Kensuke Nakajima

Tsunamis are commonly generated by earthquakes beneath the ocean floor, volcanic eruptions, and landslides. The tsunami following the Tonga eruption of 2022 is believed to have been excited by atmospheric pressure fluctuations generated by the explosion of the volcano. The first, fast-traveling tsunami was excited by Lamb waves; however, it has not been clarified observationally or theoretically which type of atmospheric fluctuations excited more prominent tsunami which followd. In this study, we investigate atmospheric gravity waves that possibly excited the aforementioned subsequent tsunami based on observations and atmosphere-ocean coupling simulations. The atmospheric fluctuations are classified as Lamb waves, acoustic waves, or gravity waves. The arrival time of the gravity wave and the simulation shows that the gravity wave propagated at a phase speed of 215 m/s, coinciding with the tsunami velocity in the Pacific Ocean, and suggesting that the gravity wave resonantly excited the tsunami (Proudman resonance). These observations and theoretical calculations provide an essential basis for investigations of volcano-induced meteotsunamis, including the Tonga event.

We investigate ionospheric disturbances using the total electron content (TEC) data obtained from the radio signals emitted from three satellites after the foreshock of the 2011 Tohoku Earthquake on March 9, 2011. Co‐seismic ionospheric disturbances (CIDs) appeared to extend from an onset point concentrically in all of the satellite data. We have found, however, that the geographic coordinates of the onset points did not coincide if the observed CIDs were assumed to occur at one altitude. Admitting that the altitudes of the onset points are different, we searched for coinciding geographic coordinates of the onset points by changing the altitudes and identified the altitude of the three onset points to be 107.8, 131.8, and 133.3 km and the onset time at these altitudes. As a result, the vertical velocity of an acoustic‐gravity wave is estimated to be 448 m/s from the travel time between the altitudes of 107.8 and 131.8 km and 370 m/s between the altitude of 107.8 and 133.3 km. This is comparable to the speed of sound calculated using the empirical model NRLMSISE‐00. The present study provides a method of determining the altitude and geographic coordinates of the onset point from the ionospheric TEC analysis independently of the seismic data. Furthermore, the source location of the acoustic‐gravity wave is simultaneously determined because its wavefront propagating vertically arrives earliest at an altitude that TEC disturbance is detectable.

Ishiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) investigated multiple equilibrium solutions of a gray atmosphere for various values of solar constant, utilizing two types of models, namely, a one‐dimensional energy balance model and a general circulation model (GCM) with simplified physical processes, both of which permit existence of the runaway greenhouse state. The study was retracted, since there was a bug in their GCM that affected quantitative aspects of the study. Here, we revise the study with re‐performing all of the GCM experiments using an appropriately corrected model. The results of re‐experiments show that the main features of the climate regime diagram drawn in the solar constant ‐ ice line latitude plane are mostly unchanged, except that the ice‐free equilibrium state, which existed in Ishiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) now disappears. It is confirmed that there are the partially ice‐covered state, the globally ice‐covered state, and the runaway greenhouse state. These three states coexist for intermediate values of solar constant. The existence of the large ice cap instability is also confirmed, although the critical latitude of the partially ice‐covered state extends equatorward. Also same as before, the small ice cap instability does not seem to appear.

Mt. Shinmoedake, a part of the Mt. Kirishima cluster of volcanoes in Kyushu, Japan, erupted on 10 March 2018. Our infrasound sensor network located at a distance of more than 200 km from the source detected signals emitted by an explosive eruption of Mt. Shinmoedake. The arrival time of the signals is divided into three time intervals. To reveal how the observed infrasound signals propagated from the source to the sensors, we carry out three-dimensional ray tracing on the basis of the Hamilton equations including the vertical profiles of the temperature and wind around the ray path. We present formulas for calculating travel time and distance of infrasound from a source to an observation site and its turning altitude in the atmosphere. We have identified four kinds of signals, namely, the waves propagated in the troposphere undergoing multiple refraction and those refracting from the stratosphere, the mesosphere, and the lower thermosphere. Brief discussion is devoted to some of the unidentified signals.

Direct-imaging techniques of exoplanets have made significant progress recently and will eventually enable monitoring of photometric and spectroscopic signals of Earth-like habitable planets. The presence of clouds, however, would remain as one of the most uncertain components in deciphering such direct-imaged signals of planets. We attempt to examine how the planetary obliquity produces different cloud patterns by performing a series of general circulation model simulation runs using a set of parameters relevant for our Earth. Then we use the simulated photometric lightcurves to compute their frequency modulation that is due to the planetary spin–orbit coupling over an entire orbital period, and we attempt to see to what extent one can estimate the obliquity of an Earth twin. We find that it is possible to estimate the obliquity of an Earth twin within the uncertainty of several degrees with a dedicated 4 m space telescope at 10 pc away from the system if the stellar flux is completely blocked. While our conclusion is based on several idealized assumptions, a frequency modulation of a directly imaged Earth-like planet offers a unique methodology to determine its obliquity.

The relationship between the size change of tropical cyclones (TCs) and radial distribution of precipitation was investigated for TCs occuring over the western North Pacific Ocean with satellite observation data. The size of a TC is defined as the radial extent of 15 m s⁻¹ winds (R15). The spatial distribution of precipitation was investigated by extracting two subperiods, which are the size‐increasing period and the size‐stationary period, as determined from R15 evolution. The results show a contrast that during the size‐increasing period, precipitation falls both inside and outside the R15, whereas during the size‐stationary period, it is concentrated near the center of the TC and falls less around and outside R15. This contrast is also confirmed statistically by composite analysis using 64 TC cases. Further analysis reveals that this contrast does not depend on the intensity or the change in intensity of the TC, which indicates that the area of precipitation related to the intensity or intensity change of TCs is different from that related to the size change of TCs. Finally, we examined the radial distribution of inertial stability in TCs and found that precipitation during the size‐stationary period occurs in a region with a higher inertial stability than that which occurs during the size‐increasing period.

Based on observations from the Hubble Space Telescope and the Subaru Telescope, we have discovered that Europa, Ganymede, and Callisto are bright around 1.5 μm even when not directly lit by sunlight. The observations were conducted with non-sidereal tracking on Jupiter outside of the field of view to reduce the stray light subtraction uncertainty due to the close proximity of Jupiter. Their eclipsed luminosity was 10^–6-10^–7 of their uneclipsed brightness, which is low enough that this phenomenon has been undiscovered until now. In addition, Europa in eclipse was Spitzer Space Telescope, but it was not detected, suggesting a significant wavelength dependence. It is still unknown why they are luminous even when in the Jovian shadow, but forward-scattered sunlight by hazes in the Jovian upper atmosphere is proposed as the most plausible candidate. If this is the case, observations of these Galilean satellites while eclipsed by the Jovian shadow provide us with a new technique to investigate the Jovian atmospheric composition. Investigating the transmission spectrum of Jupiter by this method is important for investigating the atmosphere of extrasolar giant planets by transit spectroscopy.