Structure and Evolution of Developing and Non-developing African Easterly Waves During National Aeronautics and Space Administration African Monsoon Multidisciplinary Analyses (NAMMA)

A modeling and data impact study was performed using the NOAA AOML/HRD experimental Hurricane Weather Research and Forecasting (HWRFx) model with the aim to find distinguishing factors to better discriminate between possible developing and non-developing African Easterly Waves (AEWs) into tropical cyclones (TCs). Two AEWs from the 2006 hurricane season were examined. One AEW was the precursor of Hurricane Helene (HAEW). The second AEW preceded Helene (NDAEW), however it failed to intensify into a TC.;The effect of convection, nesting and initial conditions in the forecast of the AEWs were evaluated. New initial conditions were produced by the Hurricane Ensemble Data Assimilation System (HEDAS) and had assimilated dropwindsonde data gathered in NAMMA not ingested into NOAA/NCEP models in real time. Nested simulations had a static parent domain with grid spacing of 9 km and higher resolution-moving nest of 3 km. Un-nested simulations were run with the 9-km static parent domain. Physics options applied to the model were semi-operational or aligned as close as possible to the operational HWRF.;The HWRFx implicit convection in the 9-km domain and the 3-km domain nesting capability had a strengthening effect on the AEWs. Assimilation of additional dropwindsondes data for the NDAEW case greatly improved the initial state of the model and produced a more accurate forecast while the HAEW case demonstrated that additional data sometimes could result in intensity forecast degradation. A detailed analysis of the dynamic-thermodynamic evolution of the environments and structures of the two AEWs provided favorable and un-favorable conditions for TC development and showed that the cyclogenesis process occurs from approximately 700 hPa to the surface or the lower troposphere. A distinctive signal for a developing system consisted on the formation of a relative vorticity (RVORT) core at the mean of the locations of wind speed minima (MLWSM) and extending from the surface to at least 850 hPa. The core of RVORT did not coincided with the location of the RVORT maximum during cyclogenesis. This emphasizes the importance of evaluating the RVORT structure horizontally and vertically in order to accurately predict cyclogenesis.