African Easterly Wave Evolution and Tropical Cyclogenesis in a Pre‐Helene (2006) Hindcast Using the Model for Prediction Across Scales‐Atmosphere (MPAS‐A)

Abstract

Abstract Tropical cyclogenesis (TCG) remains an elusive phenomenon partly due to the limited understanding of complex water vapor‐convection‐wave interactions. The Model for Prediction Across Scales‐Atmosphere (MPAS‐A) was used to study the TCG of the African easterly wave (AEW) that became Hurricane Helene (2006). The two main objectives were: (a) evaluate the capability of MPAS‐A to simulate TCG from an AEW by comparing MPAS‐A—initialized with the Integrated Forecasting System (IFS) and the Global Forecast System (GFS)—with observations together with reanalysis and, (b) use the hindcast to investigate the role of moisture in the mechanisms that led to Helene's TCG. The more intense GFS‐initialized pre‐Helene was slower propagating and was associated with a wetter and stronger monsoon when compared to both the IFS‐initialized simulation and observed. TCG occurred when net moisture flux within the boundary layer toward the center of the wave increased persistently. The reanalysis pre‐genesis top‐heavy vertical mass flux profile transitioned to a bottom‐heavy profile during TCG, whereas the simulations had top‐heavy and bottom‐heavy profiles simultaneously, resulting from a more‐intense and fast‐occurring TCG than in the reanalysis. Moisture‐vortex instability helped explain the vertical mass fluxes and the co‐location of convection, moisture and wave vortex demonstrating to be an applicable theoretical model for TCG. Moisture mode was tested as a diagnostic tool for AEW evolution and TCG. The case exhibited some moisture mode properties, and it is proposed that AEWs become more moisture‐mode like once reaching western Africa and during TCG. An AEW TCG pathway is proposed.

Keywords

• tropical cyclogenesis
• African easterly wave
• MCSs/convection
• moisture mode
• MPAS
• water vapor