Scientific Online Letters on the Atmosphere (SOLA) is a fully Open Access journal under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License, and aims to make a first decision within one month and a decision upon resubmission within a further month.
Last year, we published 44 papers including those in the Special Editions of Research on Extreme Weather Events That Occurred around East Asia in 2017-2021 and of the Frontier of Atmospheric Science with High Performance Computing. There is also a new special edition on Advances in Studies of Torrential-Rain-Producing Quasi-Stationary Band-Shaped Precipitating Systems, or ‘Senjo-Kousuitai’, which currently welcomes submissions. Those who are interested in this topic are encouraged to submit papers to this special edition.
The Editorial Committee of SOLA presents the SOLA Award to one or two outstanding papers published in each year. The SOLA Award in 2022 were presented to two papers: Maejima et al. (2022) and Nasuno et al. (2022). The SOLA Award in 2023 will be announced shortly, by the end of January 2024.
There is a change in the word count limitation for papers submitted in 2024 and afterwards. This is because Journal of the Meteorological Society of Japan, the sister journal, will cease publishing "Note" paper. In order to accommodate needs to publish short articles, SOLA will increase the word count limitation up to 3500 at the time of the acceptance. The current word count limitation of 3100 words still applies at the time of the initial submission, based on the total words included in the abstract and in the main body of the manuscript; but with the title, authors’ names and affiliations excluded in the word count.
We hope that SOLA is one of your preferred platforms where you can disseminate your research achievements. SOLA welcomes submission from the international community in meteorology, atmospheric sciences, and the related fields.

抄録全体を表示

The Editorial Committee of Scientific Online Letters on the Atmosphere (SOLA) presents the SOLA Award to one or two outstanding papers published each year. We are pleased to announce that The SOLA Award in 2023 will be given to the paper by Drs. Sachie Kanada and Akira Nishii, entitled "Observed Concentric Eyewalls of Supertyphoon Hinnamnor (2022)" (Kanada and Nishii 2023) and to the paper by Dr. Kenji Suzuki et al., entitled "Development of a New Particle Imaging Radiosonde with Particle Fall Velocity Measurements in Clouds" (Suzuki et al. 2023).

抄録全体を表示

Although data assimilation originates from control theory, the relationship between modern data assimilation methods in geoscience and model predictive control has not been extensively explored. In the present paper, I discuss that the modern data assimilation methods in geoscience and model predictive control essentially minimize the similar quadratic cost functions. Inspired by this similarity, I propose a new ensemble Kalman filter (EnKF)-based method for controlling spatio-temporally chaotic systems, which can be applied to high-dimensional and nonlinear Earth systems. In this method, the reference vector, which serves as the control target, is assimilated into the state space as a pseudo-observation by ensemble Kalman smoother to obtain the appropriate perturbation to be added to a system. A proof-of-concept experiment using the Lorenz 63 model is presented. The system is constrained in one wing of the butterfly attractor without tipping to the other side by reasonably small control perturbations which are comparable with previous works.

抄録全体を表示

The TRMM PR and GSMaP products are analyzed to examine the variability in different types of precipitation under three rain intensity classes for investigating the evolution of tropical convection over the tropical western Pacific (WP) and eastern Pacific (EP). Composite time series are constructed around the GSMaP precipitation maxima for different precipitation types identified from the TRMM PR. The composite evolution is further broken down into weak, moderate, and strong rains, divided by applying quartile-based thresholds to rainfall maxima. The results show that the evolution of deep convection reaches a precipitation peak earlier by a few hours than stratiform evolution precipitation for strong and moderate rain regardless of the region. Deep convective rain still predominates in the WP weak rain, while shallow precipitation dominates the weak rain in EP. The composite time series of vertical velocity is overall physically consistent with the expectation from these findings. It is implied that the convective-stratiform lag and tilted updraft structure for strong rain are shared between WP and EP, while the attribution of weak rain can differ significantly between the regions.

抄録全体を表示

In this study, the characteristics of the South and Southeast Asian summer monsoon onset in ENSO years were re-investigated using clustering analysis. Our approach focuses on detecting synoptic patterns, including identifying the pattern that best represents the monsoon onset condition. By projecting the real atmospheric conditions and their temporal evolutions into a recognizable and interpretable “weather pattern space” by using Self-organizing Maps and k-means, we can track the changing patterns and identify the initial occurrence of deep convection driven by large-scale circulation, which is considered the monsoon onset. The ENSO signal was defined by anomalous sea surface temperature over the Nino 3.4 region. The results revealed that the mean onset timing is later in warm phases than in the cold phases of ENSO. The onset in the warm phases is characterized by greater persistence of the subtropical high over the Vietnam East Sea (VES) and drier conditions over the equatorial Indian Ocean. Additionally, monsoon westerlies and deep convection are found to be weaker during the onset of the warm phases. The proposed approach can be considered a new option, an alternative to enhance the existing monsoon onset detection method and explore its characteristics.

抄録全体を表示

A novel dropsonde, the iMDS-17, has been developed since 2017. The dropsonde was designed to be launched from an aircraft flying at an altitude of 13,000 m and ground speed of 720 km h⁻¹. Owing to their light weight of approximately 130 g, parachutes are not necessary when observing upper-air atmospheric conditions throughout the troposphere. Approximately the same technology as that of the Japan Meteorological Agency (JMA) operational radiosonde (iMS-100) was used, except that the pressure was measured directly using a pressure sensor. To evaluate the performance of the iMDS-17, intercomparison experiments were conducted on 27 and 28 March 2024 on the southern island of Japan using a balloon-borne shooter system. The dropsonde and radiosonde were lifted by the same balloon and released at altitudes between 9,000 and 13,000 m. Comparison between data obtained from the experiments confirmed the adequate performance of iMDS-17 in temperature and wind observations; maximum differences between the dropsonde and radiosonde were less than 1 K and 2 m s⁻¹ in temperature and wind, respectively, in most altitudes between 9-2 km. Meanwhile, a dry bias was identified for humidity, particularly in moist layers. The causes of the dry bias are discussed.

