13:45 〜 15:15
[AAS04-P01] Dynamical Processes of the Weakening in the Extratropical Transition of Numerically Simulated Typhoon Hagibis (2019)
キーワード：メソスケール気象、熱帯低気圧、数値モデリング
Tropical cyclones (TCs) have warm air at the storm center and low-level severe wind at the meso-alpha scale over the tropical ocean. A TC, which developed in high sea-surface temperature (SST) and weak vertical shear of environmental winds in the tropics, often experiences weakening of the warm anomaly and horizontal expansion of the severe-wind area, associated with the decrease in the SST and increase in the vertical shear in moving toward mid-latitudes. This structural change is known as the extratropical transition (ET) of TC. After ET, TCs re-intensify as an extratropical cyclone or decay. Dynamical processes of the inner-core structural change of the storm during ET remain an open question because numerical simulations with the cloud-resolving scale are required to capture the processes. Particularly, there are few studies of the weakening processes.
The purpose of the present study is to examine the weakening processes of the TC structure in Typhoon Hagibis (2019), which made landfall in Japan, based on a numerical simulation with a nonhydrostatic regional model. The simulation can reasonably capture the weakening after reached to the lifetime maximum intensity of the storm, compared with the best track from the Japan Meteorological Agency. In the weakening period, Hagibis made landfall in Japan, and then the storm moved toward the mid-latitude ocean again. During the landfall, the storm intensity (i.e., the inner-core wind speed and the central pressure of the storm) was rapidly weakened due to not only a change to the extratropical environments but also an increase in surface friction. The rapid weakening corresponded to the decrease of the temperature at the storm center in the upper troposphere.
In the present study, we focus on the weakening period in which the storm was over the ocean. In the weakening period, the simulated storm exhibited a decrease in the temperature at the storm center in the lower and middle troposphere. According to a potential temperature budget analysis, the cooling near the storm center is mainly due to adiabatic processes associated with updrafts at the storm center. In contrast to moist updrafts near the storm in the tropics, the updraft is very dried, so the adiabatic cooling in the updrafts is not canceled by the diabatic heating due to condensation. The dried updrafts are related to low-level jets in the north-northwest sides of the storm, based on backward trajectory analysis. Unlike typical asymmetries of horizontal winds exhibited in moving TCs (i.e., severe winds in the right side of the moving direction), these low-level jets in the north-northwest will be a feature in ET. The present study suggests that the unique structure of asymmetric winds during ET can be due to the weakening of the inner-core structure in Hagibis.
The purpose of the present study is to examine the weakening processes of the TC structure in Typhoon Hagibis (2019), which made landfall in Japan, based on a numerical simulation with a nonhydrostatic regional model. The simulation can reasonably capture the weakening after reached to the lifetime maximum intensity of the storm, compared with the best track from the Japan Meteorological Agency. In the weakening period, Hagibis made landfall in Japan, and then the storm moved toward the mid-latitude ocean again. During the landfall, the storm intensity (i.e., the inner-core wind speed and the central pressure of the storm) was rapidly weakened due to not only a change to the extratropical environments but also an increase in surface friction. The rapid weakening corresponded to the decrease of the temperature at the storm center in the upper troposphere.
In the present study, we focus on the weakening period in which the storm was over the ocean. In the weakening period, the simulated storm exhibited a decrease in the temperature at the storm center in the lower and middle troposphere. According to a potential temperature budget analysis, the cooling near the storm center is mainly due to adiabatic processes associated with updrafts at the storm center. In contrast to moist updrafts near the storm in the tropics, the updraft is very dried, so the adiabatic cooling in the updrafts is not canceled by the diabatic heating due to condensation. The dried updrafts are related to low-level jets in the north-northwest sides of the storm, based on backward trajectory analysis. Unlike typical asymmetries of horizontal winds exhibited in moving TCs (i.e., severe winds in the right side of the moving direction), these low-level jets in the north-northwest will be a feature in ET. The present study suggests that the unique structure of asymmetric winds during ET can be due to the weakening of the inner-core structure in Hagibis.