総観ならびに対流スケールにおける線状降水帯の出現特性に関する研究

Line-shaped rainbands (LRBs) are a meteorological phenomenon which describes that multiple cumuliform clouds line up in a linear pattern, causing rainfall areas to stagnate in a specific region and producing heavy rainfall. In meteorological terms, this phenomenon is characterized by horizontal spatial scales between 20 and 200 km, and is considered as a meso β-scale heavy rainfall event composed of convective clouds with horizontal spatial scales between 2 and 20 km (i.e., meso γ-scale). Furthermore, meso β-scale phenomena are included in the environmental fields with horizontal spatial scales ranging from meso α (approximately 200 to 2000 km) to the total observation scale (approximately 2000 to 20000 km). Some examples of LRBs are rainfall events occurred under the influence of fronts (e.g., the July 2008 torrential rains and the July 2020), as well as the September 2014 case in the Ishikari and Iburi regions in Hokkaido, and the September 2015 Kanto and Tohoku torrential rains (which occurred in a large-scale convergence field caused by typhoons). In particular, the 2015 torrential rainfall events are the focus of this research. This study is based on analyzing meteorological phenomena of multiple spatiotemporal scales. Specifically, this study systematically analyzes the meso-β scale LRBs by considering the multiplicity of meteorological phenomena with respect to spatiotemporal scales. Based on past observations, the relationship between the long-term occurrence characteristics of LRBs and the environmental field is clarified. The characteristics of future changes of(in) LRBs and the environmental fields in the latest climate projection information are also shown. Moreover, the study also presents the relationship between the spatiotemporal distribution of precipitation cells constituting the LRBs and the three-dimensional winds fields estimated using the variational principle. In particular, the LRBs are quantified using a high-resolution radar network. The purpose of Chapter 2 is to clarify the regional characteristics of LRBs, such as the number of occurrences, the timing of occurrence, and the shape of rainfall areas, using nationwide and long-term observations. We set thresholds for the amount and shape of rainfall and stagnation, including past disaster cases in Hokkaido, and extracted LRBs from nationwide and long-term data using analytical rainfall data. The analysis of the obtained statistical analysis shows that the western Japan region including Kyushu has many east-west extending LRBs that occur in the rainy seasons from June to July, while the Sea of Japan side of Hokuriku and western Tohoku has many northwest to southwest extending LRBs. Although there is no significant difference in the shape and the distribution of the duration of stagnation among the regions, it is found that most of the heavy rainfall events are linear with the ratio of major to minor axes ranging from 2.5 to 3.5. The extracted cases presented in this chapter will be the subject of analysis The purpose of Chapter 3 is to clarify the characteristics of meteorological fields on the quasi-seasonal and daily scales in the years of high or low linear precipitation over Hokkaido and its surrounding areas. In this study, we focus on cases of linear precipitation events that occurred in Hokkaido and its surrounding sea area among the LRBs selected in Chapter 2. Since the number of linear precipitation events tends to be concentrated in July and August in this region, we analyze the characteristics of the two-month mean fields for the years of high and low frequency in the past. The characteristics of the summer mean field are as follows: the Pacific High dominates the region, and the amount of water vapor flux is high due to the edge flow of the anticyclone from the southwest in the region; the amount of water vapor flux is high over Hokkaido. The summer mean field is characterized by the predominance of a Pacific high pressure system, high water vapor flux due to the edge flow of a high pressure system from the southwest in the region, and cold air inflow over Hokkaido moving southward due to westerly wind meandering, which was seen as a clear anomaly. The spatial distribution of the oligohydrocline contrasts with that of the multiple-year event, which is a circulation field dominated by the Okhotsk Sea anticyclone. In order to objectively analyze the occurrence characteristics of daily weather fields with a large number of variables, we used a classification method that combines statistical methods and machine learning to reduce the amount of data while preserving the characteristics of weather fields. As a result, it is found that either the pattern of cyclone passage, stagnation of fronts, or the pattern of approaching tropical cyclones are more frequent in frequent occurrence years than in normal years. The purpose of Chapter 4 is to clarify the relationship between the characteristics of occurrence of LRBs (LRB occurrence) and meteorological fields using the latest climate forecast information. Based on the definition in Chapter 2, LRBs are extracted from a large ensemble of (model?) data for Hokkaido and its surrounding sea areas. The results show that comparing to the past climate in the same region, the number of LRBs increases 1.5 times and 2.1 times for a 2-degree and 4-degree rise respectively in global mean temperature in the future climate. In addition, we defined the years with more or less LRBs for each climate condition, and conducted the same analysis as in Chapter 3. The results show that the characteristics of summer mean fields under the two future climate conditions are consistent with that for the past climate. In the oligopause year, the three patterns that are prone to the occurrence of LRBs on the daily scale obtained in Chapter 3 appear approximately 1.3 times more frequently, and the occurrence rate of LRBs in the same weather field pattern is 1.5 times higher in the oligopause year than in the normal year. The purpose of Chapter 5 is to clarify the spatiotemporal distribution of precipitation cells constituting LRBs. Of the 2379 cases of LRBs extracted in Chapter 2 that occurred from 2014 to 2019, 411 cases were found to be within the observation range of two or more X-band MP radars, and 396 cases were found to have high-quality observations with a data deficit of less than 15% of the duration time . We also developed a method of tracking precipitation cells that compose LRBs using high-resolution radar information, and compared it with the results of previous analyses. By applying the newly developed precipitation cell tracking method to all of the above-mentioned cases, it was confirmed that the movement speed of precipitation cells constituting LRBs in the past was approximately between 10 and 25 m/s, their intervals were concentrated in the range of 10 to 15 km, and the lifetimes of rain clouds were longer than those of ordinary isolated cumulus clouds. Some of the rain clouds were observed to have a lifetime of more than 60 minutes, which is longer than that of ordinary isolated cumulus clouds. Based on the combination of the direction of the precipitation zone and the direction of movement of precipitation cells, the formation types of LRBs that have occurred in the past are classified. The purpose of Chapter 6 is to clarify the relationship among the velocity of movement of precipitation cells, rainfall intensity, and three-dimensional wind speed in the case of LRBs. In this chapter, the principles and principles of meteorological observation using Doppler radar are described in detail. The same principles are applied to the observation of LRBs, and the three-dimensional precipitation cells and wind fields are calculated from time to time for several cases of LRBs after verifying the combination of multiple radars and the existence of topographical effects as boundary conditions. As a result, it was found that the movement speed and direction of precipitation cells in the case of the September 2014 linear precipitation event in Hokkaido and the September 2015 linear precipitation event in Kanto coincided with the horizontal wind speed at approximately 2 km to 3 km height. Furthermore, it was found that vertical vortex structures that promote convection existed in a high percentage at the time and location of heavy precipitation events during the life cycle. As described above, the LRBs that have occurred in Japan in the past are considered from the viewpoint of the general observation scale. The environmental fields on quasi-seasonal and daily scales that tend to generate LRBs are clarified from the perspective of the total observation scale. In addition, the latest climate forecast information on LRBs is used. The results also provide important information on the future changes of LRBs in the latest climate projection data. Furthermore, from the viewpoint of convective scale, the spatiotemporal characteristics of precipitation cells and three-dimensional wind fields that compose LRBs are shown from the viewpoint of convective scales.