Influence of the Saharan Air Layer on Hurricane Nadine (2012). Part I: Observations from the Hurricane and Severe Storm Sentinel (HS3) Investigation and Modeling Results

This study uses a model with aerosol-cloud-radiation coupling to examine the impact of Saharan dust and other aerosols on Hurricane Nadine (2012). In order to study aerosol direct (radiation) and indirect (cloud microphysics) effects from individual, as well as all aerosol species, eight different NU-WRF simulations were conducted. In several simulations, aerosols led to storm strengthening, followed by weakening relative to the Ctrl simulation. This variability of the aerosol impact may be related to whether aerosols are ingested into clouds within the outer rainbands or the eyewall. Upper tropospheric aerosol concentrations indicate vertical transport of all aerosol types in the outer bands but only vertical transport of sea salt in the inner core. The results suggest that aerosols, particularly sea salt, may have contributed to a stronger initial intensification, but that aerosols ingestion into the outer bands at later times may have weakened the storm in the longer term. In most aerosol experiments, aerosols led to a reduction in cloud and precipitation hydrometeors, the exception being the dust-only case that produced periods of enhanced hydrometeor growth. The Saharan Air Layer (SAL) also impacted Nadine by causing a region of strong easterlies impinging on the eastern side of the storm. At the leading edge of these easterlies, cool and dry air near the top of the SAL was being ingested into the outer-band convection. This midlevel low equivalent-potential-temperature air gradually lowered toward the surface and eventually contributed to significant cold pool activity in the eastern rain band and in the northeast quadrant of the storm. Such enhanced downdraft activity could have led to weakening of the storm, but it is not presently possible to quantify this impact.

Satellite imagery and products of the 16–17 February 2020 Saharan Air Layer dust event over the eastern Atlantic: impacts of water vapor on dust detection and morphology

On 16–17 February 2020, dust within the Saharan Air Layer (SAL) from western Africa moved over the eastern Atlantic Ocean. Satellite imagery and products from the ABI on GOES-16, VIIRS on NOAA-20, and CALIOP on CALIPSO, along with retrieved values of layer and total precipitable water (TPW) from MIRS and NUCAPS, respectively, were used to identify dust within the SAL over the eastern Atlantic Ocean. Various satellite imagery and products were also used to characterize the distribution of water vapor within the SAL. There was a distinct pattern between dust detection and dust masking and values of precipitable water. Specifically, dust was detected when values of layer TPW were approximately 14 mm; in addition, dust was masked when values of layer TPW were approximately 28 mm. In other words, water vapor masked infrared dust detection if sufficient amounts of water vapor existed in a column. Results herein provide observational support to two recent numerical studies that concluded water vapor can mask infrared detection of airborne dust.

The Discovery of African Dust Transport to the Western Hemisphere and the Saharan Air Layer: A History

CAPSULEOver fifty years ago, African dust was serendipitously discovered in the Caribbean in three separate efforts one of which led to a fundamental understanding of the phenomenon.

When Does the Saharan Air Layer Impede the Intensification of Tropical Cyclones?

We investigate the circumstances under which the Saharan air layer (SAL) has a negative impact on the intensification of tropical cyclones (TCs) over the North Atlantic Ocean. Using hurricane tracking, aerosol optical depth (AOD) data, and meteorological analyses, we analyze the interaction of the SAL with 52 named TCs that formed over the east and central Atlantic south of the Cape Verde islands between 2004 and 2017. Following the categorization of negative SAL influences on TC intensification by Dunion and Velden, only 21% of the investigated storms can be classified (28% of all storms that encountered the SAL), and 21% of the storms continue to intensify despite the presence of the SAL. We show that among TCs that encounter the SAL, there is evidence supporting a weak negative correlation between the magnitude of TC intensification and the ambient AOD. However, above-average Saharan dust abundance in the vicinity of TCs is not a good independent indicator for storm nonintensification. To better understand the specific processes involved, a composite study is carried out, contrasting storms that intensify in the presence of the SAL against those that do not. We find that sheared air masses on the north side and drier air from the northeast of the storm early on during its lifetime, in addition to higher AOD, are associated with TC nonintensification in proximity to the SAL.

Satellite Imagery and Products of the 16–17 February 2020 Saharan Air Layer Dust Event over the Eastern Atlantic: Impacts of Water Vapor on Dust Detection and Morphology

On 16–17 February 2020, dust within a Saharan Air Layer (SAL) from western Africa moved over the eastern Atlantic Ocean. Satellite imagery and products from ABI on GOES-16, VIIRS on NOAA-20, and CALIOP on CALIPSO along with retrieved values of layer and total precipitable water (TPW) from MiRS and NUCAPS, respectively, were used to identify dust within the SAL over the eastern Atlantic Ocean. Use of various satellite imagery and products were also used to characterize the distribution of water vapor within the SAL. There was a distinct pattern between dust detection and dust masking and values of precipitable water. Specifically, dust was detected when values of layer or TPW were approximately 14 mm; in addition, dust was masked when values of layer or TPW were approximately 28 mm. In other words, water vapor masked infrared dust detection if sufficient amounts of water vapor existed in a column. Results herein provide observational support to two recent numerical studies that concluded water vapor can mask infrared detection of airborne dust.

Is aerosol optical depth a good metric to map dust properties? Lessons learned from AER-D

<p>The main observable quantity used on a global scale to map aerosols is aerosol optical depth (AOD), from ground-based and satellite remote sensing. It is at the same time an optical property and a vertically integrated quantity, and it is commonly used as the main metric towards which to pull aerosol models, through data assimilation, verification, and tuning. Here we introduce a few reflections on how to better constrain our knowledge of the Saharan Air Layer and its associated mineral dust, following results from the AER-D campaign.</p><p>AER-D was a small field experiment in the Eastern Atlantic during August 2015, based on the opportunity given by the simultaneous ICE-D experiment. The purpose of AER-D was to investigate the physical properties of the Saharan Air Layer, and to assess and validate remote sensing and modelling products. The FAAM atmospheric research aircraft was used as a flying laboratory, and it carried a full set of instruments aimed at both in-situ sampling and remote sensing.</p><p>A broad distribution of particle sizes was consistently observed, with a significant giant mode up to 80 µm, generally larger than what was observed in previous experiments: we ascribe this to the set of instruments used, able to capture the full spectrum. We will discuss the representation of the particle size in operational models, and we will show that despite predicting an extinction coefficient of the correct order of magnitude, the particle size is generally underestimated. We will also discuss the implication of the giant particles for the ground-based remote sensing of columnar size-distributions from the SKYNET and AERONET networks (Sunphotometer Airborne Validation Experiment, which was a component of AER-D).</p><p>We will present the vertical structure of the Saharan Air Layer, and in particular one episode when the structure was very different than the one generally accepted in the conceptual model. Moreover, the comparison with the operational models showed that they can predict a correct aerosol optical depth (AOD, a vertically integrated quantity) despite missing the vertical distribution.</p><p>These findings lead to a series of reflections on how to better constrain our knowledge of the Saharan Air Layer and its representation in operational models. Size-resolved properties and the vertical distribution are essential companions of the global AOD observations commonly used operationally. We will also discuss objectives and ideas for future field experiments.</p>