Diagnosis and analysis of thermodynamic factors influencing the sudden northward turn of Typhoon In-Fa
Typhoons can cause significant damage,and accurate forecasting of their paths is crucial for mitigating their impact.This study investigates the sudden northward turn of Typhoon In-Fa(2021),which veered nearly 90 degrees northward,using the WRF model to perform accurate numerical simulations combined with analyses of upper atmospheric circulation and basic airflow patterns.The findings indicate that the northward turn of Typhoon In-Fa was driven by several key factors:1)As Typhoon In-Fa approached Taiwan,its track shifted northward due to the strengthening and westward expansion of the western pacific Subtropical high,combined with the deepening of the upper-level long-wave trough over China and the blocking influence of a low-pressure vortex near Japan,which prevented further westward movement.Concurrently,the northward surge of cross-equatorial flow altered the basic airflow from a northeasterly to a southwesterly direction,enhancing the northward movement trend.2)The continuous eastward shift of the mid-latitude longwave trough over China and the eastward retreat of the sub-tropical high led to the formation of a significant positive potential vorticity center north of the center of Typhoon In-Fa,accompanied by counterclockwise positive potential vorticity advection on the northwest side of the center.This altered the structure of the potential vorticity field near the typhoon center,slowing its westward trajectory and steering it northward.3)The vertical shear of the ambient wind near the typhoon center was unfavorable for westward movement but facilitated northward movement due to the distribution of high and low values,with lower values north of the typhoon center.4)Both simulation experiments indicated persistent wind speed asymme-try around the typhoon center,with the 500 hPa wind speed maximum shifting from the west to the east side of the typhoon center.The presence of southerly winds on the right side of the typhoon increased the northward com-ponent of its movement,significantly contributing to its abrupt northward turn.5)In the unstable energy field,a low-energy tongue developed on the west side of the typhoon,promoting movement into the unstable atmospheric junction region.These findings highlight that the sudden northward turn of Typhoon In-Fa was driven by a combi-nation of thermodynamic factors,providing valuable insights for improving typhoon track forecasts.
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