Comparison of Saharan Dust Surface Mass Observations and Lidar in Miami, FL, to the MERRA2 Reanalysis

Saharan dust is the dominant aerosol in summertime southern Florida, peaking climatologically in July. Daily filter-based dust concentrations, at times size-resolved, and micropulse lidar-derived dust vertical structure from June-September of 2014-2016, Miami, Florida, are compared to the dust representation in NASA’s Modern-Era Retrospective analysis for Research and Application-Version 2 (MERRA2). Observed surface dust loadings reaching 80 µg^m-3 occur episodically, with some dust present at almost all times, indicating a consistent aerosol presence within the marine boundary layer. MERRA2, through its assimilation of satellite and surface-based aerosol optical depth, adequately captures day-to-day Saharan dust variability. The daily MERRA2 near-surface dust mass concentrations are highly correlated with the in-situ measurements, but overestimated. The MERRA2 dust mass is overestimated for diameters between 2.0 to 6.0 µm, and underestimated for particles with diameters between 6.0 to 20 µm and less than 2.0 µm compared to size-resolved measurements. This is thought to reflect MERRA2’s parameterized size-distribution which is based on data gathered near dust sources, while the Miami dust observations are made on dusty air masses that have undergone deposition. Scanning Electron Microscopy (SEM) reveals some particles exceed 20 microns, which is the maximum diameter accommodated by the MERRA2 dust size. MERRA2 surface-layer sea salt mass concentrations are also overestimated, despite realistic wind and relative humidity within the reanalysis. MERRA2 places most dust below 5 km altitude, while lidar observations place most of the free-tropospheric dust somewhat lower, below 3 km. A better understanding of the MERRA2 dust representation, which is used to initialize dust transport forecasts, will increase confidence in reanalysis-derived dust impact studies and subsequent forecasts.