抄録全体を表示

Numerical simulations of convective boundary layers (CBLs) based on the stability categories of the Pasquill–Gifford (PG) chart were conducted using a large-eddy simulation (LES) model. We studied the simulation settings for different combinations of wind speeds and sensible heat fluxes that could quantitatively generate turbulences satisfying the requirement of each stability category. Additionally, we determined appropriate CBL depths for desired turbulence generation and verified the effectiveness of the velocity scale (w^*) in turbulence generation simulations. This study identified wind speed and sensible heat flux combinations that could generate turbulences under the atmospheric stability categories B (unstable), C (weakly unstable), and D (neutral). However, the results did not follow the scaling law based on w^* in category D when the CBL depth was 600 m. A shallower CBL (300 m depth) should be set in category D to generate turbulence under the scaling law based on w^*. In category B, the transient layer disappeared due to active thermals in the CBL when the depth was 300 m. These results indicated that we should set appropriate CBL depths in addition to horizontal wind speeds and sensible heat fluxes to generate desired and scalable turbulences based on the PG chart.

抄録全体を表示

The diurnal variation of boundary layer wind speed in Japan was statistically investigated using wind profiler data for 2002-2013. The analysis was made for 17 stations where the diurnal variation in surface air temperature had an amplitude of 2°C or more. It was found that the wind speed in the upper boundary layer (985 m above the surface) had a minimum in early afternoon in all seasons, in agreement with the general understanding that winds in the daytime boundary layer are reduced due to enhanced frictional force. However, the wind speed in the lower boundary layer (394 m) had a maximum in the afternoon at some stations in contrast to the general feature mentioned above. This afternoon maximum is more conspicuous in summer than in winter and in southern Japan than in northern Japan, and appears to be due to thermally driven local circulations. Nevertheless, the ratio of wind speeds at 394 m with respect to surface had a minimum during the daytime in all stations and seasons, indicating the daytime reduction of surface-relative boundary layer wind speed as a common feature over land.

抄録全体を表示

Soil physical properties are critical to the energy and water balance between land and atmosphere interactions. Accurate soil data inputs could improve the simulations in land surface models and numerical weather models. However, further efforts are required to access the impact of soil data changes on global long-term simulations for climate system models. The Flexible Global Ocean-Atmosphere-Land System Model: Grid-Point Version 3 (FGOALS-g3) in an Atmospheric Model Intercomparison Project (AMIP) - style configuration with two different soil texture datasets is employed to investigate the role of soil texture in the long-term simulations of hydrological and related atmospheric variables. The results show that the difference in sand and clay content between the two datasets is slight in the global mean but exhibits regional heterogeneity. Updating soil texture data considerably reduced the deviation of global annual mean surface soil moisture, with significant improvements occurring in regions with the most remarkable changes in sandy soil content. However, there is almost no improvement in runoff, precipitation, and temperature on the global annual mean scale due to the complexity of the impact factor. Simulations of long-term soil moisture would be enhanced with more accurate data on soil texture.

抄録全体を表示

During typhoons, direct, reliable experimental observations of the atmosphere and sea surface are difficult. The target storm for the present experiment in 2023 was typhoon Khanun, a Category-4 storm. Two wave gliders, used as autonomous surface vehicles, were improved after assessing problems encountered during a 2022 storm. These improvements reduced equipment vibration and sensor damage on the wave gliders and resulted in uninterrupted data acquisition. Changing atmospheric and oceanographic phenomena were continuously observed before and after passage of the typhoon on both sides of the typhoon's course, and inside the storm zone. Meteorometers were mounted redundantly to evaluate sensors with different specifications and to assess the reliability of acquired data. Data collected at the sea surface during typhoons should enhance understanding of interactions between the atmosphere and ocean.

抄録全体を表示

We have developed a compact ground-based microwave radiometer (MWR) for estimating water vapor. The MWR observes radio wave intensity at frequencies between 17.9 and 26.4 GHz across 34 channels and estimates precipitable water vapor (PWV) and the profile of water vapor density using machine learning methods. Data from the Global Navigation Satellite System (GNSS) and radiosonde (SONDE) collected at the Meteorological Research Institute of the Japan Meteorological Agency were used to train and evaluate the machine learning models. Data from June 2021 to March 2022 were used for training, and data from April 2022 to March 2023 were used for evaluation. As a result, the maximum root-mean-square errors (RMSEs) of MWR-derived PWV compared to GNSS-derived PWV and MWR-derived water vapor density compared to SONDE at the lowest layer of the atmosphere were 2.7 mm and 2.4 g m⁻³, respectively. Analysis of the error characteristics of water vapor estimation showed that both PWV and water vapor density profiles had errors in the presence of cloud water, as determined by infrared radiometer, and high accuracy in the absence of cloud water. The estimation accuracy was also affected by fog and water vapor inversion layer.

抄録全体を表示

This study reproduced the meteorological conditions, including typhoon movements near Japan and wind changes over Tokyo, during the 1923 Great Kanto Earthquake, using a numerical simulation model (Weather Research and Forecasting v4.3) and the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the 20th century (ERA-20C). The reproduced meteorological conditions coincided in many respects with weather analysis maps and observations produced by the Central Meteorological Observatory. Strong southerly winds around noon on the day of the earthquake were associated with a typhoon on the coast of the Sea of Japan and appear to have had a significant impact on the spread of fires immediately after the earthquake. However, the strong evening and nighttime winds observed at the Tokyo station are likely to have been local phenomena associated with the fire spread, which caused severe damage in Tokyo after the earthquake.

