Unexpected vertical structure of the Saharan Air Layer and giant dust particles during AER-D

Abstract

Abstract. The Saharan Air Layer (SAL) in the summertime eastern Atlantic is typically well mixed and 3–4 km deep, overlying the marine boundary layer (MBL). In this paper, we show experimental evidence that at times a very different structure can be observed. During the AERosol properties – Dust (AER-D) airborne campaign in August 2015, the typical structure described above was observed most of the time, and was associated with a moderate dust content yielding an aerosol optical depth (AOD) of 0.3–0.4 at 355 nm. In an intense event, however, an unprecedented vertical structure was observed close to the eastern boundary of the basin, displaying an uneven vertical distribution and a very large AOD (1.5–2), with most of the dust in a much lower level than usual (0.3–2 km). Estimated dust concentrations and column loadings for all flights during the campaign spanned 300–5500 and 0.8–7.5 g m−2, respectively. The shortwave direct radiative impact of the intense dust event has been evaluated to be as large as -260±30 and -120±15 W m−2 at the surface and top of atmosphere (TOA), respectively. We also report the correlation of this event with anomalous lightning activity in the Canary Islands. In all cases, our measurements detected a broad distribution of aerosol sizes, ranging from ∼0.1 to ∼80 µm (diameter), thus highlighting the presence of giant particles. Giant dust particles were also found in the MBL. We note that most aerosol models may miss the giant particles due to the fact that they use size bins up to 10–25 µm. The unusual vertical structure and the giant particles may have implications for dust transport over the Atlantic during intense events and may affect the estimate of dust deposited to the ocean. We believe that future campaigns could focus more on events with high aerosol load and that instrumentation capable of detecting giant particles will be key to dust observations in this part of the world.

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

“…As a result of these developments, observational campaigns have now shown that coarse and giant dust particles are far more prevalent, and transported further and higher than previously thought. Fennec, the Saharan Mineral Dust Experiment 1 20 (SAMUM1), Saharan Mineral Dust Experiment 2 (SAMUM2), Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE), AERosol Properties -Dust (AER-D) and Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) have all reported a significant presence of coarse to giant dust particles, despite the sampling locations of Saharan dust ranging from very close to sources to thousands of kilometres away Weinzierl et al, 2011;Ryder et al, 2013b;Ryder et al, 2018;Denjean et al, 25 2016;Marenco et al, 2018).…”

Models transport Saharan dust too low in the atmosphere compared to observations

“…Flights B923 and B924 both took place on the 12 th August flying between Praia and Fuerteventura to sample the outflow from a dust uplift event that had happened on the 10 th August in Northern Mali. These flights were able to reach the main dust plume, which means that highest AOD's and extinction coefficients of the campaign were measured on this day (Marenco et al, 2018). The 335 two flights sampled the same plume at different times during the day, and only B923 is shown here as results for flight B924 are similar.…”

“…AERosol properties -Dust (AER-D) was a collaborative campaign led by the Met Office in collaboration with the Universities of Reading and Hertfordshire (Marenco et al, 2018). It was held at the same time as the Ice in Clouds Experiment -Dust (ICE-D), a larger collaborative campaign involving the Met Office, the, National https://doi.org/10.5194/acp-2020-57 Preprint.…”

“…In addition to vertically-resolved information, we also highlight the importance and need for better constrained size-resolved properties of dust, needed to reproduce the correct relationship between concentration and extinction coefficient. Particle size-distributions, both in the model representation and in the observations, should cover the whole size spectrum, including the giant mode (Marenco et al, 2018;Ryder et al, 2019). Ideally, these 585 observations should be coordinated, vertically resolved, and established across a number of locations downstream from sources, e.g.…”

The Universal Cloud and Aerosol Sounding System (UCASS): a low-cost miniature optical particle counter for use in dropsonde or balloon-borne sounding systems

“…(c) Effective diameter with respect to altitude, the embedded cloud layer at 4 km is evident due to the steep increase in effective diameter. The blue trace shows raw data, whilst the orange trace shows smoothed data using a 10 point moving average.3.1.2 Upsonde systemThe UCASS was used during the AERosol properties -Dust (AER-D) & Sunphotometer Airborne Validation EXperiment -Dust (SAVEX-D) campaigns over Cape Verde during August 2015(Marenco et al, 2018). A UCASS was launched as part of a balloon-based system in tandem with a GRAW DFM-09 during a Saharan dust event on the 25 th August, from Instituto Nacional De Meteorologia E Geofisica (INMG), Espargos, Sal island.…”