Radar observations of convective system variability and interaction with African easterly waves in the West African Sahel region
Rosana Nieto-Ferreira, East Carolina University, Greenville, NC; and T. M. Rickenbach, N. Guy, and E. R. Williams
The African Monsoon Multidisciplinary Activities (AMMA) Intensive Observational Period took place during the summer of 2006 providing a wealth of information about the onset and development of the monsoon season over the Sahel region of West Africa. Surface radar observations near Niamey, Niger during AMMA documented the structure, motion, and precipitation of cloud systems during the monsoon season for the first time in that part of the Sahel. In this study, these unique observations for that part of the Sahel were combined with Global Precipitation Climatology Project (GPCP) satellite rain estimates, National Center for Environmental Prediction (NCEP) reanalysis winds, and sounding data to study the interaction between convective systems and African easterly waves (AEW).
The 2006 monsoon season was in the average range (within one standard deviation of the mean) with respect to the climatology of precipitation and AEW activity in the Sahel, and thus convective system activity may be considered representative of the climatology. A total of 28 squall line mesoscale convective systems (SLMCS) passed through Niamey between July-September 2006. According to both radar and GPCP estimates, these westward moving systems were the most important rainmakers in Niamey producing about 90% of the rainfall despite being present only about 17% of the time. Convective-stratiform partitioning showed that although convective rain occupied only 12% of the total SLMCS area, it accounted for 55% of the total rainfall associated with these systems. The stratiform rain fraction was higher later in the season during Aug-Sep when more and stronger AEW crossed 2.5°E, thereby producing synoptic scale dynamic forcing that favored stratiform rain production. Infrared imagery and a composite of GPCP precipitation suggested that these westward propagating systems originated over elevated terrain in southern Niger hundreds of kilometers to the east of Niamey. Consistent with the early morning time of arrival of SLMCS at Niamey, radar observations indicated a strong early morning peak in the Niamey diurnal cycle of rainfall. Local isolated convection modulated by afternoon heating produced a weaker rainfall peak in the afternoon, leading to an overall bimodal diurnal rainfall variation.
NCEP Reanalysis 700 mb relative vorticity, wind, and GPCP rain composites combined with the analysis of an SLMCS case suggested that the SLMCS may act to strengthen the African easterly jet (AEJ) in a positive feedback. Accelerating rear inflow behind the squall line leading edge strengthened the AEJ, which both increased low-level wind shear and enhanced positive vorticity in the AEW trough. Thus the AEW and SLMCS are coupled in that the stronger AEJ, enhanced by SLMCS rear inflow, augments SLMCS formation and maintenance. Detailed case studies and model simulations of SLMCS events are needed to further explore this hypothesis.
Session 5, Monsoons - II
Tuesday, 13 January 2009, 11:00 AM-12:00 PM, Room 129A
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