A quasi-stationary convective band that persisted for approximately ten hours caused precipitation in the northern part of Kyushu Island, Japan on 5 July 2017. The extreme amount of rainfall produced by this convective band caused a number of landslides and flash floods and resulted in a severe disaster. The Weather and Research and Forecasting (WRF) model was used to perform numerical simulations and to clarify the genesis and maintenance processes of the convective band. A full-physics WRF simulation successfully reproduced the observed features of the convective band and extreme precipitation. It was shown that a quasi-stationary convergence zone in the low level played a crucial role in generating and maintaining the convective band. Trajectory and frontogenesis analyses showed that low-level confluent flows due to the blocking effects of a high pressure system located over the Sea of Japan were responsible for the formation, intensification, and sustenance of the convergence zone. Furthermore, the frontal structure of the convergence zone was intensified due to the land-sea thermal contrast between Kyushu Island and the Tsushima Strait. Two additional experiments, namely a simulation with flattened topography of Kyushu Island and a simulation without considering raindrop evaporation also reproduced the observed band well. These results indicate that topography and a cold pool due to raindrop evaporation played only minor roles in the genesis and maintenance of the convective band.
In August 2016, a total of eight typhoons occurred in the western North Pacific, and four of which landed on northern and eastern Japan. These typhoons were associated with heavy rainfall and strong winds and caused massive damages in the Japanese archipelago. Moreover, five of the eight typhoons underwent extratropical transition (ET), which was more frequent than an average of 2.1 typhoons per year during August. To clarify the characteristics of the typhoon tracks that caused such unusual landfall and frequent ET in August 2016, we conducted k-means cluster and cyclone phase space (CPS) analyses for typhoons in August and September. Composite analysis and case study were also conducted to clarify the synoptic environments around the typhoons. To examine the unusual characteristics in August 2016, we compared the results of the analyses for this period with those in August from 2001 to 2015 and those in September 2016. The k-means cluster analysis showed that the direction of the typhoon tracks in August 2016 was more northward than that of the typhoons in August from 2001 to 2015. Moreover, the CPS analysis revealed that ET in August 2016 was characterized by a more indistinct structural change from a warm-core structure to a cold-core structure with a shorter duration than ET in August from 2001 to 2015. The synoptic environments around the typhoons in August 2016 were characterized by enhanced undulations of the upper-tropospheric jet stream, increased amplitudes of the mid-tropospheric trough, and relatively warm air around the typhoons in the lower troposphere. These synoptic environments explained the unusual landfall of typhoons with a more northward track and the more frequent ET and more indistinct structural evolution of ET in August 2016.
In this study, the impacts of Typhoon Morakot's (2009) vortex structure on the extreme rainfall in Taiwan were investigated using piecewise potential vorticity (PV) inversion. The control (CTL) experiment, starting at 0000 UTC 7 August or 15 h before landfall, reproduced the event realistically and was validated against the observations. By altering the PV perturbation inside 750 km from its center, we conducted sensitivity experiments in which the size and circulation strength of TY Morakot were reduced/weakened in the initial field in several different ways.
In the sensitivity tests, particularly those in which the initial PV within the inner core (≤ 250 km) was significantly weakened, the storm made landfall earlier, stayed over land longer, and exited Taiwan later. Such track changes were accompanied by a contraction and spin-up of the inner core at the early stages of the integration, caused by convection/latent heating within the inner core under large-scale, low-level southwesterly flow. As a result, Taiwan received an overall rainfall amount either comparable to or even more (up to 12 %) than that of the CTL in all tests. Thus, a weaker TY Morakot does not necessarily lead to less total rainfall over Taiwan, and the strong southwesterly flow and its moisture supply were bigger factors than the vortex structure in this event.
On the other hand, the rainfall in the southern Central Mountain Range on 8 August, which was the most-rainy area and period in reality, tended to decrease by up to 40 % with the contraction and a weaker outer circulation. Thus, the rainfall patterns and evolution in the sensitivity tests were considerably different from those in CTL, indicating that the vortex structure plays a significant role in the rainfall in this region.
Using the Climate Forecast System Reanalysis, Joint Typhoon Warning Center best track, and Tropical Rainfall Measuring Mission precipitation data, two long-lasting synoptic-scale wave trains in 2004 and 2006 are selected to investigate the atmospheric factors controlling the structures of westward-propagating synoptic-scale disturbances over the tropical western North Pacific. The essential difference between these two wave trains is found in their vertical structures. In 2004, the maximum perturbations occurred from the middle to lower troposphere with an equivalent barotropic structure; however, in 2006, they primarily occurred in the upper troposphere with a prominent tilt regarding height. Distinct configurations of the monsoon troughs, the tropical upper-tropospheric troughs (TUTT), and associated vertical wind shear caused such structural differences. In 2004, the TUTT shifted eastward, creating an easterly sheared environment to confine synoptic-scale waves in the lower troposphere. Then, the monsoon trough enhanced the wave activity through barotropic energy conversion in the lower troposphere. In contrast, while the TUTT shifted westward in 2006, synoptic-scale waves prevailed in the upper troposphere by the environmental westerly shear. Meanwhile, the disturbances developed in the upper troposphere through to the conversion of kinetic energy from the TUTT, exhibiting a top-heavy vertical structure. The coherent movement of the monsoon trough and the TUTT modulate the vertical structure and the development of the synopticscale waves.
