The maintenance and propagation of African easterly waves across northern Africa a case study and analysis of the environment /

by Robertson, Katie Ellen

ROBERTSON, KATIE ELLEN. The maintenance and propagation of African easterly waves across northern Africa: A case study and analysis of the environment (Under the direction of Dr. Yuh-Lang Lin) This research focuses on the formation and maintenance of an African Easterly Wave (AEW) that developed over Northern Africa in 2000. Specifically, the pre-development period of Hurricane Alberto that initially formed over the Ethiopian Highlands (EH) region is studied in conjunction with this AEW disturbance. Based on EUMETSAT infrared (IR) satellite imagery and ECMWF 0.5o observational model (EOM) analysis data, we identified four convective genesis periods (G-I, G-II, G-III, and G-IV) and three lysis periods (L-I, L-II, and L-III). Based on the EOM analysis of the pre-Alberto system, we found that moisture and vertical velocity played a major role in the generation and maintenance of the convective cycles associated with the pre-Alberto system as it traveled westward across Northern Africa. Initially, orographic effects induce vertical velocity and moisture, and as the Alberto system travels westward these parameters are governed more by environmental factors. The National Center for Atmospheric Research (NCAR) Regional Climate Model Version 3.0 (henceforth RegCM3) was used to run the control case and several sensitivity simulations. We found that the EH is important for focusing and organizing AEW features by producing a consistent stationary wave mode, generating lee side vorticity, and initially providing sufficient vertical moisture flux for convection. Planetary boundary layer (PBL) effects over the EH cause the initial convection in response to diurnal diabetic heating. As this convection propagates downstream, it merges with the orographically generated mesovortex (MV) on the lee of the EH and a mesoscale convective complex (MCC) results. PBL effects and moisture availability are important for propagating and maintaining AEW disturbances as they travel to the west and supporting sufficient moisture by allowing for surface moisture fluxes to affect these systems and aid in convective development. The AEW is slightly modulated by orography downstream from the EH region. Both the EH region and the PBL effects are essential in the proper development and propagation of the AEW and its subsequent disturbances.