抄録全体を表示

On 5 February 2024, an extratropical cyclone passing the south coast of Japan brought 8 cm of snow to Tokyo, while in Tsukuba, located about 50 km from Tokyo, the snow cover was 0 cm. We investigated the mechanism generating the inhomogeneous distribution of snow cover using a numerical weather model with 1 km grid spacings. Our numerical simulation shows that a coastal front is located on the eastern coast of the Kanto Plain. On the other hand, a relatively warm area broadly spreads from the leeward side of mountainous areas including Mt. Tsukuba. A strong downward flow related to a gravity wave caused by the mountainous area brings adiabatic warming on the leeward side of the mountains. This adiabatic heating accelerates snowfall melting at the low troposphere, and the melting layer is higher on the leeward side of the mountains. Also, the adiabatic heating reduces relative humidity and decreases total precipitation amount. As a result, there is much less snowfall on the leeward side of the mountainous areas, including Tsukuba observational station, as compared with neighboring areas. A numerical simulation with 5 km grid spacings cannot simulate the local-scale snowfall distribution around the mountainous areas.

抄録全体を表示

 The interaction of airflow with mountain ranges in a stable atmosphere generates internal gravity waves, leading to wind deceleration on the windward side and acceleration on the lee side. Recent studies have explored airflow over the bended mountain range, characterized by convexity on the windward side and concavity on the lee side. In this study, we have computed linear analytic solutions for three-dimensional mountain waves over such terrains, and examined the surface winds (u and v), and horizontal divergence.

 Our analysis reveals that when the terrain features convexity on the windward side and concavity on the lee side, surface wind speed amplifies within the area of concave region through the low-level convergence. In the bell-cosine mountain range, the maximum downslope wind exceeds that predicted by the analytic linear solution for the two-dimensional bell-shaped mountain range (U + NH/2). However, it does not surpass the maximum wind observed for the 2-dimensional bell-cosine mountain range. The presence of the convex bend in the mountain range yields flow splitting in the upwind side and does not promote downslope wind and wave breaking.

 The presence of concavity in the lee side amplifies the downslope wind by low level convergence in the lee side and convexity in the windward side of a mountain range has the potential to enhance downslope winds when the terrain slope becomes asymmetric. Our findings shed light on the potential enhancement of downslope winds in mountain ranges exhibiting such terrain features.

抄録全体を表示

Turbulence interferes with aircraft operations and occurs frequently near transverse bands. Forecasters from the Japan Meteorological Agency have reported that these transverse bands with wave-like patterns on cloud tops (WPC) tend to generate strong turbulence. We analyzed the rates of turbulence associated with transverse bands in the upper troposphere near Japan at different altitudes, focusing on the presence or absence of WPC. The results show that a significantly higher proportion of turbulence was associated with transverse bands with WPC. Moreover, turbulence occurs above and below these transverse bands. Regarding transverse bands with WPC, moderate turbulence was observed within 4,000 ft (≃ 1.2 km) above and below the cloud, indicating that turbulence area extends over a wide vertical range.

抄録全体を表示

We present weather classifications for high temperatures in major Japanese cities. We created a self-organizing map (SOM) of summer pressure patterns in Japan and generated frequency distributions of hot days on the SOM node space for each city. Through hierarchical clustering, we identified several weather zones, which tended to coincidently experience hot summer days with daily maximum temperatures of 30°C or more, or extremely hot days with daily maximum temperatures of 35°C or more. The obtained weather zones for hot summer days partially followed a local climate classification previously proposed in geography. In contrast, weather zones for extremely hot days lacked geographical continuity and were rather related to whether the sites are inland or coastal. This suggested that extremely hot days were frequently caused by the foehn phenomenon.

抄録全体を表示

The Mozambique Channel trough (MCT) is one of the weather systems that affect Southern Africa including Mozambique, but has not received much attention. Through the empirical orthogonal function (EOF) analysis applied for austral summer-mean sea level pressure (SLP), interannual variability of the MCT is categorized into that of its intensity and zonal shift. The MCT intensity is significantly correlated to ENSO but is not correlated to regional precipitation in Mozambique. In contrast, the zonal shift of the MCT is not significantly correlated to ENSO but is correlated to regional precipitation there. A westward shift of the MCT is accompanied by the positive subtropical Indian Ocean dipole (SIOD) and strengthened Mascarene High (MH) that enhance moisture convergence over the Southern Indian convergence zone, inducing increased precipitation in southern and central regions of Mozambique. An eastward shift of the MCT is not necessarily accompanied by the SIOD but accompanied by weak suppression of the MH and precipitation there. However, if the eastward shift occurs simultaneously with the negative SIOD, it accompanies El Niño and prominent weakening of the MH, which should lead to stronger precipitation decrease in southern and central regions and increase in the northeast region of Mozambique.

抄録全体を表示

Pakistan experienced two major flood events in July 2010 and July–August 2022, resulting in over 1,500 fatalities. The 2010 floods primarily affected Northern Pakistan, while the 2022 floods severely impacted Southern Pakistan. This study examines rainfall patterns, moisture transport, and their connection to European heatwaves and blocking highs. In 2022, unprecedented rainfall in Southern Pakistan contrasted with the northern-focused rainfall in 2010. An intense cycle of low-pressure systems in 2022 transported moist air from the Indian Ocean, facilitated by southeasterly and southerly winds, and triggered deep convection, while in July 2010, southerly winds were dominant. Rossby wave breaking events in late July 2010 and late August 2022 caused the southward movement of upper-level vorticity perturbations, leading to heavy rainfall. Additionally, the heavy rainfall in Southern Pakistan in late August 2022 was influenced by concurrent intense low-pressure systems.

抄録全体を表示

This study investigates the dependency of the shortwave three-dimensional radiative transfer (3D RT) effects upon the grid width when simulating an isolated cumulus. A meteorological model coupled with a 3D RT model enables us to investigate the dependency through simulations which consider feedback of the 3D RT to the atmosphere. Using the coupled model, simulations of the isolated cumulus were conducted for grid widths ranging between 25 m and 500 m. The same series of simulations were also conducted using a conventional one-dimensional (1D) radiative transfer model and were compared with the results by the 3D RT model. The comparison clarifies that the discrepancy in liquid water path between the two RT schemes becomes more prominent with smaller grid widths. The comparison also indicates that the difference in the radiative heating is negligible for grid spacing of 500 m but was substantial for finer grid spacings. These results suggest that the 3D RT model is required to accurately simulate the feedback between clouds and shortwave radiation for an isolated cumulus with grid widths of at least 250 m or smaller, when the lateral edge of the cloud is spatially resolved.

