HRD seminar – Ghassan “Gus” Alaka, Colorado State University – 8 September 2014

Gus presented a seminar on “Intraseasonal Variability of African Easterly Wave”.

Abstract

African easterly waves (AEWs) and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer. Since AEWs are seed disturbances to Atlantic tropical cyclones, oscillations in AEW activity in West Africa directly impact tropical cyclogenesis in the Atlantic Ocean. AEWs also provide vital rainfall for self-sustaining agriculturalists in the African Sahel. This study performs an all-encompassing analysis of the 30-90-day variability of AEWs and associated energetics, utilizing both reanalysis products (i.e., ERA-Interim, CFSR) and output from the Weather Research and Forecasting (WRF) model. Specifically, the PKE and perturbation available potential energy (PAPE) budgets are used to understand the factors that contribute to PKE maxima in West Africa and the extent to which these surges of AEW activity are modulated by the Madden-Julian oscillation (MJO). Of course, internal forcing of 30-90-day AEW activity might be important as well, governed by active/break cycles in the monsoon. Thus, sources of PKE are analyzed as a function of an MJO index and a local 30-90-day West African PKE index. While the evolution of PKE is similar in the two indices, the MJO index describes a small (yet still significant) fraction of the local 30-90-day variance. WRF sensitivity tests support this finding, with the intraseasonal power of PKE almost unchanged after the removal of all 30-90-day input into the model boundary conditions. Previous studies have shown that East Africa (e.g., east of Lake Chad) is an initiation zone for AEWs and that increased PKE in this region precedes increased downstream AEW activity on intraseasonal timescales. Reanalysis products reveal that baroclinic energy conversions help initiate intraseasonal AEW activity in East Africa, while barotropic energy conversions have this role in WRF. In West Africa, barotropic energy conversion is vital in both reanalysis and model data. Additionally, the reanalysis products and WRF output agree that the primary role of diabatic heating is to destroy PAPE in a negative feedback to West African baroclinic energy conversions. Unlike in the reanalysis products, however, baroclinic overturning in WRF destroys PKE.

A video recording of the presentation is available on the anonymous ftp site:

ftp://ftp.aoml.noaa.gov/hrd/pub/blog/seminars/2014/Alaka_HRD_seminar_20140908.mp4

Alaka's presentation

Alaka’s presentation

 

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