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This two‐part study presents a comprehensive analysis of (1) the vortex structure, (2) the inner core planetary boundary layer (PBL) wind profile, and (3) the overland surface winds of Typhoon Faxai(2019) during landfall observationally (Part I) and in high‐resolution Weather Research and Forecasting Model (WRF) simulations (Part II). Part II presents a framework for evaluating the wind field in the WRF simulations of Typhoon Faxai (2019) by comparing them with the observations presented in Part I. First, WRF simulations with two different surface roughness z0$$ {z}_0 $$ tables are presented. The simulation with the default z0$$ {z}_0 $$ table in WRF is shown to largely overestimate the surface wind speed. The overestimation is largely mitigated by using larger z0$$ {z}_0 $$ based on the climatology for urban and forest areas in Japan. Next, using the modified z0$$ {z}_0 $$ table, WRF simulations with three different PBL parametrizations, namely, Mellor–Yamada–Janjić, Yonsei University, and Mellor–Yamada–Nakanishi–Niino schemes are presented, and their impacts on the simulated vortex structure, inner core PBL wind profile, and overland surface winds are evaluated. In particular, the simulated surface winds are in good agreement with the observations outside the inner core of Typhoon Faxai. However, during the passage of the eyewall, the maximum surface winds are underestimated in the simulations. We show that the underestimation of the inertial stability, associated with the excessively large inner core of Typhoon Faxai in the simulations, led to the overestimation of the eyewall PBL jet height. This, in turn, caused the underestimation of the peak surface winds, as based on the empirical model of the wind reduction factor F2 km$$ {F}_{2\;\mathrm{km}} $$ proposed in Part I. [ABSTRACT FROM AUTHOR]

This two‐part study presents a detailed analysis of (a) the vortex structure, (b) the inner core planetary boundary layer (PBL) wind profile, and (c) the overland surface winds of Typhoon Faxai (2019) during landfall observationally (Part I) and in high‐resolution Weather Research and Forecasting model simulations (Part II). Part I presents a comprehensive observational analysis of (a), (b), and (c) of Typhoon Faxai (2019) based on radar dual‐Doppler analysis and spatially dense surface wind observations over the Kanto region of Japan. An empirical model for the wind reduction factor F2 km$$ {F}_{2\;\mathrm{km}} $$ is proposed and validated with surface observations and radar velocity–azimuth display analyses at Narita and Haneda airports, both of which were affected by the passage of Faxai's inner core. Two distinct PBL jets associated with the inner rain band and the eyewall were observed over Narita Airport, which was located to the right of Faxai's track. Based on the empirical model of F2 km$$ {F}_{2\;\mathrm{km}} $$, it is found that the maximum surface wind speed observed at Narita Airport was caused by the peak of F2 km$$ {F}_{2\;\mathrm{km}} $$ due to a strong PBL jet associated with the eyewall located as close as ∼$$ \sim $$0.3 km from the ground. In addition, the wind swath analysis shows that a large area far inland on the right side of Faxai's track experienced strong surface 10‐min winds due to the combination of the passage of the eyewall PBL jet and the small surface roughness length around agricultural land such as paddy fields and farm fields. [ABSTRACT FROM AUTHOR]

