Validation of the Saharan dust plume conceptual model using lidar, Meteosat, and ECMWF data

Lidar observations collected during the Lidar In-space Technology Experimen t experiment in conjunction with the Meteosat and European Centre for Mediu m-Range Weather Forecasts data have been used not only to validate the Saha ran dust plume conceptual model constructed from the GARP (Global Atmospher ic Research Programme) Atlantic Tropical Experiment data, but also to exami ne the vicissitudes of the Saharan aerosol including their optical depths a cross the west Africa and east Atlantic regions. Optical depths were evalua ted from both the Meteosat and lidar data. Back trajectory calculations wer e also made along selected lidar orbits to verify the characteristic anticy clonic rotation of the dust plume over the eastern Atlantic as well as to t race the origin of a dust outbreak over West Africa. A detailed synoptic analysis including the satellite-derived optical depths , vertical lidar backscattering cross section profiles, and back trajectori es of the 16-19 September 1994 Saharan dust outbreak over the eastern Atlan tic and its origin over West Africa during the 12-15 September period have been presented. In addition, lidar-derived backscattering profiles and opti cal depths were objectively analyzed to investigate the general features of the dust plume and its geographical variations in optical thickness. These analyses validated many of the familiar characteristic features of the Sah aran dust plume conceptual model such as (i) the lifting of the aerosol ove r central Sahara and its subsequent transport to the top of the Saharan air layer (SAL), (ii) the westward rise of the dust layer above the gradually deepening marine mixed layer and the sinking of the dust-layer top, (iii) t he anticyclonic gyration of the dust pulse between two consecutive trough a xes, (iv) the dome-shaped structure of the dust-layer top and bottom, (v) o ccurrence of a middle-level jet near the southern boundary of the SAL, (vi) transverse-vertical circulations across the SAL front including their poss ible role in the initiation of a squall line to the southside of the jet th at ultimately developed into a tropical storm, and (vii) existence of satel lite-based high optical depths to the north of the middle-level jet in the ridge region of the wave. Furthermore, the combined analyses reveal a complex structure of the dust p lume including its origin over North Africa and its subsequent westward mig ration over the Atlantic Ocean. The dust plume over the west African coastl ine appears to be composed of two separate but narrow plumes originating ov er the central Sahara and Lake Chad regions, in contrast to one single larg e plume shown in the conceptual model. Lidar observations clearly show that the Saharan aerosol over North Africa not only consist of background dust (Harmattan haze) but also wind-blown aerosol from fresh dust outbreaks. The y further exhibit maximum dust concentration near the middle-level jet axis with downward extension of heavy dust into the marine boundary layer inclu ding a clean dust-free trade wind inversion to the north of the dust layer over the eastern Atlantic region. The lidar-derived optical depths show a r apid decrease of optical depths away from land with maximum optical depths located close to the midlevel jet, in contrast to north of the jet shown by satellite estimates and the conceptual model. To reduce the uncertainties in estimating extinction-to-backscattering ratio for optical depth calculat ions from lidar data, direct aircraft measurements of optical and physical properties of the Saharan dust layer are needed.