In this study, the effect of the zonally-elongating monsoon trough (MT) on the interaction of binary tropical cyclones (BTCs) is investigated by using data analysis and idealized simulations. The interaction of BTCs is found to be sensitive to the relative orientation of the two tropical cyclones (TCs) embedded in the MT. When the two cyclones are lined up in a northeast–southwest (NE–SW) orientation, the MT steers the two cyclones to approach each other and promotes the Fujiwhara effect. In contrast, when the initial cyclones are oriented in the northwest–southeast direction of the MT, they will move away from each other under the large-scale steering flows.

Idealized simulations are conducted to understand how the MT and the β-effect influence the BTC interactions, focusing on NE–SW oriented pairs. The steering flows at different stages are examined by partitioning them into the one from the MT and the other cyclone in the pair. The analysis shows that the binary TCs' motions are mainly controlled by the large-scale steering flows in the initial stage. In the case of BTCs with a NE–SW orientation, the MT can promote two TCs to approach each other, thus increasing the possibility of binary interactions.

The sensitivity of the interaction of BTCs to their intensities, the strength of the MT, and the β-effect are examined. The stronger the MT, the stronger its large-scale steering flows will be, thus making the two NE–SW oriented TCs merge faster. Furthermore, the binary interaction is stronger on the β-plane compared with that on the f-plane. It is likely due to the β-induced Rossby wave energy dispersion. As the MT evolves into a monsoon gyre (MG)-like pattern, a pronounced southwesterly flow emanates in the southeast quadrant of MG. This southwesterly flow acts as a steering flow to help the western TC move northeastward, accelerating to reach the critical distance.

We examined the essential features and formation mechanism of the strong local “Suzuka-oroshi” winds, which are located leeward of the Suzuka Mountains in Japan. This area features favorable topography for downslope windstorms. Climatological analysis revealed that Suzuka-oroshi mainly occurred after an extratropical cyclone with a cold front and passed the Sea of Japan (55 % of all occurrences). Additionally, inversion layers (1–5 km level) were observed in 74 % of cases. Climatological analysis using spatially dense observational data revealed that the strongest winds tended to blow in the northern part of the plain on the leeward side. Numerical simulations for one case by the Weather Research and Forecasting (WRF) model with a 1 km grid increment supported this finding. Simulation results with and without the Suzuka Mountains demonstrated that the strong Suzuka-oroshi in the northern part of the plain comprised downslope windstorms with a transition of flow regime (the internal Froude number was lesser than 1.0 at the windward of mountains and greater than 1.0 above the leeward slope). Additionally, the differences in the height of the mountains between the northern and southern parts resulted in greater wind speed in the northern parts compared to the southern parts.

This study reveals the potential roles of the sea surface temperature (SST) warming associated with the Pacific Meridional Mode (PMM) and the Indian Ocean (IO) warming on tropical cyclone genesis (TCG) in the North Pacific (NP) by focusing on the super El Niño event that occurred in 2015. We used the global non-hydrostatic model to conduct perpetual experiments by integrating for 30 months to obtain a climatological condition for July 2015 and examine sensitivities to SST in the warming region of PMM and IO on TCG over NP. We showed that if SST associated with PMM is warmer, the monsoon trough in the western North Pacific (WNP) and vertical wind shear over the eastern North Pacific (ENP) become weaker, reducing TCG in the WNP and increasing TCG in the ENP. We also show that if SST over IO is warmer, the monsoon trough in the WNP becomes weaker, although the vertical wind shear over the ENP does not change appreciably. We found that with SST warming associated with PMM or over IO, the anticyclonic anomalies over WNP intensify. We confirmed that if SST is warmer for PMM in the absence of the El Niño forcing, the cyclonic anomalies over WNP intensify as in previous studies. The present results imply a non-linear response for the forcing of the warm SST associated with PMM and El Niño.

