Climatology of convective density currents in the southern foothillsof the Atlas Mountains

摘要

Density currents fed by evaporationally cooled air are an important dust storm–generating feature and can constitute a source of moisture in arid regions. Recently, theexistence of such systems has been demonstrated for the area between the High AtlasMountains and the Sahara desert in southern Morocco on the basis of case studies. Here, aclimatological analysis is presented that uses data from the dense climate station networkof the IMPETUS project (An Integrated Approach to the Efficient Management of ScarceWater Resources in West Africa) for the 5 year period 2002-2006. Objective criteriamainly based upon abrupt changes in wind and dew point temperature are defined toidentify possible density current situations. The preselected events are then subjectivelyevaluated with the help of satellite imagery and surface observations to exclude causesfor air mass changes other than moist convective cold pool formation. On average, 11 ±4 density currents per year are detected with the main season lasting from April toSeptember. Density currents occur mainly in the afternoon and evening due to thediurnal cycle of moist convection. Mean changes at the leading edge are increases in2 m dew point temperature and wind speed by 5.4°C and 8.2 m s-1, respectively, and adecrease in 2 m air temperature of 2.3°C. The High Atlas and Jebel Saghro are found tobe the most important source regions, while only a few systems originate over theSaharan lowlands. Labilization of the atmosphere due to upper-level troughs overnorthwest Africa and an enhanced moisture content favor density current formation. Inaddition, detailed case studies representative of different density current types arepresented.