MIT radar observations of the evolution of the West African Monsoon during the AMMA IOP

The onset and evolution of the West African Monsoon during the 2006 AMMA Intensive Observation Period (IOP) is studied using ground-based radar, GPCP rainfall, and NCEP reanalysis fields. The Massachusetts Institute of Technology (MIT) radar, funded by NASA, made continuous measurements of precipitation systems (organization and rainfall intensity) in the vicinity of Niamey, Niger from July to September 2006. Radar reflectivity data were processed and quality-controlled, and show the evolution of the three-dimensional structure of precipitating systems at ten-minute time intervals.

Analysis of radar observations suggest that most of the rainfall in Niamey during the AMMA IOP was associated with large convective systems, observed to begin in the second week of July as the monsoon was established. These large systems were organized as squall line mesoscale convective systems (SLMCS) with large trailing regions of stratiform rain and propagated westward every 2-3 days. The radar observations also show that during the IOP, SLMCS usually passed through Niamey in the early morning hours. SLMCS observed with the MIT radar in Niamey corresponded well with rainfall events in the GPCP rainfall dataset. In the large-scale, the NCEP Reanalysis 700 mb vorticity fields suggest a strong connection between African easterly waves and the westward propagating SLMCS observed by the radar.