抄録全体を表示

In this study, we detected and analyzed cold air damming (CAD) and precipitation occurrences spanning 40 years from 1980 to 2019 in the Kanto region. We explored the climatological relationship between CAD and heavy precipitation. Of the 13.5 CAD events per year, 4.0 were associated with 24-h precipitation of 100 mm or more. In the southern Kanto region except for the western mountainous area, more than 25% of heavy precipitation was related to CAD, reaching over 40% in parts of the Boso Peninsula. CAD events with heavy precipitation exhibited consistent characteristics throughout seasons, including (1) a pronounced pressure gradient between the well-developed low-pressure off the southern coast of western Japan and the high-pressure on the northeast side of Japan, and (2) a strong moisture influx toward the Kanto region. These features highlight the crucial and general mechanisms of heavy precipitation formation through significant moisture ascent over the cold air dammed over the Kanto region.

抄録全体を表示

In this study, we examine the characteristics of drop size distributions (DSDs) in the midlatitude maritime environment. This study is based on two years of observational data collected using a disdrometer installed on Hegura-jima Island, a remote island in the Sea of Japan. The analysis revealed that there are two primary types of DSD regimes over the midlatitude maritime region: “larger drop size type” (D-type) and “larger number density type” (N-type). The D- and N-type regimes are characterized by larger raindrop sizes and a higher number density of raindrops at a given precipitation intensity, respectively. Stratification with reference to the satellite-derived brightness temperature indicates that N-type regime is associated with warm rain processes, whereas D-type precipitation events occur through ice-phase processes. In addition, this study offers a methodology for better comprehending the holistic view of precipitation processes by integrating satellite and ground-based observational data.

抄録全体を表示

Based on previously reported Asian-Australian Monsoon indices, this study characterizes the intensity of the southwest (SW) monsoon associated rainfall and its variability in different subregions of the western Philippines. Reanalysis and satellite-based datasets are utilized to derive these monsoon indices, which include the southerly and westerly wind shear indices, and outgoing longwave radiation-, and mean sea level pressure-based indices, spanning from 1991 to 2020. Subsequently, these indices were integrated to develop a local SW monsoon index (LSWMI) in the Philippines, which was compared and assessed with climatological gridded- and ground-based rainfall datasets to quantitatively describe the spatiotemporal dynamics of the SW monsoon season over the 30-year period. Results show that the proposed LSWMI can sufficiently capture the occurrences of heavy rainfall events in western Philippines. Moreover, the LSWMI is also capable in describing the evolution (onset, peak and decay) and distinct spatiotemporal characteristics of the SW monsoon, as it propagates from northern to southern Philippines. Overall findings demonstrate the significance of utilizing the LSWMI in characterizing and quantifying the SW monsoon, which ultimately provides new insights on advancing the monsoon monitoring and forecasting capabilities in the country.

抄録全体を表示

This study examines the seasonal characteristics of the interannual stratospheric variability that impact the polar tropospheric climate in the northern hemisphere winter, herein referred to as the Arctic Stratospheric Oscillation (ASO). The westerly wind anomalies associated with the ASO begin in the middle stratosphere around 60°N in early winter, gradually strengthen to reach their maximum in the upper stratosphere in January, and then move downwards with a decrease in intensity to the lower stratosphere in March. The seasonal progression of the ASO is found to be associated with increasing negative sea level pressure anomalies at the polar cap, reaching their maximum in March. It has been determined that the main driving force for ASO is planetary waves, with a major contribution from non-stationary waves. It is suggested that the ASO could have an impact on the occurrence of Sudden Stratospheric Warmings and Vortex Intensifications until midwinter. November signal of the ASO is found to be a promising candidate for predicting the polar climate for the subsequent winter season in both the troposphere and the stratosphere.

抄録全体を表示

In this study, we analyzed lightning distribution in the Malacca Strait area from 2009 to 2021 and explored its underlying physical mechanisms. Lightning activity in this area was higher in the strait than over the land. Both land and sea areas exhibited distinct diurnal cycles. Lightning at sea was prevalent during the night and early morning, whereas lightning over land was prevalent during the late afternoon and evening. Seasonal disparities in lightning occurrences were also noted, with the movement of the Intertropical Convergence Zone influencing the relatively high lightning occurrences in Southeast Asia during Mar–May and Sep–Nov. Furthermore, lightning in the eastern terrestrial area of the strait preceded the western terrestrial area by approximately 1 to 2 h. Additionally, lightning tends to initiate earlier in the more southward parts around the strait. Lightning occurrences strongly correlated with surface airflow convergence, highlighting the diurnal land–sea breeze cycle as the primary mechanism underlying the lightning formation in the Malacca Strait.

抄録全体を表示

To examine how the Kuroshio large meander (KLM) changes the intensity of a distant tropical cyclone (TC), we conducted a control simulation of Typhoon Neoguri, which approached the Kuroshio during the KLM event in October 2019, and sea surface temperature (SST) sensitivity experiments regarding the SST warming near the Tokai District and SST cooling south of the Kii Peninsula (KP) during KLM periods. Comparisons between the control and SST sensitivity runs revealed that the KLM can impact the intensity of Neoguri, even being far away from the Kuroshio. KLM-induced SST warming (cooling) enhanced (weakened) the moisture influx toward Neoguri through changes in the surface evaporation in the KP–Tokai area, indicating the penetration of wetter (drier) air parcels into the TC inner core region. The relatively wet (dry) environment in the inner core modulated the latent heating around the TC center, leading to enhancing (weakening) the distant TC. In October 2019, the KLM acted in the direction of strengthening Neoguri because the warm (cool) SST anomalies near Tokai (KP) were remarkable (insignificant). This study suggests that the KLM could have contradictory impacts on the intensity of a distant TC, depending on the KLM-induced SST anomaly patterns in the KP–Tokai area.