Obstacles on railway tracks are a factor hindering safe and stable operation. For this reason, obstacle detection systems have been introduced at level crossings, slopes and under platforms. However, they have been introduced only in limited sections for limited applications. Therefore, the authors considered to introduce an obstacle monitoring system that combines 90 GHz band millimeter‐wave and Radio over Fiber (RoF) technology, as an obstacle monitoring system over tracks to monitor the entire line. Since the millimeter‐wave band has a wide bandwidth, high precision detection is possible when they are used in radar. In addition, since radar control units can be grouped in one place by introducing the RoF technology, we can build a system at a low cost. In order to study the feasibility of introducing a new obstacles monitoring system combining 90 GHz band millimeter‐wave and RoF technology in railway environment, a field test using prototype equipment was conducted on a railway track. As a result, it was confirmed that the prototype radar system could detect humans and non‐metallic objects entering a track at a distance of about 200 m from the location of the prototype system. These results shows that the proposed system has the potential to be introduced as a new railway obstacle monitoring system using 90 GHz band millimeter‐waves and RoF technology. © 2023 The Authors. IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Using potentially best available rainfall data sets for the entire country of Japan (spatial scales of 1‐ and 20‐km), we analyze the 1–24 hr and city‐scale (1–400 km2) extreme rainfalls for both current (2006–2020) and future periods (2081–2109) at 1.5 K global warming scenario, complementing previous work that focuses on either coarse spatial and temporal scales or other warming scenarios (e.g., RCP and 2 K warming scenarios). A peak‐over‐threshold (POT)‐based approach is applied to compute areal reduction factor (ARF) for subsequently establishing intensity‐duration‐area‐frequency (IDAF) curves. Our results reveal that ARF values generally decrease with increasing area size and increase with longer duration and are affected by multiple underlying physical phenomena. Moreover, we find a greater increase in the rainfall intensities for shorter durations and higher return periods, ranging from 9.4% (1‐hr) to 6.2% (24‐hr), averaged for all return periods and 8.3% (25‐year) to 7.3% (2‐year), averaged for all rainfall durations. Spatially, extreme rainfall intensities are projected to increase by 8.9% in northern Japan (albeit with a less intense rainfall intensity in the current period), which is greater than the rest of the country (6.8%), underscoring the need to focus more on infrastructure designs in northern Japan. The projected IDAF curves further display an increase in the frequency of extreme events at city‐scale, for example, 25‐year extreme rainfall events in 2006–2020 would likely be 5‐10‐year events in 2081–2109. Our results with the state‐of‐the‐art data and implementable approach can be utilized for policymaking to reduce the warming‐induced risks in Japan and beyond. Key Points: For the first time, we analyze the city‐scale and sub‐daily extreme rainfall for the entire country of Japan by using high‐quality data setsAreal reduction factor values generally decrease with increasing area size and increase with longer duration and are affected by multiple physical phenomenaUnder 1.5 K warming, extreme rainfall will increase more for shorter durations and higher return periods, especially in northern Japan [ABSTRACT FROM AUTHOR]

High‐frequency (HF) radar monitors the sea surface current velocity and provides information for tsunami early warning. SeaSondeR, an HF ocean radar system in the eastern Tsugaru Strait, Japan, measured the tsunami‐induced current velocity during the 2022 Tonga volcanic tsunami. As an air‐coupled tsunami, the generating mechanism was complex, making it difficult to predict coastal tsunamis using traditional early warning methods. We adopted the tsunami data assimilation (DA) approach, which reconstructs the tsunami wavefield using offshore data and does not require source information, to forecast the coastal tsunami waveforms. Observations from the HF radar and offshore bottom pressure gauges (OBPGs) were used as the input for tsunami DA. The assimilation process started at 09:00 (UTC, hereafter) and forecasts were made at 14:00 and 15:00. The surface current velocity recorded by the HF radar reached the maximum (∼0.25 m/s) at 13:00, which corresponded to a negative phase of ∼2 cm sea level variation observed by OBPGs. We compared the forecasted waveforms with the observed waveforms at Hakodate and Shimokita tide gauges. The assimilation results obtained using HF radar showed a better performance in tsunami forecasting than those using OBPG in this case study. At 14:00, the forecasting accuracy indices were 91% and 67% for the next 2 and 6 hr, respectively. At 15:00, it was 63% and 70% for the next 2 and 6 hr, respectively. We suggest that HF radar could be a good supplement to OBPG for tsunami early warning purposes. Plain Language Summary: On 15 January 2022, the eruption of the Hunga Tonga–Hunga Ha'apai volcano generated a tsunami that arrived in Japan. The tsunami was detected by a high‐frequency (HF) ocean radar system in the eastern Tsugaru Strait. The HF radar recorded the velocity of tsunami‐induced sea surface currents. We used these velocity data to forecast the tsunami waveforms at coastal tide gauge stations. The forecast method, called tsunami data assimilation, does not require tsunami source information but accurately forecasts the coastal tsunami waveforms. We demonstrate that the HF radar data can be successfully applied to tsunami early warning systems. Key Points: High‐frequency (HF) radar observed surface current velocity in the eastern Tsugaru Strait during the 2022 Tonga volcanic tsunamiHF radar observations were used for data assimilation to forecast tsunami waveforms at coastal tide gaugesThe assimilation results using HF radar had an accuracy of over 63% for Hakodate and Shimokita tide gauges [ABSTRACT FROM AUTHOR]