This study investigates the global drop size distribution (DSD) of rainfall and its relationship to large-scale precipitation characteristics using the Dual-frequency Precipitation Radar (DPR) onboard the Global Precipitation Measurement (GPM) Core Observatory. This study focuses on seasonal variations in the dominant precipitation systems regarding variations in DSD. A mass-weighted mean diameter (Dm), which is estimated based on the dualfrequency information derived from the GPM/DPR, is statistically analyzed as a typical parameter of the DSD. Values of the annual mean Dm, in general, are larger over land than over the oceans, and the relationship between Dm and precipitation rate (R) is not a simple one-to-one relationship. Furthermore, Dm exhibits statistically significant seasonal variations, specifically over the northwest Pacific Ocean, whereas R shows insignificant variations, indicating the variations in R cannot explain the distinct seasonal changes in Dm. Focusing on the seasonal variation in Dm over the northwest Pacific Ocean, the results indicate that the variation in Dm is related to the seasonal change in the dominant precipitation systems. In the summer over the northwest Pacific Ocean, Dm is related to the organized precipitation systems associated with the Baiu front over the mid-latitudes and tropical disturbances over the subtropical region, with relatively higher precipitation top heights, composed of both stratiform and convective precipitations. Contrary to the summer, larger Dm over the mid-latitudes in winter is related to extratropical frontal systems with ice particles in the upper layers, which consists of more stratiform precipitation in the storm track region. The smaller Dm over the subtropical northwest Pacific Ocean in winter is associated with shallow convective precipitation systems with trade-wind cumulus clouds and cumulus congestus under the subtropical high.
Based on observations, the number of tropical cyclones (TCs) approaching the southern coast of Japan, including Tokyo, has increased over the last 40 years, and these TCs are increasing in strength when they approach land. The environmental conditions for TC development have become more favorable, with warmer sea surface temperature, less vertical wind shear, and more moisture in the atmosphere. In addition, the translation speed of TCs has decreased, which indicates a longer influence time. Comparison of the synoptic environment during July–October between the first (1980–1999, P1) and second (2000–2019, P2) 20 years shows that the sub-tropical high is strengthened in P2, where the western and northern edge of the high extends further into the west and north, respectively. Also, the westerly jet is weakened in P2 over and south of Japan in the middle-to-upper troposphere. These changes in the synoptic environment are considered to play a role in the increase in the number of TCs approaching Tokyo and also in the creation of more favorable conditions for TC development. The relationship between the changes in TC characteristics over the last 40 years and global warming is unclear. As the Pacific Decadal Oscillation is in a positive phase in P1 and in a negative phase in many years of P2, decadal oscillations may have played some role in the increase in the number of approaching TCs and in the changes in the synoptic environment.
Yamaguchi and Maeda (2020) The above paper was press released. (25 Aug. 2020)
Press release document (in Japanese)
Observed surface air temperature (SAT) warming at urban stations often contains both the signal of global warming and that of local urban heat island (UHI) effects; these signals are difficult to separate. In this study, an urban impact indicator (Uii) developed by the authors was modified to represent the extent to which the observed temperature from a given station was influenced by UHI effects. The Uii of a city was calculated by simplifying the city's shape to a circle. In addition, a modified Uii (MUii) was calculated by considering the realistic horizontal distribution of the urban land. We selected 45 urban stations in mainland China, along with an adjacent station for each to give a station pair. Background climate changes across each pair were near-homogeneous. Thus, differences in the trends of annual averaged daily mean SAT (Trendmean), maximum SAT (Trendmax), and minimum SAT (Trendmin) between the urban and adjacent stations (ΔTrend) could be mainly attributed to differences in MUii changes between the urban and adjacent stations (ΔMUii). Several linear regressions between ΔTrend and ΔMUii for the 45 station pairs were calculated to estimate UHI effects on Trendmean (UTmean), Trendmax (UTmax), and Trendmin (UTmin). The results showed that the mean MUii of the 45 urban stations increased from 0.06 to 0.35 during 1992–2013. Positive correlations between ΔMUii and ΔTrend for the 45 station pairs were significant at the 0.001 significance level (except for Trendmax). The average UTmean and UTmin of the 45 urban stations during 1954–2013 were approximately 0.05 and 0.11°C decade−1, respectively, accounting for 18 % and 31 % of the overall warming trends, respectively. The UTmin estimated in this study is about twice that of previous results based on regression equations between Uii and SAT trends.
