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 a 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 showed 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 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 transition of flow regime (internal Froude number was less than 1.0 at the windward of mountains and larger than 1.0 above the leeward slope). Additionally, differences in height of the mountains between the north and south parts results in the 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 of 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, causing reduced TCG in WNP and increased TCG in ENP. We also showed that if SST over IO is warmer, the monsoon trough in WNP becomes weaker, although the vertical wind shear over ENP does not appreciably change. 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.
This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019. Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the tropical disturbance that developed into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS (Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatological mean; it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The Pre-Faxai area with high Grid-EW scores moved westward because of the background easterly flow over the ENP, then entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that the important environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds associated with high EW scores; they were also caused by upper-troposphere westerly winds associated with an upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai 10 days before the typhoon arose, an indication that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.
In this study, the effect of the zonally-elongating monsoon trough (MT) on the binary tropical-cyclones (TCs) interaction is investigated by using data analysis and idealized simulations. The binary-TCs interaction is found to be sensitive to the relative orientation of the two 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 orientated in the northwest-southeast (NW-SE) 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 binary TC 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 binary TCs 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 binary TCs interaction 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 to 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 (TCW) move northeastward, by which accelerates to reach the critical distance.
Ultra Violet (UV) parasols are a reasonable countermeasure against heat stress as they are portable and inexpensive. This study compared the heat stress mitigation effect of a UV parasol with that of street trees and dry-mist spraying on a hot and humid summer day in Japan. We observed meteorological elements and calculated the universal thermal climate index (UTCI) and wet-bulb globe temperature (WBGT) under UV parasol, street trees, dry-mist spraying, and direct sunlight. The observed UTCI and WBGT under the UV parasol were lower than those in direct sunlight by 4.4 and 1.3°C, respectively, because of the decrease in black globe temperature caused by the reduced downward shortwave radiation. This demonstrated that UV parasol reduced heatstroke risk by one level. The effect of the UV parasol was equal to or greater than 75 % of that of the street trees from the perspective of UTCI. The street trees reduced the UTCI and WBGT by 5.9 and 1.9°C, respectively, compared to those in direct sunlight, resulting in the reduction of heatstroke risk by one level. In contrast, dry-mist spraying did not mitigate heat stress in conditions with moderate winds. Although the results of this study were obtained from observations on a single day, comparison with earlier studies confirms that the values observed in this study are representative results on summer days in Japan.
A significant fraction of tropical cyclones develop in baroclinic environments, following tropical cyclogenesis “pathways” that are characterized by dynamical processes often associated with higher latitudes. This study investigates whether such storms are more likely to undergo subsequent extratropical transition than those that develop in more typical, non-baroclinic environments. We consider tropical cyclones globally in the period 1979-2011 using best-track datasets, and define the genesis pathway of each storm using McTaggart-Cowan's classification: non-baroclinic, low-level baroclinic, trough-induced, weak and strong tropical transition. In each basin, we analyze the total number and the fraction of storms that underwent extratropical transition, their seasonality, and storm tracks, according to their genesis pathways. The relationship between the pathways and extratropical transition is statistically significant in the North Atlantic and Western North Pacific, where the strong tropical transition and the trough-induced pathways have a significantly greater extratropical fraction compared to all other pathways, respectively. Latitude, longitude and environmental factors such as sea surface temperature and vertical shear were further analyzed to explore whether storms in these pathways happen to be in environments conducive to extratropical transition, or whether a “memory” of the genesis pathway persists throughout the storm life cycle. After controlling for genesis latitude, the relationship between the strong tropical transition and trough induced pathways, and extratropical transition occurrence remains statistically significant, implying a lasting effect from the pathway on the probability of an eventual extratropical transition.
The stratospheric polar vortex and its breakup are important dynamical phenomena. In previous studies, three diagnostics for vortex breakup have been suggested by using the potential vorticity (PV) and zonal wind for the lower stratosphere. These three diagnostics, however, cannot be applied for the upper stratosphere, since the evolution of the polar vortex is more complicated and, therefore, it is more difficult to prescribe key parameters. Here we define the dates of the breakup and formation of the polar vortex by finding the maximum peaks in the averaged rates of change in the equivalent latitude, PV, and wind speed at the vortex edge. By applying our new definition to the ERA-Interim reanalysis data, the breakup and formation dates of the Arctic and Antarctic polar vortices for the whole stratosphere were obtained for 1979-2018. Our newly-defined vortex breakup date is compared with the date of the stratospheric final warming, which is defined as the timing of zonal mean westerlies changing to easterlies without recovering to a westerly exceeding the threshold westerly wind speed. To see if our definition is consistent with atmospheric transport near the vortex edge, the dates of the formation and breakup of the polar vortex are compared with the mixing ratios of long-lived trace species. It turns out that the newly defined dates well match the changes of concentrations of trace gases in the stratosphere for the winter of 1996-1997. Considering all the above observations, our definition of the vortex formation and breakup appears to be applicable to the whole stratosphere.