抄録全体を表示

This study evaluated the impact of climate change on the intensity of blowing-snow events across a wide probability spectrum including extreme events by a dynamically-downscaled meteorological dataset with a large number of ensembles called d4PDF. Focusing on four sites in Hokkaido, the hourly snow transport rate (STR) was estimated from wind speed, temperature, and snowfall. The historical experiment of d4PDF can reproduce the observed distribution of STR. The +2K experiment of d4PDF indicated that the severe blowing-snow events became rarer. Moreover, the monthly maximum STR exhibited a decrease, yet it showed significant spatial differences and seasonal variations. The monthly maximum STR and its drifting term in the mid-winter was the most significantly reduced at a site along the Pacific coast. At this site, the mean snow-covered duration (SCD) from December to February was shorter than that of the other sites. Such a decrease in STR would be due to the shortening of the SCD and the substantially related to the critical temperature at the freezing point.

抄録全体を表示

A bias-corrected downscaled 1-km mesh future climate dataset across Japan called NIES2020, based on five global climate models (GCMs) selected from the Coupled Model Intercomparison Project Phase 6 (CMIP6), has been used for regional impact assessments and adaptation studies under various emission scenarios. However, it is not yet revealed what determines the scenario dependence of the Japanese precipitation changes unscaled with global mean temperature changes. Here, we disentangled the inter-scenario differences in precipitation changes averaged across Japan. In the CMIP6 GCMs, the ensemble mean precipitation increases more in the mid-21^st century under low-emission scenarios than higher-emission scenarios, consistent with NIES2020. In the low-emission scenarios, rapid reductions of anthropogenic aerosol emissions from East Asia enhance the surface downward shortwave radiation around Japan, promoting evaporation and precipitation. Such high precipitation sensitivity per degree of global warming is confirmed regardless of the season. In contrast, the precipitation increase is most suppressed under a high-emission scenario with weak air pollutant mitigation. Therefore, future precipitation changes across Japan are more constrained by aerosol emission changes than global warming levels, especially in the mid-21^st century. This suggests climate response to air pollutant mitigations needs to be considered for implementing impact assessments and adaptation strategies in Japan.

抄録全体を表示

To identify and characterize the synoptic-scale precipitation systems causing widespread heavy precipitation events over Japan and to evaluate their possible future changes, annual maximum of area-averaged daily and 5-day accumulated precipitation for 720 years were analyzed for both historical and 4 K warming climates using a large ensemble dataset with 5 km horizontal resolution. According to statistical cluster analysis, the approach of tropical cyclones is the primary factor causing widespread heavy precipitation events in both the historical and 4 K warming experiments, although the Baiu front and migratory extratropical cyclones also contribute to event occurrence. The frequency of tropical-cyclone-associated events is lower in the 4 K warming climate compared with the historical experiment because the occurrence frequency of tropical cyclones is lower over the western North Pacific. The decrease in frequency of tropical-cyclone-associated events leads to a relative increase in the frequency of events associated with other precipitation systems (i.e., the Baiu front and migratory extratropical cyclones) under the warming climate. The anomalous moisture supply in the 4 K warming experiment causes the widespread heavy precipitation derived from the Baiu front and migratory extratropical cyclones to intensify to reach a magnitude comparable to that of historical-climate tropical-cyclone-associated events.

抄録全体を表示

The interannual variation in the number of heavy rainfall events in Japan in 1976-2022, extracted from 3-hour accumulated precipitation (P3H) data exceeding 130 mm, showed a relatively large relationship (correlation coefficient 0.45) with sea surface temperature (SST) around the Japanese Islands. In the Kyushu area during the rainy season (June–July), the correlation coefficient with SST became considerably smaller, while a relatively large relationship (correlation coefficient 0.45) was found with the appearance frequency of 500m-height water vapor flux above 250 g m⁻² s⁻¹, suggesting that the interannual variation could be considerably influenced by the synoptic scale pressure pattern. Diurnal variations in the number of heavy rainfall events, including long-term increasing trends, were also investigated. Although less significant diurnal variations were observed on the annual basis, the events in the Kyushu area were more frequent in the morning (7-9 JST: JST = UTC + 9 hours) during the rainy season. In the Kyushu area, the 47-year long-term trend of heavy rainfall events in 4-9 JST was a 7.47-fold increase in June and July, while it was only a 1.35-fold increase in the other months.

抄録全体を表示

In this study, we quantitatively evaluated the shape and fall velocity of precipitation particles in convective clouds observed by Rainscope to better understand graupel formation processes. Rainscope is a newly developed particle imaging radiosonde that provides much clearer precipitation particle images than those obtained by a conventional videosonde. In addition, it can measure particle fall velocities in clouds. Rainscope was launched into a convective cloud with active lightning and gusts on 25 June 2022. The particle images captured by Rainscope provide detailed information on particle shapes, surface conditions, and contours, facilitating the quantitative evaluation of particle shape. The observed circularity, defined as a function of the particle circumference, and aspect ratio* indicate that graupel just above the freezing level, which coexisted with frozen particles, differs from graupel with an ice crystal as an embryo. The particle fall velocity of graupel in the lower layer was smaller than that of frozen particles and larger than that of general graupel, which forms from an ice crystal. Therefore, graupel in the lower layer likely originated from a frozen particle, which was formed by freezing a raindrop lifted by updrafts and then rimed.

抄録全体を表示

Based on the station observations and reanalysis data, this study investigates the temporal variation characteristics of winter extreme snowfall events over Northeast China (NEC) and the possible causes involved. In recent four decades, the snowfall amount over NEC has a significant increasing trend, especially during the 21^st century, which is dominated by its extreme component. On the contrary, the snowfall days over NEC exhibit an opposite variation trend, showing a rapid decrease during the research period. The opposite variation trends suggest a rapid increase of extreme snowfall events in this region. Composite results of 39 extreme snowfall cases reveal that the dominant circulation pattern causing the extreme events is the enhanced local meridional circulation over the north NEC, and significant relationships can be found between the northeast cold vortex (NECV) and extreme snowfall event. During the 21^st century, both the 500 hPa geopotential height and the 850 hPa air temperature present negative tendencies over the middle and high latitudes of Asian continent. This is beneficial for stronger and more frequent northerly winds behind NECV to cause more intensified low-level convergence over this region and finally trigger more extreme snowfall events.

