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Initiation of deep convection in the Sahel in a convection-permitting climate simulation for northern Africa

Initiation of deep convection in the Sahel in a convection-permitting climate simulation for northern Africa

Maurer, V., Bischoff-Gauß, I., Kalthoff, N., Gantner, L., Roca, R. and Panitz, H.-J.


Q.J.R. Meteorol. Soc., 143, 806–816, doi:10.1002/qj.2966

Datum: 2017

A convection-permitting climate simulation was performed for Northern Africa for the year of 2006 and a domain of 2561 × 1721 × 50 grid points. The general performance of the simulation was evaluated for the Sahel region for July and August, where the land-surface–atmosphere feedback has a strong influence on convection initiation. The simulation showed reasonable skill.

A tracking algorithm was applied to identify the initiation locations of mesoscale convective systems (MCSs). The aim of the investigation was to distinguish the different processes of MCS initiation during the afternoon and particularly to assess the influence of spatial land-surface anomalies on convection initiation. On average, for all initiations considered, a clear spatial signal of the ratio of sensible to latent heat flux at the land surface (Bowen ratio), indicative of a land-surface influence, was not detected. Only when certain initiations were selected using objective as well as subjective criteria did a signal of the land surface become apparent. The largest number of afternoon initiations in the Sahel could be attributed to convergence caused by gust fronts of approaching systems (35%). Still, almost 16% of the initiations that were identified were preferentially downstream from a region with an elevated Bowen ratio. About 9% were regenerated systems and 19% of the MCSs were not assigned to one of these classes, including systems that formed by the merging of smaller cells developing at multiple locations in a region of large-scale favourable conditions, e.g., due to large-scale convergence.

The results confirm the importance of local convection initiation downstream of spatial land-surface anomalies in the Sahel. They also highlight the mechanism of convection initiation by cold pools and related gust fronts.