An Evaluation of the Parallel Ensemble Empirical Mode Decomposition Method in Revealing the Role of Downscaling Processes Associated with African Easterly Waves in Tropical Cyclone Genesis

In this study the parallel ensemble empirical mode decomposition (PEEMD) is applied for an analysis of 10-yr (2004-13) ERA-Interim global reanalysis data in order to explore the role of downscaling processes associated with African easterly waves (AEWs) in tropical cyclone (TC) genesis. The focus of the study was aimed at understanding the downscaling process in multiscale flows during storm intensification. To represent the various length scales of atmospheric systems, intrinsic mode functions (IMFs) were extracted from the reanalysis data using the PEEMD. It was found that the nonoscillatory trend mode can be used to represent large-scale environmental flow and that the third oscillatory mode (IMF3) can be used to represent AEW/TC scale systems. The results 1) identified 42 developing cases from 272 AEWs, where 25 of them eventually developed into hurricanes; 2) indicated that the maximum for horizontal shear largely occurs over the ocean for the IMF3 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 that the downscaling process was active in 13 hurricane cases along their tracks; and 4) showed that among the 42 developing cases, only 13 of the 25 hurricanes were found to have significant downscaling transfer features, so other processes such as upscaling processes may play an important role in the other developing cases, especially for the remaining 12 hurricane cases. In a future study, the authors intend to investigate the upscaling process between the convection scale and AEWs/TCs, which requires data at a finer grid resolution.