Wind gusts associated with severe convective clouds are mainly classified into tornadoes, which occur with updrafts, and downburst and gust fronts, which occur with downdrafts. It is known that wind gusts often occur in quasi-liner convective system. During the Baiu season, quasi-liner convective system develops along the Baiu front. However, the genesis mechanisms of wind gusts that occurs in the Baiu frontal zone are not cleared, while the number of tornadoes and downbursts during Baiu season is not large. The purpose of this study is to clarify the genesis mechanisms of the tornado and downburst which occurred along the Baiu frontal zone. For the purpose, we performed a high-resolution simulation of these wind gusts and examined the developing process with the cloud resolving model CReSS.

The cases we studied are wind gusts that occurred on July 8, 2020 in the Baiu frontal precipitation zone. According to the JMA C-band radar observations, a tornado occurred in Kuwana City, Mie Prefecture around 06:30 JST, and a downburst occurred in Yaotsu Town, Kamo-gun, Gifu Prefecture around 07:00 JST on the same day. It shows that different types of wind gusts occurred in the same Baiu frontal zone.

Town, Kamo-gun, Gifu Prefecture around 07:00 JST on the same day. It shows that different types of wind gusts occurred in the same Baiu frontal zone. To simulate the wind gusts, firstly a 500 m-resolution experiment was conducted. After the experiment, one-way nested 50 m-resolution experiments in two regions were conducted using the result of the 500 m-resolution experiment. The outer 500 m resolution experiment reproduced the Baiu frontal precipitation zone formed along the Baiu front and showed a circulation field with strong precipitation in the northern part of the Baiu frontal zone. The first 50 m resolution experiment reproduced a tornado at the center of the circulation field in the northern part of the Baiu frontal zone. Around the reproduced tornado, the convergence and updraft areas were present near the surface, and the vertical vorticity and cyclonic structure were prominent from the surface to an altitude of about 1000 m. These characteristics indicate that this tornado is a non-supercell type. However, compared with cold frontal non-supercell tornado, this simulated tornado differs in terms of occurring between the west side of cold moist air and the east side of the partial warm dry air. The partial warm dry air heated by latent heat of nearby convection, flowed from southwest of heavy precipitation area. The result of the vorticity budget analysis indicated that enhancement of stretching of vertical vorticity at lower level had the largest contribution to development of the tornado. In the second 50 m resolution experiment, which reproduced the downburst, strong downdrafts exceeding 10 m/s corresponding to the downburst were reproduced at the edge of the strong precipitation area on the northern side of the Baiu frontal zone. The vertical structure around the time of the downburst indicated that the downdraft intensification occurred with dry air inflow from the rear side of the Baiu front. The result of the sensitivity experiment without the process of the raindrop evaporation did not show strong downdraft in same area of control experiment. These results suggest that the raindrop evaporations due to the inflow of dry air contributed to the occurrence of the downburst.