The maximum storm surges caused by Typhoon Jebi (2018) were examined using a storm surge model, and by track ensemble simulations based on a meteorological model and a parametric tropical cyclone (TC) model. The storm surge at Osaka Port was estimated more accurately by the meteorological model than the parametric TC model. The differences between both models were due to a “wind setup effect”, where the topography enhanced surface winds over Osaka Bay. The typhoon track ensemble simulations demonstrated that the maximum storm surge was dependent on perturbation of the track of Typhoon Jebi along the entire coast of the Japanese Islands, including the main island, Kyushu, and Shikoku. Open shallow bays had maximum storm surges exceeding 2.50 m. In coastal areas where larger maximum storm surges were estimated, the longitudinally perturbed “worst-case course” appeared 0.4-0.8° west or east of the “hit course”, indicating that the wind setup effect was an important factor in the maximum storm surge. The distance of the worst-case course from the hit course was almost the same as the radius of maximum wind of Typhoon Jebi. Although the models had similar worst-case courses for each coastal area, the meteorological model estimated a slightly higher simulated maximum storm surge than the parametric TC model. For the main island, Kyushu, and Shikoku, approximately 6 % of the maximum storm surges exceeded 2.00 m. Although these values may differ for other typhoons and sampling points, it is important to estimate the maximum storm surges and worst-case courses at all coastal areas, including regions where storm surges by typhoons is unknown yet occurred because this will provide important information enabling effective disaster prevention and risk management.
The present study uses an object-based evaluation metric to examine the precipitation bias over the Maritime Continent in the global cloud-resolving models. We specifically focus on the difference between the models that directly resolve convection and those using convection parameterization. The 40-day hindcast experiments of the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domain (DYAMOND) intercomparison project are evaluated against the high-resolution satellite rainfall products. The hindcast of the Central Weather Bureau Global Forecast System (CWBGFS) under the DYAMOND protocol is also included. The results show that most models simulate insufficient numbers of large precipitation system (object-based precipitation system (OPS), > 370 km in scale), indicating weaker convection organization. The observation indicates that the maximum precipitation within the OPS intensifies with increasing object size. All of the models capture this positive relationship, but most of them overestimate the sensitivity. Most of the models overestimate both frequency and intensity of small OPS (< 160 km), except the models with convection parameterization (i.e., CWBGFS, European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS)-9km). Although most of the models can reproduce the observed peak time of diurnal precipitation over the land area in the Maritime Continent, the simulated fractional contribution of different sizes of OPS to the total precipitation varies from model to model, and their peak times do not follow the observed ones with delayed peak times as the size of OPS increases from small, mid-size to large categories. Most of the models reasonably capture the mean diurnal cycle peak time, but only the models with convection parameterization and Model for Prediction Across Scales (MPAS) can represent the diurnal evolution of fractional contribution from different OPSs. The implications of current results to the upscale processes of the tropical convection systems in the global models are also discussed.
Robust and uncertain sea-level pressure patterns over summertime East Asia in the future global warming projections and their causes are studied by applying the inter-model empirical orthogonal function (EOF) analysis to the multi-model experiments in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and focusing common features with the previous CMIP5 analysis. The ensemble average and the first to third EOF modes associated with future pressure changes are similar to the corresponding ones from CMIP5. The first and second modes represent strengthened and weakened high pressure systems in subtropical and northern East Asia, respectively. The third mode is the reverse anomaly of the climatological pressure pattern over summertime East Asia, indicating weakened southerly monsoon winds. The second mode pattern makes positive contributions to almost all the CMIP6 future pressure changes, representing a robust future projection pattern. The robust mode is the result of surface warming over the northern continents and neighboring seas that is stronger than the global average. The first and third modes are considered to be uncertain (but major) patterns in the ensemble projections because the signs of their contributions to the future changes are dependent on the model used. Suppressed vertical motion over the equatorial (northern) Indian Ocean caused by the vertically stabilized atmosphere under the global warming scenario is the source of the first (third) mode, together with the counter vertical motion anomaly over the equatorial (northern) Pacific. The above characteristics of the modes are essentially similar to those identified in the CMIP5 analysis while different sea surface temperature anomalies are related to the secondary structures of the modes. Some uncertainties in the future projections can be attributed to the systematic differences in the model climatology of the present-day precipitation, which determines the distribution of the suppressed vertical motion under the future warmer climate.
This study assesses heatstroke risk in the near future (2031-2050) under RCP8.5 scenario. The developed model is based on a generalized linear model with the number of ambulance transport due to heatstroke (hereafter the patients with heatstroke) as the explained variable and the daily maximum temperature or Wet-Bulb Globe Temperature (WBGT) as the explanatory variable. With the model based on the daily maximum temperature, we performed the projection of the patients with heatstroke in case of considering only climate change (Case 1), climate change and population dynamics (Case 2), and climate change, population dynamics, and long-term heat acclimatization (Case 3). In Case 2, the number of patients with heatstroke in the near future will be 2.3 times higher than that in the baseline period (1981-2000) on average nationwide. The number of future patients with heatstroke in Case 2 is about 10 % larger than that in Case 1 on average nationwide despite of population decline. This is due to the increase in the number of elderly people from the baseline period to the near future. However, there were 21 prefectures where the number of patients in Case 2 is smaller compared to Case 1. Comparing the results from Cases 1 and 3 reveals that the number of patients with heatstroke could be reduced by about 60 % nationwide by acquiring heat tolerance and changing lifestyles. Notably, given the lifestyle changes represented by the widespread use of air conditioners, the number of patients with heatstroke in the near future was lower than that of the baseline period in some areas. In other words, lifestyle changes can be an important adaptation to the risk of heatstroke emergency. All of the above results were also confirmed in the prediction model with WBGT as the explanatory variable.