On The Uncertainties in Potential Vorticity Tendency Diagnosis of Tropical Cyclone Track

A vortex can be treated as a positive potential vorticity, which is linked to potential vorticity tendency (PVT). PVT diagnose is an approach to estimate vortex propagation with moderate accuracy. Individual physical processes can also be quantitatively estimated by PVT according to their contributions to the wavenumber one component of PVT. In this study, idealized simulations in Weather Research and Forecasting (WRF) Model with 3-km horizontal resolution are conducted to further investigate the vortex track and motion vector diagnosed by PVT.

A vortex approaching an idealized terrain with significant track deflection is analyzed using PVT. Previous studies demonstrated that vortex motion corresponds well with PVT. However, this study suggests non-negligible discrepancies between PVT and the track. Since the least square method is used to estimate the motion vector (Cx and Cy), uncertainties using this method needs to be addressed. Errors of the least square method of different calculation time are shown to quantify the uncertainty of PVT.

Although PVT diagnosis does not fully capture the TC track, it can generally identify the tendency of vortex motion. However, we show that the vortex areas chosen for the calculation would have large impact on the results. This study suggests that PVT within 100-km, averaged at 850hPa-400hPa around the TC center, most aligns with the TC motion. As for individual processes, horizontal advection is the main contributor to PVT, as compared to vertical advection and diabatic heating.

Further investigation comparing the results of PVT of westward moving and northward-turning TCs are conducted. Whether PVT can be applied in various cases and its limitation will be discussed in the study.