Hurricane Helene (2006) and the Saharan air layer

The aim of this study is to investigate the structure of the Saharan Air Layer (SAL) and its interaction with Hurricane Helene (2006). A particular focus is placed on the role of the dust radiation feedback. We use the model system COSMO-ART (Aerosols and Reactive Trace gases) in which the dust emission and dust transport as well as the radiation feedback are implemented. With the help of COSMO-ART we analyse the radiative effect of the dust on the convective systems and their environment.

On 9 September 2006, a low-level positive vorticity anomaly associated with the westward extension of the Saharan heat low occurred over West Africa at around 5-4W, 19-21N. It moved along about 18°N, crossed the West African coast line and then moved towards the southwest, where it merged with a positive vorticity maximum associated with the monsoon depression. When this positive vorticity anomaly was collocated with the vorticity maximum of the AEW, out of which Hurricane Helene (2006) developed, the development of the pre-Helene tropical depression was initiated. This secondary heat low circulation led to strong wind speeds near the surface and, thus, to the emission of significant amounts of mineral dust. . Dust was also emitted by the gust fronts of the convective systems over land, and due to orographical effects at the Algerian Mountains. The dust was transported over the Atlantic in the SAL. High values of SAL occurred north and northwest of the convective systems that developed into a tropical depression, and were present in the vicinity of the storm during the whole genesis period of Helene. When Helene was a mature Hurricane, bands of dry air spiralled in towards the centre. They were observed by TPW satellite images and seen in model simulations.

Model runs using COSMO-ART with a horizontal resolution of 28 km were conducted with and without the radiation feedback of the dust. Based on these model runs, the structure of the SAL and its sensitivity to the dust-radiation-dynamics interaction are analysed. The transport of mineral dust is analysed by calculating trajectories. We aim to assess how the dust influences the environment of convective systems embedded in the AEW out of which Helene developed.