Foreword to special section on the Puerto Rico Dust Experiment (PRIDE)

Abstract

[1] One of the more significant difficulties in determining the meteorological and climatic impacts of airborne dust is the development of consistent optical, microphysical, and transport models. Unlike some aerosol species that can be relatively well characterized in terms of size, chemistry, and optical properties, the complicated nature of airborne dust defies easy parameterization or categorization (e.g., see review by Sokolik et al. [2001]). The complexity of dust has forced researchers to make frequent assumptions and simplifications in their research. Often the assumptions of one study contradict those of another. For example, a particular satellite system or model may reproduce the correct optical depth for airborne dust in a given region, while the underlying numerical, microphysics, and radiation algorithms can often be unphysical. More simply stated: We may be deriving “reasonable” answers with the wrong physics. Thus the purpose of the Puerto Rico Dust Experiment (PRIDE) was to better characterize the microphysics, optics, and composition of airborne dust. [2] Being the largest dust source in the world, North Africa has rightfully been the focus of much of the research on dust by the scientific community. Dust from North Africa is frequently transported over the Mediterranean, south subtropical Atlantic Ocean and Caribbean [e.g., Carlson and Prospero, 1972; Ganor and Mamane, 1982; Swap et al., 1996; Moulin et al., 1998; Prospero, 1999]. Monthly average midvisible optical depths from dust on the borders of Africa are frequently above 0.4 [e.g., Holben et al., 2001; Tanre et al., 2001]. With such consistent dust emissions, North Africa and surrounding ocean regions are a very important natural laboratory for studying dust behavior. On the basis of studies in North Africa and adjacent oceanic regions, a number of outstanding scientific issues have been identified. Some of the significant questions include the following. [3] 1. How well do current the conceptual models for dust transport [e.g., Karyampudi et al., 1999] compare to the large volume of currently available observational data? [4] 2. If based on the same meteorology and taken on equal footings, to what extent do the various dust source functions used in the scientific community agree? [5] 3. What are the individual biases for each in situ method used to measure dust properties? [6] 4. The chemical, microphysical, and optical properties of the dust are typically measured independently in different locations. If a complete measurement set is taken, are the measures consistent with our current understanding of airborne dust microphysics and optical properties? [7] 5. Discrepancies exist between in situ measurements, taken mostly in the surface layer, and remotely sensed measurements of microphysical and optical properties such as single scattering albedo. For example, are the satellite and inversion derived values of single scattering albedo from Kaufman et al. [2001] and Dubovik et al. [2002] more representative of dust than the more commonly used values based on Mie Theory and index of refraction values from Patterson [1981]? [8] 6. What is the effect of the nonsphericity of dust particles on dust's radiative properties and hence satellite retrievals of dust optical thickness? Or how much bias is introduced if the spherical particle assumption is used in look-up tables? [9] 7. Can the relative importance of each of these preceding issues be determined for different applications? If so, does this scaling of importance indicate where additional research effort should be expended? [10] The PRIDE field campaign was conducted in July 2000 by a group of United States Navy, NASA and university scientists interested in examining questions such as those listed above. As discussed by Prospero [1999], large quantities of Saharan dust are transported from Africa across the Atlantic into the Caribbean during summer months. An analysis of AVHRR and AERONET data suggests t