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

2003; American Geophysical Union; Volume: 108; Issue: D19 Linguagem: Inglês

10.1029/2003jd003510

2156-2202

Jeffrey S. Reid, Hal Maring,

Aeolian processes and effects

[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 that in July the midvisible optical depth in the Caribbean can vary from 0.2 to 0.7, with a mean value of ∼0.3 [e.g., Swap et al., 1996]. These findings suggested that Puerto Rico, in the Leeward Islands of the Caribbean, is an appropriate location for the scientific investigation of airborne dust from North Africa. The availability of facilities at Naval Station Roosevelt Roads made Puerto Rico an economical choice as well. [11] The PRIDE campaign had two principal objectives. First, participating scientists wished to better understand the nature of dust transport in order to evaluate/validate numerical transport models such as the Naval Research Laboratory Aerosol Analysis and Prediction System (NAAPS) (see http://www.nrlmry.navy.mil/aerosol) and the NCAR Model for Atmospheric Transport and Chemistry (MATCH) [Colarco et al., 2003a, 2003b]. This research included issues relating to how models should parameterize dust production, vertical entertainment, advection, and scavenging. [12] The second principal objective of PRIDE was to investigate the extent to which the microphysical, chemical and optical properties of dust particles need to be known before remote sensing systems can accurately determine dust optical depth and estimate radiative flux. This objective is somewhat different from most column closure experiments where some “agreement” is sought between different in situ and remote sensing systems. It was understood from the initiation of the project that in the case of dust such closure studies would be highly subjective. Hence the purpose of PRIDE was more in line with a general scale and error analysis. [13] The PRIDE intensive operations period spanned 4 weeks from 28 June through 24 July 2001 and included a surface station, two research aircraft (a Piper Navajo and a Cessna 172), the research vessel Chapman, and two Aerosol Robotic Network (AERONET) Sun photometers. Measurements were focused around the principal ground site at Cabras Island (latitude 18.21 N, longitude 65.60 W), located several hundred meters offshore east of the main island of Puerto Rico. There the University of Miami Mobile Aerosol Laboratory and a radiation site from NASA Goddard Space Flight Center were deployed. Studies utilized a number of satellite sensors including MODIS, MISR, CERES, GOES, and AVHRR. Also during a portion of the PRIDE field campaign, the NCAR C-130 operated as part of the PELTI experiment. Dust-related findings from this effort are given by Garrett et al. [2003]. [14] This special section consists of manuscripts related to the PRIDE campaign and touches all facets of airborne dust related research including sources, transport, remote sensing, microphysics, and chemistry. Further work, particularly on dust particle absorption and transport modeling, is ongoing. [15] One aspect of the PRIDE study, somewhat unusual to many, is that PRIDE was almost spontaneously formed by a diverse group of interested scientists in a synergistic manner. The team is grateful to the Office of Naval Research Code 322 and the NASA MTPE Office for working to make this mission happen. We are most grateful and give our sincerest thanks to Steven Schegrud, James Stark, Roger Hahn, and the service men and women attached to Naval Station Roosevelt Roads for their aide and hospitality. In particular, we are grateful to Daniel Eleuterio and the entire staff at North Atlantic Meteorology and Oceanography Detachment, Roosevelt Roads, without whose help this mission would not have been possible.