Large-scale tropical experiments such as BOMEX and GATE have provided us with a wealth of data to further our understanding of the structure and dynamics of atmospheric circulations at various scales. Using BOMEX data, Carlson and Prospero (1972) wrote a landmark paper on the structure and evolution of Saharan dust outbreaks that provided us a basic framework for our comprehension of the Saharan air layer (SAL) and its relationship to easterly wave disturbances. Later Diaz et al. (1976) extended this study with a detailed examination of the structure and dynamics of the Saharan air layer (i.e., the meteorological counterpart of the Saharan dust layer) with the use of BOMEX data. Subsequently, with the application of GATE data, Karyampudi (1979) studied the Saharan air layer structure and dynamics and presented a three- dimensional schematic of the Saharan dust plume conceptual model. Furthermore, Carlson and Caverly (1977) and Carlson and Benjamin (1980) have exploited the in-situ aerosol and radiation data collected during GATE to further our knowledge of the radiative characteristics of Saharan dust. On the other hand, Burpee (1975) and Reed et al. (19977) have studied the mean characteristics of easterly wave disturbances by applying compositing techniques to GATE data. Using GATE Level IIIB data set (Miyakoda et al., 1982) to initialize the Penn State/NCAR mesoscale model, Karyampudi and Carlson (1988) not only provided further insights on the structure and dynamics of Saharan dust outbreaks but also explained the dynamical influence of the Saharan air layer on the growth of some easterly wave disturbances. Westphal et al. (1988) further extended the study of Karyampudi and Carlson by including the mobilization and transport of Saharan dust from West Africa.
Although a greater understanding of the tropical weather systems involving various scales, ranging from easterly waves, the intertropical convergence zone, cloud clusters and cloud lines to individual cloud elements, has been achieved due to numerous studies conducted with the aid of the GATE data (Houze and Betts, 1981), the fundamental relationship between the SAL and the Saharan dust at various scales including the meteorological processes that are responsible for the dust outbreak and transport is not well understood. Furthermore, it is not clear how well the 3-D Saharan dust plume conceptual model, presented by Karyampudi (1979) based on GATE observations and previous studies and subsequently discussed by Prospero and Carlson (1981), Karyampudi and Carlson (1988) and Westphal et al. (1988), is applicable to individual dust outbreaks. Validation of the Saharan dust conceptual model against independent data sets has not been made due largely to the difficulty in obtaining concurrent spatial measurements of Saharan dust and SAL over the tropical Atlantic. Although sporadic dust observations were made aboard aircraft during flights of opportunity in field campaigns such as GATE, some of the large-scale features of the dust plume such as the dome-shaped structure, the anticyclonic rotation and horizontal and vertical extent of the dust plume including the occurrence of maximum dust concentration presented in the conceptual model are yet to be validated with dust measurements over wider scales. Due to lack of extensive observations, full verification of this conceptual model has been solely limited to the numerical modeling studies (e.g. Karyampudi and Carlson, 1988; Westphal et al. 1988). Fortuitously, extensive lidar observations of Saharan dust collected during NASAUs Lidar In-space Technology Experiment (LITE) in September of 1994 provided an unique opportunity to revisit the Saharan dust plume conceptual model. Using this data set in conjunction with ECMWF and Meteosat data, Karyampudi et al. (1998) have recently completed a thorough and detailed validation study of the Saharan dust conceptual model. These results show that many of the Saharan dust plume features advanced with the use of GATE data are remarkably valid. Therefore, it is the aim of this paper not only to present a historical perspective on the development of the Saharan dust plume conceptual model, but also to elucidate what we have learned since GATE in the context of validating the conceptual model with unprecedented lidar measurements of Saharan dust collected during the recent LITE experiment.