Torrential rain in Typhoon Hagibis caused a devastating disaster in Japan in October 2019. The precipitation was concentrated in the northern half of Hagibis during extratropical transition (ET). To elucidate the mechanisms of this asymmetric precipitation, synoptic- and mesoscale processes were mainly analyzed using the Japan Meteorological Agency Non-Hydrostatic Model. The present study demonstrates that the asymmetric processes were different depending on the ET stages. When Hagibis was close to the baroclinic zone at middle latitudes on around 12 October (the frontal stage), heavy precipitation in the northeastern part of Hagibis was attributed to warm frontogenesis and quasigeostrophic ascent, as reported in many previous studies. In contrast, when Hagibis was moderately distant from the baroclinic zone on around 11 October (the prefrontal stage), heavy precipitation in the northern part occurred in a slantwise northward ascending motion in the outer region. This slantwise motion developed in a region with strong westerly vertical shear, which was enhanced between Hagibis and a westerly jet stream. Based on the analyses of potential vorticity and absolute angular momentum, this region was characterized by reduced moist symmetric stability in the lower and middle troposphere accompanied by inertial instability in the upper troposphere and conditional instability in the lower troposphere. These results provide additional insights into the time evolution of asymmetric processes during ET in the absence of a distinct upper-tropospheric trough, particularly, the slantwise motion in the prefrontal stage.

Twentieth-century atmospheric reanalysis datasets are important for understanding climate in the early era of the century. This paper first compares two sets of the 20th-century atmospheric reanalyses, the NOAA-CIRES-DOE 20th-Century Reanalysis Version 3 (20CRv3) and the European Centre for Medium-Range Weather Forecasts (ECMWF) 20th-Century Reanalysis (ERA20C), as far as the summer low-level cross-equatorial flows (CEFs) over the Asian-Australian monsoon region are concerned. The results show evident regional differences in the intensity of individual branches of CEFs between the two reanalyses despite an overall agreement in the climatological seasonal mean and variability. At the interannual timescale, significant differences are observed prior to 1925 and in the 1940s. During the two periods, there are often opposite variations in Somali CEF in the two datasets, along with obvious different amplitudes in the Bay of Bengal (BOB) and Australian CEFs. At the interdecadal timescale, the two datasets have different periodicities in Somali CEF and have a greater fluctuation of BOB CEF after 1925 in ERA20C than 20CRv3, as well as an opposite decadal variation in the Australian CEF prior to 1940 and in the 1960s. As for the long-term trend, both the Somali and BOB CEFs exhibit intensification in both the datasets, but the intensification amplitude is bigger in 20CRv3 than in ERA20C for Somali CEF; the Australian CEF exhibits a weakening trend in both the datasets but is less evident in 20CRv3. To figure out which of the two datasets is relatively more reliable, the observed cross-equatorial meridional gradient of sea-level pressure index and the Indian summer monsoon rainfall index, which both have longer instrumental records, are used as benchmarks to validate the CEFs in view of their close connections. The results indicate that ERA20C is more reliable and thus more suitable for investigating decadal climate variability of the 20th century across the hemispheres.

The nonhydrostatic numerical weather prediction (NWP) model ASUCA developed by the Japan Meteorological Agency (JMA) was launched into operation as 2 km- and 5 km-resolution regional models in 2015 and 2017, respectively. This paper outlines specifications of ASUCA with focus on the dynamical core and its configuration/accuracy as an operational model. ASUCA is designed for high computational stability and efficiency, mass conservation, and forecast accuracy. High computational stability is achieved via a time-split integration scheme to compute acoustic terms and an advection scheme with a flux-limiter function to avoid numerical oscillation. Additionally, vertical advection and sedimentation are calculated together with another exclusive time-splitting technique. ASUCA adopts hybrid parallelization using Message Passing Interface (MPI) and Open Multi Processing (OpenMP) for high computational efficiency on massive parallel scalar computers. The three-dimensional arrays are allocated such that the vertical direction is the stride-one innermost dimension to effectively use cache and multithread parallelization. This is particularly advantageous for physical processes evaluated in a vertical column. To ensure mass conservation, density rather than pressure is integrated as a prognostic variable in fluxform fully compressible governing equations. ASUCA exhibited better performance than the previous operational model in idealized and NWP tests.