaged Navier-Stokes simulations and large eddy simulations for grid spacings in the terra incognita range. Plain Language Summary Meteorologists use computer simulations to predict atmospheric phenomena. When simulating the atmosphere, they divide it into small boxes and calculate the changes in wind speed, amounts of moisture and precipitation, and other important variables in each box. Here, our question is how finely we should divide the atmosphere to obtain the correct "answer" in the simulations; we call this the convergence problem. The more finely we divide the atmosphere, the more closely the simulation results approach the correct answer, but the more computational resources we need. The convergence problem is an important topic for us when carrying out accurate atmospheric simulations with limited computational power. This paper has addressed this problem. The target of our simulation is a group of cumulonimbus clouds. We performed several simulations with progressively smaller boxes to investigate how finely we should divide the atmosphere to reach convergence. We found that we should divide the atmosphere so that the width of each box is as small as 1/20 to 1/40 of the width of an upward current in an individual cumulonimbus cloud. We believe that this paper provides a new guideline for accurate atmospheric simulations. Previous studies have shown that at least a 4-km horizontal grid spacing is necessary to simulate the structure of deep convective systems plausibly (Weisman et al., 1997), a 250-m grid spacing is required to resolve
Journal of the Meteorological Society of Japan. Ser. II
During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiros... more During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiroshima and Keihanshin areas were subjected to unusual total rainfall amounts in 72 hours from 1200 UTC 4 July onward. However, the number of sediment disasters was significantly larger in the Hiroshima area. Among the possible reasons for the difference in the sediment disaster occurrences between the Hiroshima and Keihanshin areas, here, we focus on the differences in the rainfall characteristics in these two areas during the heavy rainfall event. Based on the radar observations, we investigate the characteristics of precipitation systems striking the Hiroshima and Keihanshin areas and find that significantly large precipitation systems with areas equal to or larger than 10 4 km 2 were dominant in the Hiroshima area, which caused rapid accumulation of the rainfall amount and enhanced the risk of deadly sediment disasters in this area. On the other hand, in the Keihanshin area, rainfall of moderate intensity and relatively small precipitation systems were found to be dominant. We suggest that the difference in the amount of damage between the Hiroshima and Keihanshin areas was mainly caused by the size difference of the precipitation systems striking these two areas. Statistics relating to the background atmospheric conditions for the precipitation systems in the heavy rainfall event reveal that a high vertical wind shear environment provides preferable conditions for the formation of large precipitation systems.
Atmospheric Science Letters
Regional climate projections inevitably inherit uncertainties from general circulation models (GC... more Regional climate projections inevitably inherit uncertainties from general circulation models (GCMs). We therefore propose a new approach for identifying the dominant uncertainties. This approach employs the downscaling procedure by Adachi et al. to the uncertainty problem using multiple GCM projections. The mean state of the large-scale atmospheric states and the deviation from this mean state are the two uncertainty factors considered here, which are provided by a GCM. These two factors are referred to as climatology and perturbation components, respectively. To demonstrate the effectiveness in identifying these uncertainty factors using the proposed approach, a regional projection of summertime climate in western Japan is conducted using four different future climate data that are calculated using an atmospheric GCM with different sea surface temperatures. Results show that the variability in surface air temperature projections is reasonably derived from the climatology uncertainty, whereas the variability in precipitation projections is equally influenced by the climatology and perturbation uncertainties. Both the climatology and perturbation uncertainties should therefore be considered when analysing regional climate projections.
Progress in Earth and Planetary Science
In this study, we provide a perspective on dynamical downscaling that includes a comprehensive vi... more In 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.
Nature Communications
Future changes in large-scale climatology and perturbation may have different impacts on regional... more 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.
