Exploring Multiscale Processes of Tropical Cyclone Genesis Associated with African Easterly Waves Using the PEEMD

Abstract:

In this study, we applied the parallel version of the Ensemble Empirical Mode Decomposition (PEEMD) for an analysis of 10-year (2004-2013) ERA-Interim global reanalysis data in order to explore multiscale interaction of tropical cyclone genesis associated with African Easterly Waves (AEWs) in sheared flows. Our focus was aimed at understanding the downscaling process in multiscale flows during storm intensification. To represent the various length scales of atmospheric systems, we extracted Intrinsic Function Modes (IMFs) from raw data using the PEEMD and found that the non-oscillatory trend mode can be used to represent large scale environmental flow and the third oscillatory mode (IMF3) is to represent AEW/TC scale systems. Our results: 1) identified 42 developing cases from 272 AEWs, with 25 eventually developing into hurricanes; 2) indicated that maximum shear largely occurs over the ocean for the IMF3 mode and over land near the coast for the trend mode for developing cases, suggesting shear transfer between the trend mode and the IMF3; 3) displayed opposite wind shear tendencies for the trend mode and the IMF3 during storm intensification, signifying the downscaling process in 13 hurricane cases along their tracks; 4) showed that among the 42 developing cases, only 13 of the 25 hurricanes were found with significant downscaling transfer features, so other processes such as upscaling processes may play an important role in the other developing cases, especially the remaining 12 hurricane cases. Investigating the upscaling process between the convection scale and the AEW/TC requires data from the finer grid resolution and will be the subject of a future study.