| Description |
AMMA (African Monsoon Multidisciplinary Analyses Experiment) and its NASA extension, NAMMA, provide an unusually high temporal and spatial resolution dataset over northern Africa and the East Atlantic during August and September 2006. Research flights during NAMMA provide an unprecedented opportunity to utilize aircraft data to look at the vertical wind structure of seven consecutive African easterly waves, both developing and nondeveloping, that passed through the experimental domain in the East Atlantic. Two waves developed into Debby and Helene, two waves are unambiguously nondeveloping, and three others are questionable in their role in the development of Ernesto, Florence, and Gordon. NCEP GDAS (Global Data Assimilation System) analyses, which are used to track both vorticity maxima and the mid-level wave trough associated with the waves, supplement and are compared with observational data to determine how well the GDAS analysis resolves the easterly wave wind structure. Results show an important distinction between tracking the large-scale easterly wave trough and vorticity centers within the wave. For NAMMA easterly waves, the vorticity maxima lag the wave trough over the ocean. Observations also show that wave-to-wave, the synoptic-scale circulations and troughs associated with easterly waves vary from well-defined to diffuse, and that the coherence may be impacted by subsynoptic-scale processes such as convection (which is difficult to sample). Often, multiple vorticity maxima are observed with a single easterly wave and the co-location of vorticity maxima with convection may be relevant for tropical cyclogenesis. For NAMMA, easterly waves with well-defined synoptic-scale circulations and troughs are well resolved by the analysis, have limited convection and are not observed to develop. On the other hand, easterly waves with weak synoptic-scale features that have widespread (and in the developing cases, intense) convection are not well handled by the analysis, but are observed to develop. Finally, a comparison of a high resolution WRF simulation of one developing case, Tropical Storm Debby, with aircraft data shows that the simulation lacks a tight, warm core circulation similar to that observed. As with the GDAS analysis, reasons behind the disagreement may include the location of latent heating, error in the initial analysis, the parameterization schemes, or the model resolution. |
