Convective Evolution during Tropical Cyclone Formation – Revisiting Zehr (1992)

Infrared (IR) brightness temperature (Tb) data were analyzed along with the ERA-Interim reanalysis for more than 150 named tropical cyclones over the Atlantic to examine the convective and dynamic evolution during tropical cyclone formation. The IR Tb of individual storms shows large temporal variability due to the diurnal cycle and sporadic nature of deep convection, and the two-stage conceptual model suggested by Zehr (1992) does not appear to represent a dominant mode of pre-genesis evolution over the Atlantic. The composite mean shows that convection intensifies nearly monotonically with time and that cold IR Tb becomes organized near the pouch center within 24 hours prior to genesis.

Cluster analysis identified two groups of storms with different spatiotemporal evolution of convection. The first group shows convective organization around the center of the wave pouch, while convection concentrates 2-3 degrees west of the wave pouch center in the second group. The analysis of the ERA-Interim data reveals that the second group is subject to an upper-level westerly relative flow and dry air ahead of the wave trough in the middle to upper troposphere. The dynamic and thermodynamic structures of the two groups of precursor waves were also examined using the ERA-Interim data.