Session 4C.7 Barotropic energy conversion as a predictor of development for NAMMA African easterly waves

Monday, 28 April 2008: 5:00 PM
Palms H (Wyndham Orlando Resort)
Robert S. Ross, Florida State University, Tallahassee, FL; and T. N. Krishnamurti

Presentation PDF (1.1 MB)

NAMMA African easterly waves (AEWs) will be examined in the context of analysis and simple diagnostics using NCEP FNL analyses. A major goal of the 2006 NAMMA field experiment was to better understand why some AEWs develop into depressions and storms, and why some do not develop beyond the wave stage. Seven waves were observed during NAMMA, of which two developed quickly into Debby and Helene within the NAMMA domain, three underwent complex evolutions related to the development of Ernesto, Florence, and Gordon beyond the NAMMA domain, and two were clearly non-developing waves. This research shows that the barotropic energy conversion at 700 hPa was a definitive predictor of wave development. The waves that developed into Debby and Helene both had strong and unambiguous positive barotropic energy conversions. The waves that did not develop immediately within the NAMMA domain, as well as the waves that remained non-developers throughout their life cycles, either had neutral or negative barotropic energy conversions. The barotropic instability of developing waves was found to be very active as the wave developed from the wave to the depression stage. This instability was less important after the depression formed, and presumably not important for the further development of the system into a tropical storm. It is reasonable to view this further development to tropical storm stage as being critically dependent on convective processes. However, the role of dynamics appears to be very important in the earlier stages of development. Satellite images do often show convective clouds in developing waves, but those are generally not organized on the scale of the wave itself.
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