抄録全体を表示

Large-ensemble experiments with global and regional climate models enable us to assess changes in the risks of local-scale heavy snowfall due to anthropogenic global warming. We conduct 100-ensemble historical and non-warming global climate experiments forced by oceanic conditions in 2021/22 when La Niña phenomena occurred, and conduct dynamical downscaling using regional climate models with 20 km and 5 km grid intervals. The 10-year return values of total winter snowfall decrease in most of Japan due to anthropogenic global warming, while they increase at high elevations and the northern parts of Japan. The winter-maximum daily snowfall is enhanced not only over high elevations but also over low elevations in Japan. Tsunan Town is located in an inland area of central Japan where the winter-maximum daily snowfall is enhanced by anthropogenic global warming. Composite analyses of winter-maximum daily snowfall events at the Tsunan weather station indicate that the enhancement of daily snowfall due to anthropogenic global warming is related to deeper troughs at 500 hPa and warmer and more humid air in the lower atmosphere in the historical 2021/22 winter than those in the non-global-warming 2021/22 winter.

抄録全体を表示

On 15 August 2023, Typhoon Lan (2023) struck the Kinki region in western Japan, bringing record precipitation to the Kinki and Chugoku regions. This study investigates a turbulent transport scheme that can predict precipitation more accurately using the Weather Research and Forecasting model. The turbulent transport schemes compared are the Yonsei University scheme, the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme, and the eddy-diffusivity mass-flux (EDMF) scheme, which is a blend of the MYNN scheme and a mass-flux scheme. Simulations are performed for a domain with a horizontal resolution of 5 km. The results show that the simulated track and central pressure of the typhoon over the Sea of Japan vary depending on the turbulent transport schemes, the MYNN scheme reasonably reproduces the distribution of heavy precipitation areas, the EDMF scheme even improves the quantitative prediction of precipitation, and the formulation of the turbulent length scale is also a key factor for the better prediction using the EDMF scheme. The EDMF scheme is expected to become a leading turbulent transport scheme in operational forecast models.

抄録全体を表示

 Since the end of continuous rocket-sonde observations, which had been conducted until the 1990s, direct observations at altitudes higher than 30 km have been conducted only intermittently, so there are fewer observation data than in lower altitude regions.

 In the present study, we conducted radiosonde observations with large rubber balloons to obtain vertical structures of wind velocity and temperature at altitudes higher than 30 km from 27 September to 3 October 2022 at the University of the Ryukyus, Okinawa Island, Japan.

 During the observation period, temperatures from 20 to 40 km altitude basically increased monotonically, including small perturbations. However, an observation at 1730 JST on 28 September showed a remarkable continuous decrease with altitude in temperature at 30-36 km altitude. This was also confirmed by Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC)-2 GNSS-RO temperature data observed near Okinawa Island and ERA5 reanalysis data. Using the ERA5 reanalysis and radiosonde observation data, we found that the temperature-depleted layer is caused by a planetary-scale wave and upward energy propagating inertia gravity wave.

抄録全体を表示

Climate experiments using global cloud-system resolving models (GCRMs) are expected to realistically simulate precipitation diurnal cycle (PDC) in the tropics, which is important for better representation of influences of cumulus convection on the climate system. This study examines how three series of decade-long climate experiments with Nonhydrostatic ICosahedral Atmospheric Model (NICAM), one of the GCRMs, realistically simulate the PDC over tropical coastal regions. Analyses reveal that it is more difficult to reproduce the PDC over coastal waters than that over coastal land, the former of which is characterized by nighttime offshore migration of precipitation areas. A comparison with in situ shipborne observations further reveals that biases in the offshore migration feature are associated with poor representation of convective cold pools; experiments with poor reproducibility of the offshore migration underestimate overall intensity of cold pools. The underestimation of the intensity may be associated with overestimation of environmental moisture in the lower free troposphere. As reproducing the environmental field is a difficult task particularly for climate experiments with global models, it seems more challenging for the climate experiments to realistically simulate the PDC over the coastal waters than for short-term experiments and regional climate experiments.

抄録全体を表示

The observation of wind over oceans remains challenging. This results in difficulty in predicting the wind speed and direction. In this study, we examined the accuracy of upper-tropospheric wind speed forecasts along the flights between Tokyo International Airport and Los Angeles International Airport. We compared the Global Spectral Model data from the Japan Meteorological Agency (as forecast data) with the observation data from the aircraft's Quick Access Recorder (as true values). The forecast errors are highest over the North Pacific Ocean, not at the end of flight when the elapsed hours are longest, with meridional winds having a larger forecast error than zonal winds. Analysis of the meteorological field where a large meridional wind forecast error occurred using ERA5 indicates that the convergence-divergence of the jet stream or the blocking of westerly winds by the upper trough may have affected the large forecast error.

抄録全体を表示

Afternoon thunderstorms, mesoscale convective systems, and other short-duration rainfall events threaten property and transportation. Recent deep learning techniques have been proven effective in nowcasting for rainfall accumulation (rain maps), but predicting occurrences of intense convective cells can add additional value to decision-making procedures. This study develops a deep-learning model that predicts the locations of cell occurrences in the next 60 minutes. The training data include reflectivities from the Taiwanese radar network and convective cell trajectories from the System for Convection Analysis and Nowcasting (SCAN). The label is the SCAN cell occurrence (1 or 0) within a 7.5 × 7.5 km² area in the next hour. In addition to providing occurrence probabilities, the post-analysis procedure deploys a threshold mask to convert the probabilistic forecast into deterministic forecasts; it achieves a ∼40% improvement in the critical success index compared with the baseline method. Furthermore, the new model informs users about the risks under the chosen threshold selected based on their risk tolerance. This study provides proof of concept that replacing the predicting objectives (“cell occurrence” instead of “rainfall”) of the model may help forecasters' decisions and the integration of deep learning into operational forecasting.