Atmospheric Science Letters
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Progress in Earth and Planetary Science
The success of sub-kilometer global atmospheric simulation opens the door for resolving deep conv... more The success of sub-kilometer global atmospheric simulation opens the door for resolving deep convections, which are fundamental elements of cloudy disturbances that drive global circulation. A previous study found that the essential change in the simulated convection properties occurred at a grid spacing of about 2 km as a global mean. In grid-refinement experiments, we conducted further comprehensive analysis of the global-mean state and the characteristics of deep convection, to clarify the difference of the essential change by location and environment. We found that the essential change in convection properties was different in the location and environment for each cloudy disturbance. The convections over the tropics show larger resolution dependence than convections over mid-latitudes, whereas no significant difference was found in convections over land or ocean. Furthermore, convections over cloudy disturbances [(i.e., Madden-Julian oscillation (MJO), tropical cyclones (TCs)] show essential change of convection properties at about 1 km grid spacing, suggesting resolution dependence. As a result, convections not categorized as cloudy disturbances make a large contribution to the global-mean convection properties. This implies that convections in disturbances are largely affected organization processes and hence have more horizontal resolution dependence. In contrast, other categorized convections that are not involved in major cloudy disturbances show the essential change at about 2 km grid spacing. This affects the latitude difference of the resolution dependence of convection properties and hence the zonal-mean outgoing longwave radiation (OLR). Despite the diversity of convection properties, most convections are resolved at less than 1 km grid spacing. In the future, longer integration of global atmosphere, to 0.87 km grid spacing, will stimulate significant discussion about the interaction between the convections and cloudy disturbances.
Progress in Earth and Planetary Science
This article reviews the major outcomes of a 5-year (2011-2016) project using the K computer to p... more This article reviews the major outcomes of a 5-year (2011-2016) project using the K computer to perform global numerical atmospheric simulations based on the non-hydrostatic icosahedral atmospheric model (NICAM). The K computer was made available to the public in September 2012 and was used as a primary resource for Japan's Strategic Programs for Innovative Research (SPIRE), an initiative to investigate five strategic research areas; the NICAM project fell under the research area of climate and weather simulation sciences. Combining NICAM with high-performance computing has created new opportunities in three areas of research: (1) higher resolution global simulations that produce more realistic representations of convective systems, (2) multi-member ensemble simulations that are able to perform extended-range forecasts 10-30 days in advance, and (3) multi-decadal simulations for climatology and variability. Before the K computer era, NICAM was used to demonstrate realistic simulations of intra-seasonal oscillations including the Madden-Julian oscillation (MJO), merely as a case study approach. Thanks to the big leap in computational performance of the K computer, we could greatly increase the number of cases of MJO events for numerical simulations, in addition to integrating time and horizontal resolution. We conclude that the high-resolution global non-hydrostatic model, as used in this five-year project, improves the ability to forecast intra-seasonal oscillations and associated tropical cyclogenesis compared with that of the relatively coarser operational models currently in use. The impacts of the sub-kilometer resolution simulation and the multi-decadal simulations using NICAM are also reviewed.
Journal of the Meteorological Society of Japan. Ser. II
The environmental conditions for tropical cyclone genesis are examined by numerical experiment. W... more The environmental conditions for tropical cyclone genesis are examined by numerical experiment. We focus on the case of a non-developing disturbance showed features for tropical cyclone genesis in the Pacific Area Long-term Atmospheric observation for Understanding climate change in 2010 (PALAU2010) observation campaign over the western North Pacific. We clarify the importance of the presence of abundant moisture around the disturbance for continuous convection and demonstrate that the collocation of a mid-level vortex and a low-level vortex, i.e., the persistence of an upright structure of vortices, is important in tropical cyclone genesis. We conduct two numerical experiments using the Weather Research and Forecasting Model Advanced Research WRF model in double nested domains with a horizontal grid space of 27 km and 9 km for the outer domain and the inner domain, respectively. The first experiment is based on reanalysis data (a control experiment) and the second includes increased water vapor content over the northwestern dry area of the disturbance. In the control experiment, the disturbance did not develop into a tropical cyclone in spite of the existence of the mid-level and low-level vortices. In contrast, the sensitivity experiment shows that a tropical cyclone was formed from the disturbance with increased water vapor content. The presence of persistent upright vortices was supported by continuous convection until the genesis of the
Journal of Geophysical Research: Atmospheres
SOLA, 2016
Resolution dependence was found in the simulated diurnal precipitation cycle over land in the tro... more Resolution dependence was found in the simulated diurnal precipitation cycle over land in the tropics. We conducted a series of grid refinement experiments of the atmosphere from 14 km to 0.87 km using a global high-resolution model without any convection parameterizations. In the high-resolution experiment, the peak of the cycle was earlier and precipitation at the peak was higher. The characteristics of the simulated diurnal precipitation cycle changed at a grid spacing of around 2−3 km. The precipitation started to increase in the morning in the high-resolution experiments, suggesting that small-scale moist convection became active in the late morning. In the lower-resolution experiments, convection and precipitation began in the late afternoon. As well as the enhancement of moisture transport from the boundary layer to the middle troposphere, the rapid formation of rain can also be attributed to the difference in diurnal precipitation cycles between lower and higher resolution experiments.
