Impact of the upper-tropospheric cold low on the genesis of Typhoon Hagupit (2020)

Typhoon Hagupit (2020), which formed unexpectedly close to land, posed great challenges for forecasters and risk mitigation. During its genesis, there was a west-moving upper-tropospheric cold low (UTCL) in the north. This study investigated the impact of this UTCL on the genesis process using numerical simulation and sensitivity experiments. In the semi-idealized experiment with this UTCL removed (run-Rcold), pre-Hagupit develops faster, but its track drifts southward in the later stage compared with the control experiment (run-cnl). In the experiment with enhanced UTCL (run-Ecold), the simulated track is similar to that in run-cnl, but pre-Hagupit does not develop into a tropical storm. In run-cnl and run-Ecold, the environmental vertical wind shear is larger than that in run-Rcold in the first two days, and the simulated pre-Hagupit experiences two prominent dry air intrusions in the middle and upper troposphere. At the second intrusion, when the weakened UTCL has moved within 2° of pre-Hagupit, the convection in both experiments decays significantly, and the development of the mid-level vortex begins to lag behind that in run-Rcold, and so does the vertical alignment of the low- and mid-level vortices. The UTCL influences the movement of pre-Hagupit by modifying the large-scale steering flows, especially those above 600 hPa. In run-Rcold, due to the absence of the northerly component of wind fields related to the UTCL circulation, pre-Hagupit starts to move west-northwestwards instead of northwestwards as in run-cnl and run-Ecold. Therefore, this UTCL leads to the northward drift of pre-Hagupit and its eventual landfall in the mainland of China, and the UTCL is detrimental to the vortex development.