抄録全体を表示

Climate change impact modelling studies often require not only mean temperature and precipitation but also other climate variables (e.g., solar radiation and wind speed) and extreme indices as input data. However, studies on observational constraints (emergent constraints) about these variables and indices are limited. Based on linearities of future climate change as functions of global warming levels and biases in recent global mean temperature trends in the simulations of 40 Earth system models (ESMs), the upper bounds of uncertainties in future changes of various variables (annual mean temperature, annual maximum daily maximum temperature, mean specific humidity, mean downward longwave radiation and specific humidity on days when annual maximum daily precipitation (Rx1day) events occur) are successfully lowered in most regions of the world. We can also reduce inter-model variances of regional changes in mean precipitation, Rx1day, mean downward shortwave radiation, mean sea level pressure and mean surface wind speed in some areas. These results would be useful for climate change impact studies to consider whether they should weight ESMs or exclude some ESMs to prevent possible biases in impact assessments.

抄録全体を表示

Temperature trends in Japanese cities were analyzed using data at 433 stations on the AMeDAS network from April 1979 to March 2023. It was found that urban warming, defined by a temperature increase at an urban station relative to the surrounding non-urban stations, had slowed down in the latter part of the analysis period. The deceleration of urban warming was commonly found for northern, eastern, and western Japan, and not only for stations in densely inhabited areas but also those at weakly urbanized sites where the surrounding population density was 100-300 km⁻². The deceleration was observed in all seasons and time of the day, although it tended to be more conspicuous in winter than in other seasons, and in the nighttime than in the daytime.

抄録全体を表示

An extreme rainfall event with 48-h accumulated precipitation amounts exceeding 500 mm on the north (Japan Sea) side of western Japan occurred when Typhoon Lan (2023) approached and passed over Japan in a weak baroclinic environment. The rainfall event included two local heavy precipitation peaks. In the present study, we perform numerical simulations with a cloud-system-resolving model to investigate the potential roles of two factors in the first event peak: (1) an abnormally high sea surface temperature (AHSST) anomaly (∼ +4°C) and (2) a mesoscale low formed over the Sea of Japan. The results of sensitivity experiments showed that the AHSST increased the total rainfall amount by about 100 mm. The mesoscale low, which was generated by southeasterly flows over the mountain ranges of central Japan, determined the location of the heavy rainfall by controlling the direction and intensity of low-level flows. The role of this terrain-induced mesoscale low provided new insight into the mechanisms producing heavy rainfall in association with typhoons approaching Japan in a weak baroclinic environment.

抄録全体を表示

Field observations were conducted at Ibaraki Airport, Japan, to determine the vertical development speed of shallow radiation fog. The development speed in shallow (less than several meters) fog was 3-16 cm min⁻¹, and was slower at higher wind speeds. The speed decreased when the air above the fog layer became drier, possibly due to the mixing of the fog layer with the dry air above it. The speed data presented here can be used in nowcasting fog development for aviation decision making.

抄録全体を表示

In this study, we introduce a new seasonal prediction system using an atmosphere–ocean-coupled general circulation model called CFES (hereafter referred to as CFES ESPreSSO). We compare its prediction skill of the interannual variability of the surface air temperature (SAT) and precipitation anomalies with that of the SINTEX-F2 seasonal prediction system. We find that CFES ESPreSSO has a higher skill in predicting the SAT variability in January-February-March over East Asia and northeastern North America than SINTEX-F2, while the following season (April-May-June), SINTEX-F2 provides better predictions of the SAT variability over the Maritime Continent and subtropical North Pacific. Meanwhile, CFES better predicts the SAT variability in July-August-September over Eurasia and Arctic, and it continues to be so over the following season (October-November-December) over Eurasia. However, the prediction skill of SINTEX-F2 is generally better in the tropics (e.g., SAT in the subtropical North Pacific, SAT and precipitation in the Maritime Continent). Regarding climate indices, CFES shows a better prediction skill for the Atlantic Niño and Ningaloo Niño indices, whereas SINTEX-F2 is generally better for El Niño and the Indian Ocean dipole mode. These results suggest that for improved seasonal forecasting, it is beneficial to consider a multi-model approach, leveraging the respective strengths of each model.

抄録全体を表示

Typhoon HAISHEN, Typhoon No. 10 in 2020, was weaker than forecasts as it moved north over the western coast of Kyushu. The typhoon intensity forecasting scheme called TIFS operated at the Japan Meteorological Agency (JMA) tended to predict HAISHEN's intensity more strongly than the observed one, resulting in large errors in JMA's operational forecasts. One possible reason for the large errors is that TIFS does not include the effect of ocean cooling associated with tropical cyclones. Here, we investigated whether the accuracy of the typhoon intensity predictions can be improved by replacing static sea surface temperature and ocean heat content used in the conventional TIFS by those predicted by an ocean model. The results of prediction experiments using the pseudo-ocean-coupled TIFS show that the over-intensification of HAISHEN was suppressed and that the prediction errors were significantly reduced. We also extended the evaluation to all typhoons in 2020 and found that the pseudo-ocean-coupled TIFS reduced the prediction errors by about 10% compared to the conventional TIFS for prediction times of 3 to 5 days. This indicates that pseudo-ocean coupling of the conventional TIFS can improve the accuracy of typhoon intensity forecasts.

抄録全体を表示

Drylands, which occupy 41% of Earth's land area, have large effects on Earth's climate via land-atmosphere interactions, and simulations of future climate indicate that drylands will be very sensitive to climate changes associated with global warming. Monitoring of drylands is therefore necessary to help guide sustainable development in drylands and to protect the global environment. This study examined changes of the global distribution of the aridity index from 2000 to 2020 and compared them to changes from 1951 to 1980. The regions with relatively wet climates, that is, semi-arid and dry sub-humid regions, became drier from 2000 to 2020. The largest use of land in drylands was grassland, followed by open shrubland, cropland, savanna, and woody savanna. More than 50% of dry land was accounted for by grasslands (18,651,109 km²) and dryland forests including shrubland and savanna (13,331,231 km²). The relationship between the aridity index and the normalized difference vegetation index indicated that the value of the aridity index of dryland forests and grasslands equaled the threshold for climatically stable existence, although the range of the aridity index was wide in both cases. We also made rough assessments of soil organic carbon sequestration in dryland forests and grasslands.

