Characteristics of Tropical Easterly Wave Pouches during Tropical Cyclone Formation

The pre-genesis evolution of wave pouches was examined for 164 named tropical cyclones that originated from zonally propagating tropical easterly waves over the Atlantic during 1989-2010 July-October using the ERA-Interim reanalysis, CMORPH precipitation and Infrared (IR) brightness temperature data. East of 60°W, most wave pouches (~80%) form at 700 hPa first, often extending down to 850 or 925 hPa off the coast of West Africa. By contrast, the majority of the wave pouches (~68%) over the West Atlantic (west of 60°W) form at 850 or 925 hPa first. Wave pouches become more vertically aligned approaching genesis. It was also found that vorticity at 925 hPa intensifies faster than that at 600 hPa. A warm-core structure forms at the meso-β scale near the pouch center prior to genesis but is less well defined at the meso-α pouch scale. The evolution of precipitation, IR brightness temperature and the low-level convergence suggests that convection begins to organize near the pouch center about one day prior to genesis, along with the rapid intensification of vorticity in the inner pouch region. The composites derived from the ERA-Interim reanalysis show that the inner pouch region has higher specific humidity and equivalent potential temperature, especially in the middle troposphere within one day prior to genesis. The analysis of the SSMI data is also carried out to examine the column moistening preceding the transition to sustained deep convection near the pouch center.