This study statistically investigates the characteristics of tropical cyclones (TCs) undergoing rapid intensification (RI) in the Southwest Pacific (SWP) region in the 37 years from 1986 to 2022. Among 364 TCs, 82 rapidly intensifying TCs (RI-TCs) were defined as TCs that experienced maximum wind speed increase of 30 kt (15.4 m s⁻¹) or more in a 24-h period. RI-TCs are frequently observed over the zonally elongated area around coral sea, south of Solomon Islands (Solomon Sea), Vanuatu, Fiji, Tuvalu, Tokelau and Samoa, while RI-TCs were rarely observed in areas of Tasman Sea, Tonga, northern waters of New Zealand, Cook Islands, Niue and French Polynesia. RI-TCs preferentially occur during the southern hemisphere summer season. Frequency of RI-TC occurrence shows a slowly increasing trend over the 37-year period. However, this increasing trend was not statistically significant at the 95 % confidence level. In El Niño years, TCs tend to undergo RI more frequently presumably due to the average genesis to the further north where sea surface temperature (SST) and ocean heat content were high. In contrast, RI-TCs occurred less frequently during La Niña years. The RI onset typically occurs 0-42 h after TC genesis with a peak frequency observed just after genesis (0-6 h). The RI duration is usually 1-2 days with a peak at 24 hours. The mean lifetime of RI-TCs lifetime was 7.86 days, longer than that of non-rapidly intensifying TCs (NR-TCs) (3.72 days). In terms of average intensity, RI-TCs have significantly lower lifetime central pressure and higher lifetime maximum wind speed than NR-TCs. RI-TCs tend to develop into more severe TCs as a result of formation in environments favorable for TC development such as weak vertical wind shear, deep moist layer, high SST and TC heat potential.

 The drop size distribution (DSD) of precipitation particles is a fundamental property for characterizing rainfall. This study statistically clarified the characteristics of DSDs using approximately 10 years of DSD data obtained from a ground-based optical disdrometer in Kumagaya, the eastern part of Japan. The results showed that DSDs tended to maintain their shape even as rainfall intensity (R) increased, and they tended to be distributed in a certain region, i.e., the mass-weighted mean diameter (D_m) ∼ 2.0-3.0 mm and the generalized intercept parameter (N_w) ∼ 2 × 10³–3 × 10⁴ mm⁻¹ m⁻³, of the DSD parameter space defined by D_m and N_w. The quasi-equilibrium shape of the DSDs, which is rarely observed only 16 cases in this study, was likely to be different characteristics between maritime and continental convection. Among them, the contribution to R was large when D_m or N_w was effectively increased with temporal change based on an error analysis. DSD characteristics were also identified by statistically evaluating the relationship between the specific differential phase (K_DP) and R in DSDs for C-band polarimetric weather radar. The results showed that the coefficient of the K_DP–R relation tended to be larger (> 24.0) during the warm season (from May to October) and smaller (< 21.0) during the cold season (from January to April and from November to December) when assuming a temperature of 10 degrees Celsius, whereas the exponent of the relation had no apparent trend. Furthermore, it is likely that the slope parameter, one of the DSD parameters, can be optimized for stronger rainfall events with a nearly same size distribution.

 Improving the existing limited network of observation sites and quantifying carbonyl sulfide (COS) temporal variability allows a more accurate understanding of the COS budget. A system with low-power consumption would enable COS concentration measurements at various observation sites. Therefore, we designed a continuous measurement system employing a commercially available portable laser-based analyser to measure atmospheric COS concentrations. To obtain precise atmospheric COS concentrations, (1) the temperature of the optical cell was stabilised at 0.13 ± 0.014 °C h⁻¹ using double insulation with a refrigerator and insulation material, (2) ambient air was used as a reference gas for 30 s every minute (1 cycle min⁻¹) after reducing its COS level to below 100 ppt using activated charcoal, and (3) the difference in water vapour concentration between ambient air and the reference was maintained within ± 400 ppm. The ambient COS concentrations were determined using three calibration gases with known COS concentrations prepared by the National Oceanic and Atmospheric Administration (NOAA). The analytical precision of the system was 12.1 ppt (1σ) over a 15-min, allowing for sufficient characterisation of diurnal variations of the atmospheric COS concentration. The observation in Tsukuba, Japan, showed that the observed COS concentrations in April 2023 were 410-599 ppt. Backward trajectory analysis revealed that air masses with high COS concentrations exceeding 550 ppt traversed over the Keihin Industrial Zone. This suggests that a continuous measurement system may discover potential COS sources, helping establish a COS observing network for more accurate oceanic and anthropogenic flux measurements.

 The inner core of a tropical cyclone (TC) is vital for TC energetics and often undergoes dramatic changes. This article provides a review on the understanding and operational practices of the structural changes in the TC inner core, mainly focusing on recent literature and activities. The inner core structure of a TC is generally described as an axisymmetric vortex in the vicinity of a hydrostatic and gradient wind-balanced state. However, this schematic can sometimes be oversimplified. Recent studies have documented small-scale features of the inner core, structural changes in TC rapid intensification, secondary eyewall formation, and eyewall replacement cycles using observational data, and idealized and sophisticated models. In line with the progress in understanding the inner core structure, several operational agencies have recently analyzed TC structural changes using their subjective analyses or diagnostic tools, contributing to disaster prevention. We also discuss potential impacts of climate change on the inner core structure, for which further work is required to reach a solid conclusion.

 In this study, the nonlinearity in a weather forecast was examined in an environment containing a mesoscale convective system. The nonlinearity was quantified by the relative nonlinearity as the extent to which the initial opposite-sign perturbed state vector does not keep the same magnitude and opposite direction in a forecast time. A pair of 18-h forecast experiments with initial perturbations of different signs was conducted for a heavy rainfall event in western Japan on 13 August 2021.

 Despite the initially different signs, the perturbations had random structures at convective scales over 2 h, taking the relative nonlinearity value 1.72 as previous studies have shown. However, the perturbations had the same sign on the meso-α scale at 11 h, taking the relative nonlinearity value greater than 1.72. This result suggested that this nonlinear signal was found not only on the convective scale but also on the meso-α scale. The nonlinear signal upscaled from convective to mesoscale, indicating a transition to a nonlinear regime at the mesoscale. Additional experiments showed that this meso-α scale nonlinear signal originated from the front with high convective activities in the initial field through the emission of gravity waves via the moist physics.

 Using observational data from the Program of the Antarctic Syowa Mesosphere-Stratosphere-Troposphere/Incoherent Scatter radar (PANSY radar) at Syowa Station (69.0°S, 39.6°E) over seven years, the climatology of gravity wave (GW) characteristics in the troposphere and lower stratosphere in the Antarctic were examined.

 Our analysis shows that the GW kinetic energy in the lower stratosphere is consistent with previous studies using operational radiosonde observations in the Antarctic, including an enhancement during austral spring. We derive a theoretical formula relating horizontal and vertical wind contributions to the GW kinetic energy with the GW intrinsic frequency and the aspect ratio. The vertical variation of the intrinsic frequency suggests the presence of GW sources near the tropopause in addition to those in the troposphere and near the ground. The GW momentum fluxes estimated from radar data indicate that net GW forcing is eastward in the lower stratosphere in seasons except for summer, which acts to accelerate the lower part of the polar night jet. Furthermore, we present the climatology of Eulerian-mean vertical winds elucidated from the long-term radar observations.