抄録全体を表示

In summer 2023, record-high temperatures were observed in many parts of the Northern Hemisphere, including Japan, where summer-mean temperature was the highest over the last 126 years. Under an unprecedented heatwave in late July through September, record-high temperatures were successively observed particularly over northern and eastern Japan. The late-July heatwave is attributable primarily to the markedly-intensified North Pacific Subtropical High over Japan, accompanied by the poleward-deflected subtropical jet (STJ). This situation occurred under the influence of the Pacific–Japan pattern driven by northwestward-moving enhanced tropical convection over the western North Pacific and the Silk-Road pattern. The enhanced convection was influenced by upper-level cyclonic vortices detached from the intensified mid-Pacific trough. Seemingly, it was also under the remote influence from positive sea-surface temperature (SST) anomalies in the western equatorial Pacific as well as negative ones in the central–eastern equatorial Indian Ocean, considered as remnant and delayed impacts of long-lasted La Niña until the preceding winter. The August heatwave occurred under the persistent poleward-shift of STJ as well as warm, moist low-level southerlies and their downslope-wind effects. Both extremely high SST around northern Japan and a long-term warming trend in air temperature could also contribute to the record-setting air temperature.

抄録全体を表示

An eyewall replacement cycle is often seen in tropical cyclones, when a secondary eyewall forms outside the inner eyewall, and the inner eyewall disappears. Although this cycle significantly affects the intensity of tropical cyclones, the mechanisms of secondary eyewall formation (SEF) are diverse, and most are complementary. Some studies have suggested that dry air inflow and diabatic cooling may have an important role in SEF via the mesoscale descending inflow (MDI). Here, we use numerical experiments to investigate the role of the middle tropospheric dry inflow in SEF. Idealized experiments were conducted using the plane version of the Nonhydrostatic Icosahedral Atmospheric Model. The control experiment produced SEF with a dry air inflow in the middle troposphere and associated MDI. In sensitivity experiments, in which the water vapor in the middle troposphere was increased in the outer areas of the tropical cyclone, the onset of SEF was delayed. These results reveal the two distinct processes of SEF: the angular momentum transport by MDI and the unbalanced dynamics in the boundary layer.

抄録全体を表示

Shallow coastal seawater response during the passage of near-landfall intensification (NLI) tropical cyclones (TCs) and non-NLI TCs was examined using oceanic and atmospheric reanalysis data and observations. The sea surface temperature ahead of the NLI-TC track is maintained or even increases when NLI-TC is approaching the land. The magnitude of the wind stress, which plays an important role in the NLI process, is related to the zonal surface wind on the right side of the tracks. Coastal mixed layer warming can be explained by Ekman transport under sustained wind stress due to surface wind forcing. The successive deepening of the coastal ocean boundary layer and the increase in warming in the subsurface seawater temperature by an average of 0.3°C, could maintain thermal capacity in a certain degree. This shallow coastal water response could partly explain the NLI progress in the northern South China Sea, indicating the importance of coastal ocean dynamics and air-sea interactions.

抄録全体を表示

This study investigated three-dimensional structure of an equilibrium drop size distribution within a convective system that spawned heavy rainfall over northern Kyushu in western Japan on 10 July 2023. Ground-based optical disdrometer observations showed that the drop size distribution shape became bimodal (the peaks are at 0.7 and 1.0 mm in diameter) and then reached an equilibrium state during the rapid increase in precipitation intensity. Analyses of vertical profiles of polarimetric measurements showed that within the convective system collisional coalescence was dominant mainly at 1.5-4 km height, whereas collisional breakup was dominant below 1.5 km height. These processes were inferred to enhance the precipitation intensity. The equilibrium drop size distribution continued at least one minute during the event, and its spatial scale, diagnosed by a radar-derived parameter to be several kilometers, suggested that the equilibrium drop size distribution was a meso-γ-scale phenomenon.

抄録全体を表示

At a site on the Sea of Japan side of central Japan, the concentrations of ionic components in PM_2.5 were measured semi-continuously for about 15 months using filter collection and ion chromatography (filter method). Continuous measurements of sulfate particles were simultaneously performed in summer and autumn using a commercial sulfate monitor (Sulfate Particle Analyzer, SPA). High concentrations of sulfate ion (SO₄²⁻) were sometimes observed from spring to summer, and the high SO₄²⁻ was thought to be due not only to trans-boundary pollution from the Asian continent but also the influence of volcanic plumes. A comparison between the SPA and the filter method showed that the sulfate concentrations measured by the SPA method tended to be about 20% lower than those by the filter method. High concentrations of sulfate particles were observed not only from volcanoes from the Kyushu District such as Sakurajima, but also from volcanic smoke derived from Nishinoshima in the Ogasawara Islands. In recent times, sulfate particles from volcanos may be important contributors to PM_2.5 in Japan.

抄録全体を表示

Ensemble dynamical downscaling experiments were performed to investigate the influence of East Siberian blocking on a heavy snowfall event that occurred over Fukui City, Japan, in early February 2018 and was associated with the development of the Japan Sea Polar airmass Convergence Zone (JPCZ). The downscaling experiments simulated the enhancement of the East Asian cold air stream and its flow along two routes: the western route, which runs from the Eurasian Continent via the Yellow Sea and the Korean Peninsula; and the northern route, which originates in the Sea of Okhotsk and runs via the northern Japan Sea. As a result, the location and strength of the simulated JPCZ in the downscaling experiments are consistent with those in the Japanese regional reanalysis dataset. For the sensitivity experiments, the blocking that develops over East Siberia just prior to the formation of the JPCZ was removed, and the results indicate that the East Siberian blocking contributes significantly to JPCZ development by enhancing the East Asian cold air stream along the western route. Additional data analyses based on the 20-year reanalysis revealed that East Siberian blocking can enhance both the western and northern routes of the cold air streams.

抄録全体を表示