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 2020
Journal of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), 2019
ABSTRACT: A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations i... more ABSTRACT: A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations is defined by using 850 hPa zonal wind averaged over the area (5°S-15°S, 110°E-130°E). This circulation index reflects monsoonal rainfall variability over Northern Australia and maritime continent. The index can be used to depict the seasonal cycle (for instance the onset) and measure the intraseasonal, interannual, and interdecadal variations of the Australian monsoon. The interannual variation of the Australian monsoon onset determined by the AUSMI agrees well with that derived from the rainfall and winds at Darwin in the previous studies. We found a significant anti-correlation between the monsoon onset date and the seasonal (DJF) mean AUSMI anomalies; namely an early onset is accompanied by a strong Australian summer monsoon and vice versa. These interannual variations are also strongly associated with El Niño-Southern Oscillation (ENSO). In contrast, the retreat dates are not signifi...
Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences... more Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, School of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, China, Indian Institute of Tropical Meteorology, Pune, India, Center for Climate Physics, Institute for Basic Science, and Department of Atmospheric Sciences, Pusan National University, Busan, South Korea, RIKEN Center for Computational Science, and Research Center for Urban Safety and Security, Kobe University, Kobe, Japan, Met Office Hadley Centre, Exeter, United Kingdom
In this study, the effect of sub-mesoscale topography (i.e., topographical features smaller than ... more In this study, the effect of sub-mesoscale topography (i.e., topographical features smaller than a few kilometers in size) on precipitation associated with thermally driven local circulations over a mountainous region is examined in the absence of synoptic-scale precipitation systems through a 100-m-mesh large-eddy simulation experiment. The observed effect of topography on precipitation is different to that identified in previous studies; sub-mesoscale topography is observed to induce a weakening effect on precipitation in this study, while previous studies have suggested that sub-mesoscale topography enhances precipitation. This discrepancy between studies is owing to differences in the scale of the topography and the precipitation-inducing system under consideration. Previous studies have focused on precipitation associated with synoptic-scale systems, where mechanical orographic forcing is dominant. The mechanism of the topographic effect where thermal orographic forcing is domi...
Deep moist convection plays a crucial role in transporting energy and momentum in the Earth’s atm... more Deep moist convection plays a crucial role in transporting energy and momentum in the Earth’s atmosphere. Examining the convection is fundamentally important for better understanding of various atmospheric phenomena, e.g., typhoons, as well as the convection itself. We developed a simple methodology for detecting the lifecycle of convection from discrete spatiotemporal data such as output of model simulations or reanalyses, which are often used in the Atmospheric Science research. The method includes only a few thresholds and hence it can be applied to data of global atmosphere in which environmental conditions for the convection are largely different in space and time. We tested and verified the method on a global simulation with high spatiotemporal resolution.
