Diagnosing the Evolution and Diurnal Cycle of Tropical Cyclone Yasa Using Potential Vorticity from ECMWF High-Resolution Operational Forecasts

Deep convective updrafts in tropical cyclones (TCs) transport air into the upper troposphere and lower stratosphere (UTLS), creating strong variability in mesoscale potential vorticity (PV) in high resolution ( ~ 9 km grid spacing) ECMWF operational forecasts. A statistical method for analyzing ECMWF PV along TC storm tracks is introduced, where the spatial standard deviation and mean are calculated in each of four quadrants, with a 6x6 full domain and 2x2 inner domain, at each level every 6 hours. This is used to investigate the evolution and diurnal cycle of TC Yasa, which occurred near Fiji during 12-20 December 2020. Comparison with Himawari-8 cloud-top temperature shows that UTLS curvilinear PV structures are related to convective bands.

A strong diurnal cycle in 100 hPa is found, with the maximum shifting from 6 am to midnight after reaching strength C2, similar to satellite studies of gravity wave activity. The range of ~ 5-12 PVU in the diurnal cycle in reduced to ~2-6 PVU upon reaching C2 in the full domain. A peak of ~ 30 PVU occurred in the inner domain just prior to rapid intensification. In the inner core at 100 hPa, diurnal variability in and are correlated at -0.58, consistent with a midnight peak in cooling and mixing in the UTLS. Co-located variability maxima in divergence, relative vorticity, and PV supports the interpretation that UTLS PV structures are due to updrafts hitting the bottom of the stratosphere and are a useful signature of dynamical adjustment in the model.