Through a set of ensemble experiments with an atmospheric general circulation model (AGCM), potential influence of sea-surface temperature (SST) anomalies is assessed on large-scale atmospheric circulation anomalies that induced two extreme events observed over Japan in July 2018. One is a heavy rainfall event in early July mainly over western Japan, which was primarily caused by extreme moisture inflow associated with a cyclonic anomaly to the southwest of Japan and an anticyclonic anomaly to the east of Japan. An AGCM experiment with prescribed global SST anomalies cannot reproduce the anticyclonic anomaly, leading to the failure to simulate the enhancement of the moisture inflow and thereby precipitation over western Japan. The other extreme event is heat wave in mid- and late July almost over the entire Japan, which was caused by a strong anticyclonic anomaly around Japan. The AGCM experiment with global SST anomalies can well reproduce the warm anticyclonic anomalies. The additional experiments have confirmed that SST anomalies in both the tropics and midlatitude North Pacific have potential for forcing the leading mode of the atmospheric variability over the western North Pacific that brought the heat wave. Both the tropical and extratropical SST anomalies are also found to force a poleward shift of the subtropical jet axis over the western Pacific and anomalous tropospheric warming in the midlatitude Northern Hemisphere, both of which persisted in June and July.
The effectiveness of the probability ellipse for tropical cyclone (TC) track forecasts is investigated with multiple ensembles from the Japan Meteorological Agency (JMA), the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction (NCEP), and the Met Office in the United Kingdom (UKMO). All TCs during the 3 years from 2016 to 2018 are included in the verification. We show that the multiple ensembles composed of these four global ensembles are capable of predicting the situation-dependent uncertainties of TC track forecasts appropriately in both the along-track (AT) and cross-track (CT) directions. The use of a probability circle involves the implicit assumption of an isotropic error distribution, whereas the introduction of the probability ellipse makes it possible to provide information as to which is more uncertain; the direction or speed of TC movement. Compared to the probability circle adopted operationally at JMA, the probability ellipse can potentially reduce the area by 16, 15, and 24 %, on average, at forecast times (FT) of 3, 4, and 5 days, respectively, indicating that narrowing warning areas of TC track forecasts by the probability ellipse enables us to enhance disaster prevention/mitigation measures.
Kawabata and Yamaguchi (2020): The above paper was chosen as a JMSJ Editor's Highlight. (13 Jul. 2020)
Description of this paper
We propose a new procedure for the objective identification and classification of heavy rainfall areas (HRAs) to advance the understanding of mesoscale convective systems (MCSs) in Japan. The distributions of accumulated precipitation amounts are evaluated from the radar/raingauge-analyzed precipitation amounts and characteristic features of HRAs are examined. The HRAs extracted during the warm seasons (April–November) in 2009–2018 are classified into four types (e.g., linear-stationary, linear, stationary, and others) based on their morphological features and temporal variations. HRAs are frequently observed on the Pacific sides of eastern and western Japan; 80 % of HRAs appeared from June to September and 60 % of the HRAs were observed in association with stationary fronts and tropical cyclones. Approximately 80 % of those HRAs of the linear-stationary type corresponded to typical elongated and stagnated MCSs, as suggested in previous studies.
Precipitation characteristics and their environments are compared between two heavy rainfall events affecting the Northern Kyushu area in Japan: the July 2018 heavy rainfall event (2018 case) and the 2017 Northern Kyushu rainfall event (2017 case). Both events occurred in the later stage of the Baiu season, after the passage of a tropical cyclone, to the south of a subtropical jet, and on the front side of an upper tropospheric trough. However, contrasting precipitation properties and environments are observed between these cases. In the 2018 case, long-lasting, heavy precipitation was observed over a large area with moderately tall precipitation systems. The environment was stable and moist compared to the climatology. A deep trough over the Korean Peninsula prepared a favorable environment for organizing precipitation systems through moistening the mid-troposphere by quasi-geostrophic dynamically forced ascent. In contrast, in the 2017 case, short-term, intense precipitation was observed over a small area with exceptionally tall precipitation systems. The environment was unstable and moist, compared with the climatology, but was dryer than it was in the 2018 case. In the 2017 case, a shallow trough over the Korean Peninsula destabilized the atmosphere via associated high-altitude cold air.
The observed contrast of characteristics between the 2018 and 2017 cases was similar to that found between the composites of extreme rainfall events and extremely tall convection events, included in the previous statistical study by Hamada and Takayabu (2018, doi:10.1175/JCLI-D-17-0632.1). Although the previous study analyzed the uppermost 0.1 % of extreme events, temperature anomalies and specific humidity anomalies from climatological values in the 2018 and 2017 cases are several times larger than those in the composites of the extreme events. This result indicates that the 2018 case was an extreme among the extreme rainfall events, and the 2017 case was an extreme one among the extremely tall convection events.