Session 4.6 Origin and propagation of a disturbance associated with an African Easterly Wave as a precursor of Hurricane Alberto (2000)

Monday, 21 June 2004: 4:45 PM
Yuh-Lang Lin, North Carolina State University, Raleigh, NC; and K. E. Robertson and C. M. Hill

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In this study, we propose that a mesoscale convective complex (MCC) and a mesovortex (MV), associated with a tropical disturbance that would become Hurricane Alberto (2000), were embedded within a wave-like disturbance over Northern Africa. The wave-like disturbance we observe may be classified as an African easterly wave (AEW). Based on the cloud top area and brightness values observed from METEOSAT-7 infrared satellite data, 4 genesis and 3 lysis stages are identified within a cycle of moist convection associated with the pre-Alberto disturbance. The availability of water vapor is the most essential factor controlling the convective cycle of the pre-Alberto disturbance over the African continent. The presence of significant topography contributes to the generation or decay of the associated MCCs through regulation of the water vapor supply. Analysis of satellite imagery also reveals that the incipient disturbances for 23 of 34 eastern Atlantic tropical cyclones originated from the Ethiopian Highlands (EH) region. We have found that the pre-Alberto disturbance exhibited characteristics of an AEW. At the EH, there existed 2 modes of disturbance development: a stationary mode and a propagating mode. The stationary mode corresponded with the generation of new convective systems over the EH with a period of about 2 to 3 days. These convective systems then propagated westward within an AEW train at an average speed of 11.6 m s-1. The average wavelength was roughly estimated to be about 2200 km. Idealized numerical simulations for uniform, easterly flow over idealized topography representative of the EH, utilizing a รข-plane approximation, have been performed to show that easterly waves can be generated at the EH.
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