The seasonal features of the eastward jet through the Tsugaru Strait was investigated using surface velocity data between 2014 and 2019 obtained from high‐frequency radar located in the eastern part of the strait, as well as data obtained from repeated shipboard observations from 2009 to 2019. The potential‐vorticity‐front‐model was used, and the volume transport of low potential vorticity (PV) water and the intensity of the PV gap at the front in the eastern part of the strait were estimated from these data. The influence of these parameters on the seasonal flow pattern of the Tsugaru Warm Current out to the Pacific Ocean (the summer/autumn "gyre mode" or winter/spring "coastal mode") was examined. The distribution of the parameters was consistent with the theoretical understanding, showing that in addition to the volume transport, an increase in the local PV gap can also contribute to the development of the gyre. Plain Language Summary: The Tsugaru Warm Current forms part of the coastal flow around Japan and shows remarkable seasonal changes in its flow pattern, which are similar to the Kuroshio meandering. The current flows along the coast from winter to spring, but demonstrates large anticyclonic meandering in the offshore direction, known as "gyre mode," from summer to autumn. The cause of this change in the flow pattern has been thought to be the seasonal change in volume transport. In this study, we used high‐resolution temporal and spatial data obtained from a high‐frequency radar system, as well as shipboard hydrographic data collected over a period of 10 years, combined with a theory based on the relationship between coastal waves and offshore jets with fronts to identify another important factor that affects the seasonal flow pattern of the current, namely, "potential vorticity." This study is important because it helps our knowledge of the behavior of fluids flowing out of channels and straits as well as that in relation to the connection between local phenomena and larger phenomena. Key Points: Observational verification of the seasonal flow pattern theory of the Tsugaru Warm Current based on velocity data from high‐frequency radarImportance of potential vorticity gap at the front recognized as well as the volume transport of the current on the flow pattern changeIntegration and improvement of our understanding about the behavior of fluids flowing out from channels and straits [ABSTRACT FROM AUTHOR]

Upper‐tropospheric clouds in the outflow layer of typhoons can affect the track of typhoons (tropical cyclones) through radiation effects. In this study, the microstructure of the outflow‐layer clouds of several typhoons was examined. Cloud radar observations of three typhoons around Japan revealed numerous protuberances in echoes along the base of the upper‐level clouds, which are referred to as mammatus‐like echoes. The horizontal and vertical scales of these mammatus‐like echoes were 0.5–3.0 and 0.3–1.5 km, respectively. Vertical observations revealed downward (upward) Doppler velocities in (between) the hanging echo regions. Upward and downward velocity maxima were estimated at 3 m s−1 around the mammatus‐like echoes. Neutral stratification developed in the dry layer beneath the cloud base in which the mammatus‐like echoes formed. These mammatus‐like structures may promote mixing along the cloud base that contributes to dissipation of the outflow‐layer clouds. Plain Language Summary: Heating and cooling due to radiative effects associated with upper‐level clouds spreading from a typhoon have been reported to have an impact on the actual track of the typhoon. Therefore, it is important to clarify the processes of formation and dissipation of the upper‐level clouds of typhoons. However, observations of such clouds, which are formed from relatively small ice particles, are scarce and the process of their dissipation is not well understood. In this study, we used cloud radar, which can detect clouds formed of small ice particles, to observe the upper‐level clouds of three typhoons that approached Japan. At the base of the upper‐level clouds, we found many protuberance structures with 0.5–3.0 km width and 0.3–1.5 km depth. These protuberance structures were accompanied by relatively large vertical motions of approximately 3 m s−1. The temperature and water vapor distributions near the base of the upper‐level clouds of the studied typhoons were conducive to these vertical motions. The small‐scale protuberance structures might promote dissipation of upper‐level clouds along their base. Key Points: Protuberances observed by cloud radar along the base of outflow‐layer clouds of typhoons are termed mammatus‐like echo structuresThe mammatus‐like echo structures are associated with vertical motions with velocity of several meters per secondMammatus‐like echoes occur in a layer with high potential for turbulence caused by sublimation of ice particles falling into a drier layer [ABSTRACT FROM AUTHOR]

Under strong wind caused by a low pressure with fronts passing near Honshu, I have observed weather clutter containing targets (ships) using an X‐band marine radar with a frequency of 9.41 GHz, a beamwidth of 1.0°, a pulsewidth of 0.5 μs, and a pulse‐repetition frequency of 1600 Hz. To investigate such clutter amplitude statistics, I have introduced the Nakagami‐m distribution to radar signal processing for the first time and used the Akaike Information Criterion (AIC). It has been found that the weather clutter amplitudes obey the Nakagami‐m distribution with the shape parameter of m = 5.85 for entire data and the log‐normal, Weibull, log‐Weibull, and Nakagami‐m distributions with the shape parameters of σ = 0.189 to 0.259, c = 6.52 to 7.81, c = 26.66 to 35.51, and m = 5.23, respectively, for small data within the beam width of an antenna. The number of the minimum AIC (MAIC) of the log‐normal, Weibull, log‐Weibull, and Nakagami‐m distributions to data is 4, 12, 4, and 1, respectively, from 21 small data. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

