AMMA: Convection in an African Easterly Wave over West Africa and the eastern Atlantic
Dr. J. Schwendike, Prof. Dr. S. Jones
- Project Group:
Weather Systems: Modeling and hazard analysis
The interaction between African easterly waves (AEWs) and convection are investigated by addressing the questions how does the AEW provide a favourable environment for the development of convection, and which role does the convection play in modifying the AEW. Over land the convection takes the form of mesoscale convective systems (MCS) which move across West Africa, playing a key role in the water cycle of West Africa. A number of MCSs may develop and decay within an AEW over the continent. Over the Atlantic, the organised convection within the AEW may develop into a tropical cyclone.
Convective systems over West Africa and the eastern Atlantic embedded in the African Easterly Wave out of which Hurricane Helene (2006) developed are investigated using the COSMO model with a horizontal resolution of 2.8 km. Our analysis focuses on the period between 09-14 September 2006. The model code was adapted to allow for budget calculations of the potential temperature, relative humidity, momentum, relative and potential vorticity. Based on the budget calucations, the convective systems over land and the ocean are characterised and their differences analysed.
A dust outbreak occurred on 9 September in the afternoon hours over the Sahara and a second one in Senegal on 11 September in the evening hours. The dustloaden air is transported westward by the AEW. High SAL values occur especially in the north and west of the convective system over the eastern Atlantic out of which Hurricane Helene developed. During its whole genesis high values of SAL are persent. When Helene is a mature Hurricane, dry air can be detected close to the storm's centre. COSMO-ART is used to analyse the effect of the SAL on the environment of the convective systems and the interaction between the convective systems and the AEW. Trajectory calculations are conducted to analyse the mineral dust transport.
Schwendike, J., and S.C. Jones, 2009: Convection in an African Easterly Wave over West Africa and the eastern Atlantic: a model case study of Helene (2006). Accepted for the AMMA Special Issue of the Q. J. R. Meterol. Soc..
Figure 1: Hurricane Helene on 18 September 2006. (http://rapidfire.sci.gsfc.nasa.gov/gallery/?2006261-0918/GordonHelene.A2006261.1645.2km.jpg)
Figure 2: Left: Meteosat 8 infrared images (10.8 µm channel) where convective objects are superimposed using shadings of grey above -64°C, orange–red colours between -64° and -82°C, and black below -82°C. Right: COSMO model runs at selected stages of the life cycle of the convective systems at the date and time given above the image. ST denotes the starting time of the simulation, and VT denotes the veriﬁcation time. The vertical integral of cloud water, cloud ice and humidity (kg m−2 ) as well as the horizontal wind (m s−1 ) at 975 hPa based on the 2.8–km COSMO runs are shown.
Figure 3: The dry air of the SAL on 12 September 2006 at 12 UTC is shaded in yellow and orange colours. The tropical depression which will develop into Hurricane Helene occurs over the eastern Atlantic with high values of SAL to the west and north of it. Image courtesy of Jason Dunion (http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal.html).