Multi-scale analysis of the 25-27 July 2006 convective period over Niamey: Doppler radar observations and simulations

The present study investigates the multi-scale processes associated with a sequence of convective events that occurred over Niamey (western Niger, 13.5°N, 2.6°E), during the 25-26 July 2006 period. This period corresponds to the active stage of the first intense monsoon surge over Sahel for 2006. It penetrated up to 18°N with a maximum of TCWV greater than 50 mm, and has been at the origin of flooding in the region of Tamanrasset on 26 July. It is preceded by a strengthening of the heat low (1004 hPa on the 24^th of July) allowing the reinforcement of the monsoon flux. This period is also characterized by an African Easterly Wave (AEW) that crossed the considered domain at a mean speed of 10 m s^-1 with a ~ 3000 km wave length.

During this 2-day period, two successive sequences of mesoscale convective systems (MCS) were located ahead of and in phase with the trough of the AEW. They were followed by suppressed or isolated convection behind the trough and in the vicinity of the ridge. The large AMMA-SOP dataset in particular the UHF radar and the MIT Doppler radar in Niamey are used in combination with a low resolution (5 km) cloud-resolving model to understand the convection organization and its interaction with the environment. Several initial and boundary conditions have been tested, but only the simulation starting with the ECMWF AMMA reanalysis succeeds in reproducing the observed features which emphasizes the importance of the initial state.

MCS1 is a very well-organized convective line followed by a transition zone and a stratiform part, whereas the observed southern edge of MCS2 is less organized and associated with a meso-vortex. Both observed and simulated cross-line circulations fit with the 2D conceptual model of fast moving squall line. During the afternoon of the 26 July, the local convection over Niamey resulted from some favorable factors (humidity, CAPE, CIN) that triggered convection while inhibiting factors (capping mid-levels dry layer, vanishing low-level wind shear pointing to the north, anticyclonic curvature of the streamlines at 700 hPa) prevented it to organize itself and to propagate. In particular, the low level wind shear seems of critical importance. The wind change associated with the AEW passage generates unfavorable shear conditions behind the trough.