Extended Abstract (348K)1B.7
Origin and propagation of a disturbance associated with an Africa Easterly wave as precursors of Hurricane Alberto (2000)
Yuh-Lang Lin, North Carolina State University, Raleigh, NC; and K. E. Robertson and C. M. Hill
Based on the analysis of METEOSAT satellite imagery, we have found at least 17 of 38 tropical cyclones that formed over the eastern Atlantic Ocean were triggered by mesoscale convective complexes (MCCs) that originated near the Ethiopian Highlands (EH) during the period of 1990 to 2001. In this study, we extend upon the findings of Hill and Lin (2003 GRL) to show that the pre-“Alberto” (2000) MCC originated from the EH. In addition, we also show that the pre-“Alberto” MCC and MV were embedded in a wave-like disturbance, which may be classified as an African Easterly Wave (AEW). Based on the cloud top area and brightness observed from satellite imagery, seven stages of the convective lifecycle of the pre-“Alberto” MCC/MV disturbance are identified, which include 4 genesis and 3 lysis stages. Water vapor was found to be the most essential ingredient for controlling this lifecycle of the pre-“Alberto” MCC/MV disturbance, except in the Genesis-I and Lysis-III stages where orography was more influential.
We have found that pre-“Alberto” MCC/MV disturbance exhibited characteristics of an AEW. At the EH, there existed 2 modes of MCC, namely a stationary mode and a propagating mode. The stationary mode generated new convective systems with a period of about 2 to 3 days. One can easily identify 2 to 3 regions of MCC, MV, or cyclonic circulation embedded concurrently within the mid-tropospheric flow over the African continent. These AEWs had an average westward propagation speed of about 11.6 m/s. 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 beta-plane approximation, have been performed to prove that easterly waves can be generated through influence of the EH.
Session 1B, CONVECTION, waves, and precipitation I
Monday, 3 May 2004, 8:30 AM-10:15 AM, Napoleon I Room
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