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Characteristics of Tropical Easterly Wave Pouches during Tropical Cyclone Formation

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  • 1 Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois
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Abstract

The pregenesis evolution of wave pouches was examined for 164 named tropical cyclones that originated from zonally propagating tropical easterly waves over the Atlantic during July–October 1989–2010 using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) and the Climate Prediction Center (CPC) morphing technique (CMORPH) precipitation. 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 and the low-level convergence suggests that convection begins to organize near the pouch center about 1 day prior to genesis, along with the rapid intensification of vorticity in the inner pouch region. The composites derived from ERA-Interim show that the inner pouch region has higher specific humidity and equivalent potential temperature, especially in the middle troposphere within 1 day prior to genesis.

Corresponding author address: Zhuo Wang, Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, 105 South Gregory St., Urbana, IL 61801. E-mail: zhuowang@illinois.edu

Abstract

The pregenesis evolution of wave pouches was examined for 164 named tropical cyclones that originated from zonally propagating tropical easterly waves over the Atlantic during July–October 1989–2010 using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) and the Climate Prediction Center (CPC) morphing technique (CMORPH) precipitation. 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 and the low-level convergence suggests that convection begins to organize near the pouch center about 1 day prior to genesis, along with the rapid intensification of vorticity in the inner pouch region. The composites derived from ERA-Interim show that the inner pouch region has higher specific humidity and equivalent potential temperature, especially in the middle troposphere within 1 day prior to genesis.

Corresponding author address: Zhuo Wang, Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, 105 South Gregory St., Urbana, IL 61801. E-mail: zhuowang@illinois.edu
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