Geophysical Research Letters
Atmospheric deep moist convection can organize into cloud systems, which impact the Earth's clima... more Atmospheric deep moist convection can organize into cloud systems, which impact the Earth's climate significantly. High-resolution simulations that correctly reproduce organized cloud systems are necessary to understand the role of deep convection in the Earth's climate system. However, there remain issues regarding convergence with respect to grid spacing. To investigate the resolution necessary for a reasonable simulation of deep convection, we conducted grid-refinement experiments using state-of-the-art atmospheric models. We found that the structure of an updraft ensemble in an organized cloud system converges at progressively smaller scales as the grid spacing is reduced. The gap between two adjacent updrafts converges to a particular distance when the grid spacing becomes as small as 1/20-1/40 of the updraft radius. We also found that the converged inter-updraft distance value is not significantly different between Reynolds-averaged Navier-Stokes simulations and large eddy simulations for grid spacings in the terra incognita range. Plain Language Summary Meteorologists use computer simulations to predict atmospheric phenomena. When simulating the atmosphere, they divide it into small boxes and calculate the changes in wind speed, amounts of moisture and precipitation, and other important variables in each box. Here, our question is how finely we should divide the atmosphere to obtain the correct "answer" in the simulations; we call this the convergence problem. The more finely we divide the atmosphere, the more closely the simulation results approach the correct answer, but the more computational resources we need. The convergence problem is an important topic for us when carrying out accurate atmospheric simulations with limited computational power. This paper has addressed this problem. The target of our simulation is a group of cumulonimbus clouds. We performed several simulations with progressively smaller boxes to investigate how finely we should divide the atmosphere to reach convergence. We found that we should divide the atmosphere so that the width of each box is as small as 1/20 to 1/40 of the width of an upward current in an individual cumulonimbus cloud. We believe that this paper provides a new guideline for accurate atmospheric simulations. Previous studies have shown that at least a 4-km horizontal grid spacing is necessary to simulate the structure of deep convective systems plausibly (Weisman et al., 1997), a 250-m grid spacing is required to resolve
Journal of the Meteorological Society of Japan. Ser. II
During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiros... more During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiroshima and Keihanshin areas were subjected to unusual total rainfall amounts in 72 hours from 1200 UTC 4 July onward. However, the number of sediment disasters was significantly larger in the Hiroshima area. Among the possible reasons for the difference in the sediment disaster occurrences between the Hiroshima and Keihanshin areas, here, we focus on the differences in the rainfall characteristics in these two areas during the heavy rainfall event. Based on the radar observations, we investigate the characteristics of precipitation systems striking the Hiroshima and Keihanshin areas and find that significantly large precipitation systems with areas equal to or larger than 10 4 km 2 were dominant in the Hiroshima area, which caused rapid accumulation of the rainfall amount and enhanced the risk of deadly sediment disasters in this area. On the other hand, in the Keihanshin area, rainfall of moderate intensity and relatively small precipitation systems were found to be dominant. We suggest that the difference in the amount of damage between the Hiroshima and Keihanshin areas was mainly caused by the size difference of the precipitation systems striking these two areas. Statistics relating to the background atmospheric conditions for the precipitation systems in the heavy rainfall event reveal that a high vertical wind shear environment provides preferable conditions for the formation of large precipitation systems.
Atmospheric Science Letters
Regional climate projections inevitably inherit uncertainties from general circulation models (GC... more Regional climate projections inevitably inherit uncertainties from general circulation models (GCMs). We therefore propose a new approach for identifying the dominant uncertainties. This approach employs the downscaling procedure by Adachi et al. to the uncertainty problem using multiple GCM projections. The mean state of the large-scale atmospheric states and the deviation from this mean state are the two uncertainty factors considered here, which are provided by a GCM. These two factors are referred to as climatology and perturbation components, respectively. To demonstrate the effectiveness in identifying these uncertainty factors using the proposed approach, a regional projection of summertime climate in western Japan is conducted using four different future climate data that are calculated using an atmospheric GCM with different sea surface temperatures. Results show that the variability in surface air temperature projections is reasonably derived from the climatology uncertainty, whereas the variability in precipitation projections is equally influenced by the climatology and perturbation uncertainties. Both the climatology and perturbation uncertainties should therefore be considered when analysing regional climate projections.
Progress in Earth and Planetary Science
In this study, we provide a perspective on dynamical downscaling that includes a comprehensive vi... more In 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.
Nature Communications
Future changes in large-scale climatology and perturbation may have different impacts on regional... more 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.
Atmospheric Science Letters
This is an open access article under the terms of the Creative Commons Attribution License, which... more This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Progress in Earth and Planetary Science
The success of sub-kilometer global atmospheric simulation opens the door for resolving deep conv... more The success of sub-kilometer global atmospheric simulation opens the door for resolving deep convections, which are fundamental elements of cloudy disturbances that drive global circulation. A previous study found that the essential change in the simulated convection properties occurred at a grid spacing of about 2 km as a global mean. In grid-refinement experiments, we conducted further comprehensive analysis of the global-mean state and the characteristics of deep convection, to clarify the difference of the essential change by location and environment. We found that the essential change in convection properties was different in the location and environment for each cloudy disturbance. The convections over the tropics show larger resolution dependence than convections over mid-latitudes, whereas no significant difference was found in convections over land or ocean. Furthermore, convections over cloudy disturbances [(i.e., Madden-Julian oscillation (MJO), tropical cyclones (TCs)] show essential change of convection properties at about 1 km grid spacing, suggesting resolution dependence. As a result, convections not categorized as cloudy disturbances make a large contribution to the global-mean convection properties. This implies that convections in disturbances are largely affected organization processes and hence have more horizontal resolution dependence. In contrast, other categorized convections that are not involved in major cloudy disturbances show the essential change at about 2 km grid spacing. This affects the latitude difference of the resolution dependence of convection properties and hence the zonal-mean outgoing longwave radiation (OLR). Despite the diversity of convection properties, most convections are resolved at less than 1 km grid spacing. In the future, longer integration of global atmosphere, to 0.87 km grid spacing, will stimulate significant discussion about the interaction between the convections and cloudy disturbances.