SUMMARY During winter, at Shonai Airport located at the northern part of Japan, some low-level turbulence caused by tornadoes and downbursts often interrupts takeoffs and landings. This is a primary source of economic loss. It is well-known that a weather radar system is one of the most useful devices for observing various weather phenomena. However, it is difficult for conventional radar systems with a range resolution of several hundred meters and a temporal resolution of about 5 min to detect small-scale phenomena with a horizontal scale of several hundred meters and duration of several minutes. We have been developing a meteorological Ku-band broadband radar (BBR) that can detect small-scale phenomena with high resolution (a range resolution of several meters and a temporal resolution of about 1 min). We constructed an advisory system with the BBR in order to support aircraft operation at this airport, and performed a comparison between BBR data and flight data recorder (FDR) data. [ABSTRACT FROM AUTHOR]

The article presents a study on the effects of grid resolution on simulated river peak flows and surface inundation in the Tsurumi and Yom watersheds in Japan through a process-based distributed hydrological model. It says that grid-based hydrological models are considered to be very useful flood modeling tools for flood risk analysis due to their capabilities of simulating models, radar, and satellite remote sensing. It states that the selection of a right grid size is important for the application of a grid-based model. The study suggests that grid resolution greatly affects the simulated river peak flows and surface inundation characteristics.

We explain a tracking method that uses a linear filter to estimate the true values of the position and the velocity with the target position as the radar observation. One rep- resentative example is a Kalman filter for three-dimen- sional space. The α-β filter with its easily analyzed performance is widely used in one-dimensional space. In a tracking method, the tracking precision must be evaluated when sufficient time has elapsed since the initial calculation, that is, in the steady state. The equation for calculating the variance of the errors in the predicted position in the steady state has been reported for radar observations of a constant-velocity linear motion target by an α-β filter. In addition, a Kalman filter can obtain the variance of the errors in the predicted position in the steady state by the Riccatti equation. In this case, however, a premise is the presence of drive noise that indicates the ambiguity of the motion model. There is no equation for calculating the variance of the errors in the predicted position in the steady state for the radar observations of a constant-velocity linear motion target. In this paper, we present an equation for calculating the covariance matrix of the prediction errors in the steady state of the position and velocity for the radar observations of the constant-velocity linear motion target in a 3D spatial linear filter. The results calculated in this paper present an extension of a conventional α-β filter. [ABSTRACT FROM AUTHOR]

Ground penetrating radar (GPR) and high-resolution shallow reflection seismic surveying were carried out to investigate the subsurface geology in and around the Uemachi Fault zone in the Yamato River area, Osaka, Japan. Shallow drilling in the area showed a major displacement event during the middle Pleistocene. The main Uemachi Fault plane could be clearly imaged on the seismic section, except for the most shallow 200 m. Several shallow normal fault planes with less displacement could be detected on both sides of the fault plane. GPR profiles confirmed the presence of several shallow normal faults within the area near the fault zone. These shallow faults could be followed in all of the GPR profiles crossing the fault zone. The integration of seismic section, GPR profiles and drilling data led to a conceptual model that explains the evolution of the Uemachi Fault system. The proposed model suggests the occurrence of several cycles of small vertical displacement along the deep part of the fault plane caused by the regional east–west compressional stress. The ductile nature of the shallow sedimentary cover and the absence of confining pressure in the shallow part allow for a considerable amount of plastic bending before failing in the shallow sedimentary layers. This bending generates stretching force within the shallow sedimentary cover, which in time, along with gravitational force, gives rise to the formation of the swarm of normal faults within the shallow layers near the fault zone. Some of the detected faults extend to a depth of less than 3 m below the ground surface, suggesting that the last tectonic activity along the fault plane may have occurred recently. [ABSTRACT FROM AUTHOR]

Sea ice clutter was observed with an X-band radar, and based on its statistical properties a new clutter suppression technique has been developed. The X-band radar was mounted on the roof of the Sea Ice Research Laboratory of Low Temperature Science, Hokkaido University, located in Mombetsu. This radar has a pulsewidth of 80 μs, and an A/D converter of sampling time 40 ps was developed to allow the spatial resolution of 12 m. The emitter coupled logic (ECL) was used as a high-speed IC. Sampled data were stored in a 64-kB RAN disk and transferred to a 5″ floppy disk via an 8-bit microcomputer. They were processed with a 16-bit computer, and it was found that sea ice clutter followed a Weibull distribution where the shape parameter ranged from 0.614 to 1.189. A new algorithm was developed for clutter suppression and the signal-to-clutter ratio was improved by 11.8 dB. [ABSTRACT FROM AUTHOR]