Abstract
Saharan dust loading in the Southeast United States has large daily, seasonal, and interannual variations capable of affecting air quality, human health, atmospheric radiative properties, and tropical cyclone development. The in situ dust measurements taken at the Rosenstiel School of Marine and Atmospheric Science (RSMAS), located at 25.76N, 80.19W, span 1974 to the present and are typically taken daily during the Miami Saharan dust season, May through September. This long-term record is sufficient for assessing Saharan dust variability on daily to decadal scales, despite being a point source.
Saharan dust is present at in at least trace amounts each day of boreal summer at Miami, with the majority of dust arrives in a few large events each year. The dust arrives in varying ‘layered’ concentrations between the surface and 5km, with the highest dust density at 2km. The majority of arriving particles are less then 2.5 microns in diameter, however, colossal particles greater than 10.0 microns were present. Miami in situ measurements are compared to the Modern-Era Retrospective analysis for Research and Application-Version 2 (MERRA2) and Goddard Earth Observing Model Version 5 Forward Processing (GEOS-5 FP) dust values. The MERRA2 and GEOS-5 FP reanalyses capture observed daily dust trends, however, modeled surface dust mass is overestimated by an order of magnitude. The overestimation of modeled dust mass is attributed to a skewed size distribution which places more particles in the 2.0-6.0 micron range than seen in measurements.
Saharan dust loading at Miami is highly dependent on the characteristics of dust transport by the lower tropospheric winds due to the vast distance the dusty air must travel and its northern latitude. The in situ dust mass concentration variability is linked to shifts in the North Atlantic Subtropical High (NASH). A southern and western shift in the NASH enhances tropical West Atlantic zonal wind and Florida Peninsula meridional winds which results in fast and efficient transport of dust from the Sahara to Miami. A dust-transport-efficiency index (DTE) is defined using the two key wind regions (i) zonal wind over the Tropical West Atlantic and (ii) meridional wind over the Florida Peninsula. The DTE index is moderately correlated to sub seasonal and seasonal dust mass concentrations at Miami, Florida which may be useful for dust prediction.
The predictability of week-to-week Saharan dust variability at Miami, Florida is addressed utilizing reanalysis and forecast model wind data. The DTE is calculated using the National Centers for Environmental Prediction (NCEP) reanalysis and retrospective forecast winds from the Community Climate System Model version 4.0 (CCSM4). The DTE prediction skill varies by year, with high prediction skill associated with large variability in the tropical winds and low prediction skill for near normal conditions. The CCSM4 retrospective forecasts well represent the summertime North Atlantic meteorology out to week-3 in both the seasonal mean anomalies and seasonal variability. The CCSM4 week-1 forecast predicts dust, by means of the DTE, as successfully as the reanalysis.