The influence of source landforms, antecedent precipitation, and winds on dust events in North Africa

This dissertation identifies conditions associated with dust storms in North Africa. The introductory chapter frames a conceptual model of how atmospheric dustiness is governed by static landforms, dynamic characteristics of the surface (represented by cumulative antecedent precipitation or CAP), and surface wind fields. Visibility at 225 meteorological stations across Africa, north of 10° north latitude, is used as a proxy for atmospheric dust. A map of landforms is created from a multiple endmember mixture analysis of MODIS satellite imagery to represent landforms. The ten landforms identified are alluvial surfaces, dunes, dry lakebeds, water, basaltic surfaces, mountains, regs, bedrock surfaces, sandsheets, and vegetated surfaces. Climatological and event-based statistics are used to determine relationships between parameters related to visibility, landforms, CAP, and wind speed. CAP represents dynamic changes in the erodibility of the surface including the sheltering effect of vegetation and delivery of erodible sediments. Surface wind velocities represent the winds that mobilize fine sediments to become dust. Alluvial surfaces, dunes, dry lakebeds, regs, bedrock, and sandsheets are more often associated with low visibility (<5 km) events than high. Negative correlations between median wind speed and the monthly frequency of visibility less than five kilometers (DEF-5) identify 33 stations where factors other than dust strongly influence the visibility record and these stations are removed. Results regarding the relationship between CAP and dustiness find that at Sahelian and southern Saharan locations, vulnerability to dust emissions is related to drought reducing vegetation cover. At central, northern, and eastern Saharan locations, vulnerability to dust emissions is related to wet conditions, probably as a result of fresh sediment being deposited at erodible locations in the landscape. Wind speed results find that for any dust source, there is an intermediate wind speed with minimum visibility, above which dispersion increases visibility and below which the wind is not strong enough to generate enough dust. Any study using dust measurements (visibility in this case) that are not close to the dust source will be characterizing the dust mobilization as well as the dust transport process so such measurements must be used with caution when trying to characterize dust sources.