Vertical profiling of convective dust plumes in southern Morocco during SAMUM

Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May-June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9 degrees N, 6.9 degrees W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100-400 m. Most of the plumes (typically 50-95%) show top heights < 1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3-4 km. Height-to-diameter ratio is mostly 2-10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5-26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (< 0.3) favours plume evolution. Observed surface, 1 and 2-m air temperatures indicate that a difference of 17-20 K between surface and 2-m air temperature and of 0.9-1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2-7 m s(-1).