Progress in Earth and Planetary Science
This article reviews the major outcomes of a 5-year (2011-2016) project using the K computer to p... more This article reviews the major outcomes of a 5-year (2011-2016) project using the K computer to perform global numerical atmospheric simulations based on the non-hydrostatic icosahedral atmospheric model (NICAM). The K computer was made available to the public in September 2012 and was used as a primary resource for Japan's Strategic Programs for Innovative Research (SPIRE), an initiative to investigate five strategic research areas; the NICAM project fell under the research area of climate and weather simulation sciences. Combining NICAM with high-performance computing has created new opportunities in three areas of research: (1) higher resolution global simulations that produce more realistic representations of convective systems, (2) multi-member ensemble simulations that are able to perform extended-range forecasts 10-30 days in advance, and (3) multi-decadal simulations for climatology and variability. Before the K computer era, NICAM was used to demonstrate realistic simulations of intra-seasonal oscillations including the Madden-Julian oscillation (MJO), merely as a case study approach. Thanks to the big leap in computational performance of the K computer, we could greatly increase the number of cases of MJO events for numerical simulations, in addition to integrating time and horizontal resolution. We conclude that the high-resolution global non-hydrostatic model, as used in this five-year project, improves the ability to forecast intra-seasonal oscillations and associated tropical cyclogenesis compared with that of the relatively coarser operational models currently in use. The impacts of the sub-kilometer resolution simulation and the multi-decadal simulations using NICAM are also reviewed.
Journal of the Meteorological Society of Japan. Ser. II
The environmental conditions for tropical cyclone genesis are examined by numerical experiment. W... more The environmental conditions for tropical cyclone genesis are examined by numerical experiment. We focus on the case of a non-developing disturbance showed features for tropical cyclone genesis in the Pacific Area Long-term Atmospheric observation for Understanding climate change in 2010 (PALAU2010) observation campaign over the western North Pacific. We clarify the importance of the presence of abundant moisture around the disturbance for continuous convection and demonstrate that the collocation of a mid-level vortex and a low-level vortex, i.e., the persistence of an upright structure of vortices, is important in tropical cyclone genesis. We conduct two numerical experiments using the Weather Research and Forecasting Model Advanced Research WRF model in double nested domains with a horizontal grid space of 27 km and 9 km for the outer domain and the inner domain, respectively. The first experiment is based on reanalysis data (a control experiment) and the second includes increased water vapor content over the northwestern dry area of the disturbance. In the control experiment, the disturbance did not develop into a tropical cyclone in spite of the existence of the mid-level and low-level vortices. In contrast, the sensitivity experiment shows that a tropical cyclone was formed from the disturbance with increased water vapor content. The presence of persistent upright vortices was supported by continuous convection until the genesis of the
Journal of Geophysical Research: Atmospheres
SOLA, 2016
Resolution dependence was found in the simulated diurnal precipitation cycle over land in the tro... more Resolution dependence was found in the simulated diurnal precipitation cycle over land in the tropics. We conducted a series of grid refinement experiments of the atmosphere from 14 km to 0.87 km using a global high-resolution model without any convection parameterizations. In the high-resolution experiment, the peak of the cycle was earlier and precipitation at the peak was higher. The characteristics of the simulated diurnal precipitation cycle changed at a grid spacing of around 2−3 km. The precipitation started to increase in the morning in the high-resolution experiments, suggesting that small-scale moist convection became active in the late morning. In the lower-resolution experiments, convection and precipitation began in the late afternoon. As well as the enhancement of moisture transport from the boundary layer to the middle troposphere, the rapid formation of rain can also be attributed to the difference in diurnal precipitation cycles between lower and higher resolution experiments.