[A-AS02] Advances in Tropical Cyclone Research: Past, Present, and Future

[AAS02-01] Improvement of Typhoon Intensification Simulation by Data Assimilation of a Single Dropsonde Sounding in the Eye

★Invited Papers

Nozomi Tamura¹, *Kazuhisa Tsuboki^1,3, shingo shimizu² (1. Institute for Space-Earth Environmental Research, Nagoya University, 2.National Research Institute for Earth Science and Disaster Resilience, Japan, 3.Typhoon Science and Technology Research Center (TRC), Yokohama National University)

Keywords:typhoon, aircraft observation, dropsonde, data assimilation

Tropical cyclones (TCs) cause strong winds, heavy rainfall, and storm surges. It is important for disaster prevention to forecast accurately their tracks and intensities. However, it is difficult to obtain direct observation data during the intensification phase of TCs because the observation density over oceanic regions is lower than that over land. Aircraft observations using dropsondes have been conducted to obtain direct observation data of TCs over oceanic regions. Most previous aircraft observations, however, have been limited to environmental regions or low altitude. In the T-PARCII (Tropical cyclone-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts) project, which has been conducted since 2016, dropsondes are released from aircraft flying over the upper troposphere to obtain direct observation data over a wide area, including the inner-core of TCs, in the Northwest Pacific region. It is known that there is the warm core near the center of the typhoon, where the temperature is higher than that of the environment region. Intensity of a TC is related to its warm core structure, assimilating data observed near the typhoon center by T-PARCII is expected to contribute to improved intensity forecasts. The purpose of this study is to clarify the impact of assimilating dropsonde data observed near the typhoon center by T-PARCII on TC intensification process.
In this study, TC Nanmadol (2022) was studied through both a control experiment (CTL, non-assimilation) and a dropsonde assimilation experiment (DA) using the three-dimensional variational method (3DVAR). The reproducibility of TC intensity and the thermodynamic and dynamic impacts of assimilation were investigated.
The results showed that TC intensity was better reproduced in the DA. In the DA, increments were applied to the upper troposphere (8 km–13 km) and lower troposphere (1 km–3 km). A comparison of potential temperature anomalies around the typhoon center revealed that the increase in upper-level potential temperature persisted for approximately 20 hours. In the DA, the warm core was enhanced below 5 km altitude after about 12 hours compared to the CTL experiment. This indicates that the warm core structure was changed by data assimilation. Sensitivity experiments that limited assimilation heights confirmed that the warming of the lower warm core was induced by assimilation in the upper level. Furthermore, dropsonde data observed approximately 20 hours after the assimilation time indicated that the warm core structure in the DA closely matched the dropsonde data. In addition, the DA showed the intensified tangential wind and lower-level inflow.
To clarify the factors that changed the warm core structure between the CTL and DA, potential temperature budget analysis was conducted. During the intensification phase, it was shown that heating factors for the warm core were dominated by asymmetric components in the CTL, whereas axisymmetric components dominated in the DA. Analysis of the axisymmetric advection terms suggested that downdraft in the mid-to-lower troposphere near the TC center strengthening the warm core.
This study revealed that accurately representing warm core structure during the intensification phase of the TC improves the reproducibility of its intensity. Additionally, it was found that the maintenance of axisymmetric downdraft in the eye promotes warm